Durability Concerns for Different Clear Covers Provided to Reinforcement in Concrete Structures When Considering Different Chloride Exposure Conditions for External and Internal Faces of Members

Many structures such as buildings, power plants, stacks, sea walls, bunkers, and tanks are built with reinforced concrete; and durability is a major concern. Corrosion of reinforcement is the most important process of deterioration. Reinforcement corrosion is prevented by maintaining appropriate quality of concrete mix, casting, and curing and by providing the requisite clear cover to reinforcement according to the postulated exposure. The provisions of several international codes were reviewed from these aspects. Many codes can be interpreted in favor of providing different clear covers on different faces of members in a reinforced concrete structure. The authors’ interpretation of the Australian and New Zealand codes revealed that these specifically allow the above. However, this practice might lead to severe durability-related concerns, particularly due to corrosion of reinforcement. Case studies conducted with postulated data from a few international codes revealed that internal corrosion cracks might occur earlier than external cracks in certain cases. Internal cracks can be hidden from view and consequently lead to higher damage to the structure prior to detection. For the simultaneous occurrence of corrosion cracks on the internal and external faces of a member, the internal surface chloride concentration would have to be between 18% and 73.2% of the external, with a median of 38%. These could hardly occur in practice—except for treated air facilities. If construction periods are long, this durability issue would be applicable even for treated air facilities. Higher differences in clear cover between internal and external faces, and higher external chloride exposure – both call for larger reduction of chloride exposure from external to internal. Unless the members are designed for corrosion as specified in some (Japanese) codes, the authors advocate the conservative option of using the same cover for all faces of concrete members to achieve a lower life cycle cost of the facility.

Design codes across the world can be interpreted in favor of different clear covers (to reinforcement) on different faces of concrete members according to exposure conditions. Only the Australian and New Zealand codes specifically mention different clear covers for internal and external faces, by the authors’ interpretation. This article examines the durability of concrete members with different clear covers on internal and external faces, with case studies generated from a few international codes. The results indicate that in such cases, corrosion cracks might occur on internal surfaces much earlier than the external surfaces. For cracks to occur simultaneously on both surfaces, the internal surface chloride concentration must be as low as 18% of the external, which is difficult to ensure in practice. Early internal cracks could be hidden from view on account of equipment or false ceiling/panels—thereby causing more damage before detection. They would be difficult to detect, to evaluate, and to carry out structural repairs. Therefore, it could be expensive to maintain and could adversely affect the service life of the structure. This study would sensitize the clients, regulators, and engineers to these concerns. The authors suggest that the conservative approach of the same clear cover may be adopted on all faces, according to the worst exposure on any face, to ensure the durability of the concrete structures.