Corrosion in Reinforced Concrete
Corrosion in its broadest sense is the destruction of a substance or its properties because of a adverse reaction with its environment. Corrosion of steel is an electrochemical process similar to a flashlight battery. In order for corrosion to take place in reinforced concrete, there must be an anode where oxidation occurs, a cathode where reduction occurs, an electrical conductor and an aqueous medium. Both oxygen and moisture must be present in order for corrosion to occur.
The alkaline environment of concrete provides a natural degree of protection against corrosion. The concrete reacts with the steel to form a film that passivates and protects the steel. However, the intrusion of chlorides and other ions can undermine these protective qualities and can cause steel corrosion if oxygen and moisture are also present. Carbonation can also reduce the alkalinity of concrete thereby permitting corrosion to occur. Carbonation occurs when carbon dioxide from the air reacts with the concrete to reduce concrete’s pH. It is normally a slow process, but can be accelerated by concrete cracking or inadequate concrete cover.
Chloride ions are by far the most common contributor to premature corrosion in reinforced concrete structures. Chloride is common in nature and may be introduced in concrete through admixtures, deicing salts, industrial chemicals, seawater or contamination of concrete mix water or aggregates.
Non-uniformities in moisture, oxygen and chloride levels can create electrical potentials that drive the corrosion reaction. Significant gradients in these constituents can make reinforced concrete vulnerable to “macrocell” corrosion.
Cracks in concrete and galvanic effects due to contact with dissimilar metals can accelerate corrosion. Other factors that influence the rate of corrosion include concrete resistivity and permeability, temperature, and depth of cover over the reinforcement.
When steel corrodes it forms rust that occupies a volume much greater than the steel itself. This exerts large expansive stresses on the surrounding concrete. Because the concrete is low in tensile strength, these stresses can cause cracking and spalling, which, in turn, permits faster ingress of water, oxygen and chlorides, accelerating corrosion further. In addition, rust staining can be aesthetically displeasing and can detract from the beauty of a structure.
Corrosion may adversely affect the structural performance of reinforced concrete. Corrosion can reduce the cross-sectional area of the steel and can create local discontinuities in the steel surface, thus reducing the steel’s tensile strength and its resistance to fatigue stresses.