DURABILITY DESIGN OF INFRASTRUCTURE ASSETS – WORKING TOWARDS A UNIFORM APPROACH F. Blin1, S. Furman1 and A. Mendes1 1 AECOM SUMMARY: In the construction and building industries it has become increasingly frequent for infrastructure asset owners and operators to specify design life requirements for both capital and remediation works. This trend supports the development of more sustainable design and construction practices that take a lifecycle approach, such as including operational and maintenance considerations in the design solution process, and not just focussing on minimisation of the initial capital cost. Whenever design lives are specified there is a clear need to adopt and implement uniform durability design practices throughout the project so that consistency of approach is achieved through:
Developing a durability management plan outlining the approach needed to achieve the design life requirements
Providing technical support to the design team so that durability is embedded in the design process
Supporting the construction team to manage issues that impact on the design life of the facility or structure
Providing input to handover documentation such as the asset register, inspection and maintenance plans etc so that the integrity of the key design inputs and assumptions required to achieve the specified service life are captured prior to commissioning and embedded in the operation phase.
This paper presents the findings of a literature survey of various documents that deal with durability design, especially ISO 13823 - “General principles on the design of structures for durability” which is discussed in detail. Of particular interest is the potential for ISO 13823 and the associated standard ISO 15686 - “Buildings and constructed assets - Service life planning” to be used more extensively in Australia. Furthermore, this paper discusses and proposes the terminology and template that could be used when designing for durability in an effort to standardise this important design practice. Keywords: Asset, Life-cycle, Durability, Corrosion. 1. INTRODUCTION The term durability has been used for a very long time but has been increasing in importance over recent years in major infrastructure projects. This is due to asset owners and operators placing greater emphasis on whole-of-life performance by nominating the design lives that the assets being created during the project are required to achieve and requiring the project delivery team to justify and demonstrate how the design life will be achieved. As asset owners become more sophisticated in applying asset management practices in their business it naturally generates a requirement to better assess and manage the social, environmental and economic risks associated with the construction and operation of an asset. To this end durability can be an effective performance indicator and risk management tool, which takes into account the consequences of the early failure of an asset (or its components) and tailors the materials selection to minimise the likelihood of such event occurring within the required design life.
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Similarly, the concepts of sustainability and whole-of-life-cycle costing are more commonly used when undertaking major capital works and tend to support the design and construction of more durable, less maintenance-intensive assets from the onset (i.e. allocating a higher budget in capital works to reduce operational costs during the service life). Interestingly, while there seems to be a general agreement as to the definition of durability - typically the ability for an asset to achieve its required design life in a given environment - there is at times some debate as to what level of maintenance would be considered adequate. Generally though, so-called “minor” maintenance (such as cleaning/washing of stainless steel elements or touch-ups of a coating) seems to be widely seen as acceptable as opposed to “major”, more labour-intensive and costly activities such as concrete repairs. There have been numerous papers produced on durability and there are a number of useful resources to support durability design. There is also an overall agreement on what is meant by designing for durability and our industry is fortunate to have a number of very experienced and knowledgeable practitioners in this field. However, in our experience there is not a common process to undertake durability design. Having a common approach within the industry would make the process more consistent and transparent (currently too much reliance is placed on opinion) for designers, construction teams, operators, and asset owners in general. It would also allow for more effective verification of the deliverables produced by peers. The interest in developing a common approach to durability design led the authors to assess the standards and codes across the world that support or guide durability design and propose a standard process template for the preparation of durability deliverables. 2. DURABILITY DESIGN AROUND THE WORLD The concept of durability, and its importance when designing, constructing and operating assets, has gained importance over the past decades. This is illustrated in the following examples for concrete elements. Back in 1963, the primary requirement for concrete structures was a satisfactory compressive strength [1]. A few years later, according to Neville [1], the British Code of Practice for Reinforced Concrete in Buildings (CP 114) stated as a general comment: ‘The greater the severity of the exposure the higher the quality of the concrete required’. Later, in 1973, Neville concluded: ‘Concrete of reasonable strength, properly placed, is durable under ordinary conditions. But when high strength is not necessary and the conditions are such that high durability is needed, the durability requirement will determine the water-cement ratio to be used’ [2]. In 2001 Neville [1] talked about the future in durability design and highlighted that for concrete an improvement in durability would be possible through the use of correct placing methods, compaction techniques, finishing operations and adequate curing. In addition, Neville stressed that in the future the importance of maintenance would be recognised as a way of achieving durability [1]. At present, most of the standards and codes available throughout the world have prescriptive and/or performance based recommendations, which rely on material requirements during design and their performance during construction. However, the majority of standards and codes appear to lack guidance on how to apply and manage the durability process from design, through construction and into the operational phase of an asset. Additionally, they do not seem to make recommendations on how to link durability to the preparation of inspection and maintenance plans. In Australia, a number of standards specify the minimum requirements necessary to achieve durability. For instance, galvanised and electro-galvanised zinc coatings in atmospheric exposures are covered by AS 2309 [3]. This standard specifies the corrosion rates and estimated service life for numerous systems in different atmospheric exposures and environmental aggressiveness. AS 2309 Appendix E lists the items to be considered during the preparation of a specification for metallic protective coatings, including both prescriptive and performance requirements (e.g. estimated corrosion patterns for a particular location and maintenance requirements). Furthermore, guidance for repair of metallic protective coating is provided in Appendix F. Some of the available concrete standards (AS 3600, AS 5100.5 and AS 3735) have a dedicated ‘Durability Design’ section [46]. For example, the ‘Durability Design’ section in AS 3600 [4] provides guidance for the durability design requirements of reinforced and pre-stressed concrete structures and members with a design life of 50 years ± 20% by defining the different types of exposures in Australia and the subsequent requirements (concrete strength grade and associated cover to reinforcement for the different environments) in order to achieve durability. In a similar manner, AS 5100.5 [5] provides guidance for the design for durability of concrete structures with a design life of 100 years, with the same focus as utilised in AS 3600. AS 3735 [6] provides guidance on the minimum cover to reinforcement required when the concrete is exposed to different liquids such as freshwater, seawater, corrosive liquids so a design life of 50 years ± 20% (as per AS 3600) can be met. In contrast to AS 2309, none of abovementioned concrete standards list requirements to be included in the preparation of a specification, including maintenance requirements. Another Australian standard to include a section in ‘Durability Design’ is AS 2159 [7] which addresses plain, reinforced and pre-stressed concrete and steel piles with a design life of 50 and 100 years, as well as timber piles. For concrete piles, AS 2159 specifies the following main requirements: minimum concrete strength and cover to reinforcement, limitation in crack width and selection of concrete aggregates. AS 2159 uses different terminology for the exposure environment in comparison to that used by AS 3600 and AS 5100.5 (i.e. non-aggressive, mild, moderate and severe). For steel piles, the requirements relate to corrosion allowance, application of coating systems or cathodic protection. The corrosion rate is specified as a range for each environment, e.g. 0.04-0.1 mm/year for a severe environment (seawater submerged, tidal/splash zone and cold water south of
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30’ S), which makes it difficult to select a specific rate when designing. In addition, a linear corrosion rate is assumed, not taking into account the higher rate usually observed for the first year (and mentioned for instance in ISO 9223 [8]). However, like the Australian standards related to concrete design [4-6], AS 2159 does not provide any guidance on how to structure a durability process from design into construction and maintenance during design life. For timber piles, AS 2159 discusses timber selection and treatment, while AS 5604 [9] provides life expectancy in-ground and above-ground for different classes of timber. Within Australia, state codes such as RTA B80 [10] and VicRoads Section 610 [11] also specify requirements to ensure concrete durability. The RTA B80 concrete requirements include: cement type, minimum cement content, maximum water/cement ratio, maximum water sorptivity, compressive strength, and curing regime. VicRoads Section 610 specifies limits such as maximum volume of permeable voids (VPV), minimum cementitious content and maximum acceptable crack width. A brief overview of codes and standards that were identified to provide durability guidance outside of Australia is presented below. In Brazil, the durability of concrete assets is addressed by a section of ABNT NBR 6118 [12]. In this standard, only four main exposure classifications are determined. However, in this case, the minimum cover to reinforcement is not the main requirement, but rather the focus is on the water/cement ratio and concrete strength grade. Interestingly, two additional requirements are made; one being that drainage must be considered during design to avoid accumulation of water on the concrete surface, and the shape and format of the structure must allow for easy access for future maintenance. In Colombia, the available standard that deals with concrete durability is slightly more detailed. NTC 5551 [13] specifies seven different exposure classifications including ‘no risk of corrosion’ and ‘high humidity’. As expected, the durability requirements depend on the type of exposure linked to water/cement ratio, compressive strength and minimum cementitious content as well as maximum allowable crack widths. In the United States, the ACI 201.2R-01 Guide to Durable Concrete [14] describes the different deterioration mechanisms in concrete including freezing and thawing, chemical sulphate attack, physical salt attack, carbonation and acid attack while some recommendations are made in order to achieve durability. These recommendations refer to: water/cement ratio, quality of materials, curing and attention to construction practices. A grading for severity of exposure is presented and subsequent requirements are listed. In addition, a section on evaluation of damage and selection of repair methods is presented. In South Africa, Alexander et al. [15] recently published a paper describing the South African approach to durability design for concrete elements. According to the authors, in order for the South African concrete industry to address the need for appropriate performance indicators, it developed a Durability Index (DI). DI is based on the quality of the cover to reinforcement/surface layer and a series of index tests (to cover a range of durability problems) [15]. It is stated, that the index tests are to be used for quality control purposes [15]. Also in accordance with Alexander et al. [15], correlation between indexes and actual structural performance allows for the prediction of the performance of concrete in the design environment. Furthermore, the authors also discussed the possibility of implementing such approach in India, as both countries have a extensive coastline and a similar internal geography [15]. Currently, durability of concrete in India is limited to the requirements of IS 456 [16], which prescribes a minimum grade of concrete strength, maximum water/cement ratio and minimum cement content for the different types of exposures. In Japan, Tomosawa [17] assessed Japan’s approach to concrete durability. According to the author, while issues remain unsolved the approach for durability of concrete has significantly improved over the past 20 years. In 1997 JASS 5 [18] was revised to account for global environmental issues, such as global warming, waste disposal and natural resources. This code is performance-based with compressive strength (assigned as durability design strength) being the main performance requirement. Furthermore, Tomosawa commented on the durability recommendations provided by the Architectural Institute of Japan in 2004 [19], whose recommendations established design and maintenance limit states as the criteria aiming to retain the required performance of a concrete asset throughout a defined period. Carbonation, salt attack, frost attack, alkali-silica reaction, and chemical attack are considered as key factors for deterioration, and deterioration prediction models are presented for carbonation, salt attack and frost attack [17, 19]. In China, Li et al. [20] provided a detailed review of the Chinese national guide for the durability design of concrete assets – CCES01-2004. This guide also specifies durability requirements for different types of environment. Requirements include binder type, binder content, water/binder ratio, curing condition, concrete strength, concrete cover, and crack control. Li et al. concluded that the requirements of CCES01 are generally at the same level with or stricter than codes such as EN 206-1:2000 [21] . In Europe, EN 206-1:2000 [21], like the standards described above, does not specify the need for maintenance requirements nor does it provide guidance on how to structure a durability process. Rather it details requirements for concrete and performance tests. This standard has a number of exposure classes for concrete elements varying from ‘X0’ – no risk of corrosion attack to XA1, XA2 and XA3 – chemical attack. Limiting values for the different exposure classes to avoid chemical
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attack from aggressive substances in soil and groundwater are provided. ISO 9223 and 9224 provide guidance on how to assess atmospheric corrosivity categories as well as provide ranges of corrosion rate after one year (ISO 9223) as well as 10 years and steady state (ISO 9224). On a broader level, AS 5104 (ISO 2394) [22] does not relate to any specific material but for assets in general as it provides principles to design for their reliability. As well as composition, properties and performance of the materials, AS 5104 also considers that to achieve an adequate durable asset the following must be considered: its intend use, required performance criteria, expected environmental condition, structural system, shape of members and structural detailing, quality of workmanship and level of control, protective measures and maintenance during the design life. More recently, ISO 13823 [23] recommends the use of limit state methods for the design and verification of assets for durability. It provides strategies for durability design, such as the development of maintenance/repair/replacement plan for the asset during the design phase, giving examples of how to structure procedures and communications to ensure durability is achieved. This document is discussed further in the next section. 3. TOWARDS A COMMON DURABILTY DESIGN APPROACH 3.1 Durability throughout an asset’s life-cycle As mentioned previously, durability often seems to be associated with the design phase of a project and at times even simply confined to the production of a plan or report. While the authors certainly agree that the preparation of a Durability Management Plan (DMP) is an essential step early in the project, the following are also critical to achieving the asset owner’s requirements:
Durability is embedded into the detailed design process, ensuring that all designs are compliant with the DMP and satisfy the design lives specified in the project scope and requirements. The assets are constructed in compliance and to achieve the targets set by the DMP and design packages. The materials in situ achieve the levels of quality and consistency expected by the designers when formulating the durability design. The assets are inspected and maintained in line with the requirements of the design so that they achieve their designated service life.
Durability processes need to be designed into each phase of the asset life-cycle and the specific performance requirements for each phase need to be clearly defined. Although the information that is needed in each phase of the life-cycle is different, the approach must be consistent to optimise the durability and satisfy the design life requirements. To ensure consistency in approach, an overall plan needs to be developed early in the project to map the overall durability process, the type of input information necessary to develop the durability requirements for each phase and the output or deliverable (including its timing) that will be generated. With correct planning the deliverable from the previous phase will form the input for the next phase. From the start of the project it is necessary to have well-defined design life requirement and a proposed maintenance strategy for the asset(s). In many circumstances the design life requirement will be nominated in bid documents, but the maintenance strategy will not always be as well defined. As mentioned previously, it needs to be determined whether assets/structures will be maintained and rehabilitated during their design life or will negligible deterioration and subsequent intervention be accepted. Where the design life is not specified by the client, several international standards, namely ISO 15686.1 & 2 [24, 25] as well as AS 5104 (ISO 2394) can be used for guidance on the appropriate design life. Once the design life and the maintenance strategy are specified, the durability process can be formulated or defined. It is important that the assumptions that are used in developing the durability process are clearly stated to provide transparency and simplify verification of the processes and outputs. During the preliminary design phase the general approach to durability is proposed and preliminary materials guidelines are developed. Parallel to this process, a basic asset hierarchy structure, which will later be used to create the asset register, needs to be developed into a working model that incorporates the asset and sub asset items in a logical manner. The asset hierarchy structure outlines the relationship between the broad asset category and the individual items that comprise the asset. The hierarchy can also set out how unique identifying tag numbers can later be generated for each asset item or sub asset item. These steps become the building blocks for an asset register that contains detailed durability information. Early in the detailed design phase a more comprehensive DMP needs to be produced, which defines clearly all the exposure conditions to which the materials will be subjected. Additionally, this document presents material selection guidelines with detailed information enabling the identification of durability risks that are based on the likelihood of material degradation and its consequence. The minimum durability requirements for each material in each distinct environmental exposure are specified, to minimise the impact of materials degradation to the extent required by the selected maintenance strategy for the nominated design life. As the detailed design phase progresses, interdisciplinary durability reviews are undertaken which includes considering the suitability of alternative materials. The durability team needs to be well embedded into a project during the
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detailed design phase to provide timely advice to the designers in an attempt to minimise any delays if construction has started. All these rapidly developing designs and material durability data should be captured in the evolving asset register that is progressively developed (based on the asset hierarchy structure produced in the preliminary design phase). The asset register should include the following as a minimum for each asset element: type, asset item/asset sub-item, design life, materials of construction, environmental exposure conditions, durability risk, durability issues and minimum durability requirements, unique tag number and location of the asset. In addition to the development of the asset register, construction repair procedures should be developed during this phase so the documents are available before they are needed. Towards the end of the detailed design phase the preliminary requirements for post construction inspection and repair procedures should be nominated. During the construction phase the durability team needs to respond quickly to requests for additional information, provide advice on repairing construction defects to minimise changes to proposed frequency of maintenance and long term impact on durability. Construction repair procedures may need to be modified to deal with specific site issues. The asset register needs to be updated progressively with as-built information incorporating changes to the durability of the asset item resulting from change of materials or damage and repair of construction defects. The post construction inspection and repair procedures should be fully developed during this phase and the procedures should be linked into the asset register. The nominated inspection and maintenance frequency should also be clearly nominated in the asset register. In the operations and maintenance phase durability information is needed to assess the current condition of the asset, estimate its remaining life and predict its long term durability performance. The asset register is constantly updated as condition assessments and maintenance are undertaken and more up to date information obtained. Durability requirements and deliverables for each phase during the life-cycle are summarised in Table 1.
Table 1: Summary of durability in each phase of the life-cycle Phase
Required inputs for durability deliverable
Durability deliverable
Durability information required by other disciplines
Preliminary design – concept or BID design
Preliminary environmental exposure information (e.g. preliminary soil data, process fluids information). Preliminary design life. Proposed maintenance strategy. Preliminary asset information (types and numbers of assets).
Preliminary DMP. Identify any key potential project risks in terms of materials/durability with regards to design/construction; durability compliance and/or delivery lead times. Asset hierarchy structure.
Preliminary guidelines for materials selection and Materials/Durability Risk register/mitigation;
Detailed design
Detailed soil testing data (soil type, composition/aggressive elements, permeability, groundwater composition, etc.). Proposed construction method (e.g. cast in-situ vs. precast concrete).
Detailed design DMP (due early in the phase). Design support (e.g. durability memos, review of design packages). Preliminary asset register. Preliminary requirements for post construction inspection and repair procedures. Construction repair procedures.
Detailed minimum durability requirements and materials selection guidelines.
Construction
Detailed design DMP. Design support durability memos. Preliminary asset register.
Durability responses to construction requests for information (RFIs) and non-conformance reports (NCRs). Procurement support and assessment of vendor durability data. Repair procedures. Inspection and maintenance procedures. As-built asset register.
Equivalent durability of alternative materials. Impact of construction damage on durability. Repair strategies and suitable repair materials.
Operations and maintenance
Maintenance inspection and repair procedures. As-built asset register.
Living asset register. Review of inspection and maintenance procedures.
Predicted rates of deterioration. Likely deterioration mechanisms. Consequences of materials degradation. Condition assessment guidelines.
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3.2 Understanding and using key durability standards Among the various standards and codes around the world, ISO 13823 (General principles on the design of structures for durability) and ISO 15686 (Service life planning) provide clear directions to guide the durability design process throughout the project life-cycle. ISO 13823 refers directly to ISO 15686-1 as the latter provides an overall framework and design procedure for service-life planning. Note that while AS 5104 (General principles on reliability for structures) deals with durability, it is primarily in relation to deterioration caused by actions such as gravity, wind, snow, and earthquake, rather than deterioration of material resulting from environmental exposure or action effects. This section combines an overview of ISO 13823 and, wherever applicable, ISO 15686, as well as the authors’ comments on these standards and their application in durability design. The scope of ISO 13823 “specifies general principles and recommends procedures for the verification of the durability of structures subject to known or foreseeable environmental actions, including mechanical actions, causing material degradation leading to failure of performance”. The definition of durability used in this standard aligns with that adopted across many standards, codes and guidelines, namely the “capability of a structure or any component to satisfy, with planned maintenance, the design performance requirements over a specified period of time under the influence of the environmental actions, or as a result of a self-ageing process”. ISO 13823 can be used for the structural and non-structural elements of both new and existing assets as the durability process is similar in both cases, as previously shown [26]. The key difference is the possibility for an existing asset to collect historical data, which can be used to better estimate the remaining life and thus the need to undertake remediation. ISO 15686 Part 7 [27] provides guidance on the use of information collected during performance assessments of an existing asset to estimate a service life. Note that ISO 13822 [28] is also a very useful document that specifies how to assess existing assets taking into account their reliability and the consequences of failure. At its core ISO 13823 proposes a limit-state approach to design for durability that can be summarised as follows:
Determination of the structure environment, which is defined as “external or internal influences (e.g. rain, UV, humidity, soil constituents) on a structure that can lead to an environmental action”. Examples of environments and agents are provided in Appendix B of this standard. Identification of the transfer mechanisms, which is defined as a mechanism which promotes or prevent transfer of environmental influences into agents resulting in environmental action. Transfer mechanisms are listed in Appendix C and include direct exposure, condensation, diffusion etc. Assessment of the environmental action, which is the “chemical, electrochemical, biological, physical and/or mechanical action causing material degradation of a component”. Environmental actions for structural materials and their control are provided in Appendix D and include corrosion (of metals), sulphate attack (of concrete) and chemical attack (of GRP and plastics) for instance. Based on the action effects on a component of a structure, which “include damage, loss of resistance, internal force/stress or change in appearance due to material deterioration, or displacement due to material deformation”, two limit states can be considered: o An ultimate limit state when the resistance of the structure or its components become equal or greater than what it can withstand. o A serviceability limit state when local damage or displacement affects the function or appearance of the structure or its components. Taking the above into account, durability requirements can be proposed to ensure that the structures and their components achieve their required performance over their design lives with “sufficient reliability”. ISO 13823 provides guidance on service-life predictions based on data/experience/tests, probabilistic approach using a limitstates methods or mathematical modelling. More details and guidance on service-life modelling is provided in ISO 15686-2.
Two durability examples that follow the process set out in ISO 13823 are presented in Table 2. ISO 15686-1 notes that the process of service life planning such as that listed above may need to be re-iterated a number of times in order to find the most appropriate and cost effective way to achieve the performance and maintenance requirements. Both ISO 13823 and ISO 15686 make a clear difference between the design life of an asset (the “specified period of time for which a structure or a component is to be used for its intended purpose without major repair being necessary”) and its service life (“actual period of time during which a structure or any of its components satisfy the design performance requirements without unforeseen major repair”). As mentioned previously the levels and/or possibility of maintenance activities to be undertaken need to be carefully considered to determine whether greater inherent durability or a more comprehensive maintenance program is required. Interestingly, ISO 15686-8 [29] proposes a factor method as a way to empirically estimate the service life of an element based on available information.
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Table 2: Examples of the application of the approach described in ISO 13823 Examples of durability process Process steps
Concrete slab in a contact with ground
Steel handrail on a balcony
Location
Outside
Outside – atmosphere
Influences
Water, soil constituents, spills/leaks
Rain, air constituents, contaminants, pollutants, temperature and humidity
Agents (Cause environmental action)
Sulphates, chlorides, acids, other chemicals from spills/leaks
Moisture, oxygen, acid
Transfer mechanisms
Direct exposure, capillarity/surface tension, diffusion
Condensation
Environmental action
Sulphate attack/chloride attack
Corrosion in atmospheric environment
Action effect
Expansion followed by disintegration/cracking and delamination
Failure, change in appearance, damage due to corrosion product expansion
Durability requirements
Design concrete characteristics (mix design) to resist attack and/or isolate from the environment
Drainage (avoid water traps), protective coatings
Structure environment
It has been the authors’ experience that the terms “design life” and “service life” are sometimes used as if interchangeable, i.e. that an asset could be designed so that it becomes completely unserviceable when it reaches its design life. The aim of adopting a durability approach is that the design life would be lower than the expected service life. This ensures that at the end of the design life an asset owner has adequate time to undertake inspections, estimate the remaining life and possibly plan for an extension of service life. It also provides a factor of safety for unforeseen increases in the aggressivity of an environment. As stated in ISO 13823 “materials, components and design, including detailing and other reliable measures to lengthen the life, should be chosen so that the predicted service life, with a target probability of failure, exceeds the required design life”. While materials selection typically focuses on minimising the likelihood of unacceptable material deterioration within the design life, it has to be also influenced by the consequence of any failure. ISO 13823 proposes four categories ranging from minor and repairable damage without injuries to people (1), to loss of human life or serious injuries or considerable economic, social or environmental consequences (4). An asset failure and its consequence relates to the level of service or reliability that the asset owner deems acceptable to comply with its obligations. The concept of risk management should be central to durability design and as such durability risks need to be identified as part of the process. This is done by assessing the likelihood of damage or failure of material/treatment options (from rare to almost certain) and understanding from the designer/constructor/operator/owner its consequences. In addition to the four categories proposed in ISO 13823 a fifth entitled “negligible” could be introduced to produce a symmetrical matrix that aligns with that presented in Table 6.6. of HB 436:2004 (Risk Management Guidelines - Companion to AS/NZS 4360) [30] as shown in Table 3. Table 3: Proposed durability risk matrix based on HB 436:2004 (AS/NZS 4360) Consequence of failure/damage
Likelihood of damage/failure
Negligible
Minor
Moderate
Major
Severe
Almost certain
Medium
High
High
Very High
Very High
Likely
Medium
Medium
High
High
Very High
Possible
Low
Medium
High
High
High
Unlikely
Low
Low
Medium
Medium
High
Rare
Low
Low
Medium
Medium
High
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When the durability risk is considered too high it could be reduced by either or a combination of the following:
Decreasing the likelihood of failure by proposing a more durable option (e.g. more corrosion-resistant metal, greater concrete cover to reinforcement, use of cathodic protection, more robust coating system). However, this has to be balanced by taking into account constructability and cost implications (i.e. can it be built, procured or even afforded). Putting in place mitigation strategies aimed at limiting the consequences of any damage/failure, e.g. having sufficient redundancy in the process equipment, restricting access. Tailoring the inspection and maintenance plan (and especially the frequency of these activities) to detect early signs of unacceptable deterioration in order to pro-actively plan for repair and/or replacement.
A similar approach can also be used when a number of materials options are available by rating (e.g. from 1 to 5) the risks associated with durability, constructability and cost, adding the figures and selecting the one with the lowest risk value. It is worth mentioning that if no major maintenance is allowed to achieve the required design life, any option requiring more than minor “refurbishment” should be eliminated. In Appendix E, ISO 13823 provides an example of procedures that can be used for ensuring durability throughout the project life-cycle (with references to the relevant parts of ISO 15686) and who they should be communicated/worked with (owner/user, contractor, fabricator, supplier, designer, investigator). This example aligns with some of the comments made by the authors in the previous section and is summarised below:
Design phase: o Undertake durability assessment (design life, environment, deterioration mechanisms) and materials selection o Design access to allow for inspection, maintenance and repair as well as based on constructability considerations o Prepare life-cycle cost/assessment (as per ISO 15686-5 [31] and ISO 15686-6 [32]), if necessary, revise the design o Prepare plans for inspection, maintenance, repair and replacement as well for quality control during construction Construction phase: o Review design and incorporate acceptable changes, which need to then be inspected and approved o Review procurement information (this is not in ISO 13823 but in ISO 15686-9 [33]) o Mitigate the risk of damage to assets during construction Maintenance and operation: o Ensure that the environmental exposure does not adversely change during the design life o Implement inspection and maintenance plan (including cleaning, repair, replacement and monitoring) o If damages/defects are identified determine cause, record to provide feedback for future practice (as per ISO 15686-7).
4. PROPOSED TEMPLATES FOR KEY DURABILITY DEVILERABLES As shown in the previous section, ISO 13823 and ISO 15686 can be used and referred to when undertaking durability design as part of service-life planning. Based on these standards, templates for the key durability documents listed in Table 1 (i.e. the durability management plan and the asset register) are being proposed. As mentioned previously, design packages should clearly list out durability information (e.g. design life, materials, requirements) for each asset/sub-asset and their compliance with the DMP so they can be extracted and added to the asset register. The latter can then progressively updated with as-built information prior to handover to the asset owner and/or operator. 4.1 Durability Management Plan - Durability in Concept and Detailed Design It is suggested by the authors that a standardised durability management plan (DMP) contains the general categories of information listed below. It is proposed the main report contains information on the approach taken to durability and overviews of all the aspects. The detailed scientific and engineering information should be contained in discrete appendices where readers of the report can clearly find the required substantiations, justifications, assumptions and modelling. It is believed that a report template developed in this fashion will be more user friendly not only for the designer/engineer, who needs to include durability information in each design reports, but also for the design verifier/proof engineer, contractor, operator and asset owner alike.
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The proposed categories in the DMP are: Project overview and overall durability objective: this is a general section that describes the project and the importance given to achieving durable outcomes in the contract document presenting the scope/technical requirements. a)
Approach and scope: in this section the extent and methodology of the durability design can be provided. It could refer to the process described in ISO 13823.
b) Referenced documents: a table can present the list of standards, codes, guides as well as project-specific documents upon which the durability design in the DMP is based. c)
Terminology: it is important to provide definitions for technical names, abbreviations and symbols (if used) used in the DMP. Sections 3 of ISO 13823 and ISO 15686 provide good references for definitions.
d) General overview of assets: in the concept DMP this section may only provide a basic list of assets and sub-assets while the DMP prepared at the start of detailed design may present a more detailed asset hierarchy. However, the development of a full asset register should not be in the scope of the DMP. e)
Overview of environmental exposure categories for the assets: This section can present the structure environment (location, influences and agents), possibly in a table format and in accordance (in particular the terminology) with ISO 13823 (and its Appendix B). For large projects this section could be split into geographical areas. Detailed information and pertinent test data should be provided in Appendix A of the DMP.
f)
Overview of deterioration mechanisms for construction materials in each exposure: This section focuses on the transfer mechanisms, environmental actions and action effects such as those described in ISO 13823, and in particular, Appendices C and D of this standard. Detailed information regarding the mechanisms could be provided in Appendix B of the DMP.
g) Durability requirements: This section summarises the outcomes of the materials selection based on the assessment/prediction/modelling of future deterioration and the durability risk associated with material deterioration. The focus should be on assets critical to project operation or subject to high durability risk. More detailed information could be provided in Appendix C of the DMP. The approaches (e.g. based on standards, limit-state calculations) followed should be clearly stated with appropriate references to documents as well as to the relevant appendices of the DMP, which provide greater details (see below). Appendix A – Environmental exposure classification
Nominate reference standards (e.g. ISO 13823, AS 3600, AS 2159, AS 5100.5, AS/NZS 2312, ISO 9223 and ISO 9224) Define how the site specific test data for the location such as prevailing wind and temperature trends are used to classify the various exposure conditions (including influences and agents as per ISO 13823): atmospheric, buried, immersed or tidal/splash. Include tables of relevant data
Appendix B – Materials degradation mechanisms
Provide technical information about the predicted deterioration mechanisms for the materials in each exposure category
Appendix C – Durability requirements
Provide greater details of durability requirements for all asset types (including those with low and medium durability risks) List the methods employed to produce the requirements and make adequate references to the appendices that specifically address them (see suggestions below)
Appendix D – Durability risk
Assess the likelihood of deterioration of an asset item Conduct a consequence rating assessment of deterioration based on the effect it would have on the operational capacity or performance of the asset (to be undertaken in co-ordination with the operations team and/or asset owner if possible) List the risk mitigation strategies proposed and provide technical information about those selected for us on the project (e.g. corrosion control measures)
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Appendix E – Predictive modelling / limit-state calculations
Explain the models used to predict the life of concrete assets in specific environmental exposures Outline the limitations of the models and define the assumptions used in process Provide limit-state calculations and make references to relevant standards
4.2 Asset register As mentioned above the asset register can be created at the design phase and progressively updated with the durability information extracted from the design packages, which would typically be reviewed by the durability team. In these packages such information could be presented in simple tables that demonstrate compliance with the DMP either by direct reference to a specific section (i.e. the type of asset in the particular environmental exposure is already presented in the DMP) or by the provision of detailed requirements to ensure that the design life can be achieved. Each asset register will be unique and the information contained in the register will depend on the number of fields that can exist in the asset management system for each asset, and specific contractual requirements from the asset owner. While it is not considered feasible to provide a typical template for the asset register, the list of key durability-related parameters should include the following: unique tag number, duty description, design life, materials of construction, environmental exposure, durability risk, durability issues (deterioration mechanisms), minimum durability requirements, location of asset, manufacturer (if relevant), drawing references, links to pertinent inspection and repair procedures, inspection and maintenance cycle frequencies and condition rating. The inspection procedures nominated in the asset register need to be developed to ensure that the following information is available during the service life of the asset: any safety issue (especially if relating to materials deterioration such as concrete spalling over pedestrian walkways), the current condition rating of the asset and any required repairs. In terms of maintenance activities, while the register would provide a schedule for minor maintenance (e.g. cleaning of stainless steel items), the timing for any major repair would depend on the overall maintenance strategy (and especially the intervention levels set for the project) and the evolution of the condition rating. 5. CONCLUSIONS This paper was borne out of the interest and experience of the authors in durability design. In particular not having one Australian standard specifically providing guidance on the process of designing for durability led to a review of relevant documents around the world. While, like Australia, a number of countries have codes or standards that are performance-based, prescriptive and/or have minimum requirements, ISO 13823 and ISO 15686 are focused on the actual processes of durability design and service-life planning. These documents also stress the importance of embedding durability throughout the entire life-cycle of an asset. In order to make the durability design process more consistent and transparent not only for practitioners but also for designers, construction teams, operators and asset owners alike, the authors have proposed a template for the preparation of a durability management plan, which draws from the guidelines of ISO 13823. Equally important is the compilation of an accurate asset register that adequately present the durability information including the type and frequency of inspection and maintenance activities that are necessary for the required design life to be achieved. 6. ACKNOWLEDGEMENTS The authors would like to acknowledge the works of Dr Frank Collins and Dr Marita Berndt in the field of durability planning. We would like to also thank Miles Dacre for his valued feedback and comments. 7. REFERENCES 1. Neville A (2001) Consideration of durability of concrete structures: Past, present and future. Materials and Structures 34: 114-118. 2.
Neville AM (1973) Properties of Concrete. 2nd.
3. AS 2309 - 2008: Durability of galvanized and electrogalvanized zinc coatings for the protection of steel in structural applications. 4.
AS 3600-2009: Concrete Structures.
5.
AS 5100.5-2004: Bridge design Part 5 Concrete.
6.
AS 3735-2001: Concrete structures retaining liquids.
7.
AS 2159-2009: Piling - Design and installation.
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8.
ISO 9223:1992 - Corrosion of metals and alloys -- Corrosivity of atmospheres -- Classification.
9.
AS 5604-2005 Timber - Natural durability ratings.
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RTA QA Specification B80 (2008) - Concrete Work for Bridges.
11.
VicRoads - Standard Specifications for Roadworks and Bridgeworks (2007) - Section 610.
12.
ABNT NBR 6118:2003 - Projeto de estruturas de concreto - Procedimento (in Brazilian Portuguese).
13.
NTC 5551 - 2007: Concretos. Durabilidad de estructuras de concreto (in Spanish).
14.
ACI 201.2R-01:2000 - Guide to Durable Concrete.
15. Alexander MG, Santhana M, Ballim Y (2010) Durability design and specification for concrete structures - the way forward. Int J Adv Eng Sci Appl Math 2: 95-105. 16.
IS 456:2000- Indian Code - Civil Engineering for RCC.
17. Tomosawa F (2009) Japan's experiences and standards on the durability problems of reinforced concrete structures. Int. J. Structural Engineering 1: 1-12. 18. Architectural Institute of Japan (1997) Japanese Architectural Standard Specification for Reinforced Concrete Work (JASS 5). 19. Architectural Institute of Japan (2004) Recommendations for Durability Design and Construction Practice of Reinforced Concrete (draft). 20. Li K, Chen Z, Lian H (2008) Concepts and requirements of durability design for concrete structures: an extensive review of CCES01. Materials and Structures 41: 717-731. 21.
EN 206-1:2000 Concrete Part 1: Specification, performance, production and conformity.
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AS 5104 - 2005 (ISO 2394:1998) General principles on reliability for structures.
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ISO 13823:2008 General principles on the design of structures for durability.
24. ISO 15686-1:2011 - Buildings and constructed assets - Service life planning - Part 1: General principles and framework. 25. ISO 15686-2:2001 - Buildings and constructed assets - Service life planning - Part 2: Service life prediction procedures. 26. Blin F, Law D, Dacre MC, op'tHoog C, Gray B, Newcombe R (2008) Extension of Design Life of Existing Marine Infrastructure - A Durability Perspective. ACA Conference 1-13. 27. ISO 15686-7:2006 - Buildings and constructed assets - Service life planning - Part 7: Performance evaluation for feedback of service life data from practice. 28.
ISO 13822:2010 - Bases for design of structures - Assessment of existing structures.
29. ISO 15686-8:2008 - Buildings and constructed assets - Service-life planning - Part 8: Reference service life and service-life estimation. 30.
HB 436:2004 (Guidelines to AS/NZS 4360:2004) - Risk Management Guidelines Companion to AS/NZS 4360:2004.
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ISO 15686-5:2008 - Buildings and constructed assets -- Service-life planning -- Part 5: Life-cycle costing.
32. ISO 15686-6:2004 - Buildings and constructed assets - Service life planning - Part 6: Procedures for considering environmental impacts. 33. ISO 15686-9:2008 - Buildings and constructed assets - Service-life planning - Part 9: Guidance on assessment of service-life data.
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8. AUTHOR DETAILS Frédéric Blin is a Principal Engineer in the Advanced Materials Group at AECOM. He holds a PhD on corrosion inhibitors and has worked on numerous projects, including the condition assessment of different types of structures exposed to various environments, non-destructive testing, crack and corrosion monitoring, survey of compliance with Australian Standards, review, and advice on durability issues, technical specification for infrastructure repair woks, modelling and prediction of future deterioration. He has also managed several projects in the field of civil and transport, especially maritime, infrastructure, and has authored and coauthored a number of publications, technical papers and technical reports. Sarah Furman is a Principal Engineer in the Advanced Materials Group at AECOM. She has a Master of Science in Corrosion Science and Engineering from UMIST in England. A materials and corrosion specialist with a broad knowledge of both metallic and non-metallic materials, she specialises in durability planning for new infrastructure, performance assessments of materials, materials selection, failure analysis, and cathodic protection design.
Alessandra Mendes is a Senior Engineer in the Advanced Materials Group at AECOM. She holds a PhD on fire resistance of concrete and has worked on numerous projects including condition assessment of concrete assets, durability advice and review from design phase through maintenance phase (including inspection and maintenance planning), technical specification for infrastructure repair woks, modelling, and prediction of future deterioration.
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