Studies of chloride binding in concrete exposed in a marine environment

Abstract

The concentrations of “free” chloride and hydroxide ions in extracted pore solution from concrete exposed and submerged in a marine field station were studied by the pore solution expression method. In addition, the corresponding concentrations of total acid soluble chloride in the concrete was analyzed. The relationship between total and free chlorides was analyzed and compared with similar data from laboratory-exposed cement paste and concrete. Hydroxide ions were found to be transported away from the concrete at a rate similar to the penetration rate of chloride ions into the concrete. The amount of bound chlorides was found to increase as the concentration of hydroxide ions in the pore solution decreases. As a consequence, the relationship between free and total chlorides in concrete with a chloride and hydroxide ion gradient was found to be almost linear. It was suggested that the nonlinear chloride binding relationship observed in laboratory equilibrium tests is not relevant for submerged concrete with diffusion gradients of chloride and hydroxide. However, only limited information exists on the long-term chloride binding relationship reflecting the long-term situation when all alkali hydroxides have been leached to the sea. It was speculated that the chloride binding and the transport rate depend on the available amount of mobile alkali hydroxide and thus on the thickness of the concrete member.