Concrete cancer: the warning signs, and what they actually mean

“Concrete cancer” is the industry’s least technical name for its most common defect: corrosion of the steel reinforcement inside concrete. The name stuck because the behaviour fits — it spreads, it accelerates, and the visible symptoms understate the extent.

The mechanism in one paragraph

Concrete protects embedded steel chemically: its high alkalinity keeps a passive oxide layer on the reinforcement. Two things destroy that protection. Carbonation — CO₂ slowly neutralising the concrete from the surface inward — and chlorides, from marine air, de-icing salts or contaminated aggregates. Once the passive layer fails at the steel, the steel rusts. Rust occupies several times the volume of the steel it came from, and that expansion cracks the concrete off from within.

The warning signs, in rough order of progression

1. Rust staining — brown streaks bleeding from cracks or joints. The corrosion is already active; the stain is the exhaust.
2. Cracking along reinforcement lines — straight cracks tracking the bars beneath, caused by expansion pressure. Distinct from shrinkage cracking, which is more random.
3. Drummy concrete — surfaces that sound hollow when tapped. Delamination has occurred below the surface; the spall exists, it just hasn’t fallen yet.
4. Spalling — concrete breaking away, often exposing corroded bars. By this stage, section loss on the reinforcement may already be structurally relevant.
5. Deflection or movement — rare, late, and serious. Get an engineer involved immediately.

Why the visible extent understates the problem

Corrosion is electrochemical, and the chemistry doesn’t stop at the edge of the visible damage. Carbonation fronts and chloride contamination extend across whole elements; the visible spalls are simply where expansion pressure won first. This is why testing — carbonation depth, cover measurement, half-cell potential mapping — matters more than photographs, and why repairs scoped from photographs alone are systematically underscoped.

It’s also why patch repairs fail: repair one spall in a chloride-contaminated slab and the corrosion cell frequently re-forms at the patch perimeter (the “ring anode” or incipient anode effect). A competent repair strategy treats the element’s chemistry — protective coatings, corrosion inhibitors, or galvanic/impressed-current protection where contamination is severe — not just its holes.

What to do with a suspected case

Photograph it, don’t disturb it, and get a diagnostic inspection before you get quotes. A written condition report with mapped extent turns an unpriceable “some concrete repairs” into a defined scope that contractors must price against each other honestly — and it tells you whether you have a maintenance item or a structural one.

This article is general information, not advice on a specific structure. Condition assessment of an individual building requires inspection.