What is carbonation in concrete?

Carbonation is a chemical process where carbon dioxide (CO2) from the atmosphere penetrates the concrete and reacts with calcium hydroxide to form calcium carbonate. Fresh concrete is highly alkaline (pH 12-13), which creates a 'passive layer' protecting the steel rebar from rust. Carbonation lowers this pH to around 9, destroying that protective layer. Once the 'carbonation front' reaches the steel, moisture and oxygen cause the steel to corrode, leading to expansion and spalling. We diagnose this using a phenolphthalein indicator test, which turns pink in alkaline (healthy) concrete and remains clear in carbonated areas.