Rebar Corrosion

Rebar corrosion — the oxidation of steel reinforcing bars (rebars) embedded in concrete — is one of the most prevalent and costly forms of structural deterioration worldwide. It affects a significant proportion of the reinforced concrete building stock constructed in the mid-to-late twentieth century, particularly those in coastal or de-iced environments and those built with inadequate cover depth or high water-cement ratio.

The corrosion process requires three conditions: the removal of the passive oxide film that normally protects steel in high-alkaline concrete (achieved by carbonation or chloride attack), the presence of moisture, and the presence of oxygen. When all three conditions are met, active corrosion begins. The iron in the steel oxidises to form iron hydroxides and oxides — collectively, rust — which have a volume approximately 2-4 times that of the original steel. This volumetric expansion generates outward pressure on the surrounding concrete cover, eventually cracking and spalling it.

Two main pathways initiate corrosion. Carbonation-induced corrosion occurs when the carbonation front advances through the cover to reach the reinforcement, lowering the pH below the depassivation threshold. Chloride-induced corrosion occurs when chloride ions — from de-icing salts, seawater, or occasionally admixtures used during construction — accumulate at the reinforcement surface above a threshold concentration, locally breaking down the passive film. Chloride-induced corrosion is typically more aggressive and more localised than carbonation-induced corrosion.

Visible signs of rebar corrosion include cracking parallel to the reinforcement line (caused by expansive pressure), rust staining on the concrete surface (where corrosion products have migrated through cracks or along aggregate-paste interfaces), and spalling or delamination of cover concrete. In advanced cases, the reinforcement may be fully exposed.

Structural assessment of corroded reinforcement requires evaluation of the section loss — how much of the bar's cross-sectional area has been consumed — and its effect on load-carrying capacity. Even where structural capacity has not yet been compromised, active corrosion causes progressive deterioration that will eventually impair performance if remediation is not undertaken.