In a concrete environment the steel reinforcement is chemically passivated by the alkalinity of the concrete cover, the rate of corrosion under ideal conditions is negligible, however due to the changes in the conditions around the steel reinforcement, the rates of corrosion can reach alarming proportions .
Steel reinforcements is often used in severe environments where soluble chlorides are present . Ingress of chloride in the concrete surrounding the reinforcements ( coastal environment or in regions where deicing salts are used ) disrupts the passive protective oxide layer causing the steel to rust and pit.
When Carbonation depth of the concrete reaches the reinforcements , it reduces the alkalinity of the steel environment, under these conditions the steel loses its passivity and rapidly corrodes .
Both the above deterioration causes are related to the quality of concrete / concrete practices, water cement ratio used and thus resultantly are governed by the permeability of the concrete .
At the Research & Development Division of The Associated Cement Companies Ltd, considerable amount of studies have been carried out on assessing the properties of the cement paste matrix with use of OPC & fly ash based blended cements and its influence on minimising the corrosion of the steel reinforcements. The studies include comparison of the alkalinity, permeability, degree of carbonation, sulphate resistance of the concrete and the corrosion characteristics of the steel reinforcements, in RCC made with OPC and fly ash based blended cements. These studies reveal that the Cement Paste Matrix achieved with the fly ash based cements show substantial improvements in the resistance of the concrete to the steel reinforcements.
The paper discusses in some details , the basic mechanism of reinforcement corrosion and the different avenues for minimizing the damage in concrete due to the steel corrosion . Based on the data generated at the Research & Consultancy Directorate the authors conclude that one of the most reliable and efficient option for minimizing the corrosion of steel reinforcement would be through use of an engineered Blended cements & concrete.
The paper also discusses the effects of use of certain corrosion inhibiting additives in the concrete, which could further help to minimize the reinforcement corrosion by enhancing the threshold limits of chloride ingress.