Explain the carbonation process in concrete and its effects.

Carbonation occurs when atmospheric CO2 penetrates concrete and reacts with calcium hydroxide to form calcium carbonate: Ca(OH)2 + CO2 = CaCO3. This reduces concrete pH from about 12.5 to below 9, destroying the passive protective layer on reinforcement and allowing corrosion. Carbonation depth increases with time (proportional to sqrt of time), porosity, and CO2 concentration. Prevention includes adequate cover, low w/c ratio, proper curing, and surface treatments or coatings.