Protection of steel reinforcement for concrete - A review

Premature failure of reinforced concrete structures occurs primarily due to early corrosion of steel reinforcement These reinforcements are prevented from corrosion by the formation of a passive oxide film in the high pH environment of concrete. However, pH decreases in actual conditions due to the ingress of harmful ions like carbon dioxide, sulphur dioxide, chloride, etc., leading to localised corrosion. Of these, chloride ions are considered to be the most harmful, which may come from the deliberate additions of calcium chloride in the cement as accelerator or from the environment (marine/coastal regions, de-icing salts). The corrosion products thus formed, being more voluminous, result in expanded pressure in the surrounding concrete, leading to cracking of the concrete. The formation of such cracks accelerates further corrosion. In view of the intensity of the problem, serious efforts have been made to prevent corrosion of reinforcement. These have led to the development of varying types of prevention techniques/measures including coatings, electrochemical techniques like cathodic and anodic protection, inhibitors, control of concrete mix and reinforcement with superior corrosion resistance. These preventive techniques have inherent advantages and disadvantages with respect to the type of structure and the surrounding environment. It is generally agreed that good concrete cover in terms of quality of ingredients, mix proportion, water-cement ratio, etc., has no substitute for the prevention of reinforcement corrosion. However, corrosion takes place due to the presence of fine cracks in the concrete assisting in the ingress of harmful ions. This necessitates an alternative preventive technique/measure which can be selected based on the type of structure to be protected, life expectancy, nature of environment, and cost of application. The present review provides an understanding of the process of reinforcement corrosion, its mechanism, different protection techniques being adopted or under development and their comparative analysis.