Carbonation of concrete is one of the reasons for corrosion of reinforcement. It is a process by which carbon dioxide from air penetrates into the concrete and reacts with calcium hydroxide to form calcium carbonates.
The percentage of CO2 present is air vary from place to place. In case of rural areas the concentration of CO2 in air may be about 0.03% by volume, where as in urban areas it may vary from 0.3% to 1.0%. This CO2, in presence of moisture changes into dilute carbonic acid and attacks the concrete and reduces alkalinity of concrete.
Due to reduction of alkalinity of concrete, the pH value of pore water in the hardened cement paste reduces from 13 to 9.0. When all the Ca(OH)2 has become carbonated, the pH value again reduces to about 8.3. And at this low pH value, the protective layer gets destroyed and the steel is exposed to corrosion.
Factors Affecting Rate of Carbonation
The rate of carbonation depends on the following factors.
Presence of pore water
Grade of concrete
Permeability of concrete
Protected or unprotected concrete
Depth of cover
If pores in the concrete are dry then CO2 remains in gaseous from and does not react with hydrated lime and carbonation cannot take place.
The higher the grade of concrete, the slower will be the rate of carbonation
In case of less permeable concrete the rate of carbonation is slower and vice versa.
Definitely the concrete which are protected are less prone to carbonation.