Concrete Carbonation

Concrete carbonation is a gradual chemical process in which carbon dioxide (CO₂) from the atmosphere diffuses into concrete and reacts with calcium hydroxide (Ca(OH)₂) — a product of cement hydration — to form calcium carbonate (CaCO₃). This reaction reduces the pH of the concrete from its naturally high alkaline level (around pH 12-13) toward neutral (around pH 8).

The significance of this pH reduction is its effect on embedded steel reinforcement. In high-pH concrete, a passive oxide film forms on the steel surface, protecting it from corrosion. When carbonation advances through the concrete cover to reach the reinforcement, the pH at the steel surface falls below the threshold required to maintain this passive film (approximately pH 9). The reinforcement becomes vulnerable to corrosion, and in the presence of moisture and oxygen, active corrosion begins.

The rate of carbonation depends on several factors: concrete permeability (lower water-cement ratio and better compaction give lower permeability and slower carbonation), moisture content (carbonation proceeds most rapidly at intermediate humidity — very dry concrete limits CO₂ diffusion, very wet concrete limits CO₂ availability), and atmospheric CO₂ concentration. Carbonation typically advances as the square root of time — rapidly in the early years, then progressively more slowly.

Carbonation depth is measured by phenolphthalein indicator testing: a freshly broken or ground concrete surface is sprayed with phenolphthalein solution, which turns purple in uncarbonated concrete (pH > 9) and remains colourless in carbonated concrete (pH < 9). The depth of the colourless zone is the carbonation depth. Where carbonation depth equals or exceeds cover depth, the reinforcement is at risk.

For structural inspectors, concrete carbonation is an important diagnostic parameter because it predicts, rather than just describes, deterioration risk. A building where carbonation has advanced to within 5mm of the reinforcement may show no spalling or cracking yet, but the risk of corrosion-induced deterioration in the next 5-10 years is high. This predictive information is valuable for maintenance programming and for risk-based inspection scheduling.