Carbonation
Carbonation is cement-based materials reacting with atmospheric CO2 over time: it heals lime mortar, shrinks concrete masonry, and can corrode rebar it reaches.
Ask Chalkline about this →Carbonation is the slow reaction of cement-based materials with carbon dioxide from the air over time. The atmospheric CO₂ reacts with the cement and lime in concrete, mortar, and masonry, and the effect cuts three ways depending on the material:
- In mortar (helpful): the lime in a mortar mix carbonates and lets the mortar autogenously heal hairline cracks, while it slowly gains strength and bond to the masonry. It is part of why lime is in the mix at all.
- In concrete masonry (a shrinkage to manage): concrete blocks and other cement-based units undergo carbonation shrinkage as they age, on top of drying shrinkage, which is one reason control joints matter and you do not lay units green.
- In reinforced concrete (the durability risk): CO₂ neutralises the alkaline pore water from the surface inward. When the carbonation front reaches the steel, the passivating layer that protects the reinforcement breaks down and the steel starts to corrode, with expansive rust that cracks and spalls the concrete.
This is why concrete cover is a durability requirement, not a tolerance: more cover buys more years before carbonation reaches the steel. As a guide, 25 mm of cover typically delays carbonation-driven corrosion for 50 years or more, while 15 mm delays it only 15 to 25 years. Get the cover right at the pour and carbonation stays harmless for the design life; skimp on it and it becomes the rust-and-spall problem years later.
Also known as: Carbonation shrinkage, carbonation of concrete.
Category: Materials / Concrete durability.
Related
See also
References
- AS 3600:2018 Concrete structures, Standards Australia (verified 2026-05-14)
- ABCB / CCAA: Durability of concrete structures guidance (verified 2026-05-14)
Last updated: 2026-05-30. Verified: 2026-05-14. Quarterly review for currency.