Making reinforced concrete immune from chloride corrosion

Chloride-induced corrosion is a major factor affecting the durability of reinforced-concrete structures. However, a high tolerance to chloride contamination has often been achieved under laboratory conditions. A significant factor affecting the chloride threshold level for corrosion initiation in concrete is the presence of defects, such as air voids, in the cement paste at the steel-concrete interface. To explain this observation it is noted that corrosion initiation and propagation involve the local production of acid at sites where the steel passive film breaks down in the otherwise highly alkaline concrete environment. Thus, solids with pH-dependent dissolution behaviour affect the corrosion process. Solid phase corrosion inhibitors would release hydroxyl ions to prevent this local reduction in pH. By reducing the entrapped air-void content at the steel to values below 0.2% it was possible to increase the chloride threshold level from 0.2% to above 2% chloride by weight of cement as the voids no longer dilute the inhibitive solid phases. A high tolerance to chloride contamination may also be achieved by reducing the electrolyte content of voids and by using electrochemical treatments to generate a reservoir of hydroxide-containing salts at the steel-concrete interface. Chloride contamination at levels above 2% chloride by weight of cement at a reasonable steel cover depth may not be possible in concrete exposed to many environments when the concentrating effects of evaporation are minimised. Thus, a chloride threshold level of 2% may effectively render steel immune to chloride-induced corrosion.