With hydration of cement the concrete is solidified and some calcium hydroxide is released which develop a protecting alkaline medium inhibiting galvanic cell action and the corrosion of steel is resisted. Later on, free hydroxide in concrete reacts with atmospheric carbon dioxide to produce calcium carbonate that leads to shrinkage cracks. This reaction is called carbonation of concrete and it reduces alkalinity of concrete as well as its strength as a protecting medium for reinforcement.

In high-quality solid concrete, carbonation is mainly restrained to surface layers of concrete and its depth does not surpass 20 mm in 50 years. Therefore, when concrete becomes permeable or when reinforcement is within reach of surface owing to insufficient cover, carbonation creates corrosion of reinforcement that results in cracking and disintegration of concrete in due course. Carbonation is very fast in dry atmosphere but as existence of moisture is required for galvanic action to happen, that's why, for corrosion of steel, an alternating dry and wet weather is more useful for corrosion. Cracks and voids in concrete facilitate premature carbonation. Due to pollution, the percentage of carbon dioxide remains high In industrial towns and as a result cracking occurs in concrete because of high carbonation.



Chlorides solution of high concentration can deteriorate the cement paste of concrete and can induce disrupting action in concrete close to sulphate attack.