Carbonation occurs when carbon dioxide from the air penetrates the concrete and reacts with hydroxides, such as calcium hydroxide, to form carbonates. In the reaction with calcium hydroxide, calcium carbonate is formed:
Ca(OH)2 + CO2 → CaCO3 + H2O
This reaction reduces the pH of the pore solution to as low as 8.5, at which level the passive film on the steel is not stable.
Carbonation is generally a slow process. In high-quality concrete, it has been estimated that carbonation will proceed at a rate up to 1.0 mm (0.04 in.) per year. The amount of carbonation is significantly increased in concrete with a high water-to-cement ratio, low cement content, short curing period, low strength, and highly permeable or porous paste.
Carbonation is highly dependent on the relative humidity of the concrete. The highest rates of carbonation occur when the relative humidity is maintained between 50% and 75%. Below 25% relative humidity, the degree of carbonation that takes place is considered insignificant. Above 75% relative humidity, moisture in the pores restricts CO2 penetration (ACI 201 1992). Carbonation-induced corrosion often occurs on areas of building facades that are exposed to rainfall, shaded from sunlight, and have low concrete cover over the reinforcing steel.
Carbonation of concrete also lowers the amount of chloride ions, needed to promote corrosion. In new concrete with a pH of 12 to 13, about 7,000 to 8,000 ppm of chlorides are required to start corrosion of embedded steel. If, however, the pH is lowered to a range of 10 to 11, the chloride threshold for corrosion is significantly lower—at or below 100 ppm (Montani 1995). Like chloride ions, however, carbonation destroys the passive film of the reinforcement, but does not influence the rate of corrosion.
