Chloride Attack on Concrete Structures:
Chloride attack is one of the most important aspects for consideration when we deal with the durability of concrete. Chloride attack is particularly important because it primarily causes corrosion of reinforcement. Statistics have indicated that over 40 per cent of failure of structures is due to corrosion of reinforcement.
Due to high alkalinity of concrete a protective oxide film is present on the surface of steel reinforcement. The protective passivity layer can be lost due to carbonation. This protective layer also can be lost due to the presence of chloride in the presence of water and oxygen. In reality the action of chloride in inducing corrosion of reinforcement is more serious than any other reasons. One may understand that Sulphates attack the concrete whereas the chloride attacks steel reinforcements.
Chloride enters the concrete from cement, water, and aggregate and sometimes from admixtures. The present day admixtures are generally containing negligible quantity of chloride or what they call chloride free. Chloride can enter the concrete by diffusion from environment. The Bureau of Indian Standard earlier specified the maximum chloride content in cement as 0.05 percent. But it is now increased the allowable chloride content in cement to 0.1 per cent. I S 456 of 2000 limits the chloride content as (Cl) in the concrete at the time of placing is shown in table-1.
Table-1: Limits of Chloride Content of Concrete (IS 456 of 2000)
Type of Use of Concrete
Maximum Total acid soluble chloride content. Expressed as
kg/m3 of concrete
Concrete containing metal and steam cured at elevated temperature and prestressed concrete.
Reinforced concrete or plain concrete containing embedded metal.
Concrete not containing embedded metal or any material requiring protection from chloride.
The amount of chloride required for initiating corrosion is partly dependent on the pH value of the pore water in concrete. At a pH value less than 11.5 corrosion may occur without the presence of chloride. At pH value greater than 11.5 a good amount of chloride is required.