Chlorides and concrete.
The most significant and common sources of chlorides are marine environments and from de-icing salts applied to road surfaces during cold weather (winter). Chlorides have little effect on hardened concrete but they increase the risk of reinforcement corrosion.
Corrosion of the reinforcement will be initiated when the chloride ion concentration at the steel reaches the so called ‘threshold level’. The subject of a safe level of chloride is somewhat controversial A fairly traditional view is that below 0.4% chloride by mass of cement represents a low corrosion risk, 0.4 to 1% a medium risk and above 1% a high risk. BRE Digest 444 gives a more comprehensive interpretation of all the factors involved in chloride-induced corrosion.
Measures for improving the resistance of the concrete to chloride ingress and approaches to modelling the rate of chloride ingress are given in Concrete Society Technical Report 61, Enhancing reinforced concrete durability