14 - Specification and achievement of cover to reinforcement

Abstract

This chapter examines the effect of non-achievement of specified cover to reinforcement on the properties of concrete structures, considers the factors influencing practical achievement of cover. Failure to achieve the specified concrete cover to steel reinforcement is probably the greatest single factor influencing the premature deterioration of reinforced concrete. Indeed, it can be stated that the assured provision of an adequate thickness of concrete of appropriate quality, properly compacted and cured, results in a dramatic reduction in the very large sum of money spent annually, worldwide, on repair and premature replacement of concrete structures. Reinforcement in concrete is vulnerable to corrosion if the protective alkaline environment is removed by carbonation of the concrete or if a critical level of chloride ion is allowed to build up. The protective capacity of a given concrete against carbonation or chloride ingress is broadly related to the mathematical square of the cover. So, it can be seen that durability performance can be highly sensitive to deficiencies in cover. Thus an actual cover of half the specified value could lead to a reduction in the time to initiation of corrosion of approximately 75 per cent.