Concrete cancer · mechanism

From carbonation to spalling: the corrosion chain

1

The carbonation front advances

CO₂ from the air converts calcium hydroxide in the cement paste to calcium carbonate. The pH falls from ~13 to below 9, moving inward from the surface by millimetres a year.

2

The steel loses its protection

Alkaline concrete wraps reinforcement in a passive oxide film. Once the carbonation front reaches the bars, the film dissolves and the steel is depassivated. Airborne chlorides (exposure class XS) reach the same result by their own route.

3

Rust expands inside the cover

Corroding steel occupies several times its original volume. The expansion cracks the concrete cover from the inside, along the line of the bars, letting in more air and water.

4

Spalling and section loss

Cover detaches in plates, exposing reinforcement that corrodes faster in open air and loses cross-section. Spalling with section-losing rebar is an immediate structural-engineer referral.

Portuguese durability rules treat the two attack routes as separate exposure classes: XC for carbonation, XS for coastal chlorides (LNEC E464). Where both run at once, the corrosion compounds.
Sources: LNEC especificações E464 and E465 · InspectOS methodology