CAUSES CAUSES OF CONCRE TE DETERI ORATION DESI DESI GN AND AND CO NSTR NSTR UCTIO N DEF DEF ECT S REHABILI REHABILI TATION OF STRUCTURAL STRUCTURAL CONCRETE
FOR PROF. K. A. SOUDKI SUBMITTED BY UMAR RIZWAN
Introduction Concrete is a very versatile material that can be cast in place with or without reinforcement, or precast or prestressed for achieving any required strength. At the same time it requires requires a proper understanding understanding of the behaviour and interaction interaction of its constituents. Any negligence in any of the phases of its design, manufacture, placement placement and maintenance maintenance would lead to its deterioration and ultimately failure to perform its intended function. There can be an enormous number of causes that can lead to the d istress istress and deterioration deterioration of concrete. Some of o f them are: Accidental loading, Chemical reactions (acid attack, aggressive water attack, alkalicarbonate reaction, alkali silica reaction, sulphate attack and miscellaneous chemical attacks), construction errors, corrosion, design errors (inadequate structural design and poor construction detailing), erosion (abrasion, cavitation), freezing and thawing, settlement and movement, shrinkage (plastic shrinkage and drying shrinkage), temperature temperature changes, fire and weathering. In this report only the design and construction construction defects leading to concrete deterioration deterioration and failure will be discussed. Two case histories are also presented in order to e laborate laborate these issues.
Construction defects defects Poor construction practices practices and negligen ne gligence ce can cause defects that lead to the cracking of concrete and further lead to start of concrete deterioration. deterioration. The construction defects can range from improper mixing, placing and curing to inadequate design and a premature removal of shoring. The construction defects are more likely to occur in new constructions as well as in repair and rehabilitation projects. Therefore a thorough understanding understanding of these defects along with wit h an aggressive aggressive inspection program is invaluable invaluable for avoiding serious deterioration problems.
Scaling, crazing crazing and dustin g of concr ete If additional water is added at the time of delivery of concrete in order to increase the workability of concrete it causes an increase in the water cement ratio of the concrete leading to a reduction in strength and durability of concrete. Which can be manifested in surface defects like scaling, crazing and dusting.
Improper alignment of formwork Improper alignment of formwork leads to discontinuities in the concrete surface, which can lead to the cavitation (erosion) if the concrete is exposed to high velocity water.
Improper consolidation If there were any air or water bubbles present at the surface of the formwork then bugholes would be caused. Similarly if the vibrator is not properly used during construction then honeycombing will also appear as shown in Figure 1. There may be some big holes in the finished structure if the concrete is not properly consolidated as shown in Figure 2. Cold joints are joints or discontinuities that may result from a delay in placement if sufficient time has elapsed to preclude the union of the material in two successive lifts as illustrated in Figure 2.
Bug holes Honeycombing Cold joint
Movement of for mwork When the concrete is setting i.e. hardening the movement of the formwork can cause cracking in concrete which may later become a cause for the deterioration of concrete.
Improper lo cation of r einforcing steel The improper placement of steel in concrete may cause the steel to be ineffective or structurally inadequate. It can also cause durability problem for the concrete, as the cover requirements may be violated and eventually lead to the corrosion of steel.
Figure. 3 Improper placement of reinforcing steel, From [2]
Premature removal of sh ores or r eshores If the shores are prematurely removed then the concrete may not have attained its desired strength, which can at the least cause cracking in the structure due to overstressing and can even cause major failures as mentioned in the case studies.
Improper curing Curing is the most important and unfortunately the most abused aspect of construction process. If concrete is not allowed to cure at proper humidity and temperature it will not be able to achieve the desired strength. The improper curing is manifested in cracking and surface disintegration.
Settlin g of the subgrade If the subgrade is not properly prepared and there is any settlement in the period when the concrete has not gained enough strength then it can cause severe cracking in the concrete, which would further accelerate its deterioration.
Design Defects The design errors can be broadly categorised into two types: a. Inadequate structural design b. Poor design detailing
Inadequate structu ral design The failure mechanism in this case is simple as the concrete is exposed to higher level of stresses than it is capable of carrying. These defects will be manifested in the concrete either by cracking or spalling. If the concrete experiences high compressive stresses then spalling will occur. Similarly if the concrete is exposed to high torsional or shearing stresses then spalling or cracking may occur and high tensile stresses will cause the concrete to crack. Such defects will be present in the areas where high stresses are expected. Through visual inspection it will be decided in order to proceed for a detailed stress analysis. A thorough petrographic analysis and strength evaluation will be required if rehabilitation is considered to be necessary. Such design problems can be prevented with a careful review of the design calculations and detailing.
Poor design details An adequate design does not guarantee a satisfactory function without including design detailing. Detailing is an important component of a structural design. Poor detailing may or may not directly lead to a structural failure but it contributes to the deterioration of concrete. In order to fix a detailing defect it is necessary to correct the detailing and not to respond to the symptoms only. Some of the general design and detailing defects are highlighted here.
Abrupt changes in section Abrupt changes in cross section may cause stress concentration that would result in the cracking of the members.
Insufficient reinforcement at reentrant corners and openings If due care is not taken in providing proper detailing of reinforcement then it would lead to the cracking of concrete due to stress concentration.
Inadequate provision for deflection
If deflections in excess of the expected be there then it will cause cracking of the walls and partitions.
Inadequate provisions for drainage Poor attention to drainage details can cause impounding of the structure that would lead to the deterioration of the structure
Conclusions It can be concluded that design and construction defects at the least can cause minor cracking or spalling leading to concrete deterioration and may become a s ource of a major structural failure. Therefore a great deal of attention and care is required in designing, detailing and construction of a concrete structure.
References 1. ACI 224.1R-93. Causes, Evaluation and Repair of Crack in Concrete Structures 2. Emmons, P., 1993. Concrete Repair & Maintenance Illustrated . RS Means, Kingston (Mass). 3. Feld, Jacob, Carper, L. Kenneth, 1997. Construction failures, John Wiley & Sons, Inc. New York. 4. Kaminetzky, D., 1991. Design and construction failures, McGraw-Hill, Inc., NewYork. 5. US Army Corps of Engineers, 1995. Evaluation and Repair of Concrete Structures- Engineer Manual, EM1110-2-2-2002 (URL address: http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-2-2002 )
