Concrete is the construction material of choice for all builders. However, the concrete may not be as concrete as we think it is. There may be several causes that can result in the deterioration of concrete. Of these, corrosion caused by carbonation and chlorides of steel reinforcement is the most common causes of concrete deterioration.
Corrosion occurs because of a basic electrochemical reaction caused by the formation of positive and negative charges in random portions of the steel. Concrete itself acts as an electrolyte through which these charges flow freely. Corrosion can thus occur with an increase in the ingress of acidic gas (carbon dioxide), causing carbonation, or an ingress of chlorides. Furthermore, due to the presence of water and oxygen, iron from the steel may react with hydroxyl ions, resulting in rust formation which is detrimental to the reinforcement.
Free alkalis such as calcium contained in the concrete on exposure to acidic gas such as carbon dioxide in air can undergo carbonation to form calcium carbonate or calcite. If the concrete has a high number of pores exposed to air or if the permeability of the concrete is high with a high water to cement ratio, there are chances that carbonation can occur to greater depths in the concrete. This can result in a weak, brittle structure with cracks leading to further deterioration.
Carbonation can be detected as slight discoloration of the concrete. Furthermore, phenolphthalein can be used to detect pH changes on the exposed concrete surface.
Various sources of chlorides include salts, marine aggregates, and sea water. Chlorides are acidic in nature, and if their presence is unchecked, they can cause severe corrosion, eventually resulting in premature cracking.
To ensure that the concrete does not deteriorate because of these reasons, it is essential to use good quality of concrete, with low water to cement ratio and appropriate aggregate size, and ensure that the prepared concrete is compact such that the pores are fewer and well distributed. In well prepared and resistant structures, it may take up to several years for deterioration to occur at the level of reinforcement.