Chloride ion content and chloride contamination of concrete

Coastal and marine structures, and structures subjected to de-icing salts such as car parks and road bridges, can suffer ingress of chloride ions. When dissolved in water sodium chloride forms a highly corrosive solution of sodium ions Na+ and chloride ions Cl-. The chloride ions can diffuse through concrete pores in solution and attack the passive layer protecting the steel reinforcement leading to corrosion.

In chloride contaminated concrete, the chloride concentration is determined at various depths in order to determine the likelihood of corrosion of the reinforcing steel. The chloride profile of the concrete is determined by taking dust samples from increments of depth, 5 to 25 mm, 25 to 50 mm, 50 to 75 mm etc. The first 5 mm of drilled concrete is normally discarded as being non-representative. Sample increments should relate to the determined location of the steel reinforcement. The analysis of concrete for chloride ion level is covered by BS EN 14629, Products and systems for the protection and repair of concrete structures. Test methods determination of chloride content in hardened concrete.

Having established the chloride concentration profile at a given age of the structure, the time taken for the concentration of chlorides at the level of the reinforcement to become critical can be estimated. This calculation of chloride ion diffusion is based on Fick’s second law of diffusion.