How does concrete’s carbonation process affect long-term durability outdoors?

Concrete carbonation is a natural chemical reaction where carbon dioxide from the air penetrates the concrete and reacts with calcium hydroxide, forming calcium carbonate. While this process can slightly increase surface hardness, it significantly reduces the alkalinity of concrete, lowering its pH from around 13 to below 9. This drop in pH eliminates the passive protective layer around embedded steel reinforcement, making it susceptible to corrosion when exposed to moisture and oxygen. Over time, corrosion-induced expansion can cause cracking and spalling, compromising structural integrity.

Outdoor exposure accelerates carbonation due to fluctuating humidity, temperature cycles, and direct contact with rainwater. Poor-quality concrete with high porosity or inadequate cover depth worsens the issue. To mitigate carbonation effects, proper mix design, sufficient concrete cover, and protective coatings are essential for enhancing long-term durability in outdoor environments.