Reinforced concrete is a very strong building material and is basically concrete strengthened with steel bars. Reinforced concrete will begin to slowly corrode when exposed to the natural environment. The following explains how this ‘carbonation’ happens and how it causes the steel reinforcement within the concrete to corrode or rust.Carbonation of concrete
Fresh concrete is highly alkaline because of hydration products such as calcium hydroxide. This environment protects the steel reinforcement bars from corrosion. However, carbon dioxide and moisture at the surface of the concrete can react with these products to produce calcium carbonate. Initially carbonation is restricted to a thin surface layer, but with time the carbon dioxide diffuses inward from the surface and the zone of carbonation extends gradually into the concrete.
The presence of pores and capillaries in the concrete facilitates the progress of the carbonation ‘front’. Good quality, dense concrete delays the development of carbonation and slows down the first stage of the corrosion process. Obviously the deeper the layer of concrete over the steel reinforcement the better protected it will be. This protective layer of concrete is called the ‘cover’.
Carbonation of the concrete does not in itself lead to a loss of structural adequacy, but when the zone of carbonation extends to the steel the protective action of the concrete is largely lost, primarily because the calcium carbonate has a reduced PH value. This then allows positive Fe+2 ions to form which join with dissolved oxygen in water to produce Fe2O3 (iron oxide or rust) and the process of corrosion of the reinforcing steel commences. The process of corrosion is greatly accelerated if chloride ions (from salt) are present. The chloride is not used in the chemical reaction to form rust but seems to assist in the formation of anodic and cathode regions in the metal.
The paradox of cover to steel in very aggressive environments
Large cover depths (50–75 mm) are required to protect reinforcement against corrosion in very aggressive environments, but thick cover leads to increased crack widths in flexural reinforced concrete members. Larger crack-widths (greater than 0.3 mm) permit ingress of moisture, resulting in possible corrosion of reinforcement and deterioration of concrete. Therefore, thick covers defeat the very purpose for which it is provided. There is a need for a balance of cover depth and crack width requirements.