Chloride penetration in concrete subject to wet/dry cycling: influence of moisture content

Concrete structures suffer from corrosion if chloride ions in the environment penetrate to the depth of reinforcing steel; surfaces periodically wetted with chloride solution are at most risk, when chloride ions can enter concrete by combined diffusion and absorption. BS8500-1 recommended minimum cover requirements primarily assume that chloride ingress is diffusion controlled, and absorption is only partly accounted for, so cover values may be inadequate. This paper investigates factors that influence absorption of chloride ions into concrete and how this affects distribution of chloride at different depths from the surface. Concrete cubes (CEM I cement; water/cement ratio 0.45) are subjected to repeated wetting (with 50% saturated salt solution) and drying cycles, during which their mass is monitored. The effective porosity, weight and distance sorptivity and chloride penetration depths are calculated for a number of curing, conditioning and drying conditions. Samples are extracted from the cubes at a range of depths and analysed for chloride content. Results show that the quantity of chloride entering the concrete, and in particular surface chloride content, is very sensitive to effective porosity/drying conditions immediately before wetting; also as much as 31% of the protection provided by concrete cover can be lost after exposure to just one wet/dry cycle, thereby significantly reducing time to corrosion of concrete structures.