Carbonatation is a slow process that occurs in concrete where lime (CaO, or Ca(OH)) in the cement reacts with carbon dioxide (CO) from the air and forms calcium carbonate.
The water in the pores of Portland cement concrete is normally alkaline with a pH in the range of 12.5 to 13.5. This highly alkaline environment is one in which the steel rebar is passivated and is protected from corrosion. According to the Pourbaix diagram for iron, the metal is passive when the pH is above 9.5.
The carbon dioxide in the air reacts with the alkali in the cement and makes the pore water more acidic, thus lowering the pH. Carbon dioxide will start to carbonatate the cement in the concrete from the moment the object is made. This carbonatation process will start at the surface, then slowly moves deeper and deeper into the concrete. The rate of carbonatation is dependent on the relative humidity of the concrete - a 50% relative humidity being optimal. If the object is cracked, the carbon dioxide in the air will be better able to penetrate into the concrete.
Eventually this may lead to corrosion of the rebar and structural damage or failure.