What is carbonation-initiated reinforcement corrosion?

Carbonation-initiated reinforcement corrosion is a type of corrosion that occurs because the pH value (of the pore water) of the concrete decreases as a result of the penetration of carbon dioxide (CO2) into the concrete. The carbon dioxide from the air converts the free and bound lime into calcium carbonate. This process is called carbonation.

The damage takes place from the outside, so that there is a limit in the concrete where the pH has decreased and where the steel is no longer protected by the alkaline environment of the concrete. If the steel lies within this boundary where the concrete has been affected, it can rust. Whether corrosion actually occurs depends on the humidity of the concrete and the characteristics of the steel.
The carbonation depth is not constant everywhere. The carbonation front shows an erratic gradient and therefore the carbonation depth is indicated using an average and a maximum depth.

How fast concrete carbonates depends mainly on the speed at which the pores dry out in the surface and the speed at which CO2 enters in the concrete. The more wet is the concrete, the more difficult the CO2 transports and the slower is the carbonation process. The carbonation depth is therefore highly dependent on the expose conditions. There are other important factors involved, like composition and quality of the concrete.
During new construction of concrete structures, a number of measures can be taken to prevent damage from carbonation-initiated reinforcement corrosion. These measures may include the application of a sufficiently large concrete cover, the application of high-quality concrete, the correct post-treatment of the concrete and the possible application of a coating on the surface.

Reinforcement corrosion initiated by carbonation can in practice be recognized by the following characteristics:

• Cracking parallel to the reinforcement bars;
• Hollow-sounding when knocking on the surface of the concrete with a hammer;
• Detached concrete cover;
• Rusting along the entire rod located in carbonated concrete.