Z7 04 Good Practice Through Design, Concrete Supply and Construction Draft

Concrete Durability Series

Durability Implementation Concrete Durability Series

Good Practice Through Design, Concrete Supply and Construction

Z7/04 Good Practice Through Design, Concrete Supply and Construction

Z7/04 First Edition Published 2014

Recommended Practice by Durability Committee


This Recommended Practice is the first edition and has been developed by the Durability Committee and Task Group of the Concrete Institute of Australia with contribution from the below listed. This is the fourth in a series of concrete durability guides.

The principle authors of this recommended practice were: T.Thomas - Boral F.Papworth - BCRC

The Task Group members for this Recommended Practice were: T.Thomas - Boral (Chair) Wolf Merretz - Structural Concrete Industries Gary Jackson – Private Consultant Louie Mazzarolo – DeMartin and Gasparinin Robert Landorf – Ancon Beton Craig Whitaker – Leighton Contractors Graeme Hastie – Private Consultant

Durability Committee active contributors were: Warren Green - Vinsi

Other Contributors included: Pedram Mojarrad - Sika

The CIA would like to acknowledge the valuable contribution of members from comments obtained through the peer review process. Many of the comments have been included directly into this document.


Forward The Durability Series is a set of Concrete Institute of Australia recommended practices that provide deemed to satisfy requirements applicable to all structure types based on standard input parameters for design life, reliability and exposure. The series includes details on project planning and implementation which if followed will increase the likelihood that the specification, design detailing and construction will be optimal to achieving the developer and community expectations regarding the long term performance of structures. Also included are methods for modelling degradation over time and for crack control design. Thus the series provides what is described as a unified durability design process. Prior to around 1970 concrete was generally regarded by asset owners, designers and contractors as a reliable construction material that provided long term durability with relatively little maintenance. Subsequently, premature deterioration of concrete structures, arising from changing cement characteristics, quality management and other factors, damaged this reputation. Because concrete is a complex material, research into the cause of problems and development of appropriate new rules and operational methods has taken a long time. The durability series provides recommendations that if followed will largely eradicate premature deterioration. Whilst research into concrete durability continues the knowledge on exposure significance, deterioration processes, materials properties and workmanship implications has developed significantly over the last 30 years. In addition, new cementitious materials and admixtures have been widely introduced. Much more advanced concretes are now available. New durability design practices have also been developed, including durability modelling methods, and new methods of construction have been introduced. However, to an extent at least, these developments are not fully reflected in a clear and unified manner through the Australian Standards dealing with concrete durability requirements (e.g. modelling methods, use of fly ash, slag and silica fume, use of galvanised and stainless steel). The durability series provides recommendations on durability design using a wider range of concretes and reinforcements, and details how to implement new durability design methods. Durability requirements in Australian Standards are fragmented through different standards and their commentaries dealing with concrete durability requirements for different structure types (e.g. AS2159, AS3735, AS4997, AS5100.5). Perceived conflicts between these documents (e.g. higher covers in AS 3735 than AS3600 for the same life and exposure) might sometimes be explained by the different owner requirements (e.g. reliability required) but reasons for the differences are not given and the associated assessment methods not clearly stated. To some extent the concrete industries energy for contributing to development of durability codes is squandered through maintenance of the multitude of codes that cover the same topic in variable ways. For many concrete elements in mild exposures incorporating the recent durability related developments into a unified durability design process for all structure types may make little difference to their durability design because existing code deemed to satisfy provisions often provide adequate performance. However, for elements in more severe exposures guidelines that comprehensively detail how to assess owner’s needs, environmental exposures and materials requirements; how to specify performance or prescriptive materials properties; and how to ensure construction is appropriate to the design will provide structures that meet their durability requirements more consistently. The durability series provides the required guidelines.

Z7/04 Good Practice Through Design, Concrete Supply and Construction The Concrete Institute of Australia’s Durability Committee was formed in late 2008 to review Z7. In view of the committees perceived need for a broader review of durability requirements it managed workshops around Australia in mid-2009 to review issues with concrete durability practices and standards in Australia. The outcome from these workshops, and other feedback from Concrete Institute of Australia members at the Concrete Institute of Australia National Conference in 2009, was that comprehensive and unified durability guidance was required. In response, the Durability Committee established Task Groups to produce a series of recommended practices as a major revision to Z7 that would form a durability series. The series comprises: 

Z7/01 Durability - Planning



Z7/02 Durability - Exposure Classes



Z7/03 Durability - Deemed to Comply Requirements



Z7/04 Durability - Good Practice Through Design, Concrete Supply and Construction



Z7/05 Durability - Modelling



Z7/06 Durability - Cracks and Crack Control



Z7/07 Durability - Testing

The durability that the owner and community require from structures will only be obtained if specific consideration is given to how durability requirements impacts on construction cost, inspection needs, maintenance requirements, aesthetics and operational and community costs that unplanned maintenance brings. Whist strong emphasis is placed on initial fitness for purpose, durability must be met long into the future, possibly well past the initial design life. The durability series will go a long way to providing the necessary tools for design and construction of durable structures based on the latest understanding of exposure, materials and deterioration process. Frank Papworth Durability Committee Chairman


Preface Australian concrete construction standards more generally focus on minimum design and material requirements and with the exception of a few more detailed “Hand Book” standards are unlikely to provide more informative recommendations about how to design or construct a structure to get the target life expectancy. The Concrete Institute of Australia Durability Series provides the tools for managing durability through design, construction and maintenance. As the title suggests, this document has applicability to more general concrete construction as well as concrete requiring specifically higher levels of durability. Z7/04 provides more specific detail covering areas such as the impact of specifications and the contract process, impacts of design on construction, more detailed view of the materials used in construction, material quality control processes, construction process and supervision as well as some detailing issues in common structural elements that may present potential durability issues to the designer & constructor. In addition to this an appendix section is included on reinforcement spacers and chairs as this is an area that has demonstrated to cause weakness in durable construction and is rarely adequately specified. The designer and durability planner must understand not only the intended design but must understand the material properties and consider how these properties can be delivered during the construction process. There are many elements to this delivery process that impact on the final structures durability and this Recommended Practice Z7/04 provides information that helps to highlight the more critical areas of concern from design detailing through material supply to construction of the structure for all concrete construction stakeholders. The document is intended to inform all parties involved in design and construction about the benefits of durability planning and subsequent control of implementation so they can deliver the expected level of maintenance and life of the structure to the asset owners.

Tony Thomas Z7/04 Task Group Chairman


Table of Contents TERMINOLOGY _________________________________________________________________________________________ 1 1. INTRODUCTION _____________________________________________________________________________________ 2 1.1 1.2

2.

CONTRACTUAL ASPECTS _____________________________________________________________________________ 3 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9

3.

3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12

3.13 3.14 3.15

3.16 3.17

3.18 3.19 3.20

3 3 5 5 6 6 6 6 7

Minimum Cover___________________________________________________________________________________________ 8 3.1.1 Casting Against Ground ____________________________________________________________________________ 8 3.1.2 Special Finishes __________________________________________________________________________________ 8 Tolerances for Constructability _______________________________________________________________________________ 8 Cover and Aggregate Size __________________________________________________________________________________ 9 Cover and Bar or Tendon Size _______________________________________________________________________________ 9 Quality Control of Cover Concrete ____________________________________________________________________________ 9 Testing of the Exposure ___________________________________________________________________________________ 11 Configuration and Congestion of Reinforcement ________________________________________________________________ 11 Dissimilar Types of Metals _________________________________________________________________________________ 14 Member Profiles _________________________________________________________________________________________ 14 Construction Joints _______________________________________________________________________________________ 14 Movement Joints_________________________________________________________________________________________ 15 Waterstops _____________________________________________________________________________________________ 17 3.12.1 Rubber and PVC Waterstops _______________________________________________________________________ 17 3.12.2 Swellable Waterstops _____________________________________________________________________________ 18 3.12.3 Metal Waterstops ________________________________________________________________________________ 18 Joint Fillers _____________________________________________________________________________________________ 18 Sealants _______________________________________________________________________________________________ 19 Large Concrete Elements __________________________________________________________________________________ 20 3.15.1 Maximum Temperature ____________________________________________________________________________ 21 3.15.2 Temperature Differentials Within One Pour ____________________________________________________________ 21 3.15.3 Temperature Differentials Between Pours _____________________________________________________________ 21 Post-Tensioned Ducts And Grouting _________________________________________________________________________ 21 Tie Bars _______________________________________________________________________________________________ 21 3.17.1 Tie Bars ________________________________________________________________________________________ 21 3.17.2 Tie Sleeve Holes _________________________________________________________________________________ 22 Tie Wire _______________________________________________________________________________________________ 23 Dowels ________________________________________________________________________________________________ 23 3.19.1 Dowel Bars _____________________________________________________________________________________ 23 Saw Cuts in Slabs________________________________________________________________________________________ 24

PRE-POUR PLANNING ______________________________________________________________________________ 25 4.1 4.2 4.3 4.4 4.5

5.

Varying Specifications During the Bid and Construction Process ____________________________________________________ Prescription vs Performance Specifications _____________________________________________________________________ Insitu Performance ________________________________________________________________________________________ Design Life ______________________________________________________________________________________________ Reliability _______________________________________________________________________________________________ Impact of High Level Durability Requirements on Construction Contract _______________________________________________ Conflicting Specifications – Low Emissions Concrete _____________________________________________________________ The Use of Third Party Products in Mix Specification _____________________________________________________________ Sharing of Risk ___________________________________________________________________________________________

DESIGN __________________________________________________________________________________________ 8 3.1

4.

Reference Documents _____________________________________________________________________________________ 2 Report Layout ____________________________________________________________________________________________ 2

Owners Requirements ____________________________________________________________________________________ Communication between Constructor, Contractor, Supplier and Designer ____________________________________________ Checks on Design and “Buildability” __________________________________________________________________________ Concrete Supply _________________________________________________________________________________________ Planning Construction_____________________________________________________________________________________ 4.5.1 Weather ________________________________________________________________________________________ 4.5.2 Exposure _______________________________________________________________________________________ 4.5.3 Concrete Delivered Temperature ____________________________________________________________________ 4.5.4 Time to Concrete Discharge ________________________________________________________________________ 4.5.5 Pumping _______________________________________________________________________________________ 4.5.6 Maximum Drop Height _____________________________________________________________________________ 4.5.7 Placing_________________________________________________________________________________________ 4.5.8 Plastic Cracking. _________________________________________________________________________________ 4.5.9 Compaction _____________________________________________________________________________________ 4.5.10 Finishing _______________________________________________________________________________________ 4.5.11 Curing _________________________________________________________________________________________ 4.5.12 Steam Curing ___________________________________________________________________________________

25 25 25 25 26 26 26 27 27 28 29 30 31 34 34 35 36

QUALITY OF CONCRETE _____________________________________________________________________________ 37 5.1

5.2 5.3 5.4 5.5

Permeability ____________________________________________________________________________________________ 5.1.1 Permeability and Water Ingress _____________________________________________________________________ 5.1.2 Permeability and Porosity __________________________________________________________________________ 5.1.3 Permeability and Strength __________________________________________________________________________ Sorptivity _______________________________________________________________________________________________ Diffusion _______________________________________________________________________________________________ Rapid Chloride Permeability Test ____________________________________________________________________________ Resistivity ______________________________________________________________________________________________

37 37 37 38 38 39 39 39

Z7/04 Good Practice Through Design, Concrete Supply and Construction 5.6 5.7

6.

CONCRETE MATERIALS, SUPPLY AND CONSTRUCTION _______________________________________________________ 42 6.1 6.2 6.3 6.4

6.5

7.

7.3 7.4 7.5 7.6

7.7 7.8 7.9

7.10

7.11

7.12 7.13

42 44 45 46 47 47 48 48 49 49 50 50 50 51

Quality of Local Supply ____________________________________________________________________________________ The Concrete Supply Specification___________________________________________________________________________ 7.2.1 Production and Project Assessment __________________________________________________________________ 7.2.2 Normal and Special Class Concrete __________________________________________________________________ Mix Design _____________________________________________________________________________________________ The Concrete Suppliers Quality Plan _________________________________________________________________________ Concrete Ordering _______________________________________________________________________________________ Concrete Batching _______________________________________________________________________________________ 7.6.1 Materials Storage ________________________________________________________________________________ 7.6.2 Batching Plant ___________________________________________________________________________________ 7.6.3 Truck Mixers ____________________________________________________________________________________ 7.6.4 Cement ________________________________________________________________________________________ 7.6.5 Aggregate ______________________________________________________________________________________ 7.6.6 Admixtures______________________________________________________________________________________ 7.6.7 Water __________________________________________________________________________________________ 7.6.8 Batching Sequence _______________________________________________________________________________ Concrete Delivery Time ___________________________________________________________________________________ Interval Between Batches __________________________________________________________________________________ Normal Concrete Properties ________________________________________________________________________________ 7.9.1 Slump _________________________________________________________________________________________ 7.9.2 Air Content _____________________________________________________________________________________ 7.9.3 Setting Time ____________________________________________________________________________________ 7.9.4 Compressive Strength _____________________________________________________________________________ 7.9.5 Tensile Strength _________________________________________________________________________________ 7.9.6 Shrinkage ______________________________________________________________________________________ 7.9.7 Bleed __________________________________________________________________________________________ Special properties ________________________________________________________________________________________ 7.10.1 Modulus Of Elasticity ______________________________________________________________________________ 7.10.2 Creep of Concrete ________________________________________________________________________________ 7.10.3 Resistance to Chloride Ingress ______________________________________________________________________ 7.10.4 Sulphate Resistance ______________________________________________________________________________ 7.10.5 Water Penetrability _______________________________________________________________________________ 7.10.6 Self Compacting Concrete Properties _________________________________________________________________ Management of trial mixes _________________________________________________________________________________ 7.11.1 Transport time and impact on water control ____________________________________________________________ 7.11.2 Prescription Vs Performance ________________________________________________________________________ Floatation of Cast in Voids _________________________________________________________________________________ Cold Joints _____________________________________________________________________________________________

52 52 53 53 54 55 55 55 56 56 56 56 57 57 57 60 61 62 62 62 63 63 64 64 65 66 67 67 67 68 68 68 68 68 69 69 69 69

REINFORCEMENT AND PRESTRESSING STEEL ______________________________________________________________ 70 8.1 8.2 8.3 8.4 8.5

8.6

8.7 8.8

9.

Cement ________________________________________________________________________________________________ Fly-ash, slag and silica fume _______________________________________________________________________________ Aggregate ______________________________________________________________________________________________ Admixtures _____________________________________________________________________________________________ 6.4.1 Air Entrainers ____________________________________________________________________________________ 6.4.2 Water Reducers _________________________________________________________________________________ 6.4.3 Superplasticers or High Range Water Reducing Admixtures _______________________________________________ 6.4.4 Set Retarding Admixture ___________________________________________________________________________ 6.4.5 Waterproofing Admixtures __________________________________________________________________________ 6.4.6 Corrosion Inhibiting _______________________________________________________________________________ 6.4.7 Polymers _______________________________________________________________________________________ 6.4.8 Shrinkage Reducing Admixtures _____________________________________________________________________ 6.4.9 Expansive Additives ______________________________________________________________________________ Water _________________________________________________________________________________________________

CONCRETE SUPPLY ________________________________________________________________________________ 52 7.1 7.2

8.

Compressive Strength ____________________________________________________________________________________ 40 Other Measures of Quality _________________________________________________________________________________ 40

Ordering _______________________________________________________________________________________________ Handling, condition and storage _____________________________________________________________________________ Reinforcement Fixing _____________________________________________________________________________________ Welding of reinforcement __________________________________________________________________________________ Reinforcement __________________________________________________________________________________________ 8.5.1 Carbon Steel ____________________________________________________________________________________ 8.5.2 Epoxy Coated Bars _______________________________________________________________________________ 8.5.3 Galvanised Bars _________________________________________________________________________________ 8.5.4 Stainless Steel Bars ______________________________________________________________________________ Prestressing ____________________________________________________________________________________________ 8.6.1 Prestress _______________________________________________________________________________________ 8.6.2 Post-Tensioning _________________________________________________________________________________ Fittings ________________________________________________________________________________________________ Provision for Cathodic Protection ____________________________________________________________________________

70 70 70 71 71 72 72 73 75 76 76 76 77 78

CONSTRUCTION ___________________________________________________________________________________ 79 9.1

Training & Supervision ____________________________________________________________________________________ 9.1.1 Supervision _____________________________________________________________________________________ 9.1.2 Personnel and Responsibility _______________________________________________________________________ 9.1.3 Construction Supervision __________________________________________________________________________

79 79 80 80


9.2

9.3

9.4 9.5 9.6

9.7

9.8 9.9

9.1.4 Inspection Programme ____________________________________________________________________________ 9.1.5 Notice for Inspection ______________________________________________________________________________ 9.1.6 Training ________________________________________________________________________________________ Placing ________________________________________________________________________________________________ 9.2.1 Concrete Placing Method Statement __________________________________________________________________ 9.2.2 Cleaning Out Forms ______________________________________________________________________________ 9.2.3 Selection of Concrete Workability ____________________________________________________________________ Transport From Truck to Pour ______________________________________________________________________________ 9.3.1 Direct Discharge _________________________________________________________________________________ 9.3.2 Pump __________________________________________________________________________________________ 9.3.3 Skip ___________________________________________________________________________________________ 9.3.4 Wheelbarrow ____________________________________________________________________________________ 9.3.5 Slide___________________________________________________________________________________________ 9.3.6 Tremie _________________________________________________________________________________________ Compaction_____________________________________________________________________________________________ Uncompacted high flow concrete ____________________________________________________________________________ Finishing _______________________________________________________________________________________________ 9.6.1 Mix Impacts on Finishing ___________________________________________________________________________ 9.6.2 Finish to Suit Subsequent Applications ________________________________________________________________ 9.6.3 Abrasion and Finishing ____________________________________________________________________________ 9.6.4 Falls ___________________________________________________________________________________________ Formwork and Falsework __________________________________________________________________________________ 9.7.1 Formwork_______________________________________________________________________________________ 9.7.2 Falsework ______________________________________________________________________________________ 9.7.3 Stripping of formwork _____________________________________________________________________________ 9.7.4 Controlled Permeability Formwork ___________________________________________________________________ Lifting and Loading _______________________________________________________________________________________ Adverse weather conditions ________________________________________________________________________________ 9.9.1 Hot Weather ____________________________________________________________________________________ 9.9.2 Cold Weather ___________________________________________________________________________________ 9.9.3 Wind __________________________________________________________________________________________ 9.9.4 Rain ___________________________________________________________________________________________

80 81 81 81 82 82 82 83 83 83 83 83 83 83 83 84 84 85 85 85 87 87 88 88 88 89 89 89 90 90 91 91

10. CAST INSITU CONCRETE _____________________________________________________________________________ 93 10.1 Columns _______________________________________________________________________________________________ 10.2 Diaphragm Walls ________________________________________________________________________________________ 10.3 Piles __________________________________________________________________________________________________ 10.3.1 Bored Piles _____________________________________________________________________________________ 10.3.2 Continuous Flight Auger Piles _______________________________________________________________________ 10.3.3 Piles for Use in Tunnels and Retaining Walls ___________________________________________________________ 10.3.4 Pile Reliability ___________________________________________________________________________________ 10.4 Walls – Free Standing ____________________________________________________________________________________ 10.5 Retaining Walls (including tunnels)___________________________________________________________________________ 10.6 Beams (including pile caps) ________________________________________________________________________________ 10.7 Slabs (including Stairs) ____________________________________________________________________________________ 10.8 Water Retaining Structures_________________________________________________________________________________

93 93 94 94 95 95 96 96 97 97 97 97

11. PRECAST CONCRETE _______________________________________________________________________________ 98 11.1 Design Requirements _____________________________________________________________________________________ 99 11.2 Quality of Construction ____________________________________________________________________________________ 99 11.2.1 Repetitive Procedures _____________________________________________________________________________ 99 11.2.2 Intense Compaction _____________________________________________________________________________ 100 11.2.3 Self-Compacting Concrete ________________________________________________________________________ 100 11.2.4 Rigid Formwork _________________________________________________________________________________ 100 11.2.5 Influence of Reliability Requirements ________________________________________________________________ 100 11.2.6 Measure of Quality Improvement ___________________________________________________________________ 100 11.3 Detailing ______________________________________________________________________________________________ 101 11.4 Utilities _______________________________________________________________________________________________ 101 11.5 Buildings ______________________________________________________________________________________________ 102 11.6 Earth Retaining Walls ____________________________________________________________________________________ 103 11.7 Beams________________________________________________________________________________________________ 103

12. SPRAYED CONCRETE ______________________________________________________________________________ 105 12.1 Materials ______________________________________________________________________________________________ 12.1.1 Cements ______________________________________________________________________________________ 12.1.2 Additions ______________________________________________________________________________________ 12.1.3 Fibres_________________________________________________________________________________________ 12.1.4 Admixtures_____________________________________________________________________________________ 12.1.5 Aggregate _____________________________________________________________________________________ 12.2 Mix Design ____________________________________________________________________________________________ 12.3 Applicator Assessment ___________________________________________________________________________________ 12.4 Acceptance Testing of Sprayed Concrete ____________________________________________________________________ 12.5 Substrate Preparation ____________________________________________________________________________________ 12.6 Finishing ______________________________________________________________________________________________ 12.7 Curing ________________________________________________________________________________________________ 12.8 Quality assurance _______________________________________________________________________________________

105 105 106 106 107 107 107 108 108 108 108 108 108

13. COMMON CONSTRUCTION PROBLEMS __________________________________________________________________ 110 13.1 Inadequate Cover _______________________________________________________________________________________ 110 13.2 Inadequate Strength Development __________________________________________________________________________ 110

Z7/04 Good Practice Through Design, Concrete Supply and Construction 13.3 13.4 13.5 13.6 13.7 13.8

Inadequate Curing ______________________________________________________________________________________ Early Age Restraint Cracks________________________________________________________________________________ Plastic Shrinkage Cracks _________________________________________________________________________________ Plastic Settlement Cracks _________________________________________________________________________________ Blistering/Delamination ___________________________________________________________________________________ Rectification During Construction ___________________________________________________________________________

110 110 110 110 110 111

REFERENCES _______________________________________________________________________________________ 112 APPENDIX 1 - SPACERS AND CHAIRS FOR SUPPORT OF STEEL REINFORCEMENT _________________________________________ 115 APPENDIX 2 : TESTING OF SPACERS. _______________________________________________________________________ 126

LIST OF FIGURES Figure 1 : Suggested reinforcement configuration to facilitate compaction Figure 2 : Slab setdown (for change of floor finish, etc) Figure 3 : Reinforcement tolerance – drip groove to cantilevered balcony slab, main reinforcement in top of slab Figure 4 : Slab setdown at window line Figure 5 : Reglet to window head at beam Figure 6 : Reglet to membrane turn-up on parapet Figure 7 : Reglet to window mullion at column Figure 8 : Drip groove in balcony slab, main reinforcement in bottom slab Figure 9 : Reinforcement detailing in parapets Figure 10 : A solution for conflict/congestion at beam-column joint (from Marosszeky and Gamble76) Figure 11 : Beam/blade-column intersection Figure 12 : Conduits in columns Figure 13 : Use of dissimilar metals (after Design and Detailing of Precast Concrete 63) Figure 14 : Construction Joint Details (From AS3735) Figure 15 : Movement Joints for Water Retaining Structures Figure 16 : Area Compacted by a Vibrator Figure 17 : Plastic cracking occur when there is inadequate thought given to controlling bleed and/or bleed effects Figure 18 : Bleed Rate (b) Compared to Evaporation Rate (e) for Two Applications Figure 19 : Effect of Curing on Compressive Strength Figure 20 : Results of Sulphate Testing Figure 21 : Alternative Uses of Water Reducing Admixtures (from Cook 22) Figure 22 : Assessment of Concrete Mixing Water 23 Figure 23 : Relationship between Chloride Diffusion and w/c Ratio24 Figure 24 : Allowable Discharge and Placing Time Vs Concrete Delivered Temperature Figure 25 : Example of the Impact of Ambient Temperature on Initial Setting Time of a Concrete Mix [derived from ref 33] Figure 26 : Photographs of issues with post-tensioned cables. Figure 27 : Effect of Surface Finish on Abrasion as Measured by Depth of Wear (from Fentress 49) Figure 28 : Rate of abrasion for hand finishing (from Kettle and Sadegzadeh 50) Figure 29 : Rate of Abrasion for power floating (from Kettle and Sadegzadeh50) Figure 30 : Effect of environment on the rate of evaporation from concrete (CCAA [34]) Figure 31 : CFA Pile Deterioration Mechanisms Figure 32 : Types of Spacer Figure 33 : Selection Procedure for Spacers

12 12 12 12 12 12 12 13 13 13 13 13 13 15 16 31 32 33 36 41 48 51 58 62 64 77 86 87 87 92 95 116 121

LIST OF TABLES Table 1 : Tolerance on Locations of Reinforcement and Tendons Table 2 : Dowel Sizes Table 3 : Planning For the Effects of Weather Table 4 : Bleed of various 50MPa mixes using LH cement (65% slag) Table 5 : Solutions Used in Research Undertaken on Sulphate Attack Using Small Prisms Table 6 : Types Of Cement Typically Suggested For Use In Concrete Under Different Conditions Table 7 : Effective Diffusivity (D) of chlorides ions at 25C in Table 8 : Measurement of moisture in the aggregate stockpile Table 9 : Estimate of Autogenous Shrinkage Component in an AS1012.13 Shrinkage test to 56 days Table 10 : Bleed of various 50MPa mixes using LH cement (65% slag) Table 11 - Design Life of Galvanised Steel in Different Exposures Table 12 : Maximum Tensile Properties Table 13 : Fixings In Concrete Cover Zone Table 14 : Corrosion Rate of Zinc Galvanising and Carbon Steel in Atmospheric Exposures Table 15 : Point Load test for spacers and chairs Table 16 : Durability properties of spacers

9 23 26 32 41 43 44 59 66 67 74 75 77 78 118 120


TERMINOLOGY

General terminology used in durability design are given in Z7/01. The terminology given below relates specifically to this document. Cover – The distance perpendicular to the concrete surface to a metal surface within the concrete Construction Allowance – The amount added to the specified minimum cover to give the target cover for construction. Target Cover - Minimum cover plus construction allowance. Generally taken as the spacer size when the reinforcement location is fixed by the use of spacers. Mean Cover – The measured mean of a representative sample of cover measurements over a defined area. Often this will approximate to the target cover. Minimum Cover – A term used to indicate the lowest distance perpendicular to the concrete surface to a metal surface but defined exactly as the characteristic cover. Characteristic Cover – Cover at a 95% and 99% confidence level for standard compaction and quality control heavy compaction and high level quality control respectively.

Z7/04 Durability Implementation

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1. INTRODUCTION This Recommended Practice provides guidance on implementation of the specified durability requirements through the design and construction stages. This includes design detailing, construction planning, concrete supply including concrete materials, construction methods and quality implementation. The aim of this document is to consider the current design guides and codes and provide additional information where they are limited. 1.1

REFERENCE DOCUMENTS The report draws heavily on the original work in Z07 which is being updated through the current work of the CIA Durability Committee. Other key sources have been:  Precast Concrete Handbook  Reinforcement Detailers Handbook

1.2

REPORT LAYOUT Durability design is covered in the Z7 Recommended Practices. In this Recommended Practice aspects of implementing durability through design and construction are considered. In Section 2 contractual aspects that influence durability implementation are discussed. Durability requirements are specified through the contract documents. This could be as part of the project deed, scope of works and technical criteria, drawings or specifications. The owner of the structure is drawn between a costly project that may be overly strict in durability requirements and lack of specificity such that a low quality poor durability structure is provided. Poor detailing of the structure can lead to various issues and this is considered in Section 3. This section provides typical details that will help overcome durability problems. Whist it is headed ‘Design’ the Contractor as the installer will need to understand these issues although they should all be fully detailed in the contract documents. Section 4 deals with pre pour planning. The concrete supplier cannot know if his product is fit for purpose if he isn’t aware of where and how the concrete is to be placed. As the concrete supplier is often best placed to advise on what can be provided to overcome a particular problem part of the pre-pour planning is to introduce the supplier to the proposed pours. Many experienced concrete technologists will review the contractor’s method statement and will be able to pass useful comment. Hence another part of pour planning is proper documentation of the methods to be used and dissemination of approved method statements to those that will place the concrete. Lack of Quality Management throughout the production, delivery and placing process may lead to construction of a cheap but poorly performing structure. This is discussed in Section 5. Concrete Materials and Concrete Supply are reviewed in Sections 6 and 7 respectively. Although Australian Codes AS and AS/NZS respectively provide useful advice there are a number of aspects left open to the specifier and these are discussed here. Additional recommendations on issues not covered by the Australian Codes are also provided. Reinforcements of various types are used in construction and these all impact on the potential durability of the structure. Durability of reinforcements is discussed in Section 8. Section 9 deals with general construction issues. Before items on the placing, finishing and curing of concrete it details training issues and solutions. Lack of training is one of the most common causes of construction defects and subsequent durability issues. Even the simplest of training can save the contractor and owner huge re-work and repair costs respectively. Hence some provision in the contract should be made for the contractor team to be adequately trained. Sections 10, 0 and 12 consider durability aspects of various ways in which concrete elements are constructed by insitu cast, precast and sprayed concrete respectively.


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2. CONTRACTUAL ASPECTS The specification documents, their clarity and style can influence the likelihood of achieving the durability of concrete that the designer intended. A poorly considered specification may lead to unnecessary construction expense or a structure that requires greater maintenance than expected over its design life. The aim of a construction contractor and concrete supplier is generally to provide a finished concrete structure that is as specified in the contract documents or variations. The contractor does not have insight into the designers objectives and to win a project must price it at the lowest price he can while still meeting the specification. In some cases what the designer intended in terms of the concrete structures durability is not achieved through this specification compliance. There are many potential causes of this failure. Conversely the contract documents may lead to unnecessary capital cost or undue maintenance due to the contractor compliance with the specification. A number of key issues regarding contract documentations influence on durability and the cost of achieving it are reviewed in this section. 2.1

VARYING SPECIFICATIONS DURING THE BID AND CONSTRUCTION PROCESS Contractors are often asked to bid on a project before all details are available. Client specifications may only outline general durability requirements with details being developed as part of the durability planning after award of the contract. Generally concrete suppliers will be requested to tender supply to a project by the construction contractor after award. The request may be presented along with relevant specification documents which may or may not have been available when the contractor tendered to construct the project. The varying nature of specifications causes several issues:  Causing unnecessary capital cost o Loading of contractors tender to cover uncertainty of specification requirements. Durability requirements can have a significant impact when the performance requirements are inconsistent. o High concrete supply prices due to risk involved in supplying to new specification requirements which hold new uncertainties. o High trial mix costs as new mixes have to be developed to meet new specification requirements.  Causing undue maintenance o Contractors underestimate what the specification will require and subsequently have to trim as much as possible leaving inadequate reserve in mix performance, i.e. lack of robustness in concrete quality supplied. o New untried mixes leading to unforseen difficulties (e.g. introduction of calcium nitrite inhibitors causing acceleration and reduced open times). The construction industries has struggled to ensure durable concrete supply as the increasing understanding of durability, and the lack of durability in some exposures, has led to a far greater emphasis in durability design. With the lack of a co-ordinated effort different parties have developed different preferred concrete requirements. These continue to evolve as the technical understanding develops. The problem is amplified by having various Australian Standards dealing with concrete durability requirements. The CIA’s Durability Series has endeavoured to bring all parties involved in durability specification together. Recommendation: Where a project specification has not been completed when contractors or concrete suppliers are asked to price a project, the specification shall state that the contractor may base his price on meeting the requirements of Australian Standards and the CIA Durability Series as a minimum.

2.2

PRESCRIPTION VS PERFORMANCE SPECIFICATIONS The requirements of a specification can impact on the time a concrete supplier will need to put together a conforming bid and the price of the concrete. Of particular significance is the extent to which the project specification is prescription or performance based. A prescription, or largely prescription based specification is usually much simpler for the concrete supplier or construction contractor to decide if a conforming tender is achievable. Normally a prescriptive specification will give details of conformance requirements for concrete constituents and


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will specify minimum cement content, maximum water/cement ratio and proportions of binder constituents (such as % of supplementary cementitious materials) as well as some basic minimum performance requirements as per AS1379, e.g. strength and shrinkage. Normally a concrete supplier will have standard concrete mixes that already satisfy these common requirements, or at least will know how the requirements can be satisfied by inference from other mixes in production. In view of this the supplier can usually respond quickly to such a specification with a competitive price not loaded due to uncertainty. The offer will generally be compliant with the specification thereby reducing tendering complexity associated with non-compliant offers. The key disadvantage of prescriptive specifications is the increase in the specifier’s risk. The specifier may assume certain concrete properties will be achieved if the prescription requirements are complied with. This may not be the case. Unless the specifier fully understands the nature of the concrete constituent materials available then there can be no certainty that the desired performance will be achieved based on the prescription. There is significant risk to specifier’s who “copy and paste” a prescription specification that worked on one project and apply it to another project where materials are different. Materials change between regions and this will often limit the ability to assume a certain performance from a prescription mix. If the performance properties assumed are not achieved and have not been specified then the specifier will be responsible for any resulting failure. A specification that only includes performance requirements of AS1379 is unlikely to be problematic for the supplier because the they will understand the test methods and are likely to have some idea on the performance of mixes from previous projects. If the performance requirements specified involve new or rare test methods then the supplier may be unable to provide assurance of meeting the specified criteria. Many durability tests for example do not have defined variances making it impossible for the supplier to determine target values required in order to meet the specification. This was a major concern raised during the workshops held to discuss what should be included in the CIA’s Durability Series. To address this Z7/07 ‘Durability Tests’ includes preferred durability tests for inclusion in specifications and includes recommendations on the methods to be used, appropriate criteria and methods of specifications. It is expected that by specifying only the tests in Z7/07 suppliers will be able to build a database of information on their mixes. Although Z7/07 is intended to standardise the durability tests used in Australian specifications, performance limits for these tests will still often mean that the contractor will need to carry out multiple trial mixes to establish whether a mix can be developed that gives conforming test results. On one major project at least, a performance that had been achievable in one location was specified in a location where the performance could not be reasonably achieved with local materials. The contractor assumed that the specified requirements could be met and spent a lot of time and money undertaking trial mixes. Ultimately the specified value was modified and the contractor sued the specifier for the wasted effort and obtained a significant settlement. Hence, specification of performance limits places an obligation on the specifier to ensure the specified value can reasonably be met using local materials. In some locations the performance of the concrete to Z7/07 tests may have become known and the required performance levels can be specified with confidence. If the specifier is uncertain about the status of local materials the concrete suppliers may be able to provide information on the mixes they have available. However, faced with uncertainty in achieving the required performance from a prescription mix and the risk of specifying required performance values that may not be reasonably achieved the specifier can specify a prescribed mix and that the required performance test be undertaken and the result reported. This eliminates uncertainty for the supplier and enables the consultant to check that the performance required will be achieved. Where a performance requirement is specified and the concrete supplier takes the risk of achieving the specified requirements they may reasonably qualify their bid, e.g. “the tender is made pending achievement of satisfactory test results in preliminary trial mixes”. This may assist the supplier in providing prices while meeting tight tender timelines and leave open the possibility for negotiating an appropriate resolution should difficulties be encountered. This is risky for all parties to the project and places increased reliance on the skill of the specifier, construction contractor and concrete supplier. Where a performance specification is used the Inspection and Test Plan (ITP) for ongoing supply will need to include the specification requirements so that the contractor can demonstrate that the required performance properties are achieved.


Page 5

The key advantage of the use of performance based specifications is that suitable mixes can be designed around available materials and at optimal cost for performance. The key disadvantage of this type of specification is that concrete suppliers may not have mix designs available that have appropriate testing to meet specification requirements that are not normally required by AS1379. Solving the lack of readily available and approved mix designs may take more time to arrange than originally expected by the specifier or construction contractor. From the foregoing it can be seen that there is some advantage for the specifier to engage with potential concrete suppliers early in a project specification development so as to gain greater certainty over the quality of mixes available and the time required to have confidence in a project tender. Recommendation: Where there is a clear understanding about the level of performance local concrete mixes can achieve it is recommended durability performance criteria and test intervals be included in the specification. Where there is no local performance data on how a mix will perform it is recommended that the specification include a prescriptive mix and require appropriate number and type of durability tests selected from Z07/07. It is recommended that the specifier: a) Allow a reasonable margin in the design to allow for uncertainties in the performance. A balance must be taken between the cost of over achieving the required performance and the cost of having to re-engineer due to the performance achieved being too low. b) Include testing in the trial mixes to assess the problems that might arise from the use of the mixture of concrete ingredients. c) Allow sufficient time to undertake the trials, do the testing and to approve the mix. Certain durability tests may take up to 4 months from the date of trial mix to complete the assessment and in these cases the specifier will either need to allow for construction based on indicative test results. 2.3

INSITU PERFORMANCE Where compressive strengths are not taken AS 3600 provides recommendation on how to assess the insitu strength from cores. This includes an allowance for strength insitu to be lower than the cylinder strengths because it is not uncommon for insitu strengths to be lower than cylinder because of the various imperfect processes associated with insitu concrete placing. Similarly a lower durability performance can be expected for concrete insitu and this must be allowed for in the durability design and testing of insitu concrete. The only durability test where this is formerly recognised is the Volume of Permeable Voids (VPV) test (AS 1012.21) where Vic Roads provide performance criteria for cylinders and cores with due allowance for this lower insitu performance. The VPV test was developed as a quality assurance test and the Vic Roads criteria for cores and cylinders makes it particularly useful in this role. Where cylinders are not taken, or where a cylinder result is suspect, the provision of core criteria means there is a clear way of assessing insitu performance. In Section 2.2 the problem for concrete suppliers warranting that a novel mix will achieve a given performance was discussed. The difficulties for contractors ensuring insitu performance are even greater as there is less data available. Hence, insitu performance requirement should only be provided as a means of assessing acceptability of as placed concrete where the Contractor has failed to meet the proof of performance through specified cylinder testing requirements. Z7/07 provides additional information on insitu performance criteria. Recommendation: Where the insitu concrete performance is to be measured then due allowance shall be made for the insitu performance to be lower than the test cylinders. It is recommended that a similar allowance be made for the insitu performance used in durability design.

2.4

DESIGN LIFE In many cases a specifier designing concrete for an environment requiring a higher level of concrete durability will also be designing for life expectancy beyond 50 years (perhaps 100 years or even more). If this is the case then the construction contract and specifications must be clear about this and more general references to compliance to AS3600 without further clarifications on how the longer life


Page 6

is to be achieved will ultimately lead to a structure that has not been built in accordance with the designers expectations. Z7/01 provides recommendations on standard design lives and Z7/03 deals with the prescription requirements for those design lives. 2.5

RELIABILITY Reliability is deduced from client requirements and used as part of the determination of prescriptive or performance durability requirements. Reliability requirements are not covered by current Australian codes. In the same fashion as design life, required reliability needs to be included in the contract documents. Z7/01 provides recommendations on reliabilities and Z7/03 deals with the prescription requirements for those reliabilities.

2.6

IMPACT OF HIGH LEVEL DURABILITY REQUIREMENTS ON CONSTRUCTION CONTRACT Specifications requiring higher levels of concrete durability can have outcomes that are impacted by the time allowed for review. Also, processes required to achieve the specified requirements may be unclear and/or poorly detailed. It is important to identify critical processes that need to be applied on-site where high durability concrete is required. It would be useful to detail these processes in the project specifications. These critical processes may include:  Use of special admixtures to enhance workability.  Special placement and compaction methodology.  Adjustment of concrete workability to promote full compaction.  Checking of formed concrete for specified cover to reinforcement.  The use of evaporation retardant on exposed concrete surfaces following placing and compaction.  Special curing methods to facilitate achievement of high surface performance. A common situation that arises in the construction process is that the method of placement of concrete is not specified in the contract documents. The construction contractor assumes that all concrete will be pumped into place and tenders accordingly only to discover that a special high durability concrete has been prescribed that is not suited to pump placement. This can lead to argument and delays to construction while a new mix is trialled or a revised method of placement is put in place.

2.7

CONFLICTING SPECIFICATIONS – LOW EMISSIONS CONCRETE Project specifications need to be designed to avoid conflicts arising from concrete performance and prescriptive specifications in the same element. The most common of these are found where there are limitations on binder components in a concrete mix design specification based on meeting reduced material carbon emissions but at the same time the construction contractor needs a special higher early strength to meet construction requirements (stripping, post-tensioning and other drivers for this requirement). The user of higher cement replacement to lower carbon emissions will normally reduce a concretes’ early strength development. Methods to balance conflicts need to be agreed to at tender or arguments and delays will occur.

2.8

THE USE OF THIRD PARTY PRODUCTS IN MIX SPECIFICATION Specifiers can prescribe the use of additions of materials to a specified concrete mix with the view of enhancing the concrete durability in a particular environment. In some cases a third party supplier will warrant certain properties to the concrete when their material is added in accordance with their own specification. This can lead to issues with a concrete supply contract under circumstances where any of the following conditions exist:  The third party material does not comply with AS1379 or its referenced standards and so the concrete supplier is unable to supply such concrete as conforming to AS1379.  The concrete supplier is not prepared to warrant the performance of their concrete with the third party material added. In this case the performance may include normal measures required by AS1379 and may extend to any additional performance measures attached to use of the additive by the third party supplier.  The concrete supplier will warrant concrete has complied with a special class prescription as required by AS1379 and the third party additive supplier warrants that their product has been added to the concrete in accordance with their own specification.


Page 7

 The project concrete specifier assumes responsibility for the performance of the concrete provided that the concrete supplier will warrant that concrete has complied with a special class prescription as required by AS1379 and the third party additive supplier warrants that their product has been added to the concrete in accordance with their own specification. In any of the above scenarios it can be seen that there is a substantial uncertainty and risk for the owner of the structure. The actual arrangement that is entered into must be agreed by all parties with an interest in this situation and should be addressed before tendering to avoid delays to a contract. The least risky option is to only use third party products where the concrete supplier is able to warrant the properties of their concrete to comply with those specified for the project. 2.9

SHARING OF RISK When a construction contractor sub contracts work and purchases concrete and associated construction materials there is a sharing of risk either through clearly specified accountabilities or incorporated in relevant standards that are specified. For example, where a construction contractor purchases concrete and specifies the characteristic strength, workability and maximum aggregate size in accordance with AS3600 and referred documents in AS3600 (e.g. AS1379) then there is a clear set of requirements on the concrete suppliers responsibilities as well as a reasonable set of design assumptions built into AS3600 that allow for reasonable risk of the impact of “normal” construction methods on concrete strength in place. In AS3600 there are also techniques for assessing constructed member strength to verify design (such as cores, non-destructive testing, and load testing). Unfortunately where durability levels are specified outside of those assumed in AS3600 there is little assistance with responsibility sharing. Issues are highlighted below with reference to specification of a performance not included in Australian standards. a) The ability to achieve a certain performance level for any test may vary from place to place due to variations in materials from place to place. Specifiers may take the responsibility for costs incurred by a supplier trying to achieve a performance requirement where a specified value cannot reasonably be obtained using available materials even though the specified value may have been achieved elsewhere. b) Where a performance value is achieved in a trial mix the specifier must know what variance for the test, materials and production method is expected, and how to interpret from this a characteristic value in order to know what performance value is reasonably achievable. c) When a reasonable characteristic performance value limit is established some way of sharing the risk between acceptance and rejection of concrete based on this needs to be determined. Where the concrete is rejected the specification should include specific actions to be taken. Degree of action required may be based on the degree of failure. d) Where a performance value is specified the specifier will need to define the frequency of test. e) Assuming the suppliers test value of laboratory cured samples will be the same as that of the cast concrete cover zone is not appropriate as many factors will mean the insitu performance is lower than the result for a laboratory cured cylinder. The specifier must make an assessment of how much lower the insitu performance can be if testing of insitu performance is to be permitted as a method for acceptance should cylinder results not achieve the required performance level. Recommendation: Project specifications covering durability testing need to ensure that: a) suppliers are prepared to supply to the proposed testing criteria, b) test methods, frequency and criteria are suitable defined, c) all parties involved have clear responsibilities and d) consequences for non-achievement of agreed performance requirements are practical and cost effective with appropriate consideration given to sharing risk between the owner and contractor.


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3. DESIGN Durability design is dealt with in other sections of Z7. In this section aspects of design detailing are primarily considered. 3.1

MINIMUM COVER The reinforcement in concrete must be correctly located if the design durability is to be achieved. Although considerable sums are often spent on durability design the most common cause of premature reinforcement corrosion is low cover. Modelling using the approaches in Z7/05 calculates the minimum cover required to achieve a stated design level at a stated reliability. In Z7/03 deemed to comply requirements for minimum cover are given. Minimum covers in Z7/03 and Z7/05 include no negative tolerance. To achieve these minimum covers an appropriate construction allowance must be added to give a target cover for construction. Australian codes often specify minimum covers which includes a construction allowance expressed as a negative tolerance. Hence, the actual minimum allowable cover in these codes is the minimum cover minus the negative tolerance. This means that if a higher or lower tolerance is required due to the method of construction the minimum cover and tolerance need to be modified. This can be confusing but may also be simple as the minimum cover specified becomes the cover that the contractor targets. The concept of minimum cover is generally taken as an absolute value but it is more realistic to consider cover as a probability distribution and in full probabilistic durability modelling cover is treated this way. Hence, minimum cover is better considered as the characteristic cover. In an ideal world the entire surface of an element would be tested for cover compliance but like most QA procedures this is not practical and a statistical based acceptance method has to be implemented. This can be achieved by testing at a number of locations selected at random over a defined area and accepting or rejecting the area based on a statistical analysis of these results. Recommendation: It is recommended that for the purpose of testing, minimum cover be treated as the characteristic cover requirement. Where an area fails to achieve the specification based on this assessment method it is recommended that the area be rejected unless an acceptable mitigation method is implemented. 3.1.1

Casting Against Ground

The minimum covers in Z7/03 and Z7/05 are based on constructing against formed surfaces or blinding. These covers need to be increased where concrete is cast against ground as detailed in AS3600 clause 4.10.3.5. 3.1.2

Special Finishes

Minimum covers may need to be adjusted where special finishes are applied to compensate for deleterious chemical actions or fracturing of the concrete. Recommendation: Where special finishes are applied to a concrete surface it is recommended that the minimum covers given in Z7/03 or derived using Z7/05 be increased as follows: a) Acid etched exposed aggregate finish +10mm. b) Bush hammered or broken surface finish +5mm. c) Retarded surface with water wash to achieved exposed aggregate +5mm. 3.2

TOLERANCES FOR CONSTRUCTABILITY The reinforcement location tolerances affected by durability as given in AS3600 are unchanged as they incorporate structural considerations and are listed as ‘Tolerance’ in Table 1. The ‘construction allowance’ (Table 1) is the recommended addition to the minimum cover to give the target cover. These are not the same as the negative tolerance in AS 3600. Consideration was given to a more complex array of construction allowances based on whether the cover was likely to be greater than or


Page 9

less than the spacer size however this was unwieldy and a simple system similar to AS 3600 is recommended. There is a reasonable amount of literature which demonstrates that the negative tolerances in AS3600 are not achievable in practice. In order to determine whether a larger construction allowance is required more thorough analysis of the variation in components that affect the cover is required. However, as current negative tolerances are inadequate they are increased to take a larger portion of the available tolerance Table 1 : Tolerance on Locations of Reinforcement and Tendons For durability where the reinforcement Location is not affected by a bars presence on opposite faces Tolerance

Construction Allowance (mm) AS3600 (negative tolerance)

Recommended

i)

In beams, columns and walls

-0mm, +15mm

5

10

ii)

Slabs on grade

-0mm, +30mm

10

20

iii)

In footings cast in ground

-0mm, +50mm

10

30

-0mm, +50mm

50

50

iv)

The position of ends of reinforcement

The contractor may determine to use lower or higher construction allowance in order to keep consistent spacer sizes but should be aware that ultimately from a durability perspective it is the minimum cover that must be measured and complied with. 3.3

COVER AND AGGREGATE SIZE For most building projects, the maximum nominal aggregate size will be 20 mm, and the Australian Code requirement that the cover be not less than the maximum aggregate size will not govern. In aggressive environments it is recommended that the cover be at least 1.5 times the maximum aggregate size. However as the cover will also be high this is unlikely to be a governing condition. Wherever the cover is less than 1.5 times the maximum aggregate size the contractor should consider whether compaction behind the reinforcement may be an issue.

3.4

COVER AND BAR OR TENDON SIZE Frequently, stirrups and ties will be not less than 10 mm in diameter. Thus, it is not until single bars of nominal size 32, or larger bundled bars, are used that the requirement for covers to be not less than the bar, tendon or duct size will dictate the cover, except for the case of rigid forms and intense compaction. Note that cover for durability purposes refers to both main reinforcement and tendons, and secondary reinforcement, such as stirrups and ties. Where cover is specified in AS 3600 for other purposes, e.g. fire resistance, it may apply to main reinforcement or tendons only.

3.5

QUALITY CONTROL OF COVER CONCRETE Increasing the concrete cover from 20 to 30 mm could more than double the maintenance-free service life of reinforced concrete structures. This highlights the importance of the correct choice and control of concrete cover to the long-term durability of concrete structures. Codes of practice or standards, such as AS 3600, provide a good guide to the quality and thickness of concrete cover to be used for non-aggressive exposures at reliabilities of 2.1 or less. However, as exposure severity increases in regards reinforcement corrosion, the cover/concrete performance requirements of Australian Standards may be inadequate. To a large extent this is resolved when appropriate supplementary cementitious materials are incorporated. The deemed to comply requirements of Z7/03 provide suitable durability solutions for a range of reliabilities, design lives and exposures.


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Whether an Australian code or Z7 cover is specified contractors need to ensure it is achieved by appropriate quality management. However no QC guidelines or compliance criteria for assuring adequate cover are given in the codes. Common practice is the checking of the gap between steel reinforcement and the formwork during a pre-pour inspection. The adequacy of such an approach to QC of concrete cover can be gauged by the frequency of revelations of the lack of cover as the main cause of the premature deterioration due to steel corrosion of many concrete structures. In most cases, the achieved cover is significantly less than that specified indicating the problem to be caused by the lack of adequate QC procedures37, 38. The adequacy and consistency of concrete cover can be improved by the following general considerations:  greater attention to detailing of steel reinforcement especially in areas of steel congestion that may inhibit adequate compaction (see Section 3.7);  accurate steel fabrication; and  greater attention to the erection of steel reinforcement especially in highly congested areas. A clear improvement in cover will also be obtained from the recommendations below. Recommendation: To improve the compliance of reinforcement cover it is recommended that the following are implemented: a) Construction Allowances in accordance with Table 1. b) Specification of appropriate spacer and bar chair type and distribution as detailed in Appendix 1. c) A formal reporting system of a prep-pour checking requiring zero failures of a statistically representative randomly selected sample before pour approval is given. Experience shows that post-pour cover QA checks using a covermeter are more reliable at identifying the cover in a pour than pre-pour checks. Post-pour cover checks are also valuable as they act as an incentive to improve achievement of cover and they identify areas of low cover for correction. Recommendation: To provide an effective quality assurance of cover it is recommended that: a) post pour cover checks, using a properly calibrated cover meter shown to have an accuracy of ±2%, be undertaken on the first pour of any type wherever possible. The pour shall be split into zones representing areas of potentially different cover, e.g. starter bar locations, corner bar locations, areas of no specific influence on cover. Each zone shall be checked at a minimum of 10 locations where each result is the average cover of 2 adjacent bars. If there are low covers the reason shall be identified and the cause eliminated for the next pour which shall then be checked in the same fashion. The process shall continue until no non-compliant covers are found. When there is no non-compliant covers the pour method and pour shall be accepted and checking of cover shall be based on measurement of a representative sample of pours. b) the assessment of cover be based on achieving the minimum cover at the following confidence level:  cover for normal compaction and quality control 95%.  cover. for intense compaction and high level of quality control 99%. c) where concrete is found to have low cover then:  cover < 10% under specification - coat to compensate for the lack of cover as approved by engineer.  cover < 30% under specification – breakout concrete and apply an approved patch repair and coating.  cover > 30% under specification – develop specific repair or replacement solution.


3.6

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TESTING OF THE EXPOSURE In some cases the exposure class may not be exactly clear at the time of design and additional data may be required during constructions to finalise the exposure class. Where this additional testing is required it will be determined by the design team and testing will be specified for the Contractor to arrange. In some cases this testing will be required in the early stages of the construction contract in others it may be obtained as the project proceeds. The Contractor should ensure he understands the requirements of the testing programme including number of tests, dispersion of testing, and the test method to be used. In some cases the method of sampling is particularly critical, for example pH values can change quickly as a sample oxidises after results have been taken. Prior to taking samples a test plan should be prepared detailing the tests to be undertaken. Some test methods are discussed in AS3600 Commentary section C4 and contained reference documents. Tests are also described in Z7/07.

3.7

CONFIGURATION AND CONGESTION OF REINFORCEMENT Reinforcement configurations need to be examined from a compaction viewpoint. The durability provisions envisage that the concrete, especially the cover concrete, will be able to be properly compacted. To enable this to be done, the following should be noted:  The layout should ensure that the concrete can be placed and compacted simply and effectively. Some layouts to permit effective compaction are shown Figure 1. Further discussion is provided by Potter55.  The layout should ensure that reinforcement is evenly distributed while maintaining the specified cover.  Consideration should be given to the expected accuracy of placement and the value of cover that should be specified (see previous discussion in Clause 3.5, also refer to Section 5 for other considerations that may influence the choice of reinforcement layout).  Special care should be taken in detailing inserts, which are frequently the cause of durability distress. They usually have to be placed at the surface of the member in exposed positions. Site operations frequently can destroy or impair the steel's protective coating. Inserts should be located so that their surfaces shed water, allowing them to remain in a permanently dry condition. Recessed inserts that can hold water should be avoided or be permanently filled with some protective material.  The detailing of reinforcement at set downs and to accommodate reglets and drip grooves, yet provide the required cover, needs to be carefully considered by the designer and not left to site staff to resolve. Suggestions of how to avoid these problems are shown in Figure 2 to Figure 8.  Maintaining cover in parapets where bent bars and hooks are used in thin members is difficult. Details should enable site staff to easily locate and firmly support the reinforcement. Figure 9 gives one suggestion for resolving this problem.  Congestion can also result in the loss of cover. Marosszeky and Gamble [76] report that on a number of building sites corrosion occurs because the cover is as low as 5 mm. This tends to be independent of the cover specified and reflects site practice in rectifying conflict in bar placement in areas of congestion. A series of solutions for common congestion/conflict situations are provided. One example is shown in Figure 10.  Detailing problems associated with columns involving blade columns and the incorporation of conduits in columns. Ways of resolving these are illustrated in Figure 11 and Figure 12.


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Figure 4 : Slab setdown at window line Figure 1 : Suggested reinforcement configuration to facilitate compaction

Figure 5 : Reglet to window head at beam Figure 2 : Slab setdown (for change of floor finish, etc)

Figure 3 : Reinforcement tolerance – drip groove to cantilevered balcony slab, main reinforcement in top of slab

Figure 6 : Reglet to membrane turn-up on parapet

Figure 7 : Reglet to window mullion at column


Figure 8 : Drip groove in balcony slab, main reinforcement in bottom slab

Page 13

Figure 10 : A solution for conflict/congestion at beam-column joint (from Marosszeky and Gamble76)

Figure 11 : Beam/blade-column intersection

Figure 9 : Reinforcement detailing in parapets

Figure 12 : Conduits in columns

Figure 13 : Use of dissimilar metals (after Design and Detailing of Precast Concrete63)

Page 14 3.8

DISSIMILAR TYPES OF METALS Different metals, whether reinforcement or fixings, should not be used in close proximity to each other in concrete. Where it is impossible to avoid this, the different metals should be adequately insulated from each other. Remember that the tie wire needs to be of the same material as the reinforcement, i.e. black, galvanised, etc. Combinations of materials likely to give rise to galvanic cells are shown in Figure 13. Whether or not these materials can be electrically connected without galvanic problems may be complex and this is discussed for zinc and stainless steel in Section 8.5.

3.9

MEMBER PROFILES The shape or profile of a member has a bearing on how that member weathers. This is discussed in detail by Hawes64 who outlines the principles and then provides a series of study sheets relating to various potential durability problems. However, a member's profile has less influence on its durability than do many other factors. Therefore, it generally suffices to ensure that water is positively drained off a member and is not allowed to pond on horizontal surfaces, especially in localities subject to freezing temperatures.

3.10 CONSTRUCTION JOINTS Formwork at joints should be sufficiently rigid to ensure no loss of fines, and reinforcement should be accurately positioned and properly supported. Concrete should be carefully compacted right up to the formwork. Construction joints should be made so that there is no loss of continuity in the member at the joint. Where the location of construction joints is shown on the drawings, the joints should not be relocated or eliminated without the approval of the designer. Where construction joints are required but not detailed on the drawings, reference should be made to the designer. When casting against existing concrete, and where sound bond across the joint is required, the existing concrete surface shall:  Be prepared to remove dirt, contamination and laitance. Of particular importance is preparation of the joint in the cover zone. This area is often not well defined on site and preparation in this critical area is frequently inadequate; and  Have a nominated surface profile. In order to ensure adequate preparation from an aggregate interlock perspective, the specification shall nominate a surface profile requirement. A requirement that has a 3mm depth of exposure of aggregate is commonly required to ensure no movement across the joint.  Be saturated surface dry and the time of pouring the fresh concrete or, where a bonding agent is used, have a bonding agent that is in a state that will promote bond. It shall be assumed that sound bond across the joint is required unless otherwise stated on the drawings. Construction joints are potentially a plane of weakness and potential issues of corrosion of reinforcement shall be avoided by not permitting lapping of reinforcement across a joint. In B2 and more severe exposures the surface of the concrete at construction joints shall be coated with a clear penetrative sealer for 100 mm either side of the joint. In water retaining structures if there is any doubt about a joint not being waterproof the joint shall include a waterstop or surface joint sealant groove or both. The specific design of waterproofing will reflect the pressure head, the potential joint opening and the required protection to reinforcement. Typical joint details are shown in Figure 14. The Construction Manager shall prepare a method statement detailing the steps to be undertaken to ensure the above is achieved.

Page 15 3.11 MOVEMENT JOINTS The principle issues for movement joints in non-water retaining structures are to ensure alignment across the joint such that: 

Dowels do not become a durability risk.



The edges of the joint do not break down. Figure 14 : Construction Joint Details (From AS3735)

a) Construction Joint Unsealed – Floor Slab

b) Construction Joint Unsealed – Wall

c) Construction Joint – Sealed on the Liquid Face (Not Applicable for Locations with an External Water Pressure)

d) Construction Joint – Sealed on the Liquid Face and with a Waterstop (Applicable for Locations with an external Water Pressure)

Water Retaining Structures This section is extracted form AS 3735 Commentary. All movement joints are a potential source of leakage and it is important that the methods of sealing them are carefully designed. Because of this, as well as their cost, it is usually desired to minimise the number of movement joints but this is to be balanced against the need to provide the appropriate flexibility of the structure. Isolation Isolation joints, as shown in Figure 15a), are used to provide an effective means of avoiding unacceptable cracking due to large movements or a large degree of restraint. Expansion Expansion joints, as shown in Figure 15b), are designed to accommodate all anticipated longitudinal movements in a structure. The reinforcement stops clear of the joint and a gap is left which is filled with a compressible filler. Waterstops are usually required to maintain the watertightness of the joint. Full contraction Full contraction joints, as shown in Figure 15c) and Figure 15d), consist of a discontinuity of both reinforcement and concrete across the joint. Waterstops and, where there is any possibility of debris entering the joint, joint sealants are essential. Dowel bars can be used to provide a capacity for shear transfer across a joint as shown in Figure 15e). Some means of bond reduction on the dowel bars will be required such as wrapping one end of the dowel bars with a bond breaking tape or placing the dowel bars within PVC sleeves (taped over on the end to prevent concrete entering the sleeve). Shear transfer by the use of keys within the concrete thickness are less effective than dowels due to the reduction of strength of the slab and they are also very expensive to form.

Page 16 Figure 15 : Movement Joints for Water Retaining Structures

a) Isolation Joint – Between Wall and Roof Slab

b) Expansion Joint – Wall Joint (Dowels Required if Shear Transfer Required)

c) Full Contraction Joint – Wall Joint (Dowels Required if Shear Transfer Required)

d) Full Contraction Joint – Floor Joint (Dowels Required if Shear Transfer Required)

e) Dowel Bar for Shear Transfer (No Steel Continuity Across Joint)

f) Partial Contraction Joint – Wall Joint

g) Partial Contraction Joint – Floor Joint (Applicable for Locations with an External Water Pressure

h) Hinged Joint – Between Base Slab and Wall

i) Sliding Joint – Between Base Slab and Wall – Prestressed Concrete Tank Wall on a Rubber Pad

j) Sliding Joint – Between Base Slab and Wall – Prestressed TanConcrete Tank on Membrane

Partial contraction Partial contraction joints, as shown in Figure 15f) and Figure 15g), may be constructed with the use of a stop end or by inducing a crack by a local reduction of the depth of concrete by 25% to 35% of the concrete thickness. Providing the percentage of reinforcement is reduced, this crack will release any stresses in the concrete allowing the joint to act as a contraction joint. A reduction in the percentage of continuous reinforcement through the joint allows the steel to yield locally with the resulting final effect that the joint acts as a normal contraction joint. There should be no need for dowel bars as well, because the continuous reinforcement across the joint should be sufficient to transfer any shear forces.

Page 17 Hinged Hinged joints, as shown in Figure 15h), are usually located at the intersection of the wall and floor of some tanks to fulfil the design condition of allowing rotation with minimal restraint and resisting thrust and shearing forces. A common method is to place the wall into a groove in the footing. Sliding Sliding joints, as shown in Figure 15i) and Figure 15j), have a complete discontinuity in both the concrete and the reinforcement, and allow movement with minimal restraint in the plane of the joint. The surface of the concrete should be very smooth and flat with a separating layer of a suitable material to allow movement to take place. Prestressed concrete tanks with a relatively large movement between the wall and the footing generally have this type of joint. 3.12 WATERSTOPS There are two primary types of Waterstops. 3.12.1 Rubber and PVC Waterstops This section on waterstops is largely extracted from AS 3735 Commentary. Waterstops and other associated items should be shown to be durable in the environment or usage to which they will be exposed for at least the design life of the structure. Alternatively, they should be designed so as to be maintained or replaced at suitable intervals. Suitable means of access should be available for replaceable items. Materials used for waterstops should not be susceptible to biological attack. Waterstops are usually proprietary items with determined performance characteristics. When specified, waterstops should be appropriate to the required design performance. The different applications of waterstops are illustrated in Figure 14 and Figure 15. It is essential that the concrete placed around the waterstop is well compacted and the waterstop is fixed and maintained firmly in position until the concrete placing is completed and the concrete has set. However, horizontal and near horizontal waterstops should not be wired firmly in position until the air beneath has been released and the waterstop is supported by compacted concrete. Waterstops may be divided into four categories. The first category, known as the central bulb type, is used to seal across expansion, contraction and partial contraction joints. The central bulb is positioned across the joint, and the waterstop is set parallel to the liquid surface of the concrete wall. There is a solid bulb or wing at each end of this type of waterstop, which is made of rubber or flexible plastic such as PVC. The distance of the waterstop from the nearest exposed concrete face should not be less than either half the width of the waterstop or 1.5 times the maximum size of aggregate. The second category is similar to the first category but has no central bulb. It is set in a similar manner to the central bulb type, but should be used only in partial contraction and construction joints. The third category, consisting of surface types of waterstops, should only be used on the underside of concrete slabs where there is sufficient support from the foundation soil or blinding concrete to prevent the waterstop from being forced away from the underside of the slab by hydraulic pressure in the joint. These waterstops are set into the surface of the concrete each side of a contraction or partial contraction joint that is formed. They can also be used with a central crack inducing tongue for induced contraction joints. To achieve good compaction of the concrete against the waterstop, it should be fixed to the blinding concrete. The use of a surface waterstop that is sometimes specified at construction joints. This type of waterstop is usually formed from rubber or flexible plastics such as PVC.

Page 18 3.12.2 Swellable Waterstops Swellable waterstops are promoted as replacements for central waterstops in rigid and movement joints. Their large expansion when contacted by water means the passage of water along the joint is blocked at the waterstop. However unlike the PVC type waterstops, water can flow through the concrete around the swellable waterstop and hence the local obstruction may not prevent leakage, particularly where the water is under pressure. Swellable waterstops should only be used where the water in contact with the waterstop is not under pressure. 3.12.3 Metal Waterstops The fourth category of waterstop is a rigid type and is specified where, as in construction joints, no movement is expected at the joint but a positive waterstop is required because of the pressure of the contained liquid. Such waterstops are usually formed from copper, zinc or galvanized steel strip but they are not used as often as they were in the past years because of the possibility of embrittlement due to strain hardening. Waterstops made from Grade 316 stainless steel with a large crimp or fold along the middle to permit some movement have recently been used. In some critical applications these waterstops have been used in parallel with a PVC waterstop. The design of the structure should generally provide for the continuity of the waterstop system across all joints and particularly junctions between floor and wall systems. The correct procedure for making the running joints on site using heat-fused butt welds for PVC, vulcanized or pocketed sleeve joints for rubber and brazed or welded lap joints for copper or steel, needs to be adopted. All waterstop corners should be prefabricated in the factory. With respect to non-metallic waterstops, both PVC and rubber exhibit satisfactory performance and durability; however, care should be taken when specifying PVC, to guard against the use of recycled or inferior grades of material during the manufacture of the waterstop. Rubber-based materials that provide a sealing mechanism by their ability to absorb water and expand should be used with care. Some products can generate large expansion forces that can cause local shearing failure adjacent to rebates if the sealant is placed too close to the concrete surface. Although recognizing the potential benefits of these materials, insufficient data are available to offer any guidance for their use. 3.13 JOINT FILLERS Joint fillers are usually boards of compressed cork or impregnated fibre installed across nearly the full depth of the joint, but leaving space near the surface for the sealant and a foam backing rod if required. Joint fillers:  Define the width of the joint.  Keep contamination out of the joint as it opens and closes.  Where joint sealing is required they give support to the sealant to enable it to resist pressure from the surface. Joint fillers will transmit some stress across the joint as it closes. Where stress transmission has to be minimised, expanding foam joint filler can be used, but these do not usually provide the same level of resistance to external pressure from service loads as does cork or fibreboard. Alternatively the joint thickness can be increased to reduce the degree of compression that will occur. The compressibility of the material in the joint should be specified where it is important. Typically a value of 5MPa maximum for 50% compression is used for bitumen sandwich board or corkboard while 1MPa max. may be appropriate for foam rubber used where less load transfer is required.

Page 19 3.14 SEALANTS Joint sealants are used to:  Protect joint edges from spalling.  To protect floors from chemical attack.  To keep debris out of the joint.  To make the building easier to clean.  To make joints look better. Sealants and associated items should be shown to be durable in the environment or usage to which they will be exposed and to have no detrimental effect on the retained liquid for at least the design life of the structure. Alternatively, they should be designed to be maintained or replaced at suitable intervals. The sealing performance is obtained by permanent adhesion of the sealing compound to the concrete each side of the joint only, and most sealants should be applied in conditions of complete dryness and cleanness. There are joint sealing compounds that are produced for application to surfaces that are not dry. The recommendations of the manufacturer should be followed to ensure that the sealing compounds are applied correctly to adequately prepared surfaces. It is necessary that the corners of the concrete each side of the joint are accurately cast as detailed with impermeable concrete to avoid water bypassing the sealant through the concrete. Recent experience has shown that polysulfide rubber compounds are attacked by chlorine and are not suitable for use in reservoirs containing treated water. An initial filling, with a higher than usual chlorine content for sterilization, hastens attack. Polysulfide sealants have also shown degradation under anaerobic conditions in sewage treatment works. Joint sealants are generally elastomeric elastomeric and are selected to suit design requirements that may include:  degree of flexibility required;  support required to edges of the joint;  contact with chlorine, sewage, waste water or other chemical; and  the ability to perform under water at prescribed depths. Joint sealants in most liquid retaining structures are subject to much higher hydrostatic loads than are generally acceptable in building applications as shown in the literature of manufacturers. The selection of sealant material will be based on the movement and hydraulic performance required by the designer. Guidance is given by reference to sealant manufacturers and their literature. Designers should be aware of the design differences for a sealant in Europe and, in some cases, more severe exposure situations in Australia. The designer should determine:  the size of the groove;  the shape of the groove;  when to place the sealant in relation to slab movement;  the primer type and number of layers;  the sealant material;  any action on the primer and sealant by the contained liquid; and  the life requirement and expectancy of the materials. In floor joints, the sealing compounds are usually applied to a groove along the line of the joint. The shape factor of a groove in building applications is usually 2 (width is double the depth) to keep stresses in the deformed shape to a minimum. In deep liquid-retaining structures, there is high friction from the pressure head, which does not allow the sealant to develop its most efficient deformed shape. In floor joints of the expansion type, the sealant must be supported and this is usually done with a joint filler. In many cases sealing can be delayed until just before the structure is put into service, so that the joint movement is stabilized and future movements will only be small. The groove

Page 20 should not be too narrow or too deep, which could hinder complete filling, and primer should be used before the sealant is applied. Many sealants perform better if two layers of primer are used. A bitumen-based sealant is usually applied to a groove with a practical minimum depth of 15 mm. Sloping groove sides are more effective but are not practical in most cases. The minimum width is 20 mm if the joint is not active (no movement); however if there is a movement, the joint will open and the crack will fill with sealant and, hence, a reasonable reservoir of sealant is required, resulting in a wider groove. Narrow and deep grooves can cause overstressing of the sealant and are not recommended for bitumen sealants. Shape factors of 1.0 to 2.0 are usual for bitumen sealants with perhaps higher values if large joint movements are expected. The action of an elastomeric sealant is elastic, rather than plastic for bitumen; however, the groove should be a minimum of 15 mm depth with a width of usually 15 mm to 20 mm. The shape factor should be in the range of 1.0 to 2.0. In building applications, a bond breaker rod/tape is required on the bottom of the groove to allow a concave upper and lower surface to develop when the joint is subject to a tension force, but in deep liquid-retaining structures the hydrostatic pressure forces the sealant into the bottom of the groove and it is doubtful whether a bond breaking rod/tape is effective. However, the rod/tape may be required for the situation when the structure is empty and subject to movements without the hydrostatic pressure. Vertical joints in walls should be primed where necessary and then sealed on the liquid face with a sealant that is usually pressured by gun or knife into the preformed chase. The sealants should have non-slumping properties and great extensibility. The long-term performance of a joint sealing compound depends on its formulation, the workmanship with which it is prepared and applied, the quality and finish of the concrete to which it is applied and the circumstances of the structure. It would be unwise to depend on joint sealants for liquid tightness in the long term, as that security should be provided by the waterstop. The sealing compound should also remain stable at the face of the joint and preclude the ingress of any hard objects that could impair joint movements. 3.15 LARGE CONCRETE ELEMENTS High temperatures generated during the hydration of cement should be carefully controlled to avoid excessive cracking. This is especially important in members greater than 1 m thick. The designers are to design the reinforcement for crack control in thick elements, elements with high restraint and/or elements with high cement contents in accordance with CIRIA C660 [58]. Prior to the trial mixes they shall confirm that the mixes are in line with the assumptions made for the crack control assessment and advise the Contractor of the construction limits the design is based on in accordance with CIRIA C660 recommendations. Prior to pouring concrete the Contractor/Designer shall show that based on design expectations including reasonable theoretical thermal and strength assumptions of the proposed mixes, allowable delivered concrete temperatures for the pour, expected ambient conditions at the time of pouring and the contractors proposed construction method, including insulation and insulation removal times, that: 1)

The maximum allowable temperature will not be exceeded.

2)

The internal temperature differentials assumed in the design will not be exceeded.

3)

The maximum estimated cool down assumed for external restraint analysis will not be exceeded.

Where the construction method proposed in the above analysis will produce temperatures within 5C of the maximum allowable limits based on design expectations noted above then the mix trials shall include specific heat rise data (e.g. semi-adiabatic tests). Where calculation using the measured heat rise properties shows that any of the insitu temperatures or temperature differential are within 5C of the maximum design limits then insitu temperatures and temperature differentials shall be monitored and stripping shall only occur when the measured differentials are such that, including the effect of thermal shock after insulation removal, the allowable differentials will not be exceeded.

Page 21 Precast T-Roffs have thick end sections and it has been shown that this can lead to localised high temperatures. The precaster shall provide a method statement showing how the precast beams are to be manufactured, how temperature will be controlled and what temperatures in the thickened end section are expected. Where these temperatures are at or near the specified limit the method statement shall show how the end section temperatures will be monitored and the control methods to ensure the specified limits are not exceeded. 3.15.1 Maximum Temperature High temperatures can induce microcracking and a chemical change to the cement hydrates such that the concrete strength is reduced by up to 30%. Delayed Ettringite Formation (DEF) can also lead to long term deterioration. Hence, the maximum temperature of all precast and cast insitu concrete during the first 28 days shall not exceed [77]: o 70°C for GP cement; and o 80°C for other approved cement systems incorporating SCM’s. 3.15.2 Temperature Differentials Within One Pour Where the body of one pour of concrete and the surface of the concrete heat up and cool down at different rates temperature differentials can lead to thermal strains. These strains, in conjunction with drying shrinkage and load induced strains can lead to internal restraint cracking. The maximum temperature differential for there to be no internal restraint crack with Granite aggregate shall be taken as 27°C. However, higher differentials are allowable when the crack width is considered. 3.15.3 Temperature Differentials Between Pours The largest risk of potentially deleterious cracking arises when one concrete pour is cast against another concrete pour, e.g. long walls on top of foundations. In general the new concrete is likely to be fully restrained by the old concrete. In these cases the crack widths are controlled by the reinforcement but crack widths also depend on the restrained thermal contraction and concrete properties. The ultimate width of thermal cracks may be added to by autogenous shrinkage, drying shrinkage and flexure. 3.16 POST-TENSIONED DUCTS AND GROUTING The grouting of post-tensioning ducts has been problematic internationally with some bridge failures due to poor grouting. In the USA and Europe there were major improvements in grouting techniques in the 1990’s but recent problems with grouting of ducts in the USA are being found particularly at the anchorages. No new techniques are proposed in this Recommended Practice for grouting. Recommendation: It is recommended that the latest techniques for quality grouting are considered for adoption through project specifications. It is also recommended that a formal process of quality assurance by testing for filling of grout ducts be implemented. 3.17 TIE BARS Metal tie bars are generally used to support the opposite faces of formwork on wall pours. When the supports are no longer needed the penetrations need to be dealt with such that they do not form future durability problems. On many elements the tie bars can simply be removed or snapped off beyond the cover zone. However, where there is a differential water pressure across the penetration particular attention has to be given to ensure the penetration does not lead to a long term durability issue. 3.17.1 Tie Bars Tie bars should not be allowed to remain in the cover zone. Where the tie rod is removed the residual hole must be dealt with.

Page 22 3.17.2 Tie Sleeve Holes Leakage through improperly sealed tie rod holes is a major problem in water retaining structures. Leakage through the hole and over the downstream face leads to evaporative concentration of contaminants over the leakage area so that as well as being unsightly and a possible environmental hazard it can lead to rapid concrete attack and reinforcement corrosion. Proper sealing of tie rod holes is essential but is often left as a note on the drawing that holes are to be sealed. Contractors are then bound to find the lowest cost method of achieving this. 1)

Above Water Where tie rod holes are entirely and permanently above water there is no potential for water leakage across the hole and adequate plugging of the cover zone will provide a durable structure. Adequate plugging means plugging with a material that will: a) penetrate the full cover zone. b) have insitu penetrability and chemical resistance properties at least equivalent to the surrounding concrete. Due account should be taken of the binder system and w/b ratio. c) have low shrinkage such that it will not shrink away from the surrounds and fall out. d) bond well to the sides of the hole. This includes taking account of the level of preparation of the hole sides. e) provide a reliability of success commensurate with the consequence of failure. Unless a proven proprietary system is used, then it is likely that failure will occur. Simple mortar plugs mixed on site have a high risk of failure. Site mixed mortars are unlikely to have the quality control required for this application and quality assurance costs are likely to make them uncompetitive. Precast plugs overcome many of the issues of site manufacture but the penetrability should still be evaluated. Propriety mortars, mixed in accordance with manufacturer’s recommendation are likely to be suitable where their performance is matched to the application. Mortar application generally requires a solid surface to push against in order to compact the mortar into place. Open holes do not provide the surface and some form of hole blocking will be required before application of the mortar.

2)

Below Water Tie rod holes below water are a frequent area of failure on liquid retaining structures. Particular issues are: a) Where the tie bar is fully removed and a sleeve is left, penetrability through the sleeved zone can be rapid leaving only the thin cover zones to provide real resistance to penetration. This is generally inadequate and should not be permitted. Even where the hole through the concrete is filled, leakage along the sleeve/filler interface is a risk. b) Where the tie bar is removed in the cover zone but a main portion of the bar is left embedded in the cover zone, water may penetrate along the tie bar concrete interface, particularly if high bleed leads to lenses under the bar. In the both these cases the most reliable method of sealing is to grout the hole through the wall, fill the holes through the cover zone as described for above water and use a membrane on the pressure face over a radius of 200mm from the tie bar hole.

Page 23 Recommendation: Tie rod holes in permanently above water concrete shall be plugged in the cover zone with a mortar that will have an insitu performance equivalent to the surrounding concrete. The Contractor shall prove this to the satisfaction of the Engineer with performance test results and a suitable application method statement. Tie rod holes in immersed zones shall be plugged over the full width of the hole unless. The material used for sealing shall have no net shrinkage and shall have a penetrability and chemical resistance equivalent to surrounding concrete. A membrane shall be applied to the pressure face over at least 200mm from the hole.

3.18 TIE WIRE To avoid durability issues with tie wire the following methods should be followed: a) Tie wire should have a similar composition to the reinforcing steel to prevent a galvanic cell occurring. b) Tie wire ends should be folded back so that they do not impinge on the cover zone c) Tie wire cuttings should be removed from the forms to prevent rust staining on the concrete surface. 3.19 DOWELS Dowel bars are used to transfer load across a slab joint that may open to an extent that aggregate interlock will not be effective. 3.19.1 Dowel Bars Dowel bars are usually located at the centre of a slab at 300mm spacing. The length and diameter of dowels depends on the floor slab thickness as this reflects the load that the dowel must support, refer Table 2. Table 2 : Dowel Sizes Slab Thickness (mm) 125-175 200-250 250+

Dowel Length (mm) 400 400 500

Dowel Diameter (mm) 12 16 20

For a dowel to provide load transfer but without inducing longitudinal restraint it must be free to move longitudinally and this means they must be straight, aligned correctly and debonded on one side of the joint. Potential issues are:  Dowel should be sawn rather than sheared as shearing can deform the ends so the dowel will not move freely.  Where two way movement is likely and dowel bars are only intended to move longitudinally the dowel will restrain transverse movement and may induce cracking. If on one side of the joint square dowels with compressible foam on both sides are used then transverse movement can be accommodated. Diamond dowels are designed to allow movement in two directions.  Dowels not aligned parallel to the surface and direction of movement will lock up. A Jig is generally required to ensure correct alignment.  Dowels displaced during placement can lead to unacceptable alignment. The alignment of all dowels should be checked immediately after placing is complete at the joint.

Page 24  Dowels are not properly debonded. Use of plastic sleeves ensures debond and good load transfer.  Sawn joints with dowels may not be perfectly located and hence the dowels must be debonded along a sufficient length to ensure the dowel will be debonded on one side of the joint.  In expansion joints the dowel must have a compressible end. This is achieved using a compressible cap.  Corrosion of the dowel at the joint is a potential issue. In most environments a galvanised dowel will provide adequate protection but in marine exposures a stainless steel dowel may be required.  Maintenance of the dowel at an adequate depth is required in order to prevent shear failure. 3.20 SAW CUTS IN SLABS Saw cuts are placed in slabs so that a crack emanating from the base of the saw cut will propagate through the slab. The crack forms as this is the weakest point and the surface appearance of a straight saw cut is far better than the appearance of a ragged crack. The straight saw cut is also easier to seal. Saw cuts in conventional concrete must be one quarter the depth to be effective and in steel fibre concrete they should be one third the slab depth. The cutting must take place before thermal contraction strains are sufficiently high that the slab cracks in an unplanned way. Common issues leading to surface cracks that may become a durability issue are:  Saw cut depth inadequate. Depth of cut should be checked on all projects at the time of cutting so that the cut can be made deeper before the slab cracks in the wrong place.  Slab thickness variation leads to locations where the slab is thinner than the cut thickness so that cracks form at the thin section. Designers should play close attention to the falls in slab and design the falls and sawn joints accordingly.  Slabs are cut too late. The slab strength must be sufficiently high that the aggregate is cut rather than flicked out and not so late that it’s too late. Typically cutting in the 1224hrs from casting is suitable but the timing should suit conditions. In hot weather cutting on the day of the pour may be necessary whereas in cooler weather first thing the following day may be better.

Page 25 4. PRE-POUR PLANNING It is useful to carry out an audit of the suitability of intended activities, product supplies and placement equipment prior to pours of critical elements. Development of “check-lists” and the constructor arranging for a meeting of representatives of all parties participating in the placement some time before it takes place are useful measures to ensure that the intended quality will be achieved. Some of the more common aspects requiring pre-pour planning are covered in this section. 4.1

OWNERS REQUIREMENTS The owner’s requirements have a large impact on durability design and these filter down to construction requirements through the specification. Owner’s input at the durability planning stages is considered in Z07/01 Durability Planning that forms part of this Durability Series.

4.2

COMMUNICATION BETWEEN CONSTRUCTOR, CONTRACTOR, SUPPLIER AND DESIGNER The development of controlled means of communication between the parties on site is critical to the achievement of the designed level of quality for a concrete structure. A useful means of driving this communication is through pre-pour meetings to discuss processes and find agreed solutions to any issues raised. Contractors often order a concrete grade without telling the supplier the full details of what is to be achieved. With the high level of practical concrete technology involved in modern concretes and the important aspects of quality control it is important that the concrete supplier is involved in project planning. The concrete supplier cannot be expected to be held to supplying a product that is fit for purpose if he does not know the full details of the form of the pour and how concrete is to be placed, compacted and cured. Designers also need to appreciate that the concrete supplier has a lot to offer in terms of his knowledge of producing a concrete that will be of high quality in a wide range of applications. By close liaison with the designer the supplier can advise how to specify a mix that can be practically supplied and meet the designer’s needs. Recommendation: It is recommended that the concrete supplier be involved early in the project and in pre-pour meetings.

4.3

CHECKS ON DESIGN AND “BUILDABILITY” One of the more common issues that are discovered during pre-pour reviews is a miss-match between design, reinforcement detailing and the methods of placement intended or the concrete mix specification. Some common issues may include in-appropriate formwork support for the intended placement rate, reinforcement spacing being too low to allow for the concrete aggregate size specified, no space between reinforcement to allow for vibrators to be inserted into the concrete to achieve full compaction, no consideration for tolerance of placement position for reinforcement and embedments to allow for adequate cover to exposed surfaces and inability to get access for placement equipment to allow placement of concrete close to its intended location in the structure without causing segregation. Advice on solutions for specific “buildability issues” is provided elsewhere in this document.

4.4

CONCRETE SUPPLY Prior to each placement it is necessary to check the following:  Can the concrete supplier provide the specified concrete at the required rate?  Are the delivery equipment (e.g. pump, skip etc.) and placing personnel (e.g. spreading, compaction and finishing teams) capable of handling concrete at the proposed delivery rate?  Has the Supplier provided adequate validation of the properties of the intended concrete mix as specified? This may include supply of historical test results for specified properties or acknowledgement of specified properties with warranty.  Has the agreed Inspection and Test Plan is being carried out including checking that appropriate facilities exist on site for the specified testing to be carried out?

Page 26  Have contingencies for hot or cold temperature conditions been made, including the control of concrete delivered temperature?  Have concrete delivery trucks got suitable access to the delivery point on site taking account of variable weather conditions and traffic conditions on the day of the pour?  Will concrete delivery times from the concrete plant to the delivery point on site give potential for significant loss of workability of the mix during the delivery and placement process?  Is the specified concrete suitable for the method of placement intended? The most common issues are the suitability of maximum aggregate size in the concrete to reinforcement spacing restrictions and the impact of placing concrete via a pump on the intended mix design and subsequent impacts on concrete properties. 4.5

PLANNING CONSTRUCTION The development of a clear work method statement will assist the process of planning for concrete placing, finishing and curing. This also aids communication of the process to concrete contractors working on the project and will ultimately assist with assuring the quality of the finished concrete. 4.5.1

Weather The weather can have a major impact on concrete quality and hence the Contractor should review the weather for the day of the pour and either plan on taking the measures listed in Table 3 or if these cannot be achieved to postpone the pour. Table 3 : Planning For the Effects of Weather

Event

Effects

Planning Measures

High ambient temperature

High concrete delivered temperatures

Will there be enough chilled water and ice available through the pour? Are shades for aggregate in place? Is sufficient clean water available for spraying aggregates?

Fast evaporation rates

Is the evaporation retarder available at the pour in sufficient quantities? Is their suitable equipment for repeated application of evaporation retarder? Is there a clear understanding of how the evaporation retarder can be applied? Does the placing plan have sufficient tolerance that problems will not arise due to the time from adding water to cement to: A) discharge? B) initial compaction? C) covering with the next layer and compaction through the top layer?

Short open times

Low temperatures

Long hardening times

Have adequate provisions been made for: A) Insulation of the pour? B) Adjustment of stripping time?

Rain

Damage the fresh concrete surfaces Thermal shock

Will the pour be covered while the concrete hardens?

Wind

Plastic shrinkage cracking

4.5.2

Are materials and labour available for timely installation of waterproof insulated surfaces? Will wind breaks need to be erected before the pour and if so what height will they need to be to be effective? Will wind increase the likelihood of plastic cracking that evaporation retarders will be required and if so are materials and suitable equipment available for application and re-application?

Exposure When casting concrete in severe exposures curing measures should ensure that the concrete is not unduly exposed prior to achieving pores discontinuity. In general, in severe exposures curing for seven days is required and this curing should ensure that there is no penetration of the concrete by contaminants in the curing period. The major potential issue is where unsaturated concrete pores are exposed to contaminated water leading to rapid

Page 27 ingress by sorptivity. The principle methods of preventing this are to maintain saturated concrete pores by the curing process. Less than saturated pores may occur when using curing compounds but in that case the curing compound may act as a barrier. Following completion of curing it is assumed that the concrete pore structure will be sufficiently closed to prevent abnormally high penetration of contaminants. 4.5.3

Concrete Delivered Temperature AS 1379 limits concrete delivered temperature (CDT) to 35C but many specifications (e.g. most Main Roads specifications) limit the maximum temperature to 32C. On larger projects where the concrete performance is critical there is sound reason to reduce the maximum concrete delivered temperature to 32C by the specification while for the majority of small low key projects reduction of the industry standard 35C is not practical. Samarai [78] notes that the heat of hydration increases rapidly when the curing temperature exceeds 25 to 30°C. At high temperature (>30°C) the reaction rates become increasingly more difficult to control and predict and hence great care is required if the concrete is to be placed successfully above 32°C.

Recommendation: The specifier should include an item in the specification for maximum CDT and not leave it to the default 35C in AS1379. A 32C maximum CDT is recommended for pours: a) with a minimum dimension of 600mm or greater. b) where the concrete will be in severe exposures. c) with potential for placing or finishing complications. It is recommended where CDT lower than 32 C are considered beneficial to reduce maximum concrete temperatures or maximum temperature differentials that the specification include limits for these and the contractors attention be drawn to the need for compliance by use of low CDT, insulation or cooling pipes. In other situations it is recommended that the required CDT be 35C in accordance with AS 1379. Lower limits may be applied for a project or for specific elements where thermal issues apply although from an economic perspective the specifier may also permit use of insulation or cooling pipes. The pragmatic solution may be to specify maximum insitu temperatures and temperature differentials with associated monitoring and leave the contractor to work out how best to achieve them. 4.5.4

Time to Concrete Discharge AS 1379 requires that all concrete is discharged 90 minutes from commencement of mixing or before proper placement and compaction of the concrete can be achieved, whichever comes first. Following concrete discharge it must be placed and in many cases a layer of concrete placed over it and vibrated through before it reaches a state where it cannot be compacted. In terms of the condition of the concrete at each time interval the following can be taken to apply: 1)

At discharge : No significant slump loss due to cement hydration. Slump loss may occur due to hydration of the cement or due to evaporation of the mix water. If significant slump loss has occurred due to hydration then the concrete must be rejected as it will be impossible to ensure that full compaction is achieved as well as concerns that the cement hydration will produce the required performance. If the slump loss is due to mix water evaporation there may be no harm caused to the cement hydration but the ability to fully compact the concrete will still be impacted and so the batch should be rejected. Hence it is taken that if there is slump loss at the time of discharge the concrete shall be rejected.

2)

Initial compaction: After discharge the concrete will have to be transported to the point of placing at which point it will be compacted to remove entrapped air. This transport time will be relatively short and it is generally accepted that there will have been little stiffening due to hydration over this period. No limit to this period is included in the

Page 28 codes and it is recommended that specifications limit the time between discharge and initial compaction to remove entrapped air to no more than 30minutes unless longer times are justified by trials. 3)

Through compaction: Frequently one layer of concrete is covered by the next layer and the interface has to be compacted to ensure no cold joints are formed. Codes make no provision for this time interval and the fluidity of the covered concrete cannot be assessed. For the layer of concrete being covered the purpose is not to remove. The poker vibrator will only just break the surface of the lower layer and hence some stiffening due to cement hydration will not be unduly prejudicial. Through compaction should however occur before initial set. This may be taken for practical purposes as no greater than 60minutes after discharge.

Recommendation: Specifications should state the allowable time from discharge to initial compaction and time from discharge to through compaction. Where no other mix information is available these should be 30 minutes and 60 minutes respectively. However, regardless of these limits the specification should require that: a) the concrete is properly compacted to remove air such that insitu concrete complies with the specified strength and durability requirements. b) the concrete is placed and compacted such that there are no cold joints such that the bond strength across layers is lower than that of concrete where there are no layers. AS 1379’s 90minute limit is frequently applicable at the 35C concrete delivered temperature limit given in AS1379 for a concrete with GP cement and no admixtures. However, two significant variants may need to be allowed for:

4.5.5

1)

Concrete temperature has a major effect on hydration rate and where the temperature of the concrete is less than 35C the allowable time to discharge may be extended.

2)

Most high performance concrete will contain admixtures and there is a reasonable prospect that the cement will not be GP. This can lead to accelerated or retarded hydration.

Pumping CIA recommended practice Z12 deals with pumped concrete and its recommendations should be followed to ensure pumped concrete will be durable. Historically pumped concrete was considered to be inherently durable where penetrability was the primary concern because to achieve a pumpable concrete higher fines contents are required and this generally meant high cement contents and low water/cement ratios. This may still apply to lower strength grades of concrete but higher strength grades of concrete may already have high fines contents and pumpable concrete is no guarantee of sufficiently low penetrability for severe exposures. The key pumping aspects for concrete durability are: a. Cement Systems Cement systems are a critical part of durability design. Although all cement systems can be used for pumped concrete they can have an impact on the mix design requirements. Silica fume – the fine particles reduce bleed and help lubricate the mix. Hence silica fume is particularly useful in achieving a pumpable concrete particularly where the pump line is tortuous or tall. On tall building projects silica fume has been included just to improve pumpability by reducing bleed under very high pressure. As silica fume also leads to low penetrability and high resistance to chemical attack it doubles as a durability enhancer and may be the ideal means of achieving pumpable concrete where durability is a concern. Conversely the very low bleed achieved may mean that strict measures will be required to avoid plastic cracking on flatwork.

Page 29 Ground Granulated Blast Furnace Slag (GGBFS) – the performance of GGBFS is very dependent on its fineness. Modern slags are frequently quite fine and they behave similarly to GP cement. However coarse slags can lead to high bleeds which then have to be controlled by other mix design aspects. Fly Ash – fly ash tends to have rounded particles and a reasonable proportion of fines that help pumbability. The improvement in performance are similar to silica fume but to a less marked extent. The durability performance enhancements of the fly ash are also less marked than with silica fume. Fine Fly Ash – the finer portion of the fly ash can be extracted separately. Fine fly ashes can improve concrete pumpability and provide durability benefits that approach those of silica fume. b. Admixtures Various admixtures are available that specifically improve the pumpability of the concrete. In general these only impart durability benefits where they also lead to a reduction in water/cement ratio. c. Aggregate Grading Aggregate grading is particularly important to a pump mix. In order to avoid bleed under pressure the grading should aim towards achieving effective perfect packing where the finer particles fill the gaps between the coarser ones. This tends to mean pumped mixes will have low bleed which means that they will be susceptible to plastic shrinkage cracking on flatwork. d. Pump Priming Priming of pumps with a grout mix is required to prevent grout from the concrete being naturally extracted to line the pipes which can lead to blockages. Most grouts will have lower performance than the higher durability concrete and hence a suitable grout mix to match the concrete binder, admixtures and water/binder ratio should be developed to maintain concrete quality. In the case of durable concrete construction the grout must be completely pumped out and disposed prior to placing the design concrete into its forms. e. Blockages Where blockages occur considerable time can be lost between placing successive parts of the pour leading to cold joints. These cold joints should be dealt with in the same way as other cold joints caused by delays to a concrete pour. f. Free Fall Free fall of concrete within a concrete pipe line should be treated the same as free fall as discussed in Section 4.5.6. However in general vertical drops of a concrete pump line are designed such the concrete acts like concrete in a tremie pipe and there is no free fall. 4.5.6

Maximum Drop Height Concrete placing operations are often planned to allow for the free fall of concrete. This planning must also consider any segregation that might occur when the concrete free falls into place. Techniques such as placing concrete with drop chutes or through windows in wall forms are thought to minimise the effects of concrete free fall. Using these measures unnecessarily, however, can increase concreting costs without improving the in-place quality of the concrete. Sometimes specifiers and inspectors dictate the maximum free-fall distance of concrete because they believe limiting free fall is necessary to minimize concrete segregation. Commonly they limit the free-fall distance to 1 to 2 meters, but occasionally the limit is as little as 0.5 meter. Neither ACI 301-9914 nor ACI 318-0215 limit the maximum distance concrete can free fall. ACI 304R-0016 states that "if forms are sufficiently open and clear so that the concrete is not disturbed in a vertical fall into place, direct discharge without the use of hoppers, trunks, or chutes is favourable." There is no limit on free fall in Australian standards but

Page 30 that might just be because the code writers believe it is something for a specification or an item to be determined on site when the mix and conditions are known. At least four field studies have shown that free fall from great distances doesn't reduce concrete quality. Although all the field studies have been for caissons, the results should also apply to other structural elements such as walls, columns, and mat foundations. 

In the Chicago area, contractors routinely construct concrete caissons by allowing the concrete to free fall to depths of up to 150 feet (46 meters). Full-length cores taken from more than 100 of these caissons over a 30-year period have shown no evidence of segregation or weakened concrete39.



Results of a field test by Turner40 to determine if concrete could be dropped vertically 15 metres into a caisson without segregation proved that there was no significant difference in aggregate gradation between control samples as delivered and free-fall samples taken from the bottom of the caisson.



A 1994 FHWA study42 provided test data leading the investigators to conclude that "the general expectation that (concrete) striking of the rebar cage will cause segregation or weakened concrete is invalid" and they found "no segregation or strength differences between low- and high-slump concrete mixtures".



More recent field studies indicate that free fall of concrete from heights of up to 46m directly over reinforcing steel or at a high slump - does not result in segregation of the concrete ingredients nor reduce compressive strength. The studies showed:  Segregation: no evidence of segregation from high level free falls.  Strength: no significant deviation of concrete strengths from sample cylinders made at the time of the placement.  Concrete hitting rebar: The general expectation that concrete striking the rebar cage will cause segregation or weakened concrete is invalid.  Effect of slump: There was no segregation or strength differences between the lowand-high-slump mixes.  Mixing soil with concrete: When concrete was directed into the soil sides and rebar cage in a caisson, there was a problem caused by the soil to slough off and mix with the concrete.  Cost: Restricting free fall heights decreases concrete production rates, thereby increasing owner's costs without increasing concrete quality

In 1999, the Federal Highway Administration eliminated its 25-foot (8-meter) free-fall limitation and now allows unlimited free fall of concrete as they found free fall of concrete from heights of up to 150 feet (46 meters). FHWA notes that restricting free-fall heights does decrease concrete production rates, which increases owners' costs without increasing concrete quality. Recommendation: A concrete pour height shall only be limited where: a) it may cause soil to be mixed into the concrete. b) reinforcement may act as a filter for the aggregate due to concrete flow at the base of the pour. c) it will leave a grout film on forms or steel that may dry and interfere with performance. 4.5.7

Placing Part of planning is to ensure that: 

Sufficient personnel are available for a placement to carry out the work detailed in the work method statement.



The proposed concrete delivery rate can be achieved by the batching plant given the plants output capacity, the storage and delivery of aggregates and cement, the supply of chilled water where required, the truck mixers available having made due allowances for transport times, pump capacity.

Page 31 

At the proposed concrete delivery rate, and given the sequence of placing, no cold joints will arise. On one tank project the length of wall pour and thickness of lift was such that at the maximum concrete delivery rate the return interval was so long cold joints between lifts was inevitable.



Sufficient poker vibrators will be available to compact the concrete placed. The number of poker vibrators required to compact a pour can be calculated based on the concrete delivery rate the thickness of each pass, the radius of action of the poker vibrator and an allowance of specifically known or assessed periods of insertion of the vibrator that produces the required compaction without segregation of the mix for each poker insertion. Example: Vibrator with 57mm diameter head has a radius of action of 330mm. Insert every 467mm max to get full compaction (Figure 16). To provide a margin allow for insertion every 400mm. Hence each insertion compacts 0.16m 2. If the insertion rate is every 6secs then the compaction rate is 1.6m2/min or 96m2/hr. If the concrete delivery rate is 40m 3/hr delivery and the pass is 0.4m thick then the placing rate is 100m 2/hour. If each vibrator can compact 96m2/hr then at least 1 active vibrator would be required and it would be reasonable to have one on standby. Figure 16 : Area Compacted by a Vibrator

4.5.8



that the workability will remain suitable for placing throughout the placing and compaction



It is important to note that the concrete workability (slump) and concrete thickness will impact on the vibrator insertion rate noted above.

Plastic Cracking. There are three types of plastic cracking that the designer can influence, i.e. plastic shrinkage, plastic settlement and autogenous shrinkage cracking. Plastic shrinkage cracks (Figure 17a) occur when the rate of evaporation (e, lt/m 2/hr) exceeds the rate of bleed water arriving at the concrete surface (b, lt/m 2/hr). There are many documents that suggest that if e < 1 lt/m 2/hr plastic cracking will be a low risk and evaporation retardation is not required. This may have been a reasonable approximation for typical concretes when this approximation was first developed but concrete bleed in modern concretes can be very low and the evaporation rate at which plastic cracking could occur may now be lower than 0.2 lt/m 2/hr for some concretes. Plastic settlement (Figure 17b) occurs when the solids settle so much that the surface level of the concrete drops (s, mm) and where it gets hung up by the reinforcement or bridges between formwork in walls cracks form. Settlement cracks may also occur when there is a change in section thickness and differential settlement occurs. As solids settle water rises and hence plastic settlement occurs when the bleed is higher than acceptable.

Page 32 There is no absolute upper bleed limit for all concrete as the pour thickness and concrete rise rate are also strongly influential. For diaphragm walls and bored piles an appropriate bleed limit is 1% with a target value of 0-0.2%. These deep pours are constructed to have a full height of fresh concrete under a high pressure of self-weight. These factors lead to high accumulated bleed even for low bleed concrete. High accumulated bleed can lead to reduced cover due to deep bleed channels, bleed lenses under bars that can effectively form irrigation channels through the concrete and a porous concrete at the surface of the pour (vertical and top faces). For thick concrete bases bleed might be limited relative to expected placing rate. A concrete poured at 2m depth/hr requires a lower bleed than one poured at 0.3m depth per hour. A general limit of 3% bleed seems sensible. Figure 17 : Plastic cracking occur when there is inadequate thought given to controlling bleed and/or bleed effects a)Too little bleed Plastic Shrinkage

b)Too much bleed Plastic Settlement

Concrete bleed can vary enormously. Table 4 shows selected results from exhaustive trials by a premix supplier. The results do not give any assessment of the admixtures but do show the variability that can occur by mixing admixtures, (e.g. Mix 5). Table 4 : Bleed of various 50MPa mixes using LH cement (65% slag) Mix 1

Manufacturer 1

Manufacturer 2

WR / Copolymer HRWR

2

P.Carb HRWR/ SRA WR

3

WR /Copolymer HRWR

4

WR / Mod.Nap. HRWR

5

WR/SRA/Mod.Nap. HRWR

SRA WR

Initial Slump With SP (mm)

Final Slump (mm)

Bleed (%)

230

120 @ 3.5hrs

0.3

220

200@ 5rs

1.2

240

130 @ 90mins

2.8

230

139 @ 2hrs

6.5

240

180 @ 5.5hrs

22.2

Bleed can be measured in accordance with AS1012 Pt 6 but this is seldom undertaken as it is not specified routinely and not a requirement of the current AS1379 standard and most specifiers do not know how to interpret the result. Bleed test results combined with monitoring ambient conditions are the key tools in determining the risk of plastic settlement and plastic shrinkage cracking. The principle factors affecting bleed rate (b, lt/m 2/hr) and settlement (s, mm) are: B (%) = Bleed (% free water) measured in AS 1012 Part 6 bleed test W (lts/m3) = the free water in the concrete H (m) = the thickness of pour T (hrs) = the length of the bleed period as obtained from the bleed test. P (m/hr) = rise rate of concrete For any concrete pour there are three possibilities:

Page 33 i) the concrete is placed to full thickness on one placement then s = BWH and b = BWH/T ii) concrete is placed before bleed of underlying ceases i.e. where H/PT () then s= 0.5BWPT and b = BWP The bleed rate needs to exceed the evaporation rate (i.e. b
Settlement

Case a: 2m column, 3% bleed, placing 1m/hr

7.2mm

9am 3pm Summer 9am Winter 3pm

Evap./Bleed (lt/m2/hr)

Case b: 300mm Slab, 1% bleed, placing one pass 0.5mm

1.8 1.4 1.0 0.6

0.2 Sydney Melbourne

Perth

Brisbane Canberra

Hobart

Darwin

Excessive plastic settlement does not always exhibit itself as cracking over bars. The concrete may hang up over bars without cracking while settlement channels form under the bars. Several cases have been reported where corrosion has commenced at the underside of bars as chloride contaminated water flushed through these ‘irrigation channels’. This also affects the bond and hence the efficiency of the steel in controlling crack spacing and width. It would be possible to include this assessment method in a guide and require that the construction procedures and mix be designed such that b
Page 34 similar in appearance to plastic shrinkage cracking. Like plastic cracking it tends to occur in high performance concrete (low w/c concrete with slag or silica fume) and may only be detected because extreme measures to protect against plastic cracking does not prevent them. As autogenous shrinkage cracking occurs with no net loss of water it can only be prevented by having an appropriate mix and particularly binder and admixture combination. Designers and concrete suppliers need to be sure they do not specify or supply concrete where excessive autogenous shrinkage leads to cracking. 4.5.9

Compaction A major area of rework and a high risk for potential durability failures is poor compaction. Where concrete is not compacted it may be severely honeycombed or have a high voids content. In the former case the concrete is likely to be repaired but always present a higher durability risk. In the latter, and many cases in between, it may go unnoticed until premature deterioration occurs as a result of rapid contaminant ingress in high permeability concrete. In critical work it will be important to verify that adequate compaction of concrete has taken place using the agreed work method. This assessment can be made by extracting concrete cores from the structure or a “mock-up” structure and compare the “saturated surface dry condition” density of the concrete cores with the density of standard concrete test specimens cast from the same batch of concrete in accordance with the relevant sections of AS1012. The average densities of the cores should be no less than 99% of the concrete cylinder density measured using the water displacement method.

Recommendation: To help ensure compaction is properly undertaken in the field it is recommended that the specification requires compaction follow the procedures given in CIA CPN 3343 and that the pour specific method statement include: a) calculations to show the proposed number of active vibrators, radius of actions and depth of vibration for each layer is sufficient to compact the concrete at the proposed concrete delivery rate b) number of vibrators on standby c) method of access for compaction including where necessary provision for clear drop lines, access methods for people and poker vibrators (e.g. ‘windows’ in forms) d) technical support information verifying that vibrating screeds will achieve the compaction required if this is to be greater than 200mm e) technical support information verifying the form vibrators will achieve the compaction required 4.5.10 Finishing The finishing and curing processes can have a direct impact on the finished surface durability of concrete and so planning to provide the necessary finish to the concrete also requires consideration. Correct finishing of an exposed surface of high durability concrete is about timing, i.e.: a)

Before final surface finishing takes place it is critical that concrete bleeding has ceased and that it is close to initial set of concrete but not past it. The purpose of any finishing process is to increase the density of the surface layer of concrete and also flatten it to the desired quality of finish. Processes that work water (bleed water or other sources) back into the surface layer will only serve to reduce the density and durability of the surface layer and must be avoided.

b)

If the concrete pours are sealed by finishing thereby preventing bleed water and air escaping a failure plane just below the surface can occur. Where this occurs as spots due to removing the vibrator it is called a blister but more wide spread damage due to finishing is called delamination. This phenomenon is often said to be caused by “finishing too early” but it may also be the result of an inappropriate mix. A sufficient window for finishing is required.

Page 35 c)

Finishing too late leads to tearing of the concrete surface.

4.5.11 Curing There are three primary aspects to curing, i.e.  between screeding and finishing to prevent plastic shrinkage cracking and top down setting;  after finishing to prevent interference with hydration due to water loss from the hardened concrete; and  thermal curing to reduce temperature differentials. Curing should be maintained in place for at least the period where the concrete has attained the properties that are assumed in the design including durability properties of the layer of concrete protecting the reinforcement. Early Age An effective means of minimising loss of water by evaporation is to spray exposed surfaces with an evaporation retardant such as aliphatic alcohol immediately after screeding and as required until the concrete is ready for finishing operations. If this is not achieved successfully finishing procedures may close the top of plastic cracks such that they only become apparent some time later. Long Term CIA Z0945 provides information on curing methods but gives little indication of the relative performance achieved using different curing methods. AS 3600 indicates minimum curing periods of 3 to 7 days depending on the exposure classification and these are appropriate for general construction. Alternative curing may be used provided it leads to concrete of suitable durability. While the durability provided by different curing methods may not match the performance of full water curing they may be adequate on a case by case basis. Some concrete members are cured by leaving the formwork in place, particularly on vertical surfaces and the underside of horizontal members. Forms should be left in place for such elements for at least the specified curing period. After stripping the forms from these surfaces, curing can be continued by the application of an impermeable covering to minimise moisture loss. However this may still not provide the same level of performance as water curing. Curing compounds (external and internal) may be the only practical method of curing some surfaces where early removal of formwork is required. Although curing compounds that meet the requirements of 90% water retention at 72 hours when tested in accordance with AS3799 improve concrete quality significantly relative to no curing they still do not lead to the same performance of water curing. Curing of concrete takes place after all finishing processes are complete. Suitable methods of curing include (in descending order of effectiveness): 

Maintaining water saturation of concrete.



Covering the exposed surfaces of concrete with wet absorptive material and wrapping this with sealed plastic.



Covering with sealed plastic film.



Retaining formwork in place.



Applying a curing agent complying with AS3799.

When undertaking durability designs based on performance of water cured cylinders an allowance must be made for the expected lower performance of other curing methods.

Page 36 Meeks 46 provides a comprehensive review of curing giving a wide range of data on the effect of curing on performance. This includes the work of Haque 47 shown in Figure 19 which highlights the effect on strength of different curing methods and cement systems. The curing comprised of 1 day in moulds, followed by 0, 7 or 28 days in a moist curing room followed by curing at the temperature and humidity shown until testing at 91 days. This is included to show how critical it is to take curing into account when assessing durability performance. Figure 19 : Effect of Curing on Compressive Strength 70

Compressive Strength (MPa)

GP

GP+30% FA

60 50 40 30 20 0d, 0d, 0d, 0d, 7d, 7d, 7d, 23°C, 45°C, 45°C, 23°C, 23°C, 45°C, 45°C, 95% 20% Water 45% 95% 20% Water RH RH Tank RH RH RH Tank

7d, 28d, 28d, 28d, 28d, 23°C, 23°C, 45°C, 45°C, 23°C, 45% 95% 20% Water 45% RH RH RH Tank RH

Curing Recommendation: It is recommended that when concrete performance is measured on water cured cylinders that the insitu performance be estimated based on the curing and the method of estimation and result be given in the durability plan. The estimated insitu performance should then be used in any modelling. Thermal Thermal curing refers to the control of the maximum temperature or temperature differential by curing methods. This might comprise use of insulation or cooling pipes and is discussed in Z7/07 of this durability series. 4.5.12 Steam Curing Steam curing could lead to problems arising from excessive concrete temperatures or rapid change in temperature. In order to ensure steam curing will be correctly carried out it is recommended a method statement be developed. Recommendation: It is recommended that where steam curing will be used that a method statement be required. This should include: a) how the steam curing will be undertaken to show the maximum temperature allowed in the Durability Plan will not be exceeded; b) how the temperature will be measured, recorded and the reported to the Systems Administrator promptly and in a secure fashion; c) specific measures for the first manufacture cycle; d) methods of uniform application of steam; and e) method of assessing insitu strength (e.g. maturity and/or cylinders).

Page 37 5. QUALITY OF CONCRETE Quality is defined by the Oxford Dictionary as the standard of something as measured against other things of a similar kind. Hence a high quality concrete in durability terms will be one that suffers less deterioration than other concrete in a given environment. The deterioration of concrete is determined by a combination of its resistance to: 

penetration of contaminants in the gases and liquids to which the concrete is exposed; and



deleterious reactions between the concrete and the penetrating contaminants.

Hence the quality of concrete in regards durability can be defined by its penetrability and chemical nature. Penetration of concrete by gases and liquids can be by permeability, absorption or diffusion. Permeability is penetration under a pressure head, absorption is the ingress due to capillary action and diffusion is where species migrate due to a concentration gradient. In many deterioration processes if the concrete is relatively impenetrable then it will prove to be durable as the reaction process may be a small part of the overall resistance or it may be that without penetration there is no reaction. Penetrability of concrete will determine the degree of ingress of aggressive agents from the environment. Concrete of high penetrability will allow the ingress of agents such as carbon dioxide (carbonation) and chloride ions as well as moisture and oxygen. Hence penetrability is a key factor in durability assessments but there is no direct measurement method for penetrability. It must be measured by one of many tests for sorption, permeation or diffusion. Similarly there is no standard test for concrete of its chemical resistance and again one of the many non-standard tests has to be selected for a measure of concrete quality in regards the resistance to the specific exposure. This report does not provide details of any of the durability test methods that may be used to assess deterioration as these are detailed in Z7/07. Historically permeability has been the most common measurement method used and the permeability is often taken to be synonymous with penetrability. For a given concrete mix, diffusion and sorption may be related to permeability although various factors impact. This means the relationship is not constant from mix to mix. Hence caution is required when using permeability as a general statement of penetrability, particularly when the ingress mechanism is not by permeation. Cracking may increase the permeability of concrete and the susceptibility to deterioration. This is more critical in some mechanisms than others. Hence permeability measured on uncracked concrete cylinders may have little relevance to insitu concrete where cracks are numerous. 5.1

PERMEABILITY Concrete permeability is influenced by numerous factors, the most important being the water/cement ratio and the extent and quality of curing given to the concrete. Aspects of mix design, notably cement content and aggregate proportioning are the major factors in producing a cohesive and workable mix, thus facilitating proper placing and compaction to produce dense impermeable concrete. As permeability is a common measure of a key factor in durability terms it is discussed first as a measure of concrete quality. 5.1.1

Permeability and Water Ingress In some cases water penetration through a concrete element with air one side and water under pressure on the other can lead to durability problems. Water penetration into concrete can be the source of chemical attack of the concrete and its contained reinforcement through leaching of contained lime and the presence of dissolved salts and gasses in water. As well as this the permeability of concrete to gases themselves may lead to similar durability problems.

5.1.2

Permeability and Porosity Within concrete, there are pores in both the cement paste and the aggregate. On occasions, concrete also contains larger voids caused by incomplete compaction and/or

Page 38 bleeding. Since the aggregate particles are normally completely enveloped by the cement paste and have insignificant penetrability compared to the paste, it is the permeability of the hardened paste that has the greatest effect on the permeability of the fully compacted concrete. The porosity of a material is the proportion of its volume that is occupied by voids. While permeability and porosity are often related, they are not the same property. The word 'pore' means a void, which may or may not be connected to other voids by capillaries. Therefore the word 'porous' in this document should be taken to mean having pores, and not as allowing fluid to progress easily. The extent to which the pores are interconnected as well as the volume of the pores present will be reflected in the permeability of the concrete. These are the strictly technical meanings of these terms as used in this document and generally accepted in technical publications on concrete throughout the world. Permeability of a material therefore is not a simple function of its porosity but depends also on the size, distribution and continuity of the pores. The permeability of the hydrated cement paste is controlled by its capillary porosity, which in turn depends on the watercement ratio and on the degree of hydration. As the water-cement ratio is increased, so the permeability is increased. As hydration proceeds, so permeability decreases. The permeability of concrete to liquids should be distinguished from its permeability to gases. Permeability is measured by the rate of flow under a constant pressure. The permeability to water is not a true measure of the mechanism by which moisture from rain penetrates into the concrete to the level of the reinforcement. There is no Australian standard test for this although “water sorptivity” and “the apparent volume of permeable voids” may be suitable indicators of this property. This is discussed in Z7/07. 5.1.3

Permeability and Strength The strength of concrete is also a function of its water-cement ratio with strength increasing as the ratio is reduced. All other factors being equal, the higher the strength, the less permeable and more durable is the concrete. Concrete strength is therefore generally used as an indicator of the quality of the concrete in Australian Standard AS3600. However, specification of a 28-day compressive strength satisfying structural design requirements may not ensure a concrete quality that is adequate for durability purposes. It should be noted that the compressive strengths necessary to provide durability under various exposure conditions may be significantly higher than would be necessary for loadcarrying purposes. Considering its importance as far as cracking is concerned, it would seem, for example, that the development of tensile strength, particularly at early ages, may be more relevant to durability requirements than is compressive strength.

5.2

SORPTIVITY Sorptivity of concrete is a measure of how rapidly water is pulled into concrete by capillary forces. Where pressure heads are low the ingress rate by capillary action is much quicker than by permeation. Hence its use as a quality indicator can be important in splash zones or areas where capillary rise leads to deterioration e.g. plinths. There are several tests that are related to sorptivity, e.g. absorption and porosity and volume of permeable voids but do not necessarily give the same result or results that have a constant relationship with each other. There are also different methods of measuring sorptivity itself that do not give the same value. This is discussed in Z7/05 – Modelling. Sorptivity measurements that give the rate of ingress of a fluid due to capillary action are useful in modelling deterioration as they can be used to give a depth to which there is a rapid build of contaminates.

Page 39 Other related measurements such as absorption tests give a measure of volume of pores open to capillary filing. These tests can be useful as general quality indicators. 5.3

DIFFUSION Diffusion tests are useful quality measures in several ways. a) Chloride diffusion tests give a direct measure of how fast chloride will travel through the saturated concrete cover. This is a worst case measurement of the rate of chloride ingress. This is the primary concrete quality parameter used in modelling the life of a structure in a marine environment. There are several ways of measuring chloride diffusion and the different tests do not necessarily give the same result. Test methods are discussed in some detail in Z7/07 – Concrete Durability Tests and the method of using the diffusion result in modelling is detailed in Z7/05 – Modelling. b) Oxygen diffusion tests provide a measure of the rate at which oxygen will diffuse the concrete to reach the reinforcement. The result can be used in modelling corrosion rate where oxygen availability at the cathode is principle means of corrosion control, e.g. buried and immersed elements. As corrosion in these areas is seldom a problem as existing code requirements are very conservative, oxygen diffusion tests are seldom undertaken. c) Water Vapour Diffusion can be an important mechanism of moisture transfer and is often responsible for failure of coatings and impermeable floor coverings. Tests for water vapour diffusion is normally limited to tests on coatings. Results can be used to show that the coating when applied at a designated thickness will be sufficiently breathable to let moisture out and thereby prevent blistering. In the case of impermeable floor coverings it is more common to ensure the floor has dried sufficiently before applying the covering or to use a covering strong enough and with sufficient water resistance to resist the water vapour pressure.

5.4

RAPID CHLORIDE PERMEABILITY TEST The name Rapid Chloride Permeability Test is a misnomer as it doesn’t really give an indication of chloride diffusion directly. Although originally developed as a test that was influenced by chloride diffusion and resistivity such that a good result indicated good corrosion protection in the initiation and propagation phases it has been found that the test is essentially a complex resistivity measurement. Like resistivity RCP can be used as a quality assurance test by establishing a RCP value in trial mixes and then monitoring production results to determine if they vary significantly from the trial mix result. However as resistivity is a quicker test there seems little point in specifying RCP testing.

5.5

RESISTIVITY Resistivity can be measured by the 4 probe Wenner method. It is a very quick test and the result is related to the chloride initiation and propagation phases in corrosion assessment. It is a very useful test for a) Quality assurance purposes. As the test is so simple it can be undertaken on all cylinders prior to 28 day or 56 day strength testing. Variation in resistivity results while strength results are unchanged is a strong indication in variation in SCM dosage or quality. This can be used to indicate if re-assessment of the batching and materials is required. b) As a means of measuring Rapid Chloride Permeability where it is specified. The correlation between resistivity and RCP is sufficiently strong that converted resistivity results are generally accepted as a method of obtaining RCP values. c) Quick assessment of quality in relation to corrosion protection when assessing cores from existing structures. Measurements can be taken on the cores after saturation by immersion to give a saturated resistivity which gives worst case results. Values in the 4-8kohm cm range at an age of 28 days or more are likely to indicate GP cement was used as the sole binder and the corrosion protection afforded is low while values of over 30kohm cm at an age of 28 days or more are likely to indicate a concrete with high corrosion resistance. Because of the correlation between resistivity and Rapid Chloride Permeability relationships between corrosion protection and Rapid Chloride Permeability can be used to assess resistivity results.

Page 40 5.6

COMPRESSIVE STRENGTH Compressive strength is a useful measure of concrete quality in regards durability. For concrete mixes with no supplementary cementitious materials or admixtures and with proper quality control, compressive strength will be a good general indicator of concrete mix durability. It is also a useful measure of consistency of concrete supply. One of the reasons that compressive strength is such a useful tool is that it is relatively simple to do, has a well-established methodology (and hence low variance) and established criteria for acceptance and sharing of risk. As such it is an essential part of the durability testing suite of tests. Characteristic strength is a value nominated that 95% of all results must achieve. However it is not the actual characteristic strength of the concrete supplied as suppliers tend to target a strength that will ensure they will not fail the nominated requirement and frequently durability requirements will mean target strength are set significantly higher than the nominated characteristic strength. This means that achievement of the nominated characteristic strength will be acceptable from a strength perspective but may not be acceptable from a durability perspective. For example: a) If the nominated characteristic strength is 50MPa but the concrete achieves an actual mean strength of 70MPa in trials then the required characteristic strength for the project from a durability perspective may be closer to 70MPa. The requirement will depend on other results from the trial mix. If performance values such as chloride diffusion and sorptivity have been measured on the mix with a mean strength of 70MPa and found to be just tolerable then the required characteristic strength should be 70MPa. If there is a wide margin between actual and required durability performance results then the required characteristic strength for durability purposes may be set significantly lower than 70MPa. b) If the nominated characteristic strength is 40MPa but the concrete achieves an actual characteristic strength of 60MPa in trials this may be an issue for crack control. The design of crack control reinforcement may have been undertaken based on a mean strength of 50MPa based on the nominated characteristic strength. When the actual mean strength turns out to be 70MPa it may be found that there is insufficient reinforcement to control cracking as the tensile capacity of the reinforcement must be greater than the tensile capacity of the concrete. Recommendation: It is recommended that specifiers and ongoing production to: a) review the compressive strength and durability test results from trial mixes and re-evaluate the required characteristic strength for durability purposes. Increasing the nominated characteristic strength may not be appropriate as it might bring about an unnecessary change of mix design and in that case the actual insitu strengths should be monitored. b) how the temperature will be measured, recorded and then reported to the Systems Administrator promptly and in a secure fashion.

5.7

OTHER MEASURES OF QUALITY As noted the other important component of deterioration is the chemical resistance. There are many forms of chemical attack and attempts have been made to develop tests around most of the mechanisms involved e.g: 

Chloride activation level



Acid resistance



Sodium sulphate resistance



Magnesium Sulphate resistance

However, there is less agreement on the application of these tests. Consequently they are not discussed individually here or in Z7/07. From a durability assessment perspective it is important to bear in mind that many of the tests are on small samples in accelerated conditions and hence the results may overestimate the likely insitu situation. A case in point is magnesium sulphate attack and seawater attack. Research on small prisms by Santhanam 48 showed that seawater gave quite

Page 41 high chemical attack yet in practice we know the attack is quite limited even in relatively ordinary concrete, e.g. 40MPa concrete with GP cement performs very well. Other results by Santhanam 48 using brine at 2.5 times the seawater concentrations (Table 5), a typical outflow from a desalination plant, could be taken in isolation to show strong chemical attack. However the rate of attack was similar to seawater (Figure 20). Table 5 : Solutions Used in Research Undertaken on Sulphate Attack Using Small Prisms Equivalent SO3 (ppm)

Equivalent Cl- / Mg2+ (ppm)

2233

19090 / 1250

2.5 times SWL

5583

47725 / 3125

Ground water (Typical) MgSO4 CaSO4 MgCl2

0.300% 0.040% 0.140%

2233

1043 / 950

Ground water (High Concentration)

5.0 times GWL

11167

5215 / 4750

Designation

Solution

SWL

Sea water (Typical) NaCl MgCl2 MgSO4 CaSO4 CaCl2

2.700% 0.320% 0.220% 0.130% 0.060%

SWH

Sea water (High Concentration)

GWL

GWH

Strength

Figure 20 : Results of Sulphate Testing

Compressive Strength (% of c ontrol)

120

100

80 SWL SWH GWL GWH

60

40

20

0 0

5

10

15

20

Time of Immersion (weeks)

25

30

35

Page 42 6. CONCRETE M ATERIALS, SUPPLY AND CONSTRUCTION The focus of this section is concrete supply and construction. Only materials properties as they affect concrete supply and construction and hence durability are discussed here. This section does not discuss what materials properties are required for durability as this is discussed in detail in other sections of the Durability Series of documents. There are a wide array of materials and design options available to the concrete mix designer that are not discussed in Australian Standards. 1. The Australian Standards provide only limited guidance on the use of different cement systems. a) In some cases the codes recommend the use of Sulphate Resisting (SR) cement but no more than that. In Australia SR cement is specifically a cement that has a limited expansion when exposed to sodium sulphate and this is generally achieved using a Supplementary Cementitious Material (SCM). In other countries it is more common to define SR cement as a cement with low tri-calcium aluminate (C3A) e.g. ASTM Type V cements. Low C3A cements are available in Australia and they can be useful where the concrete is to be exposed to magnesium sulphate, ammonium nitrate and some other chemicals. However, low C3A cements are not suited to use in marine or other chloride exposures as they reduce the concrete’s resistance to chloride ingress and they give a low chloride threshold value but if their use is essential due to the other chemicals cover should be increased to compensate for the low chloride performance. b) If the design lives specified are to be achieved in severe exposures it would generally be essential to use a Supplementary Cementitious Material (SCM) as part of the binder system. SCM’s are widely available materials that from a sustainability perspective should be used in concrete. They enhance concretes performance and so have a double effect of reducing the use of cement and prolonging the life of the structure. Yet the standards do not stipulate their use. c) There is no guidance on the use of SCM’s for resistance to ASR yet this is a general requirement in Queensland due to the widespread need to use aggregates that would be reactive when using GP cement. 2. AS 1379 enables designers to specify concrete with strengths of up to 100MPa but AS 3600 calls up 50MPa as the maximum strength for durability provisions. Hence High Strength Concrete with low w/c ratios is not covered in the key design standards. 3. The codes provide virtually no durability guidance on stainless steel reinforcement, galvanised reinforcement, specialty admixtures, effect of various curing methods, how to incorporate coatings into a design, or how cathodic protection (prevention) should be employed. This section discusses important aspects of materials used in concrete construction in relation to durability on a much wider basis than that covered in Australian Codes on covering concrete durability. 6.1

CEMENT Cement type has a significant influence on mix design, transport and constructability. In general GP cement is used for construction but may become unsuitable as the only binder constituent for a number of reasons. The properties of a binder combination can affect the penetrability of concrete. For the same watercement ratio, coarse cement tends to produce a paste with a higher porosity than a finer cement 24. The composition of the cement may also affect finished concrete quality in so far as it influences the rate of hydration. In mass concrete, where heat of hydration may cause thermal cracking and indirectly affect durability, the use of low heat cement (Type LH) will permit better control over heat evolution. Types of cement and blends suggested for use in concrete under certain conditions are shown in Table 6.

Page 43

Strength High Strength Very high early strength High early strength Moderate early strength Low early strength Exposure Conditions Moist and mod. AAR Inland >1km from ocean High carbon dioxide Weak acids Soft waters Sodium Sulphates <600mg/L 600-3000mg/L >3000mg/L Magnesium Sulphate Low-moderate High Ammonium Nitrate Marine and coastal Submerged Tidal or splash Coastal or marine atmospheric Construction Aspects Cold weather Coloured concrete Thermal Aspects min. dimension 0-0.4m 0.4-0.6m 0.6 to 1.5m 1.5m to 3m >3m

Off White and White

ASTM Type V + 10-12% Silica fume

ASTM Type V

Type SR

Type LH

Type HE

Type GP + 55 to 70% Slag +5% Silica fume

Type GP +5 to 10% Silica Fume

Type GP + 55 to 70% Slag

Type GP + 30 to 55% Slag

Type GP + 20 to 30% Flyash

Type GP

Condition

ASTM Type V + 15-20% Fly Ash

Table 6 : Types Of Cement Typically Suggested For Use In Concrete Under Different Conditions

  

 



 

 



 





 

 

  

  





  

    

 

 

 

 





    



 

 



   

 



 

 



  

 

 

 - preferred cement system

Certain types of cement possess improved resistance to a particular form of attack, for example, sulphate resisting cement (Type SR). Blended cements made using fly ash, ground granulated iron blast-furnace slag (slag) or silica fume can have higher resistance to acids than, say, general purpose cement (Type GP). Where concrete is in a marine environment, the effect of chloride ion diffusion, with the consequent increase in risk of reinforcement corrosion, is a major consideration. In these circumstances, it is regarded as advantageous to use a cement with greater capabilities of binding the chlorides and/or controlling the chloride ion mobility. Page18 measured the diffusivity of chloride ions in various cement pastes after two months of curing. The results are shown in Table 7. The results indicate the better performance of the blended cements, at least under the curing regime used in the test. The authors also caution against drawing general conclusions about the role of low C3A contents influencing the rate of chloride ion diffusivity.

Page 44 Table 7 : Effective Diffusivity (D) of chlorides ions at 25C in various cement pastes of w/c 0.5 (from Page 18). Type of cement

6.2

Dx109cm2/s

GP/65% BFS

4.1

GP/30% PFA

14.7

GP

44.7

Type V cement

100.0

FLY-ASH, SLAG AND SILICA FUME The use of materials, such as fly ash, slag and silica fume have long been known to improve concrete durability under certain conditions27. When used properly in concrete they can improve sulphate resistance, durability in marine environment, and reduce the damaging effects of aggregates containing reactive silica. Work at CSIRO28 showed that fly ash influenced the characteristics of the cement matrix, the effects noted being both physical and chemical. The physical effect was a reduction in water requirement for a given degree of workability. This is attributed to the largely spherical form and favourable size distribution of the fly-ash particles. The chemical effect of fly ash, it was reported, was the pozzolanic effect. This consists not only of the in-depth reaction, which may take several weeks to one or more years to occur, but also a more rapid process involving reaction at the surface of the pozzolan particles. The use of fly ash can also reduce the water requirement of the concrete due to the spherical shape of the fly-ash particles and the void-filling effect provided by their fineness. Incorporation of fly-ash in a concrete mix by way of partial replacement of the fine aggregate increases this tendency to reduce water demand29. These findings explain the contribution of fly ash to early-age strength, which would not occur if the reaction were solely a long-term classical pozzolanic reaction. Other published CSIRO research 30 reported on the effect of fly-ash and water-reducing admixtures on concrete durability. The reduction in water requirement made possible by the use of fly ash reduces permeability. In addition, the presence of fly ash in the concrete can often allow full compaction to be achieved with less effort. AS 3582.131 specifies the requirements of fly ash for use with portland cement. Whereas the chemical reaction of fly ash in concrete is that of reaction with the calcium hydroxide liberated from the portland cement during hydration, slag hydrates autogenously in a portland cement/slag mixture after first being 'activated' by the alkalies and sulphates liberated during the hydration of the portland cement32. Some reduction in water requirement may be possible by incorporating slag in a concrete mix (either preblended with the cement or added as a separate ingredient to the mix) while maintaining similar workability. However, this reduction is generally less than that afforded by the use of fly ash. The results of local work on the sorptivity of concrete, as defined by Ho 22, containing slag has been published by the ACI33. The influence of the curing environment on the properties of concrete containing various combinations of fly ash, slag and chemical admixtures has been studied. The results of this work indicated that concrete containing only Portland cement as the binder was at least as vulnerable to lack of moist curing as the other combinations 34. The use of slag in concrete is reported in detail by Hinczak 35. AS 3582.236 specifies the requirements of slag for use with portland cement. Silica fume has been used in Australia since 1977. It is a by-product of the reduction of high-purity quartz with coal in an electric arc furnace in the manufacture of silicon and ferrosilicon alloys. It derives its pozzolanic properties from its very fine particle size and high silica content.

Page 45 In Australia, silica fume has been used in concrete mainly to provide high performance such as high chloride ion resistance, and to improve the properties of shotcrete. Australian experience with the use of silica fume in concrete has been reported by Papworth37 and Burnett38. The requirements of silica fume for use with portland cement are specified in AS 3582.3 39. ACI Committee 234 has reported on the use of silica fume in concrete including effects on concrete durability33. Favourable effects in relation to durability and the use of silica fume are:  reduced bleeding tendency;  reduced permeability;  increased sulphate resistance;  minimising damage due to alkali-aggregate reaction;  increased resistance to de-icing salts and other aggressive chemicals. Possible adverse effects from the use of silica fume in concrete include:  a requirement for an increased dosage of airentraining agent for a particular air content;  tendency to increase plastic shrinkage cracking because of reduced bleeding;  increased water demand of the concrete unless silica fume content is small (use of a water reducing agent, preferably a superplasticiser, is recommended if silica fume is used in concrete). Curing of concrete, whether containing pozzolans and other similar materials or not, is important and is discussed further in Section 4.5.11. Recommendation: It is recommended that: a) where high magnesium sulphate, ammonium nitrate or other chemical exposure warrants the use of a low C3A cement that the specifier consider the specification of a Type V cement. b) where type V cement are to be used and the concrete is exposed to chlorides in water in excess of 10,000ppm then the cover should be increased appropriately. c) specifications require the use of a SCM at a sufficiently high level to ensure corrosion protection where concrete is exposed to liquids with chlorides in excess of 10,000ppm. 6.3

AGGREGATE The properties of the aggregates used in concrete can have a significant effect on its durability. The volumetric stability of the aggregate can influence the drying shrinkage and creep of the concrete. Poor shape can increase water requirements, with a subsequent increase in drying shrinkage and permeability. Attention should be paid to the possible introduction of harmful materials with the aggregates, e.g.:  organic matter, which may interfere with hydration of the cement;  clay and other fine material, which may coat the aggregate, and/or be chemically reactive, and/or form soft inclusions in the concrete, and/or increase the water demand of the concrete;  salts, such as chlorides, sulphates and nitrates;  unsound particles;  permeable material;  mica;  reactive iron pyrites and marcasite;  reactive siliceous or carbonaceous aggregate;  porous aggregates that demonstrate unsoundness in the sodium sulphate soundness test (such aggregates can increase drying shrinkage and reduce frost resistance).

Page 46 Soundness of the aggregate, as indicated by performance under conditions of wetting and drying, is important. In temperatures up to 300°C the use of siliceous and other aggregates in the concrete should be satisfactory. At high temperatures, up to 1000°C, fine-grained basic igneous rock aggregates, such as basalt or dolerite, will normally be unaffected by heat and are preferable to siliceous aggregates. At extremely high temperatures, up to 1350°C, heat-resistant aggregates and cement should be used. Suitable aggregates include crushed firebrick and other materials that result from, or are made by, a high temperature process. High-alumina cement should prove satisfactory if a relatively dry atmosphere can be maintained after the initial curing period40. It has been recommended24 that if a choice of aggregates is possible, there should be compatibility in elastic properties of the coarse aggregate and the hardened mortar. The requirements of aggregates for use in concrete are specified in AS 2758.1 41. AAR is a chemical reaction between alkali hydroxides (sodium and potassium) in the pore solution of the concrete, from cement and other sources, and certain mineral phases present in the coarse or fine aggregates. The two major types of AAR are:  alkali-carbonate reaction (ACR); and  alkali-silica reaction (ASR). Alkali-carbonate reaction is a reaction between certain dolomitic limestones and the alkalies in the concrete. This type of reaction is very rare in Australia and therefore is not discussed further. Alkalisilica reaction is a reaction of the alkalies in the pore solution of the concrete with aggregates containing certain forms of reactive silica such as amorphous silica or opaline material, cryptocrystalline quartz, chalcedony and cristobalite. As a result of the reaction, a silica gel is formed which imbibes water, causing a volume expansion which can induce cracking in concrete, particularly in high-strength-concrete members exposed to external moisture. Damage to concrete due to AAR may take the form of map cracking in unrestrained concrete or a crack pattern reflecting reinforcement or prestressing tendons in restrained concrete. Other symptoms of AAR can be exudation of gel through pores or cracks, reaction rims on affected aggregate particles, and sometimes popouts. Three conditions must be fulfilled for AAR-related damage to occur in concrete:  The presence of reactive forms of silica in the aggregate.  A sufficiently high alkali content in the concrete pore solution.  Sufficient moisture. In relation to the vast amount of concrete in use in Australia, the amount of concrete affected by AAR is small but significant. However, with the increasing trend to high-strength concrete and high performance concrete it is important to consider this phenomenon when designing future work. An Australian document on minimising the risk of damage due to AAR in concrete16 provides designers, specifiers, suppliers, and contractors with guidelines for the assessment of risk of damage due to AAR based on the type of the structure, the environment and their interaction; the assessment of aggregates for potential reactivity; and the precautions to adopt to minimise the risk of damage due to AAR. Recommendation: It is recommended that a specification include a clause that requires that the risk of ASR be assessed in accordance with HB 79 and mitigation measures follow the guidance given in HB79. 6.4

ADMIXTURES Admixtures are used to provide a particular benefit or to obtain a particular effect in the concrete. The most commonly used admixtures are chemical admixtures which modify a number of properties of concrete (e.g. setting characteristics, water demand, air entrainment and chemistry) and reduce its cost. The requirements for chemical admixtures and guidance on their use in concrete are given in AS 1478.2.

Page 47 The effects of water-reducing, set-controlling and air-entraining admixtures on the durability of concrete will depend on the way in which the characteristics of the concrete are modified, which in turn depends on the changes made in mix proportions, and the nature of the ingredients of the admixture. These admixtures normally increase the workability of the concrete and reduce bleeding. Increased workability and reduced bleeding can be expected to reduce permeability, thereby increasing durability. A positive step in reducing chloride levels has been taken in recent years by the introduction of chemical admixtures with an insignificant chloride content. These 'non-chloride' admixtures, and particularly 'non-chloride' set-accelerators, would seem to offer the benefit of water-reduction and set-control without the addition of the objectionable chloride ion. Depending on how these nonchloride admixtures are incorporated in a concrete mix, they can assist with reduction of permeability without increasing the chloride ion content. 6.4.1

Air Entrainers Air-entraining agents are able to improve the resistance of concrete to the destructive effects of freezing and thawing. When properly incorporated in the mix, air entraining agents also increase the workability of concrete. This makes it possible to reduce water requirements for a given slump, and also to reduce the fine aggregate content of the mix (thus allowing a further water reduction). With suitable mix adjustment, and correct use of an air-entraining agent, the reduction in strength that might be expected with increased air content can be minimised. On the negative side air entrainers, or for that matter any admixture that entrains air, can exacerbate the risk of delamination in floor slabs.

Recommendation: It is recommended that air entertainers, or admixtures that entrain air in some way, not be used in concrete to be used for flatwork or concrete exposed to severe exposures unless specifically approved by the Engineer. 6.4.2

Water Reducers Water-reducing admixtures can be used in three ways as illustrated in Figure 21. It can be seen that from the point of view of maintaining the same 28-day compressive strength at the same workability, the use of a water-reducing admixture will permit a reduction in cement content. The potential 28-day compressive strength increase, in this case, is generally greater than would be experienced through the reduction in water/cement ratio alone. This additional strength increase is attributed to the dispersive effect of the admixture in allowing more cement-particle surfaces to come in contact with the water in the early stages. Because they allow a reduction in the water requirement of a mix for a given slump their use can result in reduced penetrability. Where cement content is reduced, care should be exercised to ensure that the durability of the concrete does not suffer.

Page 48 Figure 21 : Alternative Uses of Water Reducing Admixtures (from Cook

22

)

When a water reducing admixture is lignin or carbohydrate based the admixture generally retards the set of the concrete. This retardation is often offset by incorporating materials such as tri-ethanol-amine or non-chloride set-accelerating compounds. Such a modification to the formulation normally has little deleterious effect on the properties of the concrete in which the admixture is used but should be checked. More recent use of Poly Carboxylic Ether (PCE) based water reducer has reduced or removed requirements for such set correction unless specifically required to retard or accelerate the set. 6.4.3

Superplasticers or High Range Water Reducing Admixtures Super-plasticising admixtures (or high-range, water-reducing admixtures) can be used to markedly increase workability or—if the consistency, as measured by slump, is constant— to significantly reduce the water requirement. In suitably proportioned mixes, it can be expected that the use of super-plasticisers would not reduce the durability of concrete, and where used to reduce total mix water, it can be expected the durability would be improved. Early superplasticisers introduced issues of segregation as they made the concrete unstable and the plasticising effects lasted for a limited time. At one stage the use of silica fume to stabilise the concrete with a superplasticiser to counter the negative effects of the silica fume on workability were considered ideal partners. Modern superplasticisers however include polymers that give the mix stability and the superplasticisers are designed to give longer slump retention times. Hence they can be used to give stable mixes with longer slump retention times. Superplasticisers are often used in conjunction with retarders to optimise the dosage of superplasticier required. In some circumstances retarders may also be required to extend setting. Because of the mixture of chemicals with potentially different cement systems trial mixes are essential if a mix with untried combinations of materials is proposed for use.

6.4.4

Set Retarding Admixture Use of set retarding admixtures at manufacturer’s recommended dosages have little effect on penetrability or chemical resistance of concrete so their primary influence on durability is to enable proper compaction and finishing over a longer period from adding water to cement. Set retarding admixtures are used in a number of ways: a) to give longer setting times, particularly in warmer climates. The extent of this extension in setting time with temperature needs to be assessed by measuring slump loss with time. When properly assessed the result can be used to permit an extension in placing time.

Page 49 b) by changing admixtures with different levels of retardation the effects of summer to winter temperatures can be allowed for. This change in admixture is not considered to constitute a change in mix performance and trial mixes are not necessary c) to maintain setting time through the day as temperatures increase and decrease. Admixture dosage for constant open time is calibrated against the concrete temperature. This can be important in order to maintain a constant finishing time across a slab. 6.4.5

Waterproofing Admixtures Appendix F of CPN28 provides details of classes of admixtures for waterproofing concrete. These admixture types work in different ways as the classes suggest and their effectiveness in regards the various mechanism of concrete penetrability will vary enormously. One of the issues with these admixture types is their actual performance in regards the penetrability mechanisms as opposed to the subjective statements often used to describe their performance (e.g. good, very good, excellent). The performance of the admixture not only depends on the admixture itself but also on the concrete quality and chemistry and the exposure. Hence, in many respects the specifier should ignore the subjective descriptions given and rely more on performance data. Where performance data is not available for the specific concrete quality or chemistry being considered then it is recommended that tests be undertaken. Suitable tests for waterproof concrete are detailed in Z7/07. These tests are designed to give either data that can be used in modelling or a general description of the tests, e.g.:  Water permeability for the rate of water penetration under a pressure head.  Sorptivity tests for the rate at which water penetrates concrete by capillary action.  Chloride diffusion to give the rate at which chloride penetrate due to a difference in chloride concentration.  Volume of permeable voids a general quality indicator of concrete penetrability.

6.4.6

Corrosion Inhibiting Corrosion inhibiting admixtures are typically one of three basic chemical types: 

Calcium Nitrite based (inorganic).



Amino Alcohol based (organic).



Combination of Amino Alcohol and inorganic inhibitor.

Calcium nitrite (CNI) inhibitors work by converting the ferrous oxide sites in the steel/concrete interface to ferric oxide. The ferric oxide is more resistant to reaction with chlorides and so leaves steel reinforcement in a passive state. The level of nitrite required is critical so the dose rate of these admixtures is dependant on the expected level of chloride at the steel/concrete interface during the life of the structure. Typically calcium nitrite admixtures contain just over 30% calcium nitrite. It has been shown that to give significant corrosion inhibition in marine structures in the splash zone a dosage of 20lt/m3 of a typical CN admixture is required. The admixture typically contains a retarder to offset the accelerating effect of the CN but this normally only has neutral effect on dosage of 10lt/m 3 and at this dosage the admixture is largely ineffective as an inhibitor in marine structures. At 15-20lt/m3 a workable concrete with sufficient open time can be obtained but additional retarder is likely to be required. As the dosage increases, corrosion inhibition improves but retention of workability becomes an increasing problem. Amino alcohol based inhibitor acts by coating the reinforcing steel and keeping chloride ions from reacting with the reinforcement. They also have an effect of inhibiting the reaction with water and oxygen at the cathodic sites on the steel which is an essential part of the corrosion process.

Page 50 6.4.7

Polymers There are three types of concrete made using added polymers, viz: polymer-impregnated concrete, polymer concrete, and polymer/portland cement concrete mixtures. Polymer/portland cement concrete mixtures (PPCC) are normal portland-cement concrete mixtures to which a water-soluble or emulsified polymer has been added during the mixing process. In Australia, this is the only way in which polymers have been used to any significant extent. PPCC applications have included, but have not been limited to, overlays of bridge decks, and parking garages, floor precasting operations, and for patching damaged concrete surfaces. The use of polymers in concrete can assist in providing greater durability. They can also be used to rehabilitate deteriorated concrete. There are limitations to the use of polymers in concrete including cost, suitability of the application, and the potential hazards of the various chemical systems in certain polymers. Further details on the use of these materials can be obtained from Polymers in Concrete44. Many other admixtures are available for use in concrete. Cook 43 summarised the effect of the use of the most commonly used admixtures on concrete durability. It is important to control accurately the quantity of admixture used, and to follow the manufacturer's recommendations generally. Only admixtures shown by trial mixes to be satisfactory for concrete should be used.

6.4.8

Shrinkage Reducing Admixtures Shrinkage reducing admixtures are used to lower the surface tension of water in the capillary pores. This reduces the shrinkage force induced by water in the capillaries as the concrete dries. The reduction in force reduces the propensity to plastic shrinkage and drying shrinkage cracks. Where these admixtures are used they may increase water demand and lead to lower strength development so should only be used after trial mixes show that the shrinkage reduction is sufficiently reduced to compensate for the strength effects.

6.4.9

Expansive Additives Expansive admixtures contain expansive calcium oxide (free lime) which reacts in the first 7 days of hydration to form calcium hydroxide. The expansive reaction offsets the subsequent drying shrinkage. In order to work the expansion must be locked into the concrete as a chemical prestress. This is achieved by providing sufficient reinforcement to restrain the expansion. As the concrete then shrinks the prestress is released. The key to success is appropriate dosage of admixture, sufficient restraint reinforcement and proper curing. These systems have been used extensively in New Zealand to provide crack free slabs. In some cases shrinkage of concrete can lead to durability and structural issues. For example pile plugs rely on the bond between the concrete and steel structurally and for corrosion protection of the steel. In these cases the use of an expansive agent can eliminate residual shrinkage as the pile provides the necessary initial restraint. Where nonshrink concrete is required it is important to use expansive aids rather than shrinkage reducing admixtures.

Recommendation: It is recommended that where admixtures are used in concrete to provide a specific property that trial mixes are undertaken with and without the admixture and representative testing is undertaken to validate the effect of the admixture on the important durability, structural and rheological concrete properties.

Page 51 6.5

WATER The simple definition of requirements for water is “Water must be clean, fresh and free from any dirt, unwanted chemicals or rubbish that may affect concrete”. It is a simple guide that conveys that if water comes from the mains and is drinkable then it can be used for concrete. However if it comes from other sources it needs to be checked. Bore water, even if drinkable, and plant recycled water should be tested and the concrete specifier should be aware of the composition and its suitability for the proposed application. Sea water should never be used for reinforced concrete and should be avoided if possible for unreinforced concrete. AS 1379 notes that water is deemed acceptable if:  Service records show it is not injurious to strength & durability of concrete or embedded items.  If service records show that when using the proposed water the strength is at least 90% of the control and initial set between -60 and +90 minutes of the control.  Water is in accordance with Table 2.2 i.e.: Sugar <100mg/l when tested to AS1141.35 Oil and grease <50mg/l when tested to APHA 5520 pH >5.0  Total dissolved solids, chloride content, sulphate content and sodium equivalent is tested and recorded. Due to concerns over the possible effects of recycled water on durability CCAA introduced a guide “Use for recycled water in Concrete Production” 23. This gives a strategy for use of water in concrete and the limits in Tables 2 and 3 of that document are a comprehensive list of parameters to consider. A suitable process for mix water assessment is given in Figure 22 below. Figure 22 : Assessment of Concrete Mixing Water 23

Page 52 7. CONCRETE SUPPLY Concrete batching is wholly within the concrete suppliers control. The supplier agrees to provide concrete to certain performance requirements and it is left largely to the supplier and hence it is important that the specifier defines requirements carefully. Australian Standard AS1379 covers most of the requirements but some important aspects are not included in the standard and have to be detailed in the specification. Hence in this section important aspects of the concrete specification related to concrete supply and durability are included. Concrete supply should comply generally with the requirements of AS 1379 and placing with those of AS 3600. Additional useful information on such items as compaction, finishing and curing can be found elsewhere 24, 83. AS 1379 is a complete guide to concrete production and delivery. Sections comprise: 1. Scope (definitions and reference documents). 2. Concrete Constituents. 3. Plant and Equipment. 4. Production and Delivery. 5. Sampling and Testing. 6. Sampling Testing and Assessment for Compressive Strength. It includes details of how to specify and produce concrete in general. It does not tell practitioners which aspects are important for different applications or for different exposures. The practitioner needs an understanding of what aspects need more detailed specification. AS1379 is independent of AS3600 and other structures codes. 7.1

QUALITY OF LOCAL SUPPLY Where high performance concrete is required away from city centres the designer needs to consider whether the local supplier will be able to produce the quality required and whether the local materials are suitable. Where the supply situation is unknown a specific supply evaluation should be undertaken early in the process. Sometimes the supply may be adequate but the suppliers may need assistance with quality documentation. A specifier who wants to achieve certain durability performance measures for a project should specify a preliminary mix acceptance process including quality plan requirements. Where there are long delivery times the concrete quality will be affected, particularly at high temperatures. A normal class 40MPa grade concrete with 80mm slump 30 minutes after batching with concrete temperature of 25°C can loose slump at 1 - 3mm per minute depending on a number of mix design factors. This slump loss is corrected for longer delivery by increased total water content or use of slump retaining admixtures.

7.2

THE CONCRETE SUPPLY SPECIFICATION The supply of higher durability concrete is unlikely to be entirely specified by general references to the key Australian Standards only (AS3600 and AS1379). It is most critical that the specifier is clear about performance or prescriptive attributes of each concrete mix that is required on the project. A simple way that is often used to express these requirements is in a form where key parameters such as water/cement ratio, cement constituent limits, durability testing limits, maximum bleeding rates etc. are tabulated. This approach to specifying highlights the mixes which are critical to the structures durability and flags potential variances from “standard clauses” often used in specifications. In addition to these mix details, a more prescriptive specification may require the raw material constituents of concrete to have properties and a testing regimen that is in excess of those required by the respective materials Australian Standard for manufacture. In this regard it is useful to

Page 53 tabulate the testing frequency and expected limits of constituent material properties where these are different to the Australian Standards Requirements. The specifier needs to exercise care when detailing both performance and prescriptive requirements which may be conflicting. Aspects of this are discussed in the section below on the transportability of specifications. 7.2.1

Production and Project Assessment Production assessment refers to the process where the quality assurance of concrete supplied to a project forms part or all of the concrete of that mix supplied from a plant. This gives the supplier greater ability to control the concrete quality and is likely to lead to lower costs. It will not necessarily lead to improved quality as part of the production assessment process may be that the supplier can reduce cement contents. However as production control only applies to N class concrete this is unlikely to be an issue. Project assessment refers to a process where the quality assurance is based only on the project concrete. This method is applied to all S class concrete.

7.2.2

Normal and Special Class Concrete There is frequently confusion over what complies N class and S class concrete. If N class is specified when an S class is required the specific properties required under S class may go unnoticed and the concrete supply may be inappropriate. N Class Where there are no special requirements for the concrete then the specification should include the following: 

Class N



Strength grade (20,25,32,40,50 MPa)



Slump (20 to 120 in 10mm int.)



Max. aggregate size (10,14,20mm)



Method of placement



If project test required



Air content to max 5%

Where N class is specified, or S class is specified and specific criteria are not nominated, AS 1379 requires that the concrete meet the following properties: 

Any AS 3972 cement with any SCM (SCM to be reported and 7 day laboratory strength >50% of 28day strength)



Density 2100-2800kg/m3



Acid soluble chloride 0.8kg/m3 (If no rebar then S class and  2kg/m3)



Acid Soluble sulphate 50g/kg cem.



Shrinkage  1000x10-6



7d mean strengths @50% f’c



No lightweight aggregate

N Class concrete is suitable for many applications and is likely to be suitable for exposure classes A1, A2 and B1. S Class Where high performance concrete is required due to the specific application requirement or due to the more severe exposure requirement the specification is to include the following: 

Class S



Strength grade (20,25,32,40,50,65,80,100 MPa)



Slump

Page 54 

Maximum aggregate size



Method of placement



If project test req’d



Strength Class (S compressive; SF flexural; ST indirect tensile)



Exposure Class (SB for B2; SC for C1 and C2; SU for U)

Where S class is specified the following additional items may be specified in accordance with the standard: 

Cement system



Early age strength



Density



Shrinkage



Chloride content



SO3 by wt cement



Flexural strength



Indirect tensile strength



w/c ratio



Admixtures and additives



Discharge time



Temperature at discharge



Testing of concrete



Other materials

AS 1379 does not mention other properties although they can also be specified as required. However when moving outside of these listed properties it would be wise to check that the supplier is prepared to supply to the specified requirements. S class concrete is extended to 100MPa although these grades may not be available from all plants. The increase in strength has been driven primarily by the use of high strength concrete to reduce the column sizes in buildings. There is little use of these higher strength grades for durability purposes although there is no reason why these grades should not be used on some projects. Before specifying these higher grades the specifier should liaise with the concrete supplier to determine if it is available to the project and if it is, the implications for cost and placeability. 7.3

MIX DESIGN The process of selection and proportioning of the ingredients of a concrete mix plays a significant part in the potential durability of the resulting concrete. Guidance on the desirable characteristics of the ingredients has been given elsewhere in the Durability Series. Specifiers should take advantage of the expertise and experience that concrete manufacturers and concrete technologists have built up in proportioning concrete mixes, especially those that will be subjected to defined aggressive influences, or used for particular purposes. A balance should be struck between the need to have adequate workability of the mix to permit placement and full compaction, and the necessity to keep the water content as low as practicable. Consideration should be given to the use of lower slumps, water-reducing admixtures, superplasticisers, fly ash, slag, and reduced fine aggregate contents to help to achieve as low a water content as possible. The quantity of water required to provide a particular workability is influenced mainly by the shape and maximum size of coarse aggregate, the proportion and grading of fine aggregate, the cement content, and the quantity and characteristics of any chemical admixture, fly ash or slag used. Thus, it is not possible to recommend precisely the maximum water content that should be used. The sand content should be kept as low as possible, compatible with the transporting, placing and finishing operations. The combined grading of the fine and coarse aggregates should be as

Page 55 continuous as possible, particularly for concrete slumps over 40 mm. In general, gap gradings should be avoided if the concrete is to be transported by pump. Gap gradings should be discouraged at slumps in excess of 80 mm unless there is sufficient cementitious material to provide cohesion. Combined aggregate grading curves are available for use in arriving at appropriate ratios of fine to coarse aggregate. Reference should be made to the CIA's Recommended Practice Pumped Concrete (Z12)45. Special consideration is needed in the selection of cement content and water-cement ratio for applications and conditions where concrete is exposed to aggressive agents. These conditions include:  Exterior or interior surfaces in contact with aggressive or corrosive liquids or gases.  Exterior surfaces of members constructed in or over water and either permanently submerged or subjected to alternate cycles of wetting and drying.  Surfaces below ground in direct contact with permeable soils and groundwater, either or both of which contain appreciable amounts of chlorides, sulphates or acids.  The concrete mix should be designed to satisfy the requirements of strength (at a workability to permit transportation), placing and full compaction of the concrete, considering the equipment to be used, and at the least water content practicable. 7.4

THE CONCRETE SUPPLIERS QUALITY PLAN Where a project requires special prescriptive or performance requirements that are outside of those covered by the relevant Australian Standards (AS3600 and AS1379) for concrete delivered to a structure, the concrete supplier needs to demonstrate the method that is intended to comply with these requirements. An effective method of providing this demonstration is via a quality plan. The concrete suppliers quality plan may include any of the following:  Specifications with applicable limits for all tests carried out on concrete constituents.  Specifications with applicable limits for each test applied to concrete.  Sampling methods and test methods required for each concrete constituent, their frequency of assessment and action on non-conformance to prevent non-conforming material being used in concrete.  Sampling methods and test methods applicable to concrete, their frequency and action on nonconformance to specification.  Verification of plant batch calibration and batch accuracy.  Verification of mixer efficiency and uniformity.  Method of manufacture and delivery of concrete including details of any processes that are in excess of the requirements of AS1379.  Details of responsibilities of suppliers staff for quality of delivered product to the project structure. It is important that the constructor and the specifier have an opportunity to review and agree to this quality plan prior to approving the supply of concrete.

7.5

CONCRETE ORDERING Concrete should be ordered from a supplier who has been evaluated and selected on his capacity to produce concrete in accordance with the contract documents and deliver the concrete at a rate consistent with the project schedule. Again, reference should be made to AS 1379. The supplier should be aware of all relevant aspects of the contract documents, and care should be taken to ensure that the supplier is aware of any particular service conditions and durability requirements affecting the mix design.

7.6

CONCRETE BATCHING This section outlines batching procedures and methods that should be employed to ensure consistent concrete properties The concrete should be mixed in a properly designed mixer, and mixing should continue until there is a uniform distribution of the materials.

Page 56 Concrete should be transported from the place of mixing to its final position as rapidly as practicable and by means that will prevent segregation or loss of any part of the mix materials. Particular care should be taken in hot, dry weather or in windy conditions to prevent evaporation or drying. 7.6.1

Materials Storage Material storage at a concrete plant should be arranged to prevent the possibility of contamination or impacts from ambient conditions such as rain or excessive heat. Every concrete plant will be designed differently. General storage requirements are covered in AS1379 Section 3.2 but in some cases where very strong levels of control on water content and possibly on delivered concrete temperature are required then it may be necessary to specify storage facilities that could include protection from rain, wind and sun for aggregates and sand. Such controls may influence plant design and must be clear in the tender process.

7.6.2

Batching Plant There are numerous designs for batching plant available. The general requirements for batching plants are contained in AS1379 Section 3.

7.6.3

Truck Mixers Truck mixers are commonly used for mixing and delivery of pre-mixed concrete in Australia. The general requirements for truck mixers are covered in AS1379 Section 3.5. This section of the standard covers assessing a mixer for mixing uniformity and the determination of an acceptable mixing time. As high durability concrete may have a low w/c ratio, it may require more energy to get full mixer uniformity in some cases. In this case it would be sensible to verify that the mix cycle proposed by the supplier does achieve the correct values of uniformity following the process given in AS1379 Appendix A.

7.6.4

Cement Cementitious material weighing can be impacted by the common means of feeding the weigh hopper by means of an air slide from the silo and once the silo valve is closed (because the correct weight has been reached) material is still in the slide. This is referred to as "in flight" material and can be corrected to some degree in automated weighing systems but will still lead to variability. The difficulty of stopping flow exactly is the same for all mix sizes and the same level of weighing accuracy is achieved for all batch sizes. An exception to this may occur if the weigh hopper is too small for the full batch of cement and it has to be batched in two parts. In that case inaccuracies may accumulate. The tolerance is set based on shutting the valve just before reaching full weight and batching systems are set for calculated "in-flight" quantities. What this means is that in a 2m3 mix trial a specified cement content of 400kg/m3 may be batched as 395-415kg/m3. That alone could mean a theoretical 0.40 w/c ranging from 0.39 to 0.41. That's without the allowable variation in water content. This needs to be considered as part of the durability assessment. The trial mix could have significantly higher or lower water content than the actual pour. Partial safety factors or full probability assessment will need to take this into account. Although these tolerances can be achieved with fly ash, slag and GP cement using air slides its more difficult to achieve these tolerances with silica fume. In a 2m 3 batch of concrete there may only be 40kg of silica fume. That's close to the tolerance for a cementitious material and can be overcome in several ways:  Pre-blend undensified silica fume with cement (generally only available in WA).  Add silica fume in bags to improve batch accuracy.  Add silica fume from a silo using a variable speed screw feed that can be more precisely controlled than the air slide.

Page 57 Because silica fume is such a small part of the mix it's more appropriate to treat it as an additive rather than a cementitious material in regards batching. 7.6.5

Aggregate Aggregates are weighed sequentially into the same weigh hopper hence the tolerance on each component is the same as the tolerance for all materials. The aggregate tolerances are higher than the cement because of the method of addition and relative proportion in the mix. Often aggregates and sand are batched by a front end loader using its bucket to try and accurately measure out final kilograms of material while the operator watches the weight read out. While the tolerance might seem high the total aggregate content is around 5 times higher than the cement content so proportionately the aggregate weighing is potentially more precise than the cement.

7.6.6

Admixtures Admixtures are batched in accordance with AS1379 unless otherwise advised. Attention needs to be given to the accuracy of admixture measuring equipment where minor changes in dose rate of the admixture used can significantly alter the concrete mix performance. Adding admixture on site can be effective but care must be taken as:  loads with and without admixture will not necessarily behave in the same manner and in slabs in particular, finishing problems may result.  a small dose of admixture may not mix uniformly through a load of concrete without significant mixing effort.

7.6.7

Water Water/cement ratio, and hence water content is one of the major aspects affecting concrete durability and hence appropriate specification, accounting and verification of water content are vital if the concrete is to be adequately durable in hostile environments. In Figure 23 (Thomas24) it can be seen that a change in w/c ratio from 0.40 to 0.45 will bring about a 54% increase in mean chloride diffusion coefficient. Hence this section goes to some depth to explain how water content should be controlled. Clause 4.2.1.2 of AS1379 defines w/c ratio as the ratio of mass of total free water to mass of cement and total free water as the sum of water: 

In aggregates over saturated surface-dry.



Added as part of admixtures.



Added to the mix at batching.



Added at the slump stand.



Added on site.

Page 58 Figure 23 : Relationship between Chloride Diffusion and w/c Ratio24

Water that is locked into aggregate is not available for hydration of cement so the term ‘total free water’ is used to define the water available for hydration. Very often water in the admixtures is ignored as it is too small to be of concern. However where the water content is high it should be included in the analysis. 1)

Measuring the Water in the Aggregates Water is typically measured by taking a sample of the aggregate and drying it in a microwave or standard oven. A test is typically undertaken first thing in the morning and slumps used to control water content during the day. In remote plants, slump may be the only means of water control and AS 1379 permits this unless control by w/c ratio is specified. Where control of the water content is critical and slump is not considered an adequate means of control, the aggregate moisture content will always be measured but the frequency is not given in AS1379. Rather performance requirements are given for w/c ratio. A discussion on the accuracy of water measurement in concrete is provided by Thomas25.

2)

Specifying w/c Ratio Two performance requirements can be specified, i.e. specified w/c ratio and specified max w/c ratio. In the former case the supplier must control the w/c ratio to within 10% of the specified value and within the later the maximum value shall not be exceeded. There is a significant difference in the two methods. Assuming the measurement accuracy of cement and water is at the limit given in AS1379 a supplier might treat w/c ratios as follows:  Specified w/c of 0.40 – target w/c is set at 0.42 and supply with a w/c range from 0.39-0.44.  Specified w/c of 0.40 maximum – target of 0.38 to give a range of w/c from 0.350.40. This significance in the difference in how w/c ratio is specified can be missed. As durability is strongly related to w/c ratio it is important that the specification is clear on whether the w/c ratio specified is nominal or a maximum. The potential difference of around 0.04 in real w/c could be significant in terms of performance and cost of concrete.

Page 59 A general clause stating w/c shall always be taken as a maximum so as to avoid any issues. Recommendation: Wherever a w/c ratio applies it shall be taken as the maximum w/c ratio unless specified otherwise. 3)

Target w/c Ratio Target w/c ratio is the w/c value that the concrete supplier sets as the w/c ratio used to assess the total water in the mix. The target will be determined by the specified w/c ratio, the batching accuracy and the accuracy of measurement of water in the aggregates. The supplier will generally batch to the maximum target w/c ratio consistent with maintaining either the specified maximum w/c ratio (taking account of batch accuracy) or the w/c ratio required to attain the measurable performance requirements, whichever is lower. This can lead to issues with supply w/c ratio. For example it is not uncommon to have a maximum w/c ratio of 0.40 but have a target w/c ratio of 0.35 in the trial mix in order to achieve the performance requirements. It is important that the w/c ratio used for trials is reflected in the w/c ratio limit set at design. This is achieved where the slump of the trial mix is used as the slump of the supplied concrete. However this means that where water is added at site based on an allowable w/c ratio that the allowable w/c ratio reflects that achieved on site. This can be achieved by specifying that the maximum w/c ratio shall not exceed the calculated w/c ratio of the trial mix by more than approximately 0.02.

Recommendation: Where w/c ratio is specified the designated w/c ratio of the concrete mix shall not exceed the calculated w/c ratio of the trial mix by more than 0.02 or as estimated for a specific mix and accounting for batching and water measurement accuracy.

4)

Frequency of Measuring Aggregate Water Content

The performance requirements leave room for considerable variability on concrete supply although this is not in the interests of the supplier. High variability leads to a lower target value being required in order to achieve the characteristic value. However as there is no way to check w/c ratio other than from the suppliers records it is important to have a good estimate of the aggregate water content. While measurement once a day with slump as a primary control may be adequate for general concrete it is unlikely to be adequate for HPC projects where water content can vary due to position in the aggregate stockpile rain, sprinklers, drying etc. The suppliers may determine they need more frequent moisture content analysis in order to control concrete variability but to ensure it happens at an acceptable interval it should be specified.

Recommendation: 1. Where w/c ratio is specified and aggregates are open to the atmosphere the frequency of testing moisture in aggregates shall be as given in Table 7. Table 8 : Measurement of moisture in the aggregate stockpile f’c

Interval between checking aggregate moisture content

≤40MPa

Daily

50MPa

6hr max

≥50MPa

4hrs max

In addition to the requirements in Table 5 the aggregate moisture content shall be measured after and during any rain. 5)

Water Addition at the Plant and on Site

The moisture content of the aggregates is assessed in order to determine the amount of water that should be added at to the mix.

Page 60 During batching of concrete approximately 90% of the estimated mix design added water is batched while the truck mixer is in the plant batching position. The remainder is held back and added at the slump stand based on estimated slump. The slump and a proportion of the remaining water to be added is estimated and added at the slump stand. All the added water is noted on the docket and recorded in the plant batch record. When a maximum w/c ratio is specified the total batched water is compared against the mix design allowable water and on site it is known what proportion of the allowable water has been added. Slump is measured on site and if lower than specified it may be allowable to add further tempering water up to the maximum design water. If this is allowed then it is critical that such water additions are thoroughly mixed as noted in AS1379. There is some debate over whether this 'spare' water should be added on site. One argument is that if the truck has been waiting then some slump will have been lost and adding water could take the w/c ratio higher than it should be. Others say that at the plant the slump was only judged by eye and hence it may have been judged low and the spare water only brings the water to the correct level. This debate arises because it is not certain what the water content in the aggregates was. The moisture content of aggregates is generally only measured first thing in the morning. After that the change in moisture content is assessed by the slump measurements. On critical projects more frequent assessment of the aggregate moisture content can be made. AS 1379 Clause 4.2.3 states that water may be added "no later than 75 min or 80% of an agreed extended period for completion of discharge … whichever is greater." In Australia there are two schools of thought on adding water on site: a)

No water to be added. Even though the specifiers may recognise that if all the allowable water was not added at the batch plant topping up to the maximum allowable on site would be satisfactory with proper subsequent mixing, they don't trust the sites enough to allow any water addition.

b)

If the total water added at the batch plant is less than the allowable for the mix (including recognition of the limit based on the w/c ratio of the trial mix) then the residual can be added on site provided the water added on site is measured and recorded on the docket together with total free water from the plant, the concrete is properly mixed after addition, concrete meets performance requirements and the supplier takes responsibility for the concrete.

If it is a responsible supplier they will ensure the correct water is added and will sign off on the mix if water is added correctly. They may add additional water if the customer asks but they will warn him they no longer take responsibility for the mix. As the water and cement contents of the concrete cannot be reliably measured after batching verification of the w/c ratio has to be made by reference to batch records. To ensure these are provided in a way that can be quickly digested the specification can require that records are provided of the total free water added for each batch in a tabular or graphical form.

Recommendation: The supplier shall provide monthly a separate data or graph for each mix showing w/c ratio based on the batched cement content and total free water content. The graph shall include all concrete batches that have not been rejected. Where the w/c ratio exceeds the designated w/c ratio for the mix a non-conformance shall be raised. . 7.6.8

Batching Sequence As noted in the previous section on water, the sequencing of water addition into the concrete mixer and to the concrete plant slump stand is important for achievement of effective mixing. This is the case for batching all materials. To improve mixing efficiency and reduce the chances of unmixed portions of concrete (not uncommon in higher binder content concrete) it is the aim of the concrete producer to sequence all materials loading into a mixer so that as much pre-blending is achieved before mixing starts as is practical.

Page 61 Higher dosage and high range water reducing admixtures need to be added in a sequence with a great deal of attention to their impact on mixing efficiency. An example of the complexity of this is an example of a high dosage admixture (20+ litres/m 3) being added to a mix with a recommended sequence of adding at the last stage of mixing for optimal performance. This recommendation could not be followed as leaving this amount of fluid out of the mix prevented the preliminary mixing from being effective. This also needs to be considered if HRWR is too be added on site. Another area of concern for sequencing relates to using admixtures that are compatible in the concrete mix but not suited to pre-blending prior to mixing. In this case addition of the admixtures to the mixer will need staging to prevent likelihood of pre-blending before mixing. The concrete supplier will need to account for this in their work method for the concrete mixes being supplied. 7.7

CONCRETE DELIVERY TIME It is important for durability that concrete is delivered in a sufficiently workable state that it can be placed and that it is placed within a time frame that will mean compaction is possible, compaction does not affect already developing bonds significantly and that vibration of interfaces is possible so that cold joints do not form. As 1379 Clause 4.2.5 states “Discharge of all concrete in a batch shall be accomplished within 90 min from the commencement of mixing or before proper placement and compaction can no longer be accomplished, whichever comes first. The 90 min limitation may be waived by agreement.” The standard includes a statement that in hot dry weather the 90 min limit may need to be reduced and in cold weather it might be increased. Mixes can also have retarders added so that the open time is considerably increased. Hence the 90 min guide is frequently inappropriate and needs to be considered almost on a day to day basis. The statement “…before proper placement and compaction can no longer be achieved” is open to abuse because it is a subjective assessment. Concrete should not be used if it has started to stiffen due to hydration yet it might still appear to have been properly compacted. This statement is probably included for the hot weather case where the concrete is stiffening well before 90 minutes. Time to discharge is a convenient way of rejecting a concrete from a concrete supply perspective but it still leaves open the issue of placing time. Once discharged some concrete will be placed and compacted within a few minutes whereas in other cases it may take hours. For example to place one layer of concrete and then place the next layer and vibrate through the layers to prevent cold joints. To overcome the issues discussed above, the specification can modify the delivery and placing times so that A. The discharge and placing times are related to concrete temperature for general concrete. B. Allowance is made to modify the discharge and placing time where the concrete is retarded. Recommendation: It is recommended that the following clause be included in the specification where discharge and placing times are a concern: -

Unless trial mixes show a longer open time the maximum time from adding water to cement shall be as shown in Figure 24. Discharge shall be taken as time to complete discharge and end of compaction shall include revibrating of the interface where concrete is placed in layers.

-

Trial mixes may be undertaken to demonstrate a longer open time for a particular mix. Open time shall be judged by keeping concrete in a truck and measuring the slump at 20 minute intervals. Time to discharge shall be taken as the time to when the slump of the concrete first falls outside the range permitted by AS 1379 for the designated slump. Time to end of compaction shall be taken as time to when the slump has fallen to 50mm or 50% of the designated slump whichever is greater. These limits shall apply to concrete delivered at a lower temperature than the concrete delivered temperature of the trial mix.

Page 62

Figure 24 : Allowable Discharge and Placing Time Vs Concrete Delivered Temperature 160

Time from addition of water

To discharge 140

To end com paction

120

AS1379

100

Poly. (To discharge) Poly. (To end com paction)

80 60 40 20 0 24

26

28

30

32

34

36

38

Concrete Temperature 7.8

INTERVAL BETWEEN BATCHES AS1379 has no specific requirement on the interval between batches. The primary concern is that the interface between 2 successive layers of concrete can be adequately compacted. This requirement will be taken account of if the recommendation in Section 7.7 is adopted and taken account of in pour scheduling.

7.9

NORMAL CONCRETE PROPERTIES There are various standard properties of concrete that should be reviewed by the project team to ensure the concrete supplied will achieve the design durability properties: 7.9.1

Slump 1)

Suitability For Construction Process One of the most frequent reasons for adding water on site is because the slump was not high enough. With modern admixtures it is possible to have a cohesive mix of the required strength and durability with a high slump and hence one of the best ways of ensuring water is not added is to provide a concrete mix with the slump required by the placing team.

2)

For Verification of Water Content AS1379 allows for the use of measured slump being a means of verifying that water content is controlled. Thomas25 provides a case for why slump control may be a more accurate method of controlling water than less accurate methods of plant aggregate moisture measurement in common use at premixed concrete plants.

3)

Measurement and Re-measurement Once the slump of concrete has been specified, the supplied concrete slumpt must lie within the range nominated in AS 1379 from the specified slump or be rejected. The primary reason is that an out of range slump will be an indicator that the design w/c ratio of the concrete may not being achieved in the supply. This could lead to the concrete having poor durability and low strength. Low slumps indicate that the concrete may have started to hydrate more rapidly or the concrete may not be placeable using the proposed methods.

Page 63 Slump should be measured shortly after arrival on site and the time should be representative of the time to slump measurement during the trial mixes. Delays in measuring slump are not recommended as it can be used as a means of bringing high slump concrete back to specification where the stiffening could be due to partial hydration as well as loss of water by evaporation. Where the slump of concrete falls outside of the allowable range the supplier can retest the concrete. Clause 5.2.4 of AS1379 allows for a repeat test of slump if the first test is outside the specified range and if the repeat test is acceptable then the batch is taken to be compliant. The intention of this clause in AS1379 is to recognise that within a batch of concrete there can be variations and it is not appropriate to reject concrete based on one result. These tests should be taken promptly after the first slump test and with little if any further mixing as the delays and mixing will tend to allow concrete to stiffen. The intent of the retest is not to allow the slump to reduce but to allow for variations within the mix. Where a batch with lower than target slump is delivered to site it may be permissible to add water or water reducers as described in Section 4.2.3 of AS1379 provided the maximum allowable mix water in the approved mix design has not been exceeded. 4)

Frequency of Slump Test Concrete slump testing is carried out in accordance with AS1012.3.1. The normal frequency of slump testing is provided in AS1379 along with acceptance criteria for different ranges of target slump. It should be pointed out that where high range water reducing agents are used at varying dose rates (such as on site addition) the slump loses some of its value as a means of assessing mix total water content. It is suggested that if the dose of HRWR admixture is varied on site then the concrete be specified by a slump of concrete prior to addition of the HRWR as well as a final slump and limiting maximum dose of HRWR. In cases of smaller pour sizes of critical high durability concrete it may be sensible to specify a higher frequency of slump testing than provided in AS1379.

7.9.2

Air Content It is rare in Australia for air entrainment to be specified for higher durability concrete. An exception for this will be in the case of concrete required for “freeze thaw” resistance. AS3600 Section 4.7 gives guidance on when this is appropriate and the air contents applicable. Cases such as Freezer room floors and concrete subject to wetting and drying in areas regularly subject to snow would be the more common examples of where this is recommended. In more general high durability concrete not subject to freezing it is suggested that air content be maintained at less than 3%. Air content of concrete is measured using one of the methods provided in the AS1012.4 series. AS1379 provides good guidance on the minimum test frequency and assessment criteria in Section 5.4.

7.9.3

Setting Time Setting times of high performance concrete’s are often significantly impacted by the admixtures used to achieve design properties. Where concrete is placed in forms it is usually less critical if concrete has extended set times but this can have significant impacts on finishing times for concrete slabs leading to arguments with placing contractors and increased potential for plastic cracking. Equally shorter set times will cause difficulty for placement in forms and in slabs with the potential for cold joints and poor finished surface. When assessing a concrete mix the specifier needs to be aware of set time of the design mix and account taken of the impact this can have on placing method, potential for cracking and on contractual arrangements with placing contractors. Assessing the concrete set time by site trial is worthwhile prior to full supply to a structure to avoid risks to the structures finish and durability.

Page 64 The setting time test is contained in AS1012.18 and defines initial set and final set times. AS1379 and AS3600 are silent on the assessment of setting time in concrete and its impact on construction or design. It should be noted that surface finishing processes for slabs should be completed at near to “Initial Set” time to get optimal surface finish without damaging durability. It should also be noted that set time achieved in a trial mix carried out in the laboratory (and most likely assessed at a temperature of around 23C) will be impacted on site by delivered concrete temperature and ambient conditions (refer to Figure 25 below as an example). Figure 25 : Example of the Impact of Ambient Temperature on Initial Setting Time of a Concrete Mix [derived from ref 33]

9 8 7

Set Time (Hours)

6 5 4 3 2 1 0 9

12

15

18

21

24

27

30

33

36

Tem perature ( oC)

7.9.4

Compressive Strength The compressive strength test is universally used in all strength classes of concrete as a means of assessment of concrete strength. In some cases compressive strength is also used as an indicator of other properties such as tensile strength, autogenous shrinkage, creep, modulus of elasticity (refer to AS3600). Compressive strength is generally tested in accordance with AS1012.9 and AS1012.8.1. The project testing frequency usually applied to compressive strength is provided in AS1379 Section 6.5. The testing frequency in this standard is based on a minimum of one sample per 50m 3 of concrete delivered to the construction site. In the case of critical and high durability concrete structures it may be necessary for the specifier to increase the sampling rate and perhaps even testing each batch of concrete if the total supply of a high durability special class concrete is insufficient to get statistically viable numbers of test samples. The means of assessing the compliance of the measured compressive strength of concrete with the design characteristic strength are provided in AS1379 Section 6. The specifier may need to consider alternative means of assessing compliance when the volume of test data for a specific special class concrete will be insufficient to provide meaningful production and project assessment.

7.9.5

Tensile Strength The tensile strength of concrete is generally measured by one of two methods: • The Flexural Tensile Strength (or Modulus of Rupture) tested to AS1012.11.

Page 65 • The Indirect Tensile Strength tested in accordance with AS1012.10. These two methods provide average results on a specific concrete mix that are different to each other. Both test methods have characteristically higher coefficients of variation than that of compression testing and it should be noted that AS3600 Section 3.1.1.3 suggests two relationship formulae for these tests in the absence of field data. Generally the indirect tensile strength of a specific concrete is lower than the flexural tensile strength (between 60% and 90% of the flexural strength value in general). Where specified, the testing frequency for tensile strength in this standard is the same as compressive strength and based on a minimum of one sample per 50m 3 of concrete delivered to the construction site. In the case of critical and high durability concrete structures where the designer deems that tensile strength is critical it may be necessary for the specifier to increase the sampling rate and perhaps even testing each batch of concrete if the total supply of a high durability special class concrete is insufficient to get statistically viable numbers of test samples. The means of assessing the compliance of the measured tensile strength of concrete with the design characteristic strength are provided in AS1379 Sections 5 and 6. The specifier may need to consider alternative means of assessing compliance when the volume of test data for a specific special class concrete will be insufficient to provide meaningful production and project assessment. AS1379 also suggests that an alternative to maintaining a significant test program for tensile strength is to take sufficient samples of both tensile and compressive strength to determine a relationship between the two test methods and thereafter use compressive strength along with a much reduced frequency of tensile strength testing to assess the ongoing mean tensile strength. The reason for this stems from the test variability in measuring tensile strength leading to uncertainty in the evaluation of results. This would only be of value where long term and larger supplies of this concrete are expected (such as road pavements supplied over longer periods). 7.9.6

Shrinkage AS3600 has quite rightly developed a model for shrinkage used in design that separates two forms of shrinkage: 

Autogenous Shrinkage



Drying Shrinkage

Autogenous shrinkage is considered to be that shrinkage which occurs as a result of “self desiccation” of the concrete after the setting concrete has formed some structure (following initial set). It has been a strongly held view that the majority of this autogenous shrinkage occurs in the first 7 days after concrete has been cast and is not impacted by the structures size, shape or atmospheric conditions but is impacted by w/c ratio (or by directly related strength). The widely held view to date has been that Autogenous Shrinkage is only a significant factor where concrete with w/c ratio less than 0.45 is being considered. AS3600 provides a set of formulae for estimating the value of Autogenous Shrinkage at any time after casting concrete in Section 3.1.7.2. Unfortunately there are no Australian Standard tests for Autogenous Shrinkage but there are a number of methods used internationally (Holt26) and these may need to be referenced by a specifier if assessment of Autogenous Shrinkage is considered to be critical. Drying Shrinkage has traditionally been estimated using the method of AS1012.13. Unfortunately there is a proportion of autogenous shrinkage that will be part of the shrinkage value measured by this test and so there is no clear method of using these test results to reference to “drying shrinkage” only as used in AS3600 (even though it is referenced as a test method). The common use of AS1012.13 is to measure the shrinkage value at 56 days drying. Given that the test method starts drying measurement at 7 days then the autogenous shrinkage component of interest is from 7 days to 63 days after casting. For the lack of any other method a designer may want to consider correcting an AS1012.13 test result based on the autogenous shrinkage calculated between 7 days and 63 days based on the target characteristic strength of the concrete and formulae provided

Page 66 in AS3600 Section 3.1.7.2. This correction for autogenous shrinkage is given in Table 9 below. Table 9 : Estimate of Autogenous Shrinkage Component in an AS1012.13 Shrinkage test to 56 days

F’c (MPa)

Autogenous Shrinkage Estimated using formulae from AS3600 from for 7 days to 63 days After Casting (Microstrain)

20

5

25

12

32

23

40

35

50

49

65

71

80

94

100

123

AS1379 recommends a frequency of shrinkage testing at one per 6 months. This may be inappropriate where assessment of drying shrinkage is critical to design. In this case the designer will need to specify a higher frequency of testing as appropriate. 7.9.7

Bleed Australian Standard 1012.6 is a test for measurement of concrete bleed water. AS1379 and AS3600 are both silent on the assessment of bleed water in concrete. There are good reasons for the specifier to ensure that bleed rates are specified for certain types of structure and some examples are given below. For a diaphragm wall and bored piles an appropriate bleed limit is a maximum of 1% of total water with a target value of 0 to 0.2%. These deep pours are constructed to have a full height of fresh concrete under a high pressure of self-weight. These factors lead to high accumulated bleed even for low bleed concrete. High accumulated bleed can lead to reduced cover due to deep bleed channels, bleed lenses under bars that can effectively form irrigation channels through the concrete and a porous concrete at the surface of the pour (vertical and top faces). For thick concrete bases bleed might be limited relative to expected placing rate. A concrete poured at 2m depth/hr requires a lower bleed than one poured at 0.3m depth per hour. A general limit on bleed water of a maximum of 3% of total water seems sensible. Concrete bleed can vary enormously. Table 10 shows selected results from exhaustive trials by a premix supplier. The results do not give any assessment of the admixtures but do show the variability that can occur by mixing admixtures, sometimes to the surprise of all (e.g. Mix 5 in Table 10 below).

Page 67

Table 10 : Bleed of various 50MPa mixes using LH cement (65% slag) Mix 1

Manufacturer 1**

Manufacturer 2**

WR + PCE HRWR

2

PCE HRWR+ SRA + WR

3

WR + PCE HRWR

4

WR + BNS. HRWR

5

WR+SRA+BNS. HRWR

SRA + WR

Initial Slump With SP (mm)

Final Slump (mm)

Bleed (%)

230

120 @ 3.5hrs

0.3

220

200@ 5rs

1.2

240

130 @ 90mins

2.8

230

139 @ 2hrs

6.5

240

180 @ 5.5hrs

22.2

** WR – Water Reducer; PCE HRWR – PCE based high range water reducer; BNS. HRWR. – Beta Napthalene Sulphonate based high range water reducer; SRA – Set Retarding Admixture.

Many other admixtures are available for use in concrete. Cook 22 summarised the effect of the use of the most commonly used admixtures on concrete durability. It is important to control the quantity of admixture used accurately, and to follow the manufacturer's recommendations generally. Only admixtures shown by trial mixes to be satisfactory for concrete should be used. 7.10 SPECIAL PROPERTIES There are a number of properties of concrete not covered in AS1379 as regards to the measurement of these properties or to the frequency of assessment and interpretation of results. These special properties are discussed in AS3600 in some, but not all cases. 7.10.1 Modulus Of Elasticity The modulus of elasticity (also “Young’s Modulus”) of concrete is a measure of elastic deflection of concrete under load. It is a value used routinely in design to estimate concrete slab, beam and suspended floor deflections in design as well as being used in the estimation of concrete Creep (see below). AS3600 Section 3.1.2 provides formulae for estimating the elastic modulus based on concrete typical average strength (not characteristic strength) and average plastic density. There are other alternative methods of estimating the elastic modulus used internationally and in research papers (Thomas 27) but it should be noted that AS3600 recommends the accuracy of its formulae as being within +20% and in most cases that is sufficient for design. If this is not the case then the designer should verify the proposed concrete mix modulus of elasticity by testing samples of concrete using the test method of AS1012.17. 7.10.2 Creep of Concrete Like the modulus of elasticity test and the tests for shrinkage, the values of concrete creep are used in design. Generally the creep values of concrete are related to the concrete strength, mix composition (binder type, aggregates and admixtures) and loading on the concrete as well as being time related in a similar way as is the case for shrinkage. AS3600 Section 3.1.8 provides formulae for estimating the creep of concrete based on concrete characteristic strength, modulus of elasticity, age of loading, concrete member dimensions and age of assessment. There are other alternative methods of estimating the creep of concrete used internationally and in research papers (Thomas 27) but it should be noted that AS3600 recommends the accuracy of its formulae as being within ±30% and in most cases that is sufficient for design. If this is not the case then the designer should verify the proposed concrete mix creep factor by testing samples of concrete using the test method of AS1012.16. It should be noted here that it takes at least 12 months to get a reliable value of creep factor so this needs to be taken into account by the designer where creep is going to be a critical factor.

Page 68 7.10.3 Resistance to Chloride Ingress There are many tests for measurement of the resistance of concrete to chloride ingress but none are currently available in the Australian standards. The designer will get guidance on choosing a suitable test method in other sections of the CIA Recommended Practice “Concrete Durability Series”. As noted before, this testing is not currently a requirement of AS1379 or AS3600 and so concrete producers may not readily have test information to support a mix design is compliant with a designer’s specification. As such the specifier and construction contractor need to be aware that common tests for resistance to chloride ingress may take up to 4 months to complete and take account of this in the project tendering process. 7.10.4 Sulphate Resistance Neither AS3600 nor AS1379 provide guidance on the testing of a concrete mix for Sulphate resistance. AS3600 Section 4.8.1 does give guidance on concrete cast in Sulphate and Acid Sulphate soils and that is to use “Sulphate Resisting” cement with suitable mixes based on exposure classifications. While there are no Australian Standard test methods for assessing a concrete mix resistance to sulphate attack, there are test methods for assessing a binder suitability as an SR Cement under AS3972 and generally SR Cements in Australia are blends of GP Cement with SCM’s. 7.10.5 Water Penetrability There are many tests for measurement of the water penetrability of concrete (Sorptivity, Initial Surface Absorption and Permeability methods) but none are currently available in the Australian standards. The designer will get guidance on choosing a suitable test method in other sections of the CIA Recommended Practice “Concrete Durability Series”. As noted before this testing is not currently a requirement of AS1379 or AS3600 and so concrete producers may not readily have test information to support a mix design is compliant with a designer’s specification and so time to assess this will need to be built into the project tender process. 7.10.6 Self Compacting Concrete Properties “Super-Workable concrete” and the subset of it referred to as “self compacting concrete” require a number of test methods to verify performance of the concrete. Currently there are no Australian Standards that detail the specialised test methods for this product but a number are provided in the CIA document on Super-Workable concrete28. Super-Workable concrete (SWC) and Self Compacting Concrete (SCC) are similar in specification except that SCC has to be designed so that no compaction is required to place this concrete whereas SWC may need some compaction and so the mix design, admixtures used and tolerance to segregation will be different for each type of product. Critical issues for these concrete products are measuring workability and more specifically the mix viscosity, the mix tendency to segregation under standard conditions and segregation under higher pressure (pump line and deep piles or footings). Both SCC and SWC can be very useful for high durability concrete as they reduce concern on the consistency of compaction delivered to concrete structures where there is poor access for vibrators. If compaction is used on SWC then the compaction method should be checked to ensure that it does not promote segregation. Compaction should not be applied to SCC. 7.11 M ANAGEMENT OF TRIAL MIXES Concrete trial mixes are often used to verify the less commonly tested properties of an existing concrete mix or to assess a new concrete mix. Trial mixes can be carried out as a full scale trial including potential delivery of a batch of concrete to the site. When the specifier and construction contractor are considering how to carry out the trial mix it is important that at some point the mix is verified using the standard production process. A

Page 69 laboratory trial mix is useful for comparison purposes and is more accurate in terms of constituent quantities, control of temperature and reflection of delivery timing. The danger with reliance on a laboratory trial mix only is that it may not represent the mixing process, temperature and time delays that regularly are part of delivery to site. 7.11.1 Transport time and impact on water control Concrete is composed of materials that can slowly absorb water from the concrete mix after batching at the plant. Materials such as cement (and SCM’s), some aggregates and some additives can absorb water and this will be seen during the delivery process as loss of slump with time and adds to any other water loss through evaporation from the delivery truck mixer. Ambient temperature also impacts on the rates of evaporation and the cement absorption rate with a result that the slump loss between batch plant and delivery of concrete is greater in hot conditions than in cool. Certain admixtures and cooling of concrete can slow the rate of this slump loss but most importantly the concrete supplier must understand this slump loss rate and ensure that this is factored into the final mix design (Thomas25). A risk of using laboratory trials only and no field trial to verify the concrete mix and its properties is that these delivery time and temperature related impacts of concrete delivery may be ignored prior to supply to a project. If a mix through specification has a particularly high rate of slump loss then this may also need to be factored into acceptable discharge times in the project specification for the mix. 7.11.2 Prescription Vs Performance As noted previously, where a specifier provides a prescriptive specification and accepts the performance attributes of that concrete then the concrete producers task is a fairly simple one of verifying that the prescriptive mix design elements can be put into a mix with suitable workability to be delivered and assure that the required testing is carried out. The use of an existing concrete mix or a laboratory trial is often satisfactory in this case as long as delivery times and temperatures are accounted for. Where a performance based specification is provided and contains testing that is not normally carried out (such as specific durability tests) then the supplier will need to research alternative mixes and carry out preliminary trial mixes to verify properties. As the lead time into a project may be limited it can be advisable for the supplier to test a range of mixes so that the performance parameters of the mixes can be assessed to find a suitably complying value. All test methods have associated variability and if a specifier requires a maximum or minimum performance parameter then it is important that a target for acceptance is set so that this test variability is accounted for to achieve the boundary conditions with a reasonable level of risk for the construction contractor and supplier. 7.12 FLOATATION OF CAST IN VOIDS Void formers that form part of a concrete pour will try to float. The uplift is very high because of the density of concrete. Very often this floatation is underestimated and the void former rises. In some cases this can lead to a cold joint due to cessation of the pour, low cover or breakout of concrete all of which can lead to durability problems that have to be considered and overcome on site. Recommendation: It is recommended that concrete specifications require that the concrete method statement include calculations of the floatation force and restrain force and elongation for objects that will try to float in the concrete pour. 7.13 COLD JOINTS Cold joints are a common risk in pours. Where they occur the designer will have to assess whether the cold joint is a structural concern or durability concern.

Page 70 8. REINFORCEMENT AND PRESTRESSING STEEL Depth of cover to reinforcement, and reinforcement condition, are key factors in the initiation and propagation of corrosion. Reinforcement and prestressing steels should be located and maintained in the correct position during concrete placing to ensure adequate cover. These materials should be handled and stored in such a way that they are free of harmful materials and are not damaged prior to concrete placement. The specification for the supply and fabrication of reinforcement and prestressing steel generally, and the fixing of reinforcement and prestressing strands and anchors in particular, should be in accordance with the requirements of AS 3600. 8.1

ORDERING Reinforcement should be ordered from a supplier who has been evaluated and selected on his capacity to produce the reinforcement in accordance with the contract documents, and deliver the reinforcement at a rate consistent with the project schedule. The supplier should be aware of all relevant aspects of the contract documents and in particular the reinforcement grade, cover and coating requirements. Ordering of reinforcement should take into account the scheduling of the project. Early delivery should be avoided since it may result in long periods of site storage, which could result in corrosion or deterioration of the reinforcement. In addition, large quantities of reinforcement stored on site may lead to the wrong bars being used, which, in turn, could affect cover requirements. When reinforcement is delivered to site it should be checked to ensure that it is as ordered. Particular attention should be given to bar dimensions that may affect cover, such as bend radii on fitments and stirrups, and bar length. In addition, bar grade and diameter should be checked. A larger number of smaller-diameter bars should be substituted only with the approval of the designer, otherwise this could result in difficulties with concrete compaction.

8.2

HANDLING, CONDITION AND STORAGE All reinforcement and prestressing steel should be clean and free from harmful matter such as loose mill scale, loose rust, oil, grease, retarders or similar matter. Reinforcement and prestressing steel should be stored clear of the ground and be effectively protected from corrosion and deterioration. Where covers are used, they should allow free circulation of air around the steel to prevent condensation, which could result in corrosion. All materials should be checked before use, particularly after long periods of site storage. Prepared tendons and reinforcement cages should be protected from traffic and other construction activities. Before placement, they should be checked to ensure that they are free of harmful matter as above. Associated materials used in prestressing operations, such as anchorages and ducts, should be handled and stored in such a manner that their effectiveness is not impaired.

8.3

REINFORCEMENT FIXING The major cause of reinforcement corrosion in concrete is insufficient cover to the reinforcement, resulting from poor construction practices, e.g. inadequate fixing and support of bars. Reinforcement, prestressing steels and any inserts should be securely fixed in position with the appropriate cover such that they will maintain their position during placing and compaction of the concrete. Reinforcement bar chairs and supports should be used to locate and maintain the reinforcement in position both horizontally and vertically as shown in the contract documents and within the specified tolerance. AS 3600 highlights the fact that fitments may also corrode. Therefore, cover needs to be maintained to these items, either by placing chairs under fitments or using larger chairs under the bars. Chair type should be selected such that it will not provide a corrosion path to the

Page 71 reinforcement. The need to ensure that the surface finish is not affected should also be taken into account. Appendix 1 provides guidance on the type and extent of usage of bar chairs and supports. Spacing of chairs and supports should be sufficient to maintain the reinforcement in its correct position between these supports. This includes hangers and ties, both of which should be sufficiently rigid to prevent extension during concreting. Adequate support is particularly required for reinforcement in vertical members (columns or walls) to avoid displacement of the bars while concrete is being placed. Particular care is required to provide sufficient cover over stirrups. Any slope or taper in the top surface, particularly across the width of the member, can result in insufficient cover unless appropriate bar shapes are adopted. Cover at edges and corners of members should be carefully checked since exposure is on more than one face, and restraint to cracking and spalling is often reduced. Cover to tie wire, particularly on vertical members and beams, should be maintained by bending the ends back into the body of the member. Tendons should be accurately located and maintained in position both horizontally and vertically and within the specified tolerance, as shown in the contract documents. Ducts should be supported as for reinforcement. Tendons should be stressed and the duct grouted as soon as practicable after placing the tendons in the duct. Special precautions to prevent tendon corrosion should be taken if unstressed tendons remain in ducts for any length of time prior to stressing. Such precautions include the use of oils or other corrosion inhibiting substances on the tendons. These materials should be used only if they have no adverse effect on the grout properties or the bond of the grout with the tendons. If special precautions are not taken, the tendons should be removed and inspected, and any change in the quality of the tendons determined by tests if their condition looks to have been significantly impaired. Special precautions should also be taken to prevent tendon corrosion if tendons remain in ducts after stressing and without grouting for any length of time. Such precautions include the use of materials as noted above. Unbonded tendons should be permanently protected from corrosion by encasing the ducts in concrete or mortar or other materials suited to the particular environmental conditions. Consideration should be given to the differential movement between the structure and the applied protection resulting from the effects of loads, creep, shrinkage or temperature changes. 8.4

WELDING OF REINFORCEMENT All welding should be carried out by qualified, skilled operators, and should be in accordance with AS 1554, Part 381. Reinforcement cages that require welding should be pre-assembled in a jig, rather than in the form or mould, to eliminate the risk of damage to the form or mould and consequent undesirable effects on exposed concrete surfaces. When welding coated reinforcements, any coating removed during the welding process should be made good in accordance with the designer's and supplier's requirements.

8.5

REINFORCEMENT The corrosion resistance of steel depends on its composition and surface finish. Clearly stainless steel, galvanised steel and carbon steel have different corrosion resistances. It is also recognised that different grades of stainless steel with have different corrosion resistant properties. However, not so well recognised is the potential difference in performance of different carbon steels.

Page 72 Reinforcement coatings take two forms, organic coatings that act as a physical barrier and active coatings that act as a barrier and sacrificial layer. There are many myths about the performance of coatings and so the specifier needs to be particularly cognisant of long term proven performance data. In this section some guidance is given regarding the use of different steel types and the coatings applied to steel. 8.5.1

Carbon Steel Black steel reinforcement is deemed to be consistent around the world and the quoted chloride activation level for reinforcement in concrete are deemed to be universally applicable. However, the chloride activation level can vary depending on the steel composition, i.e. alloying materials used and the surface finishing. At this stage there are no recommendations regarding use of such steels but some information is provided here should materials from overseas be introduced in the near future. Low carbon reinforcements are being introduced in the USA and Middle East with improved chloride activation levels. Some are said to have a chloride activation of 3 times that of conventional reinforcement. Use of these high corrosion resistant materials could have great benefit at allowing reduction of cover or increase in design life at the same cover. However, clear evidence of the allowable increase in chloride threshold would be required before allowing reduced covers.

Recommendation: It is recommended that use of lower covers for low carbon reinforcement only be permitted where the increased corrosion threshold has been clearly demonstrated and appropriate probabilistic modelling has been carried out to demonstrate the reliability of the low carbon steel at the reduced minimum cover. 8.5.2

Epoxy Coated Bars There was great interest in the use of epoxy coated bars for durability from around 1980 to 2000 following its introduction in the USA and subsequent wide spread use in the Middle East. Positive coating experience is limited to electrostatic spraying of epoxy powder to straight lengths of reinforcement. Performance is discussed in details in FIB 2009. There was some attempt to introduce them to Australia but a proper manufacturing plant was not established and the performance of the early attempts was shown to be poor in salt spray cabinet tests. FIB 2009 suggests epoxy coated bars gives better corrosion protection than galvanising but there are many requirements to ensure a coating that provide adequate performance and to date no facility has been developed in Australia that would give coated reinforcement compliant to the various standards. FIB 2009 outlines the applicable standards and requirements as follows. In ISO 14654 the requirements for the epoxy-coated steel are prescribed. ISO 14656 defines the requirements to the coating powder and the sealing material. These demands correspond to the test programme in ASTM A775. According to the afore-mentioned standards, the finished coating thickness shall be at least 170 and 175 μm. respectively. There shall be not more than 3 to 4 holidays (pinholes not discernible to unaided eye) per metre of bar. The adhesion of the coating to the steel is tested by bending coated bars 120° around a mandrel with diameter 8 to 10 times (ASTM) respectively to an angle of 180° around a mandrel with a diameter 4 to 6 times (ISO) that of the bar without cracking or debonding appearing in the coating on the outside of the bend. The bond stress reduction should not exceed 15% when compared with uncoated steel (ASTM A775).

Recommendation: Epoxy coated bars had very poor acceptance in Australia and it is recommended they are not used as a means of corrosion protection until such time that a proven system is accepted by the Australian construction industry.

Page 73 8.5.3

Galvanised Bars Galvanised reinforcement is widely used to give additional corrosion protection and it has a long history of successful use. Understanding the mechanism by which it works highlights how it should be designed in practice. This is detailed in CIA CPN17 81 and FIB 2009. Galvanising produces a coating on the reinforcement consisting of several layers of ironzinc alloys metallurgically linked to the base steel. It is a very tough coating with excellent resistance to abrasion and impact and the method of its application (dipping in a molten zinc bath) means there is a very low incidence of incomplete coverage. FIB2009 details the significance of the properties and thickness of the different layers but this detail is not highly important to the general understanding of how the galvanising protects the reinforcement. The corrosion mechanisms of zinc in concrete are well researched. Zinc in concrete quickly passivates and unless it is activated in some way it has a very low corrosion rate and will not provide significant sacrificial protection to black steel but it works as an effetcive barrier coat to the underlying black steel. FIB 2009 provides details of this process but the following are the key points: a) Cracks in the coating occur due to the process and bar bending post galvanising but these should not be considered problematic b) No allowances are required for changes in steel properties of galvanised reinforcement as there are practically no harmful consequences to the steel due to the process of galvanising c) Galvanising should be undertaken after welding. Where welding is required after galvanising the galvanising should be removed by grinding or grit blasting in the vicinity where a weld is to be made. Zinc rich paint should be applied to the area where the coating is lost. d) “Extensive research in the last twenty years has universally demonstrated that there was no reduction in bond strength for galvanized bars compared to equivalent nongalvanized bars.” e) At a pH of less than a threshold of 13.2 the zinc passivates in a matter of days of being embedded in the concrete. Where the concrete pH exceeds 13.2 (e.g. in alkali concrete) the zinc may corrode rapidly and disappear in a short time. Use of SCM’s is likely to resolve the issues of high alkali cements. f) The passivity of black steel breaks down at a pH of below about 11. Hence in carbonated concrete it can corrode rapidly if there is sufficient moisture. The zinc coating remains passive in completely carbonated concrete and corrodes only very slowly at a uniform rate. Theses aspects make galvanized steel reinforcements exceptionally suitable for use in carbonated concrete. g) “If the limit of the chloride content under these conditions (corrosion activation) is set at 0.5 to 1.0 % (by weight of cement) for uncoated steel, it would seem reasonable to set a limit of about 1.5 % for galvanized steel.” The time to corrosion activation of galvanised steel may typically be 2-3 times that of ungalvanised steel. h) When the galvanising does corrode there is no noticeable protection at areas of pitting corrosion from the surrounding zinc and the propagation time for galvanised steel is not significantly different to ungalvanised reinforcement The significance of galvanising on the design life in different exposures is given in Table 11.

Page 74 Table 11 - Design Life of Galvanised Steel in Different Exposures Exposure Carbonated concrete B1 and B2 chloride exposures

C1 and C2 chloride exposures

Mechanism

Design life significance

Galvanising remains passive at pH 9 and hence does not corrode The surface chloride level is unlikely to exceed the chloride activations threshold for zinc

Even at low covers 100 years design life is expected.

The surface chloride level is likely to be above the chloride activation level for zinc and hence a zinc activation front will move into the concrete with time

Galvanised fittings exposed at the surface should not be used as they will quickly corrode. Galvanised rebar at the same cover as black steel will have a much increased life or alternatively galvanised reinforcement with lower cover than black steel can be used for the same design life.

Recommendation: : It is recommended that durability design adopt points a) to h) and the lives in Table 11 . Given that the passive galvanising does not provide galvanic protection to the black steel it is important to repair damage to the galvanised coating. This could be achieved using zinc tape with a conductive adhesive backing or high build zinc rich coating which can be applied insitu. CIA CPN1781 notes that the galvanising surrounding damage will provide protection to the underlying reinforcement but theory suggests that passive zinc in concrete will not do that. CIA CPN1781 provides information on repair materials but since 2002 other materials have become available. CIA CPN1781 notes the importance of repair is due to consumption of the zinc at these defects but theory suggest it is because the black steel is at risk of corrosion at damage points while the zinc remains passive. Recommendation: It is recommended that concrete specifications permit electrical continuity between galvanised and black reinforcement. CIA CPN1781 also notes that galvanic corrosion between the black steel and the galvanising will occur unless the connection is deep in the concrete or the galvanised and black steel are isolated. This does not give the whole storey. The location of the connection between the black steel and galvanised steel is to some extent immaterial. If an electrical connection is made anywhere and the black steel and galvanised steel anywhere are in ‘close’ proximity and both are active then galvanic corrosion could occur. If either is passive and will remain so then galvanic corrosion will not occur. Typically both will remain passive in a reinforcement design but that’s not the case for fixings where the near surface galvanising may become active in high chloride environments. However galvanised fittings should not be used in these environments. Recommendation: It is recommended that concrete specifications require that: a) Galvanised fittings not be used in marine exposures. b) Galvanised fittings may be used in exposure class B provided that the fitting is isolated from the reinforcement. c) Electrical isolation between galvanised fittings and reinforcement is not required in A and B1 exposures. Insulation between black steel and galvanised fitting maybe beneficial in B2 exposures. In less severe exposure isolation of the fittings is not required. In C exposures galvanised fittings should not be used.

Page 75 The term ‘close’ was used above as it has an imprecise definition. It is really the resistance between the anode and cathode that is important. In very high resistant concrete only a few millimetres separation will provide sufficient resistance that the galvanic cell will not work. In a wet chloride environment with GP cement long line galvanic cells could enable galvanic corrosion over a meter separation or more. When using hot-dip galvanized reinforcement the following precautions should be observed: When using hot-dip galvanized reinforcement the following precautions should be observed: a) b)

Use of only galvanized tie wires and galvanised steel chairs

c)

No use of re-bent galvanized reinforcement

d)

8.5.4

The bars are passivated in a 0.2% sodium dichromate solution applied by the galvanizer not more than 60 days before casting in the concrete

No welding of galvanized reinforcement unless weld damage is made good using and appropriate

e)

Cold-worked or cold-formed deformed bars are not galvanised

f)

Hard drawn steel wire fabric may be galvanised.

Stainless Steel Bars FIB 2009 notes that stainless steel is a group of corrosion resistant highly alloyed steels and provides details of performance of different types of stainless steel reinforcement in different exposures depending on the composition of the alloy produced. It notes “It is therefore important to select steel types with an alloy content which is sufficiently corrosion resistant for the job to be done and having sufficient mechanical properties and weldability as well.” FIB 2009 reports on specifications for stainless steel reinforcement and provides details of UK specifications as follows: “BS 6744 specifies stainless hot-rolled and cold-worked steel bars to achieve characteristic strength levels of 500 N/mm2 or higher. Strength grades are defined in Table 12. Table 12 : Maximum Tensile Properties Strength grade

0.2 % proof strength Rp0.2 (N/mm2)

stress ratio Rm/Rp0.2

Elongation at fracture A3 (%)

Total elongation at maximum force Am (%)

Nomina l size (mm)

500

500

1.10

14

5

6.50

650

650

1.10

14

5

3.25

In the UK, stainless steel is currently produced from the austenitic and ferritic-austenitic materials 1.4301, 1.4436, 1.4429, 1.4462, 1.4501, 1.4529. They are listed from left to right in order of increasing corrosion resistance and, consequently, of increasing initial cost. In most situations, standard austenitic grades 1.4301 or 1.4436 will provide an acceptable solution when designing against corrosion. The higher grade austenitic and ferriticaustenitic steels should be considered when the possibility of high levels of chloride buildup in concrete over time is anticipated (e.g. marine structures or traffic structures heavy contaminated with deicing salts). These materials are typically available in all three strength grades; however the duplex steel designation 1.4462 is only available in 650 grade.” FIB 2009 notes “The corrosion properties appear to be extremely dependent on the state of the steel surface. In particular, all scale and temper colours can aggravate pitting corrosion and therefore the usual welding procedure will lead to a significant reduction in the corrosion resistance; it reduces the level of chloride contamination at which corrosion can take place. This problem can be overcome by using a more highly alloyed the steel or by removing mill-scale and temper colours by pickling or shot blasting.”

Page 76 There is no Australian Standard for Stainless Steel but Queensland’s MRTS71A provides an excellent guide specification for supply, fabrication and placing. 8.6

PRESTRESSING 8.6.1

Prestress A compounding issue with corrosion of prestressed cables is that they can fail without warning Patterson(9) quotes examples of: 

Laval highway overpass in Quebec where five died when a bridge collapsed even though a visual inspection had identified no problems a year earlier.



Botany Bay Bulk Liquids Berth where a catwalk collapsed when a worker walked across it. The investigation showed there was no sign of spalling or surface rust staining although corrosion initiation was at a weak point in the cover.



Botany Bay Jetty which was demolished after 15 years due to risk of sudden collapse.

In view of the low likelihood of early visual warning of catastrophic failure prestressed elements should be designed: 1)

With a high reliability against corrosion activation; or

2)

Corrosion monitoring that will give early warning of failure.

Recommendation: It is recommended that prestressed concrete shall be designed with a reliability of 3.6 for time to initiation of prestressing steel where design life modelling is undertaken in accordance with Z07/05. Where deemed to comply requirements of Z07/03 are used the applicable reliability shall be selected. Where Australian codes are used, the designer should take account of the extent to which a higher reliability might already be included in the code and the actual reliability required for the element.

8.6.2

Post-Tensioning The principle issues with post tensioning are: 1)

Poor anchorage protection leading to corrosion of anchor plates, wedges and cables (Figure 26a). The protection of the ends of anchorage is a difficult task as effectively a small area of topping is being applied. Failure of such toppings over time is not surprising as there is no mechanical key between the topping and anchorage and there can be high differential movements. In order to avoid failure a material with a low modulus physically anchored to the parent concrete would provide a higher degree of success.

2)

Poor grouting leading to voids in the tendon ducts (Figure 26b). Poor grouting may be a bit of a misnomer as grouting requirements for ducts has been lifted considerably in the last 15 years. However in the USA failures of tendons have occurred even with these new standards. It seems that grouting to eliminate duct voids requires an extremely high standard of materials, testing, grouting and quality assurance.

Durability design of post tension systems became a major issue in Europe and North America after the collapse of two bridges. The issue was considered so serious that the Department of Transport in the UK banned the use of Post Tensioned bridges until improved guidance became available. European codes of practice 74,75 now provide significant guidance on methods of ensuring durability and this is developed by increasing levels of corrosion protection for increased levels of exposure severity. It is recommended that these documents are used as the basis of specification for post-tensioned systems.

Page 77 Figure 26 : Photographs of issues with post-tensioned cables.

a) Condition of a prestressing anchorage after removal of the concrete protection. 8.7

b) Boroscope picture of void in a prestressing cable

FITTINGS To prevent the occurrence of galvanic cells, bars and inserts with metallic coatings should not be placed in contact with uncoated bars or inserts; nor should they be placed in contact with bars or inserts having a different metallic coating. In unavoidable cases, non-metallic spacers (e.g. polyethylene) may be used to provide local insulation, but this should be done only under careful site supervision. Tie wires used with coated bars and inserts should have similar coatings. Recommendation: It is recommended that protections of fixings in concrete shall comply with Table 12.

Table 13 : Fixings In Concrete Cover Zone Exposure

Item

General

All

A1

All

A2, B1, B2

Bolts

Anchors

C

All

Protection Required for Atmospheric Exposure AS 2312 provides useful information on the expected life of zinc coatings in all environments including marine & severe marine. Malleable cast iron or electroplated zinc are suitable for 50 year life. HDG as AS 1214. 375g/m2 is average requirement but might be difficult for smaller fasteners if centrifuged – see AS 4680 Table 2. Hot dip galvanised 600g/m2 avg. Ability to achieve this level of HDG is dependent on size of anchor – usually greater than 6mm thickness – see AS 4680 Table 1. Stainless steel - PREN of approximately 24 (e.g. type 316) is required for marine atmospheres for 50 year life while in severe marine atmospheres for 100 year life may require PRE of approximately 34 Duplex stainless such as 2205. Fasteners of specific types may not be available in this grade. The higher the PREN, the greater the corrosion resistance. AS 4312 talks a little on stainless steels in atmospheric environments.

Galvanised fixings in concrete are passivated by the uncontaminated concrete and when connected to the reinforcement which is also passive and will not corrode. In chloride environments the passivation on galvanised fittings will break down as chlorides ingress and an active cell

Page 78 between electrically connected reinforcement and the galvanising would lead to accelerated loss of the galvanising. In carbonated concrete the galvanising remains passive and no active galvanic cells develops. Hence in all B2 exposures galvanised fittings shall be electrically isolated from the reinforcement. This can be achieved by the use of insulation tape. Stainless steel fixing will not develop a significant potential difference between the reinforcement and the stainless steel unless the black steel corrodes in which case a significant potential difference will develop. However as corrosion of the black steel would in any event be a design failure electrical isolation is not required.

Table 14 : Corrosion Rate of Zinc Galvanising and Carbon Steel in Atmospheric Exposures

Corrosivity Category ISO 9223 C1 - Very Low (Inside buildings) C2 - Low (Dry rural areas) C3 - Medium (Coastal areas low salinity) C4 – High (Coast. 50m to 1km inland) C5 - Very High (Beachfront and offshore)

Corrosion rate (µm/yr) Zinc Galvanising Carbon Steel (EN ISO 14713) (EN ISO 12944-2) <0.1 <1.3 0.1-0.7 1.3-25 0.7-2.1 25-50 2.1-4.2 50-80 4.2-8.4 80-200

From Table 14 it can be seen that an 80 µm galvanising layer would be lost due to corrosion in less than 10-20 years in beachfront exposures and 20-40 years in many coastal exposures. 8.8

PROVISION FOR CATHODIC PROTECTION Concrete elements should be designed on the basis of achieving their design life without application of cathodic protection. Cathodic protection (CP) is an effective way of controlling corrosion in severe environments should durability become an issue in the future and any element in B2 or worse exposure classes should be designed for future CP with a feasibility of such being outlined. This should include the need to make reinforcement electrically continuous, how anodes would be deployed and how systems would be monitored. The maintenance management plan shall include inspection and monitoring requirements and this shall consider the requirements to assess if installation of a CP system is required. For example this might require an electro-chemical condition evaluation at 20 years to determine how the structure is performing.

Page 79 9. CONSTRUCTION No matter how good the design and detailing or the materials specified for a concrete structure, it is the construction practices that will substantially determine its durability. It is essential for the production of durable concrete that the contractor ensures satisfactory workmanship on site and adopts appropriate quality control procedures. Particular emphasis should be placed on ensuring the correct cover to reinforcement and tendons, and satisfactory compaction of the concrete and the adoption of efficient curing regimes. These factors are generally recognised as being the most important in construction for durability. In addition, adequate construction inspection by the designer, combined with supervision of construction procedures by the contractor, are essential to ensure a level of construction consistent with the design assumptions and details. The roles of the designer and contractor should not be considered as totally separate tasks. Cooperation between the design team, including architect, structural engineer, services engineers, and the contractor and various sub-contractors is a necessary part of the building programme. Potential problems should be identified and solved at an early stage and at a professional level, with input both from design consultants and contractor. Even if the mix is well designed, and the concrete properly cured, low penetrability will be achieved only if the concrete is properly compacted. This section discusses aspects of placing and compaction that impact on concrete durability. Porous concrete resulting from unsatisfactory compaction can permit penetration of aggressive agents to the reinforcement, with consequent corrosion problems. Proper compaction is essential to expel air entrapped in the concrete during placement, to consolidate the concrete without causing segregation, to reduce the risk of settlement cracking and to ensure good bond between concrete layers and between concrete and reinforcement. 9.1

TRAINING & SUPERVISION Rostram17 notes that most durability failures relate to a known process and could have been avoided and hence “The most important element in preserving and improving the quality of structures and their performance is efficient continuing education, where new theories, new technologies and experience gained can be spread to a sufficiently large number of people involved in the design, construction and upkeep of building structures.” This is as true today as it was in 1989. This section provides guidance on the training (content and method) to be given to the concrete team to minimise durability issues arising through lack of knowledge. However no publication can give a complete guide and it is necessary for all engineers to develop knowledge through education. As durability failures occur in the long term the designers, construction engineers and concreters often do not see the results of the errors they have made and hence their education is particularly important. 9.1.1

Supervision Supervision of the work to ensure appropriate quality on site is the responsibility of the contractor. Inspection of the work to ensure that construction procedures and materials meet the requirements of the contract documents is the responsibility of the designer. The contractor is responsible for construction, but must also accept some responsibility for the quality of the finished product and its durability. In recent times, the contractor's role has tended to change from that of a builder who employed and was in control of labour and materials to that of a manager coordinating sub-contractors. The five concrete trades of formwork construction, reinforcement supply, reinforcement fixing, concrete supply, and concrete placement are often undertaken by different sub-contractors, sometimes with more than one subcontractor for the one trade.

Page 80 Working under these extended lines of control, with the added pressure of cost and time constraints, it is obvious that the contractor requires a high level of application and organisation to produce a concrete product of satisfactory quality and durability. The designer is responsible for producing suitable documentation for the works but must also accept the responsibility of inspecting the works to ensure that the construction meets the design requirements. The designer's role in inspecting the work has also tended to change in recent times, with responsibilities sometimes being placed on the builder or an independent representative of the client who may have little knowledge of the design intent. Cost and time factors may also play a part in the level of inspections. It is important for the various parties to recognise the changing roles of both the contractor and designer. The level of supervision and inspection should be appropriate to the project and should be agreed on between the parties before construction starts. 9.1.2

Personnel and Responsibility Construction of the work is the responsibility of the contractor and should be carried out only under the supervision of the contractor's personnel who are suitably qualified and experienced in concrete work. This supervision should ensure that the requirements of the design as contained in the contract documents are achieved in the construction. A programme of inspection by the designer, or a suitably qualified and experienced representative of the designer, should be established and implemented to ensure that construction procedures and materials meet the requirements of the contract documents. This does not relieve the builder of his responsibility to institute a level of supervision that will meet the requirements of the contract documents. Agreement should be reached between the client, contractor, architect and engineer prior to the start of construction as to the role each is to play in supervising/inspecting the works. If value is to be gained from an inspection programme it is essential that the appropriate person carries out the inspections. This may not necessarily be the client's representative. Where specific construction activities are required by the designer, such activities should be verified by the designer in accordance with the contract documents.

9.1.3

Construction Supervision The extent of supervision by the contractor will depend largely on the degree of difficulty or complexity of the work, together with the importance attached to the achievement of the design requirements – in general, and the concrete durability in particular. Particular areas of supervision of importance to concrete durability include the following: 

Formwork and falsework – design, materials, accuracy and watertightness of the system adopted.



Reinforcement, tendons and fittings – the condition of these elements when supplied, storage requirements on site and accuracy when positioned.



Tolerances – on formwork, reinforcement, tendons and fittings.



Concrete – the mixed concrete, placing, compacting, finishing and curing.



Protection – of the concrete during the curing period.



Removal of the formwork – stripping times and procedures for form removal.

In addition, the contractor should ensure that correct job records are maintained, and data on materials used in the construction are collected. This information should be held during the progress of the work and for a period following completion, as set out in the contract documents. 9.1.4

Inspection Programme The extent of inspection by the designer should be related to the level of the designer's confidence in the contractor and to the nature of the work. (This assumes that the designer has been engaged to carry out inspection.) While some items of inspection will be common

Page 81 to most projects, the designer and construction supervisor should agree on the inspection programme before construction starts. The inspection programme should include written check lists for items of construction, together with criteria for acceptance or rejection. Provision should be made for the specific verification and acceptability of implemented field changes. Minimum inspection programme requirements should include: 

the formwork system;



reinforcement installation;



concrete quality;



concrete placement;



compaction; and



curing. Other factors that may be included are:



formwork removal;



climatic conditions;



sampling and testing;



finishing operations; and



grouting operations.

Photographs documenting construction sequence, job progress and construction details may be appropriate to some operations. Inspection records should be maintained during the progress of the work, and for a period following completion, as set out in the contract documents. Depending on the project, a full-time representative of the designer and/or client may be required on site. The role of such an inspector should be carefully documented and in this regard the procedures contained in the ACI's Hot Weather Concreting 79 are considered useful. 9.1.5

Notice for Inspection The contractor should give adequate notification of when inspections are required, as set out in the contract documents. For inspection of reinforcement, sufficient time should be allowed for carrying out the inspection and making any alterations or additions that may be necessary following the inspection.

9.1.6

Training While experience is essential it is also important that the concrete team have formal training in concrete placing. For supervisors it is appropriate that they have been trained at CCAA or TAFE courses on concrete. For the placers and finishers it is important that they be given facts to replace the myths they often work with. These facts can be imparted by:

9.2



Toolbox training where key facts are introduced together with details in the pour specific method statement.



Circulation of method statements which include critical facts.



Use of posters in the work rooms that highlight key issues.

PLACING Placing of concrete is often undertaken as an ad-hoc process where the concrete arrives and the concrete team places it with little formal planning. Planning is a very important aspect. Concrete should be deposited as close as practicable to its final position, and in such a manner as to avoid segregation due to re-handling or flowing. The rate of concrete placement should permit proper compaction.

Page 82 Placing methods should be selected to avoid undue loads on reinforcement or its supports. For example, pump lines or barrow runs should be supported on separate chairs and not directly on reinforcement. Concrete should be placed in a continuous operation so that fresh concrete is always placed against plastic concrete to produce a monolithic mass. 9.2.1

Concrete Placing Method Statement Concrete shall be placed, compacted, finished and cured so as to: 

ensure concrete is placed and compacted within the times allowed;



prevent segregation or loss of materials;



prevent premature stiffening;



prevent displacement of reinforcement, fitments or embedments;



produce a dense homogeneous product which is monolithic between planned joints and/or the extremities of members, or both;



completely fill the formwork to the intended level, expel entrapped air, and surround all reinforcement, tendons, ducts, anchorages and embedments;



achieve the specified finishes ; and



prevent plastic and drying shrinkage cracks, and prevent thermal cracking.

The placing and compaction method statement shall detail methods to be utilised to ensure proper placing and full compaction and shall specifically include: 

minimum equipment requirements;



experience required;



supervision requirements;



thickness of concrete layers; and



methods and limits of form vibration.

The concrete placing method statements shall also detail methods to be utilised to ensure that defective concrete does not result from:

9.2.2



moving concrete excessively by the use of vibrators;



dropping concrete from height without adequate provision;



placing at excessive temperatures;



placing during rain;



plastic settlement;



plastic shrinkage; and



placing in water.

Cleaning Out Forms Just prior to placement of concrete the interior of the forms need to be inspected for contaminants (drink cans, cigarette butts, bundles of tie wire, sawdust,etc) that will have an impact on the uniformity of the placed concrete. All such items shall be removed.

9.2.3

Selection of Concrete Workability Concrete should be deposited as close as practicable to its final position, and in such a manner as to avoid segregation due to re-handling or flowing. The rate of concrete placement should permit proper compaction. Placing methods should be selected to avoid undue loads on reinforcement or its supports. For example, pump lines or barrow runs should be supported on separate chairs and not directly on reinforcement. Concrete should be placed in a continuous operation so that fresh concrete is always placed against plastic concrete to produce a monolithic mass.

Page 83 9.3

TRANSPORT FROM TRUCK TO POUR In this section factors that affect concrete quality during transport are discussed. 9.3.1

Direct Discharge Direct discharge is where the chute from the truck discharges directly into the pour. In principle there is nothing wrong in this approach, in fact as it removes a transport process it may lead to improved concrete quality. However the following key aspects must be considered:

9.3.2



That placing in this method does not lead to the need to move concrete using vibrators.



That reinforcement cover is maintained where it has to be fixed during the pour because during early stages of the pour the truck is driven over areas which will be the later stages of the pour. Reinforcement should always have proper spacers and supports.

Pump Use of pumps for concrete placing is discussed elsewhere in this report.

9.3.3

Skip Where skips are used the following precautions are required:

9.3.4



The skip seals so that there is no grout loss.



The skip is clean and contains no water yet the surface should be pre-wetted with fresh water.



Use of skips does not delay the placing such the open time of the concrete is exceeded.

Wheelbarrow Wheelbarrows have been used to place high quality, high performance concrete on offshore structures constructed in Norway using farm labour. Issues using wheelbarrows are similar to skips.

9.3.5

Slide Slides can be used to move concrete from the truck to the pour. In essence they are an extention to the truck. Issues are similar to those of the skip.

9.3.6

Tremie In tremie concrete the principle is that the end of the tremie is within the fresh concrete and a head of concrete builds up in the tremie such that it becomes more akin to a pump line than a drop pipe. The main issue is to ensure the end of the tremie is kept in the concrete and the tremie is kept full.

9.4

COMPACTION Concrete should be thoroughly compacted by vibration or other suitable means and thoroughly worked around reinforcement and fixtures so as to form a solid mass, free of voids. Particular care should be taken to ensure that there is sufficient equipment with appropriate capacity to compact the concrete. The capacity will depend on the geometry of the concrete element, access to the element, restrictions imposed by the reinforcement, the workability of the concrete, placement rate and rate of supply. High-strength, low slump concrete or concrete with large aggregate requires considerable effort for proper compaction. Even if the mix is well designed, and the concrete properly cured, low penetrability will be achieved only if the concrete is properly compacted. Porous concrete resulting from unsatisfactory compaction can permit penetration of aggressive agents to the reinforcement, with consequent corrosion problems.

Page 84 Proper compaction is essential to expel air entrapped in the concrete during placement, to consolidate the concrete without causing segregation, to reduce the risk of settlement cracking and to ensure good bond between concrete layers and between concrete and reinforcement. Concrete should be thoroughly compacted by vibration or other suitable means and thoroughly worked around reinforcement and fixtures so as to form a solid mass, free of voids. Particular care should be taken to ensure that there is sufficient equipment with appropriate capacity to compact the concrete. The capacity will depend on the geometry of the concrete element, access to the element, restrictions imposed by the reinforcement, the workability of the concrete, placement rate and rate of supply. High-strength, low slump concrete or concrete with large aggregate requires considerable effort for proper compaction. Internal vibrators should not be used to move the concrete laterally. Care should be taken to ensure that reinforcement is not displaced by prolonged contact with such vibrators. Thorough compaction is particularly important at edges and corners, where the concrete is exposed on more than one face. When concrete is placed in a number of layers, the internal vibrators should penetrate into the layer below that being placed. When concrete is being placed in high lifts, the discharge stream should avoid displacing the reinforcement. Where necessary, the concrete should be deposited in its final position through chutes or tremie pipes. Particular care is required in areas of congested reinforcement, where placing and compaction is difficult, as such areas can often exhibit segregation or voids if not carefully compacted. Where plastic shrinkage or settlement cracks occur these can often be closed by re-vibrating the concrete. 9.5

UNCOMPACTED HIGH FLOW CONCRETE High flow concrete is frequently used in piles and diaphragm walls and the concrete is not vibrated to remove air. If the concrete were true self compacting concrete then it would be reasonable to assume the concrete insitu has adequate performance based solely on the standard test results obtained for self-compacting concrete. Results from standard cylinders from high flow concrete however cannot be taken to be representative of the insitu concrete unless samples are taken because the degree of air exclusion insitu will be uncertain. Recommendation: Where high flow uncompacted concrete is used samples should be taken from concrete representative of insitu concrete during trials from the works to validate its performance compared to tests on standard cylinders.

9.6

FINISHING Concrete is generally provided with some form of finishing process on at least one surface following placing and compaction. This process must match the properties of the hardening concrete and should not detract from the aim of achieving a suitable level of durability. This section discusses aspects of achieving a suitable finish. A basic rule for finishing is that no operation should take place while there is an excess of free moisture on the surface. The object of finishing should be to achieve a uniform, dense, compact surface of the dimensions and finish specified in the contract documents. In particular, the correct cover to reinforcement, tendons, ducts and inserts should be maintained, and the surface texture should be appropriate to any subsequently applied finish and usage. The density of the surface layer is critical in providing a barrier to penetration of aggressive agents, which could cause corrosion of reinforcement. Absorbent formwork is not as effective in creating this protective layer as non-absorbent formwork. In addition, trowelling too early, or while bleed water remains on the surface, will result in the formation of a weak layer. If the trowelling is prolonged, the surface will be weakened by the presence of laitance. This can lead to early failure of floor surfaces subject to traffic.

Page 85 Where special surface treatments affect or remove the surface layer, increased cover should be provided. Such treatments include exposed aggregate, bush hammering, sand blasting and water jetting. 9.6.1

Mix Impacts on Finishing A concrete mix design will also affect the finishability of concrete. Over sanded mixes may be easy to finish as they allow a lot of ‘fats’ to be worked up but they can lead to poor quality surfaces. The mix can also affect the risk of delaminations.

Recommendation: Mix designs for flat work should be tailored to achieving a high quality surface performance rather than ease of finishing. However finishability is also important and trials should be undertaken of mixes to show the finished surface performance and finishability as part of the trial mix process. 9.6.2

Finish to Suit Subsequent Applications The effect of the surface finish of a member on its durability is frequently overlooked. It has been suggested that the cement-rich dense surface layer formed against steel formwork provides a strong penetration of moisture65. Absorbent formwork is not as effective in creating this protective skin. On the other hand, in the case of impermeable formwork (e.g. steel, hard-faced plywood), during compaction some entrapped air and a proportion of the mixing water migrates towards the formwork and is trapped at the concrete/formwork interface. This may result in the occurrence of blowholes at the formed concrete surface. Recent developments in Japan have led to the use of permeable formwork also known as controlled permeability formwork (CPF). The principle underlying CPF is that it acts as a filter through which air and bleed water can pass, while minimising the loss of the cement. The improvements in the appearance and durability of concrete surfaces that can result from the use of CPF are demonstrated and reviewed by Collins66. Steel trowelling of the cast-concrete surfaces results in similar surface densification effects to casting against steel formwork. However, the timing of the operation and the technique employed can greatly affect the result. Trowelling too early or while bleed water is on the surface or over trowelling will result in the formation of a weak surface layer (e.g. laitence) that will abrade easily under traffic (see Clause 2.3.2). Surface treatments that remove the outer layer of hardened concrete, e.g. bushhammering, or that lowers its protective capacity, e.g. acid washing, should be avoided where possible, or increased thicknesses of cover should be specified to compensate for the loss (see Clause 4.3.1).

9.6.3

Abrasion and Finishing A basic rule for finishing is that no operation should take place while there is an excess of free moisture on the surface. Proper surface finishing procedures and curing are essential to produce a concrete surface of high quality to resist abrasion.

Figure 27 indicates the improvement in performance gained by hard-steel trowelling as compared with wood floating. Hard-steel trowelling was carried out by hand and involved three steel trowelling operations, the last exerting heavy pressure at the edge of the trowel and carried out after a reasonable delay (in excess of 8 hours after placement).

Page 86 Figure 27 : Effect of Surface Finish on Abrasion as Measured by Depth of Wear (from Fentress49)

Kettle and Sadegzadeh50 have examined the effect of modern construction practices on abrasion resistance and confirmed the above results. They reported as follows: ‘Power finishing significantly increased abrasion resistance, with the benefits being directly related to the number of applications of power trowelling. This is attributed to surface compaction and to reduction of the water-cement ratio of the surface matrix. Indeed, repeated power trowelling reduced the influence of mix design on the abrasion resistance, with all specimens achieving a similar surface matrix. Vacuum dewatering also increased the abrasion resistance with the applied suction leading to a reduction in the water-cement ratio and an increase in the cement content of the surface layer. This process was most effective on concrete with a high, initial water-cement ratio. ‘Efficient curing significantly increased the abrasion resistance. While no significant differences were detected between the abrasion resistance of slabs cured by wet burlap or plastic sheeting, the plastic sheeting method was less susceptible to error. Curing compounds were very beneficial, with their efficiency being very dependent on the texture of the applied surface. In summary, while mix design clearly influenced the abrasion resistance, attention to both construction procedures and curing could achieve similar changes in abrasion resistance’. These results are illustrated in Figure 28 and Figure 29 for a concrete mix containing 345 kg/m3 of portland cement and a water/cement ratio of 0.54. In this context, repeated power trowelling/ floating is the typical standard of finish required for industrial floors in Australia.

Page 87 Figure 28 : Rate of abrasion for hand finishing (from Kettle and Sadegzadeh 50)

Figure 29 : Rate of Abrasion for power floating (from Kettle and Sadegzadeh 50)

A detailed discussion of all aspects relating to industrial pavements can be found in T48 51. 9.6.4

Falls Ponding of water on concrete surface leads to accelerated deterioration. Hence the upper horizontal surface on all elements shall be laid to a fall to ensure water shedding. Where this is not possible for structural or operational reasons the exposure class shall be lifted by one category.

9.7

FORMWORK AND FALSEWORK Formwork and falsework should have sufficient strength to carry all applied loads, including the dynamic effects of placing and compacting the concrete, without loss of shape. Design of formwork and falsework should generally be in accordance with the requirements of AS 3600 and AS 361053. Antill52 provides further discussion on the design of formwork and falsework and sets out appropriate design procedures.

Page 88 9.7.1

Formwork Formwork should be sufficiently rigid to maintain the forms in their correct position and to the correct shape and profile such that the finished concrete conforms to the specified lines, levels and shapes and reinforcement is located with the correct cover as shown in the contract documents. Forms should be designed for the appropriate method of placing and compacting, and be constructed from non-reactive material suitable to give the specified surface finish. Formwork should be arranged such that it can be stripped and removed from the concrete without shock, disturbance or damage. Forms should be mortar-tight to ensure no loss of fines during compaction, which could result in areas of porous concrete; in particular, care should be taken where holes are needed in forms to allow projecting reinforcement or embedments to be placed. Provision should be made in the formwork for cleaning out and inspection prior to concrete placement. Care should be taken to ensure that any form hardware, such as form ties, that is to be left in the concrete is provided with at least the minimum cover specified for the reinforcement. Specialist design - Where forms are required to provide a particular finish, such as in facade panels, it may be necessary to engage a specialist to carry out the design. Similarly, large or complicated forms (such as those for bridge superstructures) are often major engineering tasks that could require specialist design. Moulds in precast work required to resist the action of pre-tensioning will also require special design considerations. Tolerances - Forms should be capable of producing elements to the specified shape and within the specified tolerances. In general, these tolerances should be one-half of those required on the elements they are to produce. Resistance to movement - Forms may require the incorporation of removable or compressible elements to relieve restraint stresses and avoid damage to the finished concrete element. Forms in prestressed work are a particular case. Here, allowance should be made for shortening and movement during the application of prestressing forces.

9.7.2

Falsework Falsework, being the structure on which the formwork rests, must provide adequate support without any sagging or displacement, which could result in cracks developing in the concrete. Depending on the nature of the work, it may be necessary to engage specialist designers for the falsework; this includes obtaining advice on foundation conditions specific to the loads being carried.

9.7.3

Stripping of formwork Formwork should not be stripped or removed until the member has attained sufficient strength to safely support its own weight, together with any applied loads. Stripping times should be specified in the contract documents. AS 3610 provides details of suitable stripping times. For some elements, the only curing available is that provided by the formwork. It is therefore desirable to leave formwork in place for as long as possible and avoid early stripping, to assist curing of the concrete. However, care should be exercised to ensure that formwork does not offer restraint to the member and lead to cracking. Particular care should be taken in stripping of forms to ensure that concrete will not be cracked or chipped or otherwise damaged. Stripping should be carried out uniformly

Page 89 throughout the project. Where colour control is specified, delays in stripping adjacent sections should be avoided, particularly on external surfaces. The period of time before stripping should be increased if the average temperature over the curing period is less than 5°C. AS 3600 provides guidance on the amount of increase in these periods depending on the ambient temperature. Where early age stripping is required for productivity reasons use of matched curing or maturity provides the best estimate of insitu strength. This can be used to enable stripping or stressing with a high degree of assurance that the required insitu strength has been achieved. In cold climates it avoids the risks associated with stripping too early when laboratory cylinders are cured at a higher temperature than the insitu concrete achieves. On one USA project a block of flats collapsed after a cold spell and forms were stripped on the normal 3 day cycle. More frequently these methods enable early curing as they take into account the rapid instu strength gain in thick sections. Where the methods show high early strength development in the cover zone they can also be used to permit reduction of curing times. Criteria used in steam curing for early cessation of curing can also be used for insitu concrete. Typically achieving 75% of the characteristic strength would be taken as sufficient to stop curing. 9.7.4

Controlled Permeability Formwork Bleed water from the concrete migrates in all directions. Where it hits an impermeable barrier such as formwork it collects leading to a localised higher water/cement ratio. This can lead to a lower performance concrete in the cover zone than might otherwise be the case. Controlled Permeability Formwork comprises a fabric on the surface of the forms that permits water arriving at the formed surface to drain away leading to a lower w/c ratio and hence higher performance concrete in the cover zone. It also eliminates air voids and weak laitance at the surface leaving a slightly textured surface. Quite extensive testing has been undertaken to show that the surface performance is enhanced but the degree of enhancement has not been related to bleed characteristics or other mix characteristic so the degree of improvement cannot be predicted. The improvement could be established by instigating trials on blocks cast during trial mixes. Blocks typically have to be at least 600mm high in order to assess the effect as at lower pressure heads the concrete performance may not be enhanced. Where coatings are proposed CPF eliminates the issues associated with a weak laitance layer and the need to fill air voids as well as providing the enhanced surface performance. The combined benefits may make its use cost effective.

9.8

LIFTING AND LOADING The most likely source of damage to an element during construction is the application of loads to parts of the structure before they have gained sufficient strength. This can lead to cracking of concrete, which may result in moisture penetration and subsequent reinforcement corrosion. Loads may be from either construction traffic or construction materials. A typical example is the stacking of bricks or other materials for following trades. The designer's approval should be sought before any construction loads are applied to any part of the structure. Too early an access for construction traffic to floor slabs or pavements should be avoided as this can cause abrasion. Traffic can affect the durability of these elements by weakening the surface layer and reducing protection for the reinforcement.

9.9

ADVERSE WEATHER CONDITIONS The contractor should be made aware in the specification of the effects of temperature extremes, humidity and wind during placing and curing and of what action should be taken, and its timing. Some guidelines may be found in Neville54, AS 1379, Potter55 and Hot Weather Concreting56.

Page 90 9.9.1

Hot Weather Temperature - High temperatures during and soon after concrete placement can lead to lower compressive strength at later ages than similar concrete cured at more normal temperatures and Delayed Ettringite Formation in the long term. In addition, temperature stresses combined with shrinkage can result in cracking of the concrete. The curing method can help to control excessive temperature differentials by providing external insulation. Examples of this include: 

the use of ply wood either as formwork or sheets placed on the surface to prevent the surface from cooling thereby reducing temperature differentials. The top surface of the pour may be insulated using polystyrene, thermal blankets or water ponding.



avoidance of spraying the surface with water that will cool the surface and give a high temperature differential. Where water curing is required the water must be applied immediately after pouring before the concrete heats up. The water is used to saturate a geotextile and then polythene pulled over to prevent drying out.



leaving the formwork in place for longer to give the centre time to cool thereby reducing the risk of thermal shock when forms are removed. The time required increases with thickness of pour as an approximate guide 1 week per m thickness is typical.



use of cooling pipes through the centre of the pour to minimise the temperatures at the centre of the pour. This is particularly attractive where there is a ready source of water that can be pumped through the pipe. Use of 75mm plastic drainage pipe zigzagged at around 750mm centres is typically adequate. This is very useful on thicker pours as the stripping time for shutters can be reduced significantly which may eliminate the need for additional shutters.

Hot weather - For concreting under hot and dry conditions the following precautions are recommended:

9.9.2



To keep them cool, formwork and reinforcement should be continuously sprayed with cold water in advance of the concreting.



Suitable barriers should be provided to protect freshly placed concrete from the evaporative influence of wind.



Although AS1379 requires that the concrete delivered temperature not exceed 35°C a lower delivered temperatures may be required. The benefits of low concrete delivered temperatures can be calculated. Frequently a temperature not exceeding 32°C is specified as this helps significantly with concrete open time but lower temperatures are often required to ensure the maximum curing temperature specified is not exceeded. This may mean placing at night so concrete materials are cool before mixing.



Concrete should be mixed, transported, placed and compacted as rapidly as practicable.



Concrete should be covered with an impervious membrane or wet hessian until moist curing begins. Curing compounds may be used but are not as effective as moist curing methods.



Precautions should be taken to avoid premature stiffening of the fresh mix and to reduce water loss by evaporation and by absorption into the subgrade, etc.



Excessive temperature rises should be reduced by shading the concrete from direct sunlight and/or covering with a reflective sheet during curing.

Cold Weather For concreting under cold conditions the following precautions are recommended: 

The concrete should have a temperature not lower than 10°C when placed in the forms.



The concrete should be prevented from freezing at any time during the curing period. Salts and chemicals should not be used for this purpose.



The concrete should be maintained at a temperature not less than 5°C, as measured at the surface, until the end of the curing period, which should be at least seven days.

Page 91 9.9.3

Wind Excessive air movement over concrete surfaces can result in moisture losses, especially when combined with adverse temperatures and low humidity. Rapid loss of moisture from the surface of the concrete will usually result in plastic shrinkage cracking. Figure 30, used by ACI Committee 305 Hot Weather Concreting35 and CCAA Hot Weather Concreting34+, is used to estimate the likelihood of plastic shrinkage cracking occurring. It is noted that where air temperature, relative humidity, concrete temperature and wind velocity combine to produce a rate of evaporation greater than 1 kg/m 2/h, plastic shrinkage cracking is likely and precautions should be taken. However, where the concrete bleed water arriving at the surface is low (e.g. concrete with high fines contents, concrete placed slowly so the rise rate is low and thin slabs) the critical evaporation rate could be much lower. Failure to prevent excessive evaporation can cause plastic shrinkage cracks, and the formation of a weak, porous surface layer. The effects of air movement due to wind can be reduced by the use of temporary wind breaks or by spraying a compound such as an aliphatic alcohol over the surface after screeding. These compounds form a surface film, reducing evaporation from the surface. They are not curing compounds since the surface film is only temporary.

9.9.4

Rain Rain can cause damage to freshly placed concrete by reducing the protection to reinforcement. In addition, the water-cement ratio at the surface can be raised, and this may reduce durability and affect the quality of the concrete surface. Concrete placement during rain should be carried out only under cover, and suitable protection should be continued until the surface has hardened sufficiently to resist rain pitting. To avoid possible surface erosion, running water should also be prevented from traversing concrete surfaces during the curing period.

Page 92 Figure 30 : Effect of environment on the rate of evaporation from concrete (CCAA [34])

Page 93 10. CAST INSITU CONCRETE This section reviews types of cast in place elements giving specific durability considerations for each. 10.1 COLUMNS The principle durability issues arising with columns are: a) Lack of cover where it is not possible to do a pre-pour check on the actual cover achieved before closing the form. Apart from particular attention to checking the number and size of spacers and geometry of the forms the QA engineer should ensure adequate post pour checking. b) Lack of cover to starter bars is another common problem. This may be checked using templates before erecting forms and failing that as part of the post pour check. Care is required as pulsed eddy current cover meters will give erroneous readings due to the mass of steel at laps. In these areas ground penetrating radar measurements are likely to be more accurate. c) Inadequate preparation of the joint. Adequate preparation of the joint in the cover zone should be checked before erecting forms. d) Bleed can be a major problem. If the bleed of the concrete is high and the concrete is placed quickly then the whole column may bleed at once creating bleed channels, bleed streaking on the surface, lower quality in the cover zone, poor bonding of reinforcement and a weak up layer of concrete. The allowable pour rate can be assessed based on the bleed of the concrete. After pouring the appearance of bleed water should be checked for 30 minutes after placing and before curing is commenced. If bleed is excessive the quality of the as placed concrete may need to be verified. e) Level of bob bars for subsequent slab pour should be carefully checked to ensure they will not give low cover in the slab soffit after underlying reinforcement is located. f)

Poor compaction in the cover zone as the vibrators are not located close enough to the surface and poor compaction over height as the methodology for compacting over a tall column is not adequately planned and instigated.

g) Segregation of the cover concrete and the concrete around bars when vibrators are placed against the reinforcement. h) Deposition of concrete pump lubricating mixes within the column. 10.2 DIAPHRAGM WALLS Diaphragm walls represent unique concrete construction because they use uncompacted, high flow highly retarded concrete tremied to displace the temporary bentonite fill in tall pours. Where exposure is benign durability issues are reduced but in more severe exposures the durability of the concrete does becomes a major consideration. It is not only the exposure and concrete quality that are a concern but joint leakage and bentonite contamination may be an issue. There is no code covering diaphragm wall construction and hence for a large part it is the concrete specification that must detail the requirements that will ensure the diaphragm wall durability. In 2012, CIA Z17 57 was first published providing a clear insight into the requirements for achieving high performance concrete in deep foundations. It notes that tremie concrete meeting requirements for flow and stability would provide a concrete which de-aerates and compacts as a result of concrete pressure head without any external vibration.

Page 94

Recommendation: It is recommended that concrete specifications require that: a) The contractor shall provide a full method statement for approval for materials, plant, operation and testing. b) The contractor provide a joint sealing method statement that includes 3 forms of sealing the joints. c) All joint leakage and leakage through cracks in the wall 1 year after construction to be sealed. d) Bentonite shall comply with API Standard 13A. e) The pH of the Bentonite be maintained at 9.5-12 when measured using electrical pH meter or pH indicator strips. f) The flow of the bentonite immediately prior to placing concrete shall have a Marsh cone flow time of 40-75 seconds. g) Samples of Bentonite taken from approximately 0.2m above the bottom of the trench immediately before placing concrete shall have a density of no greater than 1.3g/ml when tested using a mud balance. h) The concrete shall utilise water reducers, superplasticisers, set retarding admixtures and stabilising admixtures as required to achieve the required flow characteristics and stability. i) The aggregate grading be in accordance with Section 3.3 of CIA Z17. j) A trial mix of the final concrete shall have a slump, slump flow and time to reach the end of an L box and Bauer filtration in accordance with Table B1 of CIA Z17. k) Acceptance tests and frequency of testing shall comply with the recommendations in Table 2 of CIA Z17. l) All concrete test methods shall comply with Appendix A of CIA Z17. m) The bleed affected concrete at the top of the pile shall be cut off. The extent of this shall be established by testing of a trial pour. n) QA forms similar to those in Appendix D of CIA Z17 be required to be completed and supplied to the client. o) The contractor shall install inclinometers, concrete strain gauges, steel strain gauges, piezometers, pressure cells, corrosion monitors and settlement monitoring points as appropriate to the project. p) Cross hole sonic logging between six points be included in the first two panels and 20% of subsequent panels shall also be tested at regular intervals.

10.3 PILES 10.3.1 Bored Piles Bored piles where bentonite is used to stabilise the pile walls and a tremie concrete used to displace the Bentonite will follow the same general procedures as the diaphragm wall and details are not repeated here.

Page 95 10.3.2 Continuous Flight Auger Piles CFA piles are cast by pumping concrete into a space left as an auger is removed. A maximum aggregate size of 10mm keeps the concrete with an acceptable paste content (too much paste increases shrinkage and increases penetrability) while placing remains practical. The concrete is not vibrated and must flow freely so many of the issues are similar to those noted for Diaphragm walls and CIA Z1757 includes comments on requirements for durable concrete. The reinforcing cage is lowered through the fresh concrete. Potential durability issues are: 

Concrete pumping pressures are high so the use of stabilising admixtures is likely to be essential where ground will absorb water from the mix, i.e. dry sands.



Inadequate control of the concrete pumping and withdrawal rate can lead to voidage. Monitoring of pumping rate and withdrawal rate are essential



Vibration from installation of adjacent piles can lead to disturbance of the hardening adjacent concrete piles.

10.3.3 Piles for Use in Tunnels and Retaining Walls Where ground is excavated around a pile specific corrosion mechanisms can develop as shown in Figure 31. Figure 31 : CFA Pile Deterioration Mechanisms

Shotcrete

Ground

Leakage through poor pile shotcrete interface

O2

Chloride ingress exacerbated by evaporative concentration

O2 B

O2 C

Pile

O2 A

2(OH)-

D

2e-

O2

Atmospheric exposure to carbon dioxide and chlorides

O2 O2 Corrosion current

Chloride ingress by diffusion exacerbated by low cover Position A in the pile is on the soil covered face. Where the soil moisture contains chlorides corrosion activation could occur due to chloride diffusion to the steel. In high performance concrete with high cover this is unlikely to occur in the design life. However in piles where cover is reduced and concrete quality is low due to construction problems this could be an issue. In fully buried piles oxygen starvation gives an insignificant corrosion rate. In piles where a face is exposed however the atmospheric face may act as the cathode so that the corrosion at A will not be mitigated. A full analysis of each situation will be required to determine if resistance control or activation control can be relied on over the design life. Monitoring of potentials and corrosion rate at A on selected piles in critical

Page 96 locations would verify if corrosion of the steel is significant. Monitoring using a rebar corrosion rate probe (half cells plus counter electrode) cast into a mortar block tied to the reinforcement cage are appropriate if a reliability analysis indicates corrosion could be critical. Position B is on the outer face of the pile next to the shotcrete joint. The quality of this joint is critical. Leakage through the joint will lead to evaporative concentration of contaminants at the surface at B. This could lead to rapid chloride diffusion through the cover zone due to very high surface chloride level. There is a ready supply of oxygen and hence corrosion damage at B could occur in a relatively short period. Dependent on the leakage this could be the most risky point for corrosion and emphasises the need for waterproofing of the joint on the upstream side of dowel bars. The chloride ingress rate in evaporative concentration situations is difficult to predict. The best protection is to require that all leakage is sealed. Dowel bars should be galvanised as a minimum. In very aggressive conditions stainless steel dowels should be considered. Position C is the on the open air face of the pile. The exposure would be the same as any uncovered element on the structure. The quality of the concrete can be checked by taking cores from the cover zone and testing for Volume of Permeable Voids. Cover can also be checked on a 100% basis if required. Hence monitoring is unlikely to be necessary. 10.3.4 Pile Reliability Piles are a generally a critical structural element and being constructed insitu underground they are not open for inspection so deterioration will not be apparent, i.e. they must have a very high reliability. Australian codes do not make provisions for high reliability in durability design but this is discussed in Z7/01 and guidance on reliability design is given in Z7/03. It is probably because of the potential quality risks and high reliability required that the bridge code does not permit CFA piles in exposures more severe than B1. In addition MRWA Specification 81480 requires a test pile be constructed using the approved trial mix and in accordance with the Pile Installation Plan. Recommendation: In addition to use of applicable items in the recommendations in Section 10.2 it is recommended that specifications for insitu cast piles require that: a) Pile durability be designed to a reliability commensurate with the consequence. A high reliability may be required where there is no monitoring and no observable evidence of deterioration. b) The contractor undertake a trial pile installation exposing the face of the completed pile over at least the top 10 meters to demonstrate that the expected performance is achieved. This demonstration shall include visual inspection, core samples and NDT in a suitable combined program c) Where piles are to have a face exposed the exposed face will be tested to demonstrate it will be durable over the design life. Testing shall include extensive cover testing such that the cover of the piles is reliably defined in terms of a mean and standard deviation and performance tests of the concrete. The engineer should determine and define in the specification whether performance tests of cores shall be for strength and / or durability (e.g. VPV test). 10.4 WALLS – FREE STANDING Being a vertically cast element walls suffer from some of the same issues as columns (See Section 10.1). However because walls are restrained at their base over a long length they suffer the additional issue of edge restraint cracking. They may also be subject to end restraint cracking where the wall is cast as an infill. This subject is complex and is covered in CIRIA C660 58 and Z7/06 will provide an Australian guide to crack control. The principle considerations are: A. Consider the pour sequence and minimise infill pours. B. Where infill pours are necessary limit them to no more than 2m long. C. Provide sufficient reinforcement to control edge restraint cracks to the specified limit. Australia Code provision for steel stress are likely to be acceptable for concrete grades 40MPa and less

Page 97 for walls no wider than 300mm and where edge restraint is less than 0.5. Outside of these conditions a design check on crack widths with specification of key criteria is required. 10.5 RETAINING WALLS (INCLUDING TUNNELS) Apart from the issues noted of free standing walls and columns, retaining walls may also have the issue associated with water retaining structures. 10.6 BEAMS (INCLUDING PILE CAPS) Beams can have the same issues as wallx in terms of edge restraint. End restraint is an even greater potential issue when the beam is supported by piles or columns. Piles may give high end restraint due to active pressure and large columns, particularly where they act in series to restrain a beam. Although beams can often cool from at least three faces thick beams are subject to high curing temperatures and this may be critical as the compressive strength of a beam is likely to be more critical than the compressive strength of a large foundation. 10.7 SLABS (INCLUDING STAIRS) Construction of durable concrete slabs is also a large topic and it is covered in detail in CCAA T4851 for industrial slabs and CCAA T5164 for residential developments with crack control being covered in CIA Z1559. Principle durability issues are:  Plastic cracking caused by poor curing although the mix design may be contributing factor.  Early age restrained cracking. Typically this will be caused by inappropriate pour sequencing or inappropriate cutting of joints (cutting too late, cutting to shallow).  Poor surface hardness leading to high abrasion. This can be bought about by a poor mix or poor placing.  Joint detailing and construction leading to joint breakdown.  Low cover due to the unformed finish leading to corrosion.  Reduced slab thickness leading to cracking.  Failure of impermeable coverings due to moisture migration from the slab. Floors are often the most critical part of a structure as they are the hard worked trafficked part of the building. Faults with floors are one of the most common causes of complaint with concrete. Hence as the topic is too large for consideration here it is recommended that the referenced documents are used for the basis of floor slab design and construction. Recommendation: Floors are often the most critical part of a structure as they are the hard worked trafficked part of the building. Faults with floors are one of the most common causes of complaint with concrete. Hence as the topic is too large for consideration here it is recommended that the referenced documents are used for the basis of floor slab design and construction. 10.8 WATER RETAINING STRUCTURES Water retaining structures have durability requirements associated with their degree of water tightness. CIA CPN2860 provides definitions related to watertightness and notes that AS3735 “sets criteria for the assessment of liquid retaining concrete structures and concrete roofs that allow some passage of water.” CIA CPN 2482 provides details of effective jointing design in water retaining structures. These documents provide a wealth of information on durability design and should be referred to for the general durability design requirements and materials for water retaining structures. Durability requirements are also dealt with in this durability series. For the face in contact with the liquid, exposures are detailed in Z7/02 and deemed to comply requirements are given in Z7/03. The durability design for the face not in contact with the face in contact with liquid the primary requirements are to design to prevent leakage such that that evaporation of the water leaves an unacceptable build-up of contaminants on the concrete surface63. The modelling methods for this are dealt with in Z7/05 and crack control is dealt with in Z7/07.

Page 98 10.9 M ASS CONCRETE STRUCTURES A massive concrete structure (e.g. thick slabs, large beams, large columns, deep foundations) is commonly defined as where its dimensions are such magnitude as to require special means of coping with the generation of heat and subsequent volume change. If these structures are of significant thickness, are made of higher strength concrete mixtures (high cement content), the impact of thermal cracking can be serious. The heat of hydration of cement is a function of its compound composition and fineness. In the event that temperature rise and the subsequent temperature differential (from surface to interior of the concrete) in the of the order of 25-30 C is judged too high from the standpoint of thermal cracking, there are several ways to lower it:  By using Low Heat cement.  By reducing the cement content of the concrete mix using admixtures or supplementary cementitious material replacements provided that this can be done without compromising the minimum strength and workability requirements needed for the job.  By achieving a cement reduction using lager maximum aggregate size.  By reducing the delivery temperature of the concrete supplied using ice replacement in mixing water or other means of concrete cooling. The traditional view of mass concrete having dimensions of approximately 2m or greater is re-defined by consideration of the temperature differential noted above. Quite small dimension structures (as low as 0.6m) have recorded significant temperature rises and maximum temperature differential in excess of 30 C when constructed from higher strength concrete. Useful guidance on managing thermal properties of concrete is given in CIRIA C66058.

Page 99 11. PRECAST CONCRETE Precast concrete is used for most applications in building and construction. Whether it be tunnelling, marine structures, buildings or infrastructure precast is synonymous with high speed of construction, attainment of superior quality and provision of exceptional surface finishes of architectural merit. Because of wide spread usage, unique construction and improved performance it may not be surprising to find that there is a book that covers virtually every aspect of this unique product. The Precast Concrete Handbook83 details design and construction requirements for the complete range of precast elements and includes many durability details. In this section extracts from Precast Concrete Handbook related to durability are included together with additional detailed information to give a comprehensive guide to durability requirements of precast concrete. Section 11.1 reviews why precast concrete can have high quality and how attainment of this quality can be verified. Section 11.3 reviews detailing of precast elements specifically as it relates to durability and the remaining sections consider specific element types. 11.1 DESIGN REQUIREMENTS The introduction of reliability requirements in Z07/01, new exposure classes in Z07/02 and new deemed to comply requirements covering a wider range of concrete materials in Z07/03 will have a significant impact on precast concrete requirements and this may make direct application difficult because standard moulds may not be suited to the requirements that these design requirements raise. Where the recommendations in Z7 are stipulated the designer should ensure that the precasters are able to comply with the requirements at reasonable cost. Recommendation: Recommendations of Z07 should only be applied to precast elements where the designer has ensured that precasters are able to supply without an undue impost on cost due to issues with standard moulds and equipment. Where there are issues, the designer should follow existing practice and find other means of proving the required durability, e.g. coatings, specialist steels or high maintenance. For most precast elements a high turnaround time is generally required and this may be achieved using high early (HE) strength cement. The durability performance of HE cement may not be the same as GP cement and this will need specific consideration, particularly in B2 and C exposures. Panels may be damaged if lifted too early or if not lifted in accordance with the designated manner. The panel designer shall provide details of the required lifting, stacking and transport methods and the concrete compressive strength requirements at each stage. The contractor shall test the panels for concrete strength where a strength limit is given. The strength shall be assessed using a Non Destructive Test method (e.g. Schmidt Hammer), matched curing, or maturity testing or compressive strength cylinders kept adjacent to the panels. Due allowance for the testing accuracy shall be made. 11.2 QUALITY OF CONSTRUCTION A major benefit of precast concrete is that the quality of the concrete is potentially significantly higher than site cast concrete. This comes about by the factory type approach to manufacture and the equipment available for production. This is largely recognised in the Australian Standards via clauses that enable lower cover for the same concrete grade ”where repetitive procedures and intense compaction or self-compacting concrete are used in rigid formwork”. 11.2.1 Repetitive Procedures Repetitive procedures means that the precast factory has the opportunity to continuously improve the processes it uses and hence the variability of construction is reduced. As durability design is based on acceptance with a limited number of failures elements that have a low variance will have lower failure rates at a given criteria even though the same mean value (e.g. strength grade) applies. Conversely the same failure rate occurs when the criteria is lowered. For the low variance to apply there must be ongoing production. Hence it clearly applies to elements like pipes where the same designs and construction methods have been used for years. It may not apply to a precaster making a certain element type for the first time

Page 100 although if the element is produced on a standard production line the basic nature of the procedures may mean that it does apply. The designer will need to ensure that supposed quality improvements that bring about the reduced covers do apply. 11.2.2 Intense Compaction When concrete is vibrated the air is expelled and a dense concrete is produced. Site concrete may be fully compacted using poker vibrators but there is always a risk that there will be areas where full compaction is not achieved, particularly in the cover zone where it is difficult to insert poker vibrators and only compaction from within the reinforcement cage is possible. Where the exposed surface of precast elements are vibrated with form vibrators on rigid forms the compaction achieved is likely to be uniformly high across the surface and a better quality cover zone will result. There is no definition of intense compaction as it could apply to different element types in different ways. It is also only relevant to the cover zone in terms of durability. Intense compaction must apply to the highest exposed face and must ensure uniform high compaction. Poker vibrators are unlikely to provide any better compaction in a precast yard than they do on a construction site and hence the term should not be applied to their use. 11.2.3 Self-Compacting Concrete As noted in Section 6.11.6 Super-Workable concrete (SWC) should not be confused with self-compacting concrete (SCC). SWC is not necessarily going to be self-compacting concrete. It may appear to not need compacting because it flows so well but this does not mean that the mix will dispel air and hence they still require compaction. Self-compacting concrete has a special mix design including materials grading and admixture use and must pass a series of specific tests to ensure capacity for segregation is limited before it can be termed self-compacting concrete. SCC is often ideal for use in precast elements because it can be placed in thin elements without fear of improper compaction. From an environment perspective it is also very much quieter as no vibration is required. As SCC does not require compaction the variability associated with normal compaction is removed and hence it is afforded the same relaxation in covers as intense compaction. 11.2.4 Rigid Formwork Rigid formwork is important as it reduces cover variability due to hogging and sagging of forms and it provides a base where form vibrators will provide uniform compaction. 11.2.5 Influence of Reliability Requirements Current Australian Standards do not specifically including any provisions for different reliabilities but design for different reliabilities is introduced throughout this durability series. As the reliability required increases the cover required increases, the lower variance associated with precast will have an even more dramatic effect on appropriate cover reductions. 11.2.6 Measure of Quality Improvement The low variance should be demonstrable in lower variance in cover and/or insitu concrete quality but a general measure of these has not been developed. Cover is relatively easy to measure and variance for different levels of construction quality could be defined. The concrete quality could also be assessed by tests such as VPV. This combination could then be used to qualify the allowance of the reduced cover allowance for precast.

Page 101 Recommendation: It is recommended that : a) Where the provisions for reduced cover are used for "repetitive procedures and intense compaction or self-compacting concrete in rigid formwork" the concrete supplier shall list the specific features of production that apply to the element that demonstrate why variance in quality of the cover zones will be lower than for normal site concrete. Unless this is clearly demonstrated then the provisions for reduced cover should not be allowed. b) On large projects or where the performance is critical the designer shall specify measurement of cover and insitu quality (e.g. core VPV tests) as appropriate to verify the improved performance. 11.3 DETAILING The recommendations for fittings and fixtures in Section 8.7 are largely drawn from the Precast Concrete Handbook and so are also applicable for precast. 11.4 UTILITIES Specific durability issues of most precast utilities are dealt with in documents dealing with the overall design of the utility. These are reviewed below.  Sleepers – AS 1085.1365 deals with the construction of railway sleepers. Sleepers have performed well in practice except for some problems with DEF. The requirements of AS 1085.13 limit the maximum curing temperature to 70C while steam curing and this should be sufficient to prevent DEF. General performance is likely to be high as the concrete is to have a minimum compressive strength of 50MPa and minimum cover tendons in the soffit of 35mm. Cover to other faces is 25mm. AS1085.14 does not mandate the use of SCM’s and this is seen as a shortcoming where sleepers are used with severe chloride exposures. As sleepers are laid on clean ballast and flooding is likely to be infrequent exposure to chlorides from groundwater is unlikely to be severe. Exposure to marine splashing is also unlikely. Hence exposure to AS 3600 exposures C1 and C2 for chlorides is unlikely but if such exposures should arise then the concrete mix should be modified accordingly (see Z7/03). Exposure to AS3600 B2 is likely. Covers of 25mm and 50MPa concrete required by AS1085.13 for B2 exposures is in accordance with AS 3600 for a 50 year life. However this is likely to only be adequate where an SCM is used. As noted in this document the failure of prestressing tendons has been problematic. Failures with no observable warning has occurred and this leads to a general recommendation for higher reliabilities when designing corrosion protection for prestressing cables. Chloride contents are limited to those in AS3600, i.e. 0.8kg/m 3. Lower chloride contents should be possible in a fabrication yard and this would enhance sleeper durability in B2 exposures. Recommendation: It is recommended that project specifications for sleepers: a) define a reliability level for the prestressed concrete. Guidance is given in Z7/01. b) require concrete and cover be specified to be in accordance with Z7/03 for a 50 year design life. Poles – AS 467666 deals with concrete utility service poles. Durability requirements are to be in accordance with AS3600 with the exception of concrete cover. AS 4676 cover requirements are given for absorption criteria as follows ≤5.5% 5.5-6.5% >6.5% 9mm 19mm AS3600 A concrete compressive strength of 40MPa is required. Use of SCM’s is not required. A list of additional actions that might be taken for B2 and C exposures are given and one or more of these must be selected to give the required life, i.e. the designer is required to determine suitable requirements based on using the covers above. Hence the requirements of AS 4676 are considered satisfactory however it places a heavy onus on the designer as the covers are lower than recommended in Z7/03 and other codes even not accounting for high reliability requirements for prestressing and if SCM’s are used. However if the concrete is coated or

Page 102 cathodic protection is applied, two of the options listed in AS 4676, durability in these exposure can be achieved at these low covers. The only significant issues found with poles in the past have been absorption from the ground causing deterioration and hence sealing in areas where the pole will be in ground water is recommended. Differential shrinkage can also cause problems where the mix is not sufficiently stable to prevent segregation across the section. A supplier who can demonstrate that the mix used is not subject to segregation should be sought. Similarly demonstration that the quality of the cover inside the pipe is not prejudiced should be checked.  Pipes – AS 4058 deals with precast pipes and is based on a long history of durability assessment of pipes in ground. Pipe may have some unique properties that set them apart from other structures in regards reinforcement corrosion mechanisms, e.g. isolated items generally within a consistent exposure and with a limited oxygen supply. Without a proven reason for the general long term high performance of pipes insitu the Z7 series has not specifically considered the corrosion of pipes and the low required covers and no recommendations are made that change cover and concrete quality requirements in AS 4058. Performance tests show that pipes are subject to the same limitations as any other concrete in regards penetrability and chemical resistance. For the same mix a pipe will have the same performance characteristics as any other concrete. From this the extensive performance of pipes can be used to provide guidance on chemical resistance of other in-ground elements and this is taken into account in Z7/03. The durability of pipes is detailed in various Concrete Pipe Association documents68,70,71. These have been used as part of the assessment of chemical attack of concrete in Z7/03. Prestressed pipes have been used in the USA and local failure of prestressing cables has led to some dramatic failure. Prestressed pipes have not be widely used in Australia and given the US experience this is fortunate. If prestressed pipe is to be used it is recommended that the corrosion protection requirements of Z7/03 are used based on an appropriate reliability requirement (e.g. approaching limit state) and design life.  Piles – AS 2159 provides details of requirements for pile durability. Requirements are independent of all other codes and use different exposure classifications even though the soil conditions and concrete quality are the same as those for concrete structures, bridges and water retaining structures. Hence it is recommended that he durability requirements for piles follow the same requirements as other in ground structures as detailed in Z7/03.  Culverts – AS 1597 deals with culverts in two parts, i.e. small and large culverts. Part 2 requires the use of SCM’s for exposure classifications B2 and C. Cover requirements are similar to those of AS 3600 but given the use of SCM’s they are likely to provide at least a 50 year life. For the 100 year life it is recommended that the requirements are checked against Z7/03 for the appropriate reliability and design life. 11.5 BUILDINGS  Wall and Façade Panels - The Precast Handbook83 contains a wealth of information on detailing of precast panels and these should be followed for precast design. Other durability requirements will be the same as for concrete for other elements as defined in the Z7 series.  Hollow Core Planks - Hollow core planks are long prestressed units typically 1.2m wide and 150400mm thick. They have large ducts running their full length that can be used for utilities and air conditioning. They can be used with toppings that act compositely. Whatever runs through the voids will influence the durability. Particular consideration shall be given to: a) the pressure, humidity, temperature and carbon dioxide concentration of the air as all could significantly decrease the initiation or propagation times and this may be of particular significance in regards the prestressing cables b) the reliability of any waterproofing of the hollow cores. There are examples of Hollow Core planks being used in splash zones with the core environment assumed to be a lower exposure than the external environment on the basis that the cores are fully sealed during construction only to find some years later that the assumption was incorrect. Where the plank is to be topped the bonding to the quality requirements for bonding are no different to any other construction joint. The plank surface will need to be laitance free and clean and should be saturated surface dry on concrete placement. Where shear at the joint is critical the profile required on the planks surface shall be specified by the designer. Where

Page 103 there is a high risk associated with delamination of toppings in the service life drummy testing one month after laying the topping will identify toppings where delamination has a high likelihood. Reflection cracking of the joints between planks is likely and the extend of this shall be specifically considered in the design. Recommendation: It is recommended that the designer should ensure that hollow core planks have sufficient cover to reinforcement at all surfaces including the hollow cores. The covers recommended in Z7/03 should be used taking account of the true long term exposures, the reliability required, particularly in regard the use of prestressed cables, and the design life.  Precast Permanent Formwork Panels - Mesh reinforced precast slabs 55-75mm thick with inbuilt trusses are laid and then topped with insitu concrete such that the precast and insitu concrete act as a composite floor. Void formers of various types can be used to reduce the floor weight. Reinforcement is typically a combination of 500L hard drawn wire and 500N bar reinforcement. It is important that the interface of the two sections provides for a composite slab. To minimise the differential movements the designer should use the same grade of concrete and same type of aggregates for precast and topping concrete so that the E-modulus are compatible. Drummy testing 1 month after completion will identify slabs that are not fully bonded. Impact echo and impulse response tests may be warranted on more critical structures. 11.6 EARTH RETAINING WALLS Where necessary precast retaining structures should follow the recommendations for water retaining structures (Section 10.8) and cast insitu retaining structures (Section 10.5).  Tunnel Segments – Tunnel segments can be conventionally reinforced, fibre reinforced or be a combination of the two. Durability design of segments should consider the guidance for fibre reinforcement. Specific aspects related to the durability of segments are: o Segments are prone to cracking during installation. Any cracks that might lead to evaporative concentration of salts on the concrete surface should be sealed. o Although joints may have been designed that allow considerable movement joints, may still leak. Typically these joints are caulked and this may just disperse the leakage. It is better to repair the joint behind the seals to give an effective permanent repair.  Mechanically Stabilised Earth Retaining Walls - AS4678 provides details of requirements for retained earth structures and considers durability aspects of various steel anchoring systems. TN/07 deals only with the precast concrete elements including cast in fittings. The concrete requirements should be based on the reliability requirements, design life of the structure and exposure class as set out in Z07/2 and Z7/03. All cast in fittings shall comply with the general requirements for fixings in Section 8.7. Fixings should be designed with a reliability of 3.8 as corrosion damage would not be seen until the collapse of a panel. Corrosion of reinforcement away from fixings may also not be evident as cracking and spalling on the observable face may often be considered a potentially catastrophic failure so reliability requirements may be high. 11.7 BEAMS  Prestressed Concrete Girders – These are used in buildings and bridges to give long spans. The durability requirements are as outlined in Z7/03 taking due account of the application of the elements and use of prestressing steel to determine applicable reliability and the design life. The following durability considerations relate specifically to T-Roff beams: o Originally Super T’s were cast with webs as thin as 90mm, but experience shows construction problems led to honeycombing. This is a durability issue but poorly compacted concrete in critical strength locations is probably even more significant. At 100mm minimum thickness there have been far fewer issues. o Ensure the maximum aggregate size is suitable to the covers proposed. 14mm maximum size aggregate is common. o Undertake a thorough pre-pour cover check, including extent and security of spacers, of the soffit and walls around the prestress locations as post pour checking will not be possible.

Page 104 o Post pour cover checking of the beams is critical. The suppliers will be working to tight cover tolerances and this combined with the low covers due to the use of high performance concrete makes the elements particularly sensitive to low covers. The prestress steel cover is also critical and a high degree of certainty that it is correctly located is important. Standard pulsed eddy current cover meters can be used in some locations but in others, particularly prestressed areas, PEC meters will not provide accurate readings due to close proximity of other steel. In these cases the post pour cover can be checked by Ground Penetrating Radar (GPR). Post pour cover checks might be at points A, B, C, D and E on the first 3 beams at 5 points along the beam length selected at random for a project.

A

F B

o

o

o o

o

o

C

D

E

Where the beams are shown to meet the cover requirement testing can be relaxed, e.g. to testing 20% of the beams produced. Where a beam fails to meet the minimum cover requirements then additional protection to the satisfaction of the Engineer should be applied and the subsequent beams manufactured tested for cover until the Engineer is satisfied cover is under control. Ensure maximum temperatures are within the maximum limit for the cement system used and temperature differentials comply with design assumptions. This should be checked on the first beam of a project by monitoring the temperature at the centre and surface of the end block. Cracks in flanges adjacent to end blocks have been observed and it has been postulated that these could be related to high temperature. The precaster shall check each beam after stripping and if there is any cracking the Engineer shall be asked to review the cracks and to work with the precaster to find ways of avoiding them in future beams. Cover to top face of the top flange may be reduced as the surface will be covered by the deck slab. Typically the cover should be 1.5 times the maximum aggregate size. The exposure class of the internal surface is debatable and the client may determine based on previous service and other aspects of the specification what cover is required. Frequently it is treated as an internal exposure. Regardless of exposure minimum cover should be at least 1.5 time maximum aggregate size Because weight is a prime concern cover is generally minimised and the specifications often call for repetitive procedures, intense compaction and rigid compaction to be used. The QA requirements should ensure this is achieved. Where a beam fails to meet the minimum cover requirement in every regard then additional protection to the satisfaction of the Engineer shall be applied.

Page 105 12. SPRAYED CONCRETE Sprayed concrete is defined in Vic Roads specification Section 684 - Sprayed Concrete84 as follows “mortar or concrete pneumatically projected from a nozzle (sprayed) at high velocity onto a receiving surface where the sprayed material undergoes simultaneous placement and compaction to produce a dense homogeneous mass. As with conventional concrete the properties of sprayed concrete can be modified and further enhanced through the addition of additives or admixtures, such as silica fume, airentraining admixtures, fibers and accelerators.” The various forms of shotcrete are also defined in specification 684 and these are summarized below: Dry mix – A mix where the dry components are blown to the nozzle where the water is added in a spray. Wet Mix – A concrete mix where the water is added at the batch plant and the mix is pumped to the nozzle where air is injected to accelerate the mix for spraying Shotcrete – sprayed concrete, typically however the maximum aggregate size is 14mm, and often 10mm, in order to reduce rebound. Gunite – Definitions differ. Some refer to gunite as a sprayed mortar with typical particle size of less than 4mm (definition used in this document) others refer to it as a drymix. The latter definition is used in VicRoads Specification 684. Sprayed concrete is not specifically covered in any Australian Standard although most factors leading to adequate performance are covered by adherence to the same principles and materials requirements as other concrete. Sprayed concrete is just concrete applied in a different manner. However, the performance is significantly impacted by the changes to the mix design needed to make it placeable and by its applications. The ensured performance of all concrete is dependent on good workmanship and quality assurance. These aspects have unique requirements with sprayed concrete and hence its performance must be covered within the project specification. Recommendation: Most aspects of specifying durable shotcrete are covered in VicRoads Specification 684 –Sprayed Concrete and it is recommended that this is used as a template for shotcrete specifications. This remainder of this section provides further explanation of aspects that affect durability of sprayed concrete or deals with specification aspects not covered by Specification 684. 12.1 M ATERIALS All materials should be delivered to the job site in an undamaged condition and stored as per manufacturer’s recommendations. Generally, durability consideration of selection, storing and handling of raw materials of shotcrete are similar to those used in conventional concrete, however, there are some more items to be considered: 12.1.1 Cements Generally speaking, there are no special requirements for the type of cement to be used in HPS. Compliance with the national standards or regulations valid in the place of use of the shotcrete is normally sufficient. One aspect that has to be considered carefully when selecting the supply of cement for a project is its compatibility with any shotcrete accelerating admixtures that are to be used. Both sodium and potassium aluminate based accelerators can exhibit variable performances, dependent upon cement chemistry. Similarly, it is understood that the recently developed alkali-free, non-caustic accelerators are also very sensitive to cement chemistry85,86. ( It is essential, therefore, that should alkali-free, non-caustic accelerators be specified by a designer or client, or selected by a contractor, on environmental, health and safety grounds, then their compatibility with locally available cements should be verified prior to the tender stage of a project, thus avoiding potential cost overruns and contractual claims.

Page 106 12.1.2 Additions The inclusion of fly-ash or blast furnace slag in shotcrete mixes is allowable and may well be beneficial to the properties of the hardened shotcrete, such as reduced penetrability, increased resistance to chlorides and sulphates and improved overall durability. When using blended cements, attention must be paid not only to accelerator compatibility, but to general mix design and extended setting times (if accelerators are not to be used). Blended cements have been used successfully in many applications87 and high early strength blended cement shotcretes have been produced in Canada 88. From an applicators perspective the use of flyash, slag and blended cements or very coarse grind cements are not desirable in non accelerated shotcrete as they may delay strength gain. An integral component of any high performance sprayed concrete is condensed silica fume (CSF). The benefits that CSF imparts to shotcrete mixes have been well documented [89,90,91,92]. Condensed Silica Fume (CSF) significantly increases the water demand of a shotcrete mix and necessitates the use of a superplasticiser to obtain the required flowability of the mix. CSF will increase the compressive strength of a shotcrete, reduce its permeability and improve the bond of the mix to its substrate (particularly important in shotcretes for rock support and repair works). In the plastic state, shotcretes containing CSF exhibit dramatically improved cohesion, reduced rebound and increased build properties. Other fine amorphous silica’s may have similar effects but their performance typically has not provided the allround benefits of silica fume in sprayed concrete. CSF is generally used as an additive in HPS mixes, at 5 to 10% by weight of cement. The Norwegian Concrete Association limits the maximum dosage of CSF in shotcretes to 15% by weight of cement but addition in excess of 10% is not cost effective unless it is necessary to provide resistance to aggressive chemical attack. CSF is normally available in three forms - undensified, densified and slurry. Densified silica fume must deagglomerate in the mix to be useful and not be deleterious and this is achieved in wet mix shotcrete. For dry mix or gunite only undensified, semi-densified (typically a bulk density of less than 350kg/m 3 is suitable) or slurry should be used. 12.1.3 Fibres Steel Fibres Steel fibres in concrete are often said to be corrosion resistant when embedded in concrete. This is true to some extent but the provisions for assessment of the durability of steel fibres given in this recommended practice should be followed in the design of shotcrete. Some fibres may have a film that provides limited corrosion protection before placing in concrete while others may have no protection. In any event during storage fibres are prone to corrosion and unlike reinforcing bar even a small amount of corrosion can have a dramatic effect on performance. Corrosion of the fibres may not be immediately apparent, particularly where bags are dispensed directly into the concrete mixer. Hence part of the quality assurance procedures should be to open sufficient bags of fibres check that the batch of fibres to be used are free from corrosion. Synthetic Fibres There are two types of synthetic fibre, structural synthetics or macro fibres, and microfibres. Structural synthetics have been developed to a point where the energy absorption of concrete made with them can be similar to that of steel fibre concrete. At low energy absorptions the synthetic fibre dosage may make them uneconomic. At high energy absorptions, i.e. where cracks widths may be high, structural synthetics may be economic and may have the benefit of high corrosion resistance at cracks. The higher the deflection

Page 107 the more economic structural synthetics becomes. However, synthetic fibres have a high creep value and this needs serious consideration in sustained load situations. Micro-synthetic fibres are used to assist with blockage and explosive spalling in the event of provide any crack control enhancement after provide some assistance in preventing early subsequent crack formation.

plastic crack control or to prevent water fires. Although micro-synthetic fibres don’t the first few hours of placing they may crack formation that act as initiators for

12.1.4 Admixtures Sprayed concrete could conceivably require the inclusion of up to six separate admixtures with the resultant quality control onus placed upon the producer at the batching plant. Dosages of the admixtures may also be high and this can have a significant effect on the physical properties of the finished concrete. Hence trials of the mix and application of the final mix are required. Where the specification allows for variation in the admixture rate then trials should be undertaken at the extreme allowable range of the admixture dosages. If accelerating admixtures are not to be used at the nozzle on application of the shotcrete, care must be taken that the combination of plasticisers and superplasticisers do not cause excessive retardation of the shotcrete. The process of applying shotcrete at high velocity to a substrate causes the expulsion of entrained air. As a high proportion of the as batched entrained air can be lost it is often necessary to batch wet-mix shotcretes at air contents in the range of 8 to 12% if the inplace shotcrete is to have adequate air-void for freeze thaw resistance93. The use of sodium silicates as shotcrete accelerators is well established. Although cheap, the high dosage rates (often > 20%) required to obtain satisfactory build and early stiffening are detrimental to the ultimate strength of shotcrete. They should not be considered for use where high performance is required. Two types of aluminate based accelerators are used in sprayed concrete, sodium and potassium. Aluminates also reduce the ultimate strength of the shotcrete. Typical dosage rates are 4 to 8%, by weight of cement. Higher dosage rates are sometimes used for very fast setting when spraying shotcrete over percolating water in rock support applications. Aluminates are sensitive to cement chemistry. Potassium aluminates are less sensitive than sodium and may be used with a wider range of cements. Alkali-free, non caustic (AF) accelerators are available in both powder and liquid form, with the latter being most suitable for use in wet-mix where high performance is required. Shotcrete containing liquid AF accelerators can achieve compressive strengths of 15 to 25 MPa at 24 hours, with only a marginal loss of strength at 28 days, compared to the same, unaccelerated shotcrete. It is important to check AF accelerators compatibility with the cement and mix. 12.1.5 Aggregate Mix gradings for sprayed concrete tend to have a high fines content and this can lead to high shrinkage relative to cast insitu concrete of a similar grade made with aggregate of the same type of rock. 12.2 MIX DESIGN A key aspect of the mix design is to ensure the sprayed concrete will adhere to the substrate and build up to the desired application thickness without sloughing off without undue rebound. Aggregate over 12mm has a high proportion of rebound and hence aggregate over this size is not normally used. Silica fume is commonly used to create a mix that is cohesive and can have high build but this also leads to a high water demand where the mix is to be pumped. Hence the use of water reducers and superplasticisers is critical.

Page 108 VicRoads Specification 684?? sets limits on drying shrinkages that are lower than that required for a N-class mix in AS 1379. These shrinkage values should be followed where possible but local materials may make this difficult. The local concrete suppliers should be able to advise on achievable shrinkage limits and where these are accepted the designer should ensure the crack widths will be adequately controlled. 12.3 APPLICATOR ASSESSMENT Competency of operation crew plays a significant role in durability aspects of sprayed concrete. This competency will initially be assessed by assessment of the previous experience of the contractors proposed application team on similar applications but will be ultimately tested by application trials of the proposed team using the proposed mix and application equipment in a trial representative of the actual application. Procedures are detailed in VicRoads Specification 684. 12.4 ACCEPTANCE TESTING OF SPRAYED CONCRETE VicRoads Specification 684 provides excellent guidance on testing and acceptance for sprayed concrete in general. However, additional testing consistent with requirements of assessing durability for the specific exposure may be required. 12.5 SUBSTRATE PREPARATION One of the key elements in having a durable shotcrete is its bonding to the substrate which requires a proper substrate preparation. Specification 684 provides sound general guidance but it should be noted that the surface of previous layers of shotcrete also need to be sound to ensure good adhesion of subsequent layers. Recommendation: In addition to the requirements of Specification 684 quality assurance procedures should be developed to ensure the following potential application defects are not left in the shotcrete: A. Sags or sloughing. B. Rebound trapped behind bars or at joints. C. Shadows behind reinforcement. D. Cold joints between layers, particularly where there is a delay in supply or at shift changes. 12.6 FINISHING The finishing and curing processes can have a direct impact on the finished surface durability of sprayed concrete. In addition, planning to provide the necessary finish to the concrete also requires consideration of mitigation of the potential for early drying of the concrete through surface protection or evaporation retardant application during placement as well as the application of a suitable curing process immediately following finishing. 12.7 CURING Sprayed concrete is unformed, has a tendency for high shrinkage and often cast in layers hence curing is more significant than for conventional concrete. Where possible normal concrete curing procedures should be followed but this is not always practical, particularly when shotcreteing in layers, and curing is often ignored leading to poor surface performance. The durability design should take this into account and it may be necessary to increase cover to allow for it. As an alternative to conventional curing an internal curing admixture may be considered. 12.8 QUALITY ASSURANCE Sprayed concretes insitu performance is particularly affected by workmanship. Hence, in addition to the quality assurance requirements applicable to all concrete, insitu testing should be undertaken.

Page 109 Recommendation: It is recommended that a representative number of samples be taken from the finished sprayed concrete for testing in a manner that will provide an indication of the likely insitu performance of the shotcrete. VPV testing may be suitable as a general assessment although testing relevant for the specific exposure may be more appropriate.

Page 110 13. COMMON CONSTRUCTION PROBLEMS In Durability Plans it is a common requirement to have the Durability Consultant review and approve solutions to non-conformances that would affect durability. In this section some of these common nonconformances are raised. 13.1 INADEQUATE COVER  Significance of cover.  Review requirement.  Pre and post pour checking.  Achieving Cover. 13.2 INADEQUATE STRENGTH DEVELOPMENT  Significance of mix design on strength development and stripping times.  Use of temperature matched cure and maturity to measure insitu strength development.  Maturity modelling.  Specification of curing regime needs to be clear.  Early strength needs to reflect real in-situ.  Early stripping impacts. 13.3 INADEQUATE CURING  Provides advice on methods of curing, i.e. implementation to maximise performance. Refers to CPN 2 on design implications of curing methods. Gives implications of steam curing and specific items to be monitored.  Quality of curing compounds and recommended dose rate.  Controlling application rate.  Supervision of application and timing.  Field trialling.  Removal of compounds. 13.4 EARLY AGE RESTRAINT CRACKS  Provides guidance on how to interpret design specification and what the contractor can do to assess and minimise issues.  Gives the significance of construction sequence and how to manage it. 13.5 PLASTIC SHRINKAGE CRACKS  Refers to CPN 5 on cause and design to avoid.  Details how and when to apply aliphatic or plastic sheets.  How to erect wind breaks and their value.  Reason for plastic crack control (durability around reinforcing).  Particular structures where this is a common issue. 13.6 PLASTIC SETTLEMENT CRACKS 13.7 BLISTERING/DELAMINATION  Lists issues and refers to comprehensive NZ document on this issue.  Protection of concrete.  Special mix requirements for this type of work.  Matching placeability and durability requirements.  Highly sensitive issue to control.

Page 111 13.8 RECTIFICATION DURING CONSTRUCTION The designer should be advised of any damaged concrete element as soon as possible after the damage is noticed to avoid possible further deterioration. Repairs that may be necessary to concrete elements during the construction period should be carried out strictly in accordance with the designer's requirements.

Page 112 REFERENCES 1. EN1992-1-1:2004. Euro-code 2. Design of concrete structures. General rules for buildings 2. Concrete Best Practice (Guidance from a European perspective). Jointly prepared by a working party of the European Concrete Societies Network. Best Practice Guides 2, 3, 5, 10 and 14. 3. Test methods and assessment of spacers (Annex), In: Merkblatt Abstanhalter, Deutscher BetonVerein EV, February 1997. 4. EN12390-3:2002. Testing hardened concrete. Compressive strength. 5. BS7973-1& 2:2001. Spacers and chairs for steel reinforcement – their specification 6. EN206-1:1997. Concrete – Part 1: Specification, performance, production and conformity 7. ENV 13670-1. Execution of concrete structures, Common. 8. EN13501-1:2002. Fire classification of concrete products 9. Patterson. A 2010 Pretensioned concrete in a marine environment” Maritime Structures Conference, Sydney, Dec 10. Baker C.N., and Gnaedinger J.P., "Investigation of the Free-Fall Method of Placing High-Strength Concrete in Deep Caisson Foundations," 1960 report 11. Turner C.D., Unconfined Free-Fall of Concrete," ACI Journal, Dec. 1970, pp. 975-976 12. STS Consultants Ltd The Effects of Free Fall on Concrete in Drilled Shafts,., report to the Federal Highway Administration, 1994. 13. Suprenant B. CI, Concrete International June 2001 14. ACI 301-99, "Specifications for Structural Concrete," 15. ACI 318-02, "Building Code Requirements for Structural Concrete," 16. ACI 304R-00, "Guide for Measuring, Mixing, Transporting, and Placing Concrete," 17. Rostram S. 1989 “Durable Concrete Structures – Design Guide” CEB Task Group 20: Durability and Service Life of Concrete Structures. Published by Thomas Telford. London. 18. Page C.L., Short N.R., E. Tarras A. “Diffusion of chloride ions in hardened cement pastes”, Cement & Concrete Research, Vol. 11, Issue 3, May 1981, ps. 395-406 19. Ho D.W.S., Hinzak I., Conroy J.J., Lewis R.K. “Influence of Slag Cement on the Water Sorptivity of Concrete”, ACI SP91-72 20. ACI 234R-06, "Guide to the Use of Silica Fume in Concrete," 21. AS HB 79. “ Alkali Aggregate Reaction –Guidelines on Minimising the Risk of Damage to Concrete Structures in Australia” – 2014 22. Cook H.K., Mielenz R.C., “Current Practice in the Use of Water-Reducing Admixtures in Concrete Construction in the United Sates of America”, RILEM, Sept 1967 23. Cement Concrete and Aggregates Australia, “Use of Recycled Water in Concrete Production” , August 2007 24. Thomas W.A., “Achieving Durability in the Construction Process” CIA Durability Workshop, 2009 25. Thomas W.A. “Controlling Water Content of Concrete for High Durability Applications” CIA Concrete 2011 Conference, Perth 26. Erika E. Holt, “Early Age Autogenous Shrinkage of Concrete” VTT Information Service, Finland 27. Thomas W.A. “The Prediction of Long Term Creep in Concrete Based on Early Age Testing” CIA Concrete 2009 Conference, Sydney 28. CIA Recommended Practice Z-40-2005, “Super-Workable Concrete”, Concrete Institute of Australia, Sydney. 29. Collins F.G. “Controlled Permeability Formwork” Concrete in Australia Vol 19, No.1, Mar 1993 ps 3-6 30. Fentress B. ‘‘Slab construction practice compared by wear tests“ ACI Jounal, 1973, 486–491 31. Kettle R.J. and Sadegzadeh M.,”Field investigations of abrasion resistance”, Materials and Structures, 1987,20,ps. 96 -102. 32. CCAA T48, “Guide to Industrial Floors”, 2009 33. A.M. Neville, “Properties of Concrete”, Longman 34. CCAA Data Sheet, “Hot Weather Concreting”, 2004 35. ACI 305R-10, “Guide to Hot Weather Concreting” 36. Main Roads Western Australia, “Specification 814 – Continuous Flight Auger Piles”, 2011 37. Sirivivatnanon, V and Baweja D Reducing the Maintenance Cost for Concrete Structures Proceedings: Australian Building Industry Conference, Queensland Master Builders Association, Gold Coast, September 1992 38. Sirivivatnanon, V and Cao, H T Quality Assurance of Concrete Structures – Analysis of In-situ Concrete Cover 'Australian Civil Engineering Transport', CE 33(2), April 1991, pp. 111–118 39. Baker C.N, and Gnaedinger J.P. "Investigation of the Free-Falll Method of Placing High-Strength Concrete in Deep Caisson Foundations," 1960 40. Turner C.D, "Unconfined Free-Fall of Concrete," ACI Journal, Dec. 1970, pp. 975-976 41. Litke S. "Concrete Free Fall Tested in Alabama Highway Department Project," Foundation Drilling, June-July 1992, pp. 14-16

Page 113 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78.

STS Consultants Ltd The Effects of Free Fall on Concrete in Drilled Shafts. Report to the Federal Highway Administration, 1994. Concrete Institute of Australia, “Compaction of concrete using immersion and surface vibrators” Current Practice Note 33, Sydney 2002 Concrete Institute of Australia, “Design of Joints in Concrete Structures” Current Practice Note 24, Sydney 2005 Concrete Institute of Australia, “Curing of Concrete” Recommended Practice Z09, Sydney 2009 Meeks K.W. and Carino N.J. “Curing of High-Performance Concrete: Report of the State-of-the-Art” National Institute of Standards and Technology Report NISTIR 6295, Maryland, 1999. Haque, M.N., 1990, “Some Concretes Need 7 Days Initial Curing,” Concrete International,Vol. 12, N2, February, pp. 42-46. Santhanam, M., “Studies on Sulfate Attack: Mechanisms, Test Methods, and Modeling,” PhD Dissertation, Purdue University, W. Lafayette, IN, USA, 2001. Fentress, B “Slab Construction Practices Compared by Wear Tests “ACI Journal”’ Vol 70 No 7 July 1973 pp 486-491 Kettle, R and Sadegzadeh, M “The influence of construction procedures on abrasion resistance” ACI SP 100’Concrete Durability’ 1987 Vol 2 pp 1385-1410. Industrial Pavements – Guidelines for Design Construction and Specification (T48) Cement and Concrete Association of Australia 1997. Antill, J M and Ryan, P W S “Civil Engineering Construction” McGraw-Hill, Sydney 1982 AS 3610 “Formwork for Concrete Structures”, Standards Australia 1995 Neville, A M “Properties of Concrete” Fourth Edition, Longman Group Ltd, London 1995 Potter, R J Prehardening Cracking in Concrete Notes on Current Practice Note 7”, CIA News October 1981. Hot Weather Concreting Technical Bulletin 95/2, Australian Pre-Mixed Concrete Association 1995. Tremie Concrete for Deep Foundations. Concrete Institute of Australia Recommended Practice Z17 Sydney 2012 Bamforth, P B, “Early-age Thermal Crack Control in Concrete” CIRIA C660, 2007 Concrete Institute of Australia “Cracking in concrete slabs on ground and pavements” Recommended Practice Z15, Sydney 2011 Concrete Institute of Australia “Watertight Concrete Structures” Current Practice Note 28, Sydney 2005 Concrete Institute of Australia “Design of Joints in Concrete Structures” Current Practice Note 24, Sydney 2005 AS 3735 “Concrete structures retaining liquids” Standards Australia, Sydney 2001 Papworth F, “Delivering a Quality Product: Design, Specification and Construction for Tunnel Durability” Australian Tunnelling Society Design and Construction Short Course, Melbourne 2012. C&CAA T51 Guide to Residential streets and paths” Cement and Concrete Association of Australia, Sydney 2004 AS 1085.14 “Railway Track Material – Part 14: Prestressed Concrete Sleepers” Standards Australia, Sydney 2001 AS/NZS 4676 “Structural design requirements for utility services poles” Standards Australia, Sydney 2001 AS/NZS 4058 “Precast concrete pipes (pressure and non-pressure)” Standards Australia, Sydney 2007 Concrete Pipe Association of Australia “Designing Permanent Pipelines” Technical Bulletin 1995 AS/NZS 2159 “Piling – Design and Installation” Standards Australia, Sydney 2007 CPAA “Concrete in Acid Sulfate Soil Conditions” Technical Brief Concrete Pipe Association of Australia, Sydney CPAA “Designing Durable Concrete Pipelines” Engineering Guideline Concrete Pipe Association of Australia, Sydney AS/NZS 1597.1 “Precast reinforced concrete box culverts- small culverts” Standards Australia, Sydney 2010 AS/NZS 1597 .2 “Precast reinforced concrete box culverts- large culverts” Standards Australia, Sydney 2013 Concrete Society Technical Report TR 72 “Durable post tensioned concrete structures” Camberley, England, 2002. fib Bulletin 33 “Durability of Post tensioning tendons” International Federation of Structural Concrete, Lausanne, Switzerland 2006. Marosszeky, M and Gamble, J Design, Detail and Construction of Reinforcement for Durable Concrete Building Research Centre, The University of New South Wales, 1987. BRE (2001) “Delayed Ettringite Formation: in-situ concrete” , Information Paper IP 11/01. Samarai M, Popovics S. & Malhotra V.M., “Effects of high temperatures on the properties of fresh concrete”. Transp. Res. Rec., 924 (1983, 42-50.

Page 114 79. 80. 81. 82. 83. 84. 85. 86.

87.

88. 89.

90.

91. 92. 93.

ACI 305R-10, "Guide to Hot Weather Concreting," Main Roads WA Specification 814 “Continuous Flight Auger Piles” Concrete Institute of Australia “The Use of Galvanised Reinforcement in Concrete” CPN17, 2008 Concrete Institute of Australia “Design of Joints in Concrete Structures” CPN24, 2008 National Precast Concrete Association “Precast Concrete Handbook” Vic Road Specification Section 684 “Sprayed Concrete” GARSHOL, K.F., “New Admixtures for High Performance Shotcrete”, Proceedings ACI/SCA Conference on Sprayed Concrete, Edinburgh, 1996; MAI, D., “Advanced Experiences with High Performance Alkali-free,Non Toxic Powder Accelerator for all Shotcrete Systems”, Proceedings, ACI/SCA Conference on Sprayed Concrete, Edinburgh, 1996 MORGAN, D.R., “Use of Supplementary Cementing Materials in Shotcrete”, Proceedings, International Workshop on the Use of Fly Ash, Slag, Silica Fume and other Siliceous Materials in Concrete, Concrete Institute of Australia, Sydney, 1988.) MORGAN, D.R., “High Early Strength Blended Cement Wet-Mix Shotcrete”, American Concrete Institute, Compilation No. 18 MORGAN, D.R., WOLSIEFER, J., “Wet-Mix Silica Fume Shotcrete : Effect of Silica Fume Form”, Proceedings, Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, 4th International Conference, Turkey, 1992; BURGE, T.A., “Fibre Reinforced High Strength Shotcrete with Condensed Silica Fume”, Proceedings, Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, 2nd International Conference, Spain, 1986; CONCRETE SOCIETY, “Microsilica in Concrete”, Technical Report No. 41, 1993; FEDERATION INTERNATIONALE DE LA PRECONTRAINTE (FIP), “Condensed Silica Fume in Concrete. State of the Art Report”, 1988 MORGAN, D.R., “Freeze-Thaw Durability of Shotcrete”, American Concrete Institute, Compilation No. 18

Page 115 APPENDIX 1 - SPACERS AND CHAIRS FOR SUPPORT OF STEEL REINFORCEMENT

1. Scope This “Best Practice Guide” covers product performance requirements for spacers and high chairs that are sufficient to achieve and maintain reinforcement cover in reinforced concrete structures. Performance requirements for spacers and chairs include material of manufacture, dimensional tolerances, point load strength, permanent deflection after loading, compressive strength, stability, fixity and durability.

2. Preface Concrete cover is of great importance for fire resistance, transfer lengths and durability of all concrete containing reinforcement. Spacers are present on every concrete surface thus there is a need to ensure that they do not become the weak link in the performance of a structure. Reinforcement should be placed with a fixing cover Cv so that there is the highest degree of probability that the minimum reinforcement cover Cmin is achieved. To achieve this, spacers are required so that they: 

ensure reinforcement cover (Cv) is maintained before and during concreting,



do not affect the durability and serviceability of the structural member after concreting.

Currently available spacers and chairs are normally made of steel, plastic and cementitious material (cast and extruded fibre concrete).

3. Terms and definitions: For the purposes of this Best Practice Guide the following terms and definitions apply. Concrete cover is the thickness of concrete between the reinforcement and the concrete surface. Cmin is the minimum concrete cover on which there is no negative tolerance and is the minimum distance between the concrete surface and the reinforcement that must be achieved with a high degree of probability. Cnom is the nominal concrete cover and is made up of the minimum cover plus an allowance for tolerances of Δc (Cnom = Cmin + Δc ) Δc the allowance for tolerances should take account of all unavoidable dimensional deviations due to bending and fixing of reinforcement, spacer type and fixing, formwork placing and the placing and compaction of concrete. The recommended figure for Δc is 10mm. While AS3600 only requires 5mm negative tolerance and testing on actual structures has shown that to achieve the minimum cover at a high reliability a tolerance of 10mm will be necessary, the contractor is still responsible for achieving the specified minim cover. Cv the fixing cover is the provided spacer cover. It is the spacing required between the outer reinforcement bars and the concrete surface. The height of the spacer is designed to this required spacing. Spacers are components which are placed to maintain the required cover between the outer reinforcement and the formwork and also between formwork faces. Spacers may be point, linear or area supporting. Chairs are used to support upper horizontal reinforcement layers in position or to separate vertical reinforcement in walls. Chairs may be basket, snake or trestle.

Page 116

4. Requirements for Spacers and Chairs: 4.1 Spacers Spacers are currently supplied in a number of different shapes and sizes and are made from plastic, steel and cementitious material (cast and extruded fibre concrete). Spacers may have differing properties in the concreted and non-concreted states. A number of the most common profiles are shown in Figure 32. Figure 32 : Types of Spacer Block Spacers

Block Spacers

Wheel Spacers

Bar Spacers

Formwork Spacers

Chairs

Area Spacers

Spacers must be fixed to the steel or positioned such that they do not move or twist during concrete placement. The profile of the spacer used should match the application. Spacers may form an unavoidable discontinuity in the cover zone but they should nevertheless not impair the impenetrability or durability of the completed structure.

Page 117 To provide cover of the correct thickness and durability all spacers must have the following properties: 

under all load and temperature conditions provide sufficient stability and load bearing capacity;



have durability and fire resistance properties that match the concrete.



where needed be able to be fixed firmly to the reinforcement.



have a negligible spring effect to avoid surface spalling after formwork removal.



be only supplied in one fixing cover (Cv) for all insitu applications. Double cover spacers may be acceptable in the controlled environment of a precast production facility.



have a profile that allows complete encasement by the concrete.



be made from a material that does not corrode and is resistant to concrete alkalinity.



be made from a material that does not provoke corrosion of the reinforcement and has no harmful or damaging effect upon fresh, hardening or hardened concrete.

4.2 Chairs Chairs are generally made from steel wire. For each application enough suitable chairs must be used to prevent any of them moving or twisting. Without deforming they must retain the reinforcement in the designed position and withstand all applied loadings. To ensure the above the chairs must be: 

able without deformation support all loads that are applied during the construction process;



stable to prevent any tipping;



capable of being firmly fixed in position; and



corrosion protected for situations when chairs rest on the formwork.

5. Types of Spacers and Chairs: 5.1 Wheel spacer – made from cementitious material or plastic 

A spacer with a circular cross-section.



Used for maintaining cover in vertical elements.



Used particularly for column formwork and for piles.

5.2 Block or point supporting spacers – made from cementitious material or plastic 

Clip-on plastic spacers - supplied with the clip being integral to the spacer.



Clip-on cementitious spacers - with one or two steel clips. Manufactured with a preformed groove for one bar whilst clipped onto the cross bar. Clips are located outside the cover zone for maximum durability.



Clip-on cementitious spacers can also be supplied with a plastic clips.



Wire-on spacers (cementitious and plastic) are attached to the reinforcement by the use of tying wire. The wire can be mild steel, galvanised or stainless.



Fixed – end spacer (plastic or cementitious) used to fit on the end of reinforcement to provide end cover.

5.3 Bar or linear supporting spacers – made from cementitious material or plastic 

Fixed - Clip-on or Wired – normally made from cementitious material and used for vertical reinforcement.



Not Fixed – made from cementitious material and plastic and used to support horizontal layers of reinforcement or horizontal continuous steel chairs.



Bar spacer should be limited to a maximum length of 350mm.

5.4 Formwork spacer (distance tube) – made from cementitious material or plastic 

Used to separate the two formwork panels.

Page 118 

Held in position by the threaded through tie bar.

5.5 Area supporting spacers – made from plastic 

Used to support light meshes in a horizontal position.

5.6 Chairs – made from steel 

Made from steel wire and supplied as baskets, snakes and trestles.



Used to support upper horizontal reinforcement layers in position or to separate vertical reinforcement in walls.

6. Permissible Tolerances Reference 2 details that cementitious spacers should have a permitted tolerance of ±2mm for all cover depths. Plastic spacers have permitted tolerances of ±1mm for cover depths of 40mm or less and ±2mm for cover depths in excess of 40mm. All chairs have a permitted tolerance of ±2mm on their height.

7. Performance Classes There are two defined performance classes for spacers and one for chairs 

Light (L) – used where reinforcement is not subject to loading from foot traffic during fixing and there are no increased requirements for load bearing capacity and stability.



Heavy (H) – used as standard spacers for all concrete construction with increased requirements for load bearing capacity and stability.



Chairs ( C )- used for basket, snake and trestles.

Reference 2 gives details on the load and permitted deformations for spacers and chairs and these are shown in Table 15. When tested in accordance with the requirements of reference 3, the values shown must be matched or exceeded. Table 15 : Point Load test for spacers and chairs Item

Category

Types

Cover/Height (mm)

Test Load

Spacer

Light (L) Heavy (H) C C

All All Basket Snake

15 to 50 15 to 100 75 to 300 50 to 300

250 N 3,000 N 670 N/m 670 N/m

Chair

Permitted Deformations Cv ≤ 20mm – 1mm Cv > 20mm – 2mm n/a n/a

8. Materials of Manufacture Spacers and chairs can be manufactured from three basic materials: plastic, cementitious material (cast and extruded fibre concrete) or steel wire. Spacers should have an equivalent durability when in place to that of the host concrete. Cementitious spacers can be either cast or extruded fibre concrete. Cast concrete spacers produced by a controlled process on site or in a factory need to be cast and cured to ensure that the actual strength and durability properties of individual spacers match those of the theoretical mix properties before use on site. Extruded fibre concrete spacers are always factory made and subject to certification of their properties and are suitable for use for all exposure conditions. All types of cementitious spacers shall be soaked with water by spraying prior to their use in the works. Due to their thermal incompatibility with, and their lack of adhesion to concrete, plastic block spacers should only be used in non-aggressive environments (exposure classes A). Plastic spacers (block, bar,

Page 119 area and formwork) shall not be used for any water retaining structure. Due to difficulties of concrete completely sheathing plastic bar spacers and the melting out of this type of spacer in fire events, the use of plastic bar spacers is not permitted under any fire exposure condition. All spacers and chairs whatever the method of manufacture shall be manufactured in an ISO9001 approved production facility. Spacers and chairs should be subject to a rigorous Quality Control and Testing regime in the manufacturing plant. The required strength and durability properties of spacers and chairs should be tested both in-house and by independent Test Houses on a regular basis. Copies of Test Certificates shall be supplied for the spacer and chair types used on site when requested.

9. Recommendations for use of spacer types 9.1 Plastic block spacers 

Plastic spacers should only be used for lightly loaded elements and for those elements exposed to environments with exposure class A.



Only spacers made from plastic with the lowest linear expansion coefficients shall be used.



Only plastic with a roughened surface shall be used to encourage bond with the concrete.



Cross sections perpendicular to the reinforcement bar should have at least 25% voids within the enclosed perimeter to avoid concrete not sheathing the spacer fully and leading to honeycombing, etc.



Shall not be used in water retaining/excluding structures for both reinforcement or formwork spacers.

9.2 Plastic bar spacers 

This type of spacer shall not be used under any circumstances.

9.3 Cementitious spacers 

Shall not be manufactured on the construction site.



Shall not be manufactured using a release agent or oil.



Shall have a minimum compressive strength when tested in accordance with EN12390-3:2002 [4].



Shall have known durability properties with minimum values as given in Table 16.

9.4 Steel chairs 

For any applications on exposed faces (exposure class A), the tips of the steel must have a plastic coating to a minimum depth of 15mm.



Except for exposure class A all chairs shall stand on the bottom reinforcement.



When concreting against any deformable layer such as insulation only chairs supported off the bottom reinforcement shall be used.

10. Special Requirements In view of the much longer life expectancy for the majority of concrete structures, certain special durability requirements are needed. For architectural concrete some simple rules should also be applied. Table 16 outlines three spacer categories to fulfil the above basic requirements plus the further requirements detailed below. This is applicable for all spacer heights. 10.1 Increased Durability Spacers need to have the durability properties that match or exceed those of the host concrete. These can be increased acid resistance, sulphate resistance, chloride resistance, abrasion resistance, etc. To ensure that durability requirements are met, spacers must be independently tested and certified. Table 16 lists an example of what should be asked for. Plastic spacers should not be used in any durability application. For improved acid or sulphate resistance a cement combination mix is recommended.

Page 120 Table 16 : Durability properties of spacers Material Quality Compressive Strength Exposure Classification

Unit N/mm2

Type A 40 X0, XC1

Water Absorption (30 mins.) I.S.A.T. (10 mins.) R.C.P. Chloride diffusion

% ml/m2/sec coulombs m2/sec.x 10-12

n/a n/a n/a n/a

Type B 50 X0, XC1-3, XD1-2 XS1-2, XF1-2 XA1-2, DC1-2 < 3.0 < 0.50 < 4,000 < 5.0

Type C 60 XO, XC1-4, XD1-3 XS1-3, XF1-4 XA1-3, DC1-4 < 2.0 < 0.25 < 1,000 < 1.0

10.2 Temperature Loading: In any concrete structure where temperature effects are prevalent such as external members, tanks, etc, no cracks or spalling should occur around any spacer. The large difference in linear expansion coefficient between concrete and plastic means that plastic spacers should not be used in such applications 10.3 Watertight Concrete: Concrete structures subject to water pressure from any side must be effective in their sealing function. All of the components of the concrete must play their part, for that reason plastic spacers should not be used in such applications. When choosing spacers attention should be given to their: 

adhesion capability to the concrete;



porosity of the material;



choosing a shape that is easily sheathed by the concrete; and



for the formwork spacer used with through ties the sealing of the hole is very important. It is recommended that only proven systems with known resistance to water pressure be used.

10.4 Fire Resistance: All spacers shall be fire resistant unless otherwise permitted. For structures requiring complete fire separation it is recommended that only proven sealing systems incorporating cementitious formwork spacers (distance tubes) with known fire resistance should be used 10.5 Architectural Concrete: If coloured, profiled, textured or exposed aggregate finishes to the concrete are specified, spacers shall be selected so that they do not disrupt the aesthetic appearance of the concrete. Spacers should be virtually invisible on the finished surface. Other points to note: 

cementitious spacers with minimal bearing areas are to be preferred;



care should be taken to ensure that regular spacing of spacers is maintained;



the formwork spacers for the tie rods should also be spaced in a regular pattern; and



sealing of these hollow spacers can often be very unsatisfactory. The use of concrete sealing plugs in conjunction with concrete formwork spacers is recommended

11. Spacer and Chair Selection 11.1 Performance The selection procedure for spacers is shown in Figure 33.

Page 121

Figure 33 : Selection Procedure for Spacers

Structural Design

Concrete Specification

Fixing Cover (Cv in mm)

Exposure Classification

Exposure Class A

Exposure Class B

Type A

Type B

Spacer Type

Fire Resistance

No

Yes

Exposure Class C

Type C

Yes or No

Performance Class (see Table 1)

Light

Light or heavy

Material Type

Plastic

Cementitious

The following should be used as a guideline for spacer selection: 

As detailed above plastic spacers are susceptible to levels of thermal variations vastly different to that of concrete and this coupled with their lack of bond to concrete makes then unsuitable for all aggressive environments and for water retaining/excluding structures.



The use of plastic formwork spacers for formwork separation (distance tubes) shall also be avoided in aggressive environments or for water retaining structures for the same reason.



Cementitious spacers are thermally compatible with the concrete and are more capable of sustaining the applied loads under physical and temperature loading. They are the preferred spacer for structural members in aggressive environments and where there are large constraining forces.



Spacers shall be selected and fixed so that the cover is guaranteed for all situations.



All block spacers shall be fixed at reinforcement nodes.



On deformable layers spacers with large supporting areas shall be used to prevent penetration during concreting.



Spacers for supporting vertical reinforcement must be chosen for their ability to allow easy encasement by concrete.

Page 122 

Linear bar spacers should only be used in maximum of 350mm lengths and always staggered across the formwork.



Linear bar spacers should only be placed parallel to the span direction in compression zones.



Linear supporting chairs must be placed continuously in the longitudinal direction.



When chairs are placed on the bottom reinforcement their spacing should match that of the spacers beneath the bottom reinforcement layer. Lines of support should lie vertically above one another.

11.2 Profile and Fixing The following should be used as a guideline for spacer selection: 

For vertical reinforcement all spacers must be firmly fixed in position.

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For all finishes consideration must be given to visibility of spacer on finished surface.

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Arched base and curved bases are best for visual concrete.

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For coverage across many bars a short triangular hooked bar is best.

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Fixing is mainly by tying wire (mild, galvanised and stainless) – note the tying wire is present in the cover zone. The type of tying wire to be used will be determined by the exposure classification and engineer’s requirements.

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Fixing can also be achieved by using a double clip spacer where the clips are located outside the cover zone – one bar is located in a groove in the spacer thus offering maximum rigidity of fixing.

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For horizontal reinforcement either block or bar spacers may be used.

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Block spacers must be used with tying wire for rigidity of fixing.

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Bar spacers do not require any fixing and are thus quicker and easier to lay.

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Considerations should be given for all spacers that they have maximum contact areas with the concrete for adhesion and bond – various profile are available that offer maximum contact area.

12. Quality Control and Testing Frequency 12.1 Quality Control 

To ensure quality standards are maintained the manufacturer must carry out continuous production control. The manufacturer or supplier must submit full details of their QC system.

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The schedule should detail test types and frequency of test for internal testing.

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The schedule should detail test types and frequency of test for external independent testing.

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Copies of documentation must be made available upon request.

12.2 Test Types and Frequency The following are a recommended internal testing frequency: 

Dimensional tolerances and shape of spacers should be checked daily.

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Point Load tests must be performed on representative spacers daily.

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Compressive Strength tests to be performed for each new production run.

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Durability tests to be performed every time the production formula or aggregate components are altered.

13. Recommendations for number of spacers and location The fixing interval is based primarily on the accepted deflection at maximum loading, e.g. when the reinforcement is subject to foot traffic especially during concreting. When using bar spacers in the tension zone, it is recommended that the bars be of short lengths (maximum 350mm) and be staggered

Page 123 across the formwork. It is also important to ensure that the spacers are resistant to extra loading from heavy reinforcement. Please also refer to references 2, 3 and 5. 13.1 Slabs Slabs - Bottom reinforcement All bottom reinforcement bars shall be supported by spacers at centres not exceeding 50d and not exceeding 1000mm. Spacers shall be staggered. Fabric reinforcement shall be supported at centres not exceeding 500mm in two directions at right angles. Slabs – top reinforcement Top reinforcement shall always be supported off the bottom reinforcement where it exists, by continuous chairs. Where there is no bottom reinforcement, the top reinforcement shall be supported by continuous chairs at centres not exceeding 50d resting on bar spacers. The spacing of all individual chairs shall not exceed 50d centres in both directions for individual bars, or 500mm for welded fabric. Slabs – edge face Vertical reinforcement – reinforcement at right angles to the top and bottom surfaces of the slab shall have spacers on alternate vertical bars Horizontal reinforcement – horizontal reinforcement parallel to the edge of the slab shall have spacers at centres not exceeding 50d and not exceeding 1000mm centres on each bar. Horizontal reinforcement – horizontal reinforcement at right angles to the edge of the slab, end spacers shall be fixed on straight bars at centres not exceeding 1000mm if no alternative exists. Hollow pot, waffle, trough and ribbed slabs Where main bars are supported by links in the ribs of hollow pot, waffle, trough and ribbed slabs spacers shall be fixed to the links at centres not exceeding 1000mm along the rib.

On each link three spacers shall be fixed: one at mid-height on each of the two vertical legs and one on the horizontal part of the link in the centre. Where the main bars are not supported by links, spacers shall be fixed to the main bars at centres not exceeding 50d and not exceeding 1000mm. 13.2 Beams General – spacers shall be attached to the links or fabric at the ends of the beam and at centres not exceeding 1000mm along the beam. Spacers shall be fixed on three faces of the same link. Bottom face – requirements are as follows: 

Narrow beams (< 250mm wide) one spacer shall be positioned in the middle of the bottom leg of the link.

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Normal beams (250mm to 500mm wide) two spacers shall be positioned within 50mm of the ends of the straight portion of the bottom leg of the link.

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Wide beams (> 500mm wide) two spacers shall be positioned within 50mm of the ends of the straight portion of the bottom leg of the link. Additional spacers may be required to ensure that the distance between them does not exceed 50d. Where multiple links overlap, an additional spacer positioned immediately adjacent to the vertical leg of one of the links is required.

Side face – for narrow and normal beams, one spacer shall be provided on each vertical leg of the link at the mid-height of the beam. For deep beams (> 100d deep) spacers shall be provided at centres not exceeding 50d. Spacers on each side of the beam shall be at the same level above the soffit. End face – every end termination of a bar at an exposed concrete face shall have a spacer.

Page 124 13.3 Columns Links to which the spacers are attached shall be at the top, middle and bottom of each lift of concrete and at centres not exceeding 100d. Spacers shall be placed on all sides of the same link according to the following criteria: 

For small (sides not exceeding 50d), square or rectangular columns, spacers shall be located on the two shortest sides in the middle of the link. Two spacers shall be positioned on each of the other two sides positioned within 50mm of the ends of their straight portion, i.e. six spacers per link.

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For wide (sides exceeding 50d), columns, two spacers shall be located within 50mm of the straight portion of the link. Intermediate spacers shall be provided so that the distance between spacers does not exceed 50d. Where the shorter sides of the wide columns do not exceed 50d, spacers shall be provided in the middle of the link on these sides.

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For columns with multiple links, an additional spacer shall be positioned immediately adjacent to the inner leg of one of the links on the straight part.

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Links with spacers in circular columns shall have at least four equally spaced spacers per link (one pitch for a helix) with the distance between them not exceeding 50d.

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Multifaceted columns shall contain at least one spacer per facet. Facets exceeding 50d shall have two spacers positioned within 50mm of the ends of the straight part of the link. Intermediate spacers shall be provided so that the distance between each spacer does not exceed 50d.

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Spiral links have the same requirement as circular columns.

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These requirements apply to all main cross-sectional shapes of columns, i.e. square, rectangular and multifaceted.

13.4 Foundations The requirements for reinforced strip footings, individual bases, ground beams, ground slabs and pile caps shall be the same as those for slabs and beams as above. 13.5 Walls For bars and fabric, face cover shall be maintained by spacers on the reinforcement nearest to the face, in two directions at right angles, at centres not exceeding 50d or 500mm, whichever is the greater and staggered. Spacers on opposite faces shall be coincident when viewed in elevation. Fabric or bars on opposing faces in walls up to 400mm thick shall be separated by rows of continuous chairs (preferably vertical) at centres not exceeding 1000mm and located at the same positions as the spacers. For horizontal reinforcement perpendicular to the end of the wall, end spacers shall be provide don the ends of straight bars when no other bars are available. In such cases end spacers shall be positioned at centres not exceeding 1000mm.

14. Specification Clauses 14.1 Specification - Type A spacers All spacer types shall be manufactured in a purpose made facility accredited in accordance with ISO9001:2000. Site spacer production is not permitted. Spacer properties shall comply with the recommended values for Type A spacers given in Table 16. Spacer profile shall be as given in the contract documents. Independent test certification shall be supplied upon request. Plastic and cementitious spacers are acceptable and should be chosen with reference to Figure 33. For water retaining/excluding structures and those with fire resistance requirements, plastic spacers shall not be used. 14.2 Specification - Type B spacers All spacer types shall be manufactured in a purpose made facility accredited in accordance with ISO9001:2000. Site spacer production is not permitted. Spacer properties shall comply with the recommended values for Type B spacers given in Table 16. To ensure maximum adherence between spacer and concrete no release agents shall be used in spacer production and the spacer profile shall maximise contact area. Spacer profiles shall be as given in the contract documents. Independent test

Page 125 certification shall be supplied upon request. Only cementitious spacers (cast or extruded fibre concrete) are acceptable and should be chosen with reference to Figure 33. 14.3 Specification - Type C spacers All spacer types shall be manufactured in a purpose made facility accredited in accordance with ISO9001:2000. Site spacer production is not permitted. Spacer properties shall comply with the recommended values for Type C spacers given in Table 16. To ensure maximum adherence between spacer and concrete no release agents shall be used in spacer production and the spacer profile shall maximise contact area. Spacer profiles shall be as given in the contract documents. Independent test certification shall be supplied upon request. Only cementitious spacers are acceptable and should be chosen with reference to Figure 33.

Page 126 APPENDIX 2 : TESTING OF SPACERS. 1. Compressive Strength – The compressive strength of each spacer mix shall be tested in accordance with the national code for concrete compressive strength. The values obtained shall match or exceed those shown in Table 16. The sample block to be tested must be made from the same mix as the spacers and manufactured in exactly the same manner. Initial Quality Control testing shall be performed using external approved laboratories. Internal factory testing should be performed on a three monthly basis. External confirmation testing to be performed on a quarterly basis. 2. Dimensional Tolerances – Tolerances should be measured using calibrated equipment. All tests to be carried out on production run spacers. Internal factory testing shall be performed on a daily basis. 3. Point Load Capacity – the Point Load Capacity of each spacer profile shall be tested in accordance with the requirements of Reference 3 or equivalent international tests. The values obtained shall match or exceed those shown in Table 15. All tests to be carried out on production run spacers. Initial Quality Control testing shall be performed using external approved laboratories. Internal factory testing shall be performed on a daily basis. 4. Water Absorption - Each spacer mix shall be tested in accordance with the requirements of BSEN1881-122:1983. The values obtained shall match or exceed those shown in Table 16. The block to be tested shall be made from the same mix as the spacers and manufactured in exactly the same manner. Initial Quality Control testing must be performed using external approved laboratories. No subsequent tests are required unless material changes to the mix are made. 5. I.S.A.T. - Each spacer mix shall be tested in accordance with the requirements of BSEN1881208:1996. The values obtained shall match or exceed those shown in Table 16. The block to be tested shall be made from the same mix as the spacers and manufactured in exactly the same manner. Quality Control testing shall be performed using external approved laboratories. No subsequent tests are required unless material changes to the mix are made. 6. Rapid Chloride Permeability - each spacer mix shall be tested in accordance with the requirements of ASTM C1202-05. The values obtained shall match or exceed those shown in Table 16. The block to be tested shall be made from the same mix as the spacers and manufactured in exactly the same manner. Quality Control testing shall be performed using external approved laboratories. No subsequent tests are required unless material changes to the mix are made. 7. Chloride Diffusion Coefficient - each spacer mix shall be tested in accordance with the requirements of any of the internationally recognised tests. The values obtained shall match or exceed those shown in Table 16. The block to be tested shall be made from the same mix as the spacers and manufactured in exactly the same manner. Quality Control testing shall be performed using external approved laboratories. No subsequent tests are required unless material changes to the mix are made.

Inspection and test records shall be fully traceable to specific production dates and batches. Inspection and test records shall be retained for 24 months and made available upon request.

Z7 04 Good Practice Through Design, Concrete Supply and Construction Draft

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