Concrete deterioration: warning signs and their diagnostic significance

Corrosion of the steel reinforcement embedded in concrete is commonly called concrete cancer. The term has persisted because the behaviour of the defect corresponds to it: the affected area extends beyond the point of origin, the rate of deterioration increases as the mechanism progresses, and the visible symptoms understate the extent of the underlying condition.

Corrosion mechanism

Concrete protects embedded steel chemically. Its high alkalinity maintains a passive oxide layer on the surface of the reinforcement, which prevents corrosion while the layer remains intact. Two processes destroy that protection. Carbonation is the gradual neutralisation of the concrete by atmospheric CO₂, progressing from the exposed surface inward and reducing alkalinity as it advances. Chlorides, introduced from marine air, de-icing salts or contaminated aggregates, break down the passive layer locally at concentrations above a threshold value, without any general loss of alkalinity.

Once the passive layer fails at the surface of the steel, the steel corrodes. The corrosion products occupy several times the volume of the steel from which they formed, and that expansion generates tensile stress within the surrounding concrete, which cracks and ultimately separates from the element. The rate is governed by the availability of moisture and oxygen at the bar, which is why exposed elements such as balcony slab edges, soffits and parapets deteriorate before internal elements of the same age.

Warning signs, in approximate order of progression

1. Rust staining — brown streaks emerging from cracks or joints. Corrosion is already active at this stage; the staining is the visible product of a reaction occurring within the element.
2. Cracking along reinforcement lines — straight cracks following the line of the bars beneath, generated by expansion pressure. These are distinguishable from shrinkage cracking, which follows a more random pattern and does not align with the reinforcement layout.
3. Drummy concrete — surfaces that sound hollow when tapped. Delamination has occurred below the surface, and the spall has formed but has not yet detached.
4. Spalling — concrete detaching from the element, frequently exposing corroded bars. At this stage, section loss on the reinforcement may already be structurally significant and requires measurement rather than estimation.
5. Deflection or movement — uncommon, late in the progression, and serious. An engineer should be engaged immediately where deflection is observed, and the area below the element should be isolated pending assessment.

Extent of contamination beyond the visible damage

Corrosion is an electrochemical process, and the conditions that support it are not confined to the boundary of the visible damage. Carbonation fronts and chloride contamination extend across whole elements; the locations that have spalled are those at which expansion pressure exceeded the tensile capacity of the cover concrete first, generally where cover was lowest. Adjacent areas at the same stage of contamination but with greater cover will spall subsequently.

This is the reason that testing — carbonation depth, cover measurement and half-cell potential mapping — provides information that photographic records cannot, and the reason that repair scopes prepared from photographs alone are systematically understated in both area and cost.

It is also the reason that patch repairs fail in contaminated elements. Where one spall is repaired in a chloride-contaminated slab, the repaired area becomes cathodic relative to the surrounding contaminated concrete, and a new corrosion cell frequently forms at the perimeter of the patch, a mechanism described as the ring anode or incipient anode effect. A competent repair strategy addresses the chemistry of the element through protective coatings, corrosion inhibitors, or galvanic or impressed-current protection where contamination is severe, in addition to reinstating the areas that have already failed.

Recommended action where corrosion is suspected

The affected area should be photographed and left undisturbed, and a diagnostic inspection obtained before quotations are sought. A written condition report with the extent mapped converts an unpriceable instruction to carry out some concrete repairs into a defined scope against which contractors can be compared on a common basis. It also establishes whether the condition is a maintenance item or a structural one, which determines both the urgency and the funding pathway.

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