What is carbonation in concrete and how does it relate to reinforcement corrosion?

• A. Carbonation consumes alkalinity, lowers pH, and removes the protective passive film on steel, enabling corrosion
• B. Carbonation increases alkalinity and strengthens passive film
• C. Carbonation has no relation to reinforcement
• D. Carbonation only affects cement hardness

Answer: (A)

Carbonation is the chemical reaction where atmospheric CO2 penetrates concrete and reacts with the hydrated cement paste, forming calcium carbonate and converting calcium hydroxide. This process reduces the alkalinity of the pore solution, lowering the pH from highly alkaline levels that keep the steel passivated toward a softer, less protective environment. When the pH falls enough, the protective passive film on steel reinforcement breaks down, allowing moisture and oxygen to drive corrosion of the steel. So, carbonation directly relates to reinforcement corrosion because it removes the conditions that normally prevent rust, making the embedded steel susceptible to corrosion over time, especially where cracks or moisture are present. It progresses slowly and is influenced by concrete permeability, moisture availability, and cracking. It does not raise alkalinity, nor does it strengthen the passive film, and it is not primarily about cement hardness.