CHLORIDE INGRESS INTO CONCRETE

This article outlines a British cement association (bca) current research programme to examine the roles of cement type and mix in retarding penetration of chloride into concrete, thus helping to reduce risk of corrosion of reinforcement steel. The project will provide a check on the basic data used in British standard (BS) 8110 part 1 1985 for the formulation of limits on permitted chloride level in concrete where prestressing wire and reinforced steel are present. Three possible approaches to the investigation of the movement of chlorides into cement based materials are outlined: (1) a very thin concrete barrier between 2 reservoirs, one containing chloride and one without chloride, is monitored for rate of diffusion. This method is usually confined to cement pastes and mortars, and data production requires several weeks; (2) an external electrical field is applied across water-saturated cement paste or concrete barriers separating volumes of chloride-free and chloride-containing solutions. This technique has been adopted as a standard procedure by the American association of state highway and transport offices (aashto-t227); and (3) the bca research uses an approach which allows concrete specimens to absorb chloride from the surroundings; a chloride concentration profile is obtained, a diffusion constant is calculated from this profile and used to predict the future course of chloride ingress. Specimens are usually concrete cubes or prisms, but may be full sized concrete members. (TRRL)