Most of the visible defects in reinforced concrete — cracking, cover spalling, rust staining — are the last symptom of a process that begins much earlier, often decades earlier, invisibly to the naked eye. Recognising which of the three main pathologies is at work radically changes the intervention strategy: treating carbonation as if it were chloride attack (or vice versa) means spending the maintenance budget badly and not solving the problem.
Carbonation: the slow, silent enemy
Healthy concrete has a strongly alkaline environment (pH 12-13) generated by the portlandite Ca(OH)₂ produced during cement hydration. This alkalinity forms a passivating oxide layer on the steel reinforcement, which protects it from corrosion — the same principle as stainless steel, but achieved chemically from the surrounding environment rather than from the alloy itself. Atmospheric carbon dioxide, diffusing into the porosity of the concrete, reacts with the portlandite and converts it to calcium carbonate, lowering the local pH to around 9. Below pH 10, the passivating layer breaks down and the steel can corrode if moisture and oxygen are present.
The carbonation front advances in depth according to a law roughly proportional to the square root of time — which is why carbonation damage, when it appears, is almost always found in structures at least 30-40 years old. The standard diagnostic test is cheap and immediate: a 1% phenolphthalein solution is sprayed onto a freshly broken concrete surface. Still-alkaline zones (pH > 9) turn fuchsia; carbonated zones stay colourless. Comparing the depth of the colourless zone with the actual cover thickness gives an estimate of how many years remain before the front reaches the reinforcement.
Chloride attack: localised deterioration
In marine environments or where de-icing salts are used (viaducts, car parks, road tunnels), chloride ions penetrate by diffusion and, once a critical threshold is exceeded (around 0.4% by weight of cement under EN 206), locally break down the passivating layer even though the overall pH remains alkaline. Unlike carbonation, which acts uniformly across the whole surface, chloride attack produces pitting corrosion — localised points of very deep corrosion surrounded by steel that's still intact — often more insidious because it's less visible in its early stages. Diagnosis requires taking powder samples at various depths and chemically titrating the chloride content, or electrochemical half-cell potential measurements (ASTM C876) that map areas at risk of active corrosion without breaking out the cover.
Alkali-silica reaction (ASR): the enemy within
ASR is less common but more insidious because it originates in the aggregate, not in the external environment. Certain reactive siliceous aggregates (some flints, opals, certain volcanic rocks) react with the cement's alkalis (Na₂O, K₂O), forming a hygroscopic gel that, as it absorbs water, expands and generates internal pressures strong enough to crack the concrete from within. The typical crack pattern is map cracking: a network of irregular cracks across the whole surface, often accompanied by whitish gel exuding from the wider cracks. A definitive diagnosis requires petrographic microscope examination of thin sections (UNI EN 12407 standard) to identify the reaction products and distinguish them from other causes of cracking. Prevention, far more effective than cure, relies on using cements with a low equivalent alkali content (Na₂O eq < 0.60%) or partially replacing cement with blast-furnace slag or fly ash, which reduce the availability of free alkalis.
From diagnosis to intervention
Once the pathology is identified, the intervention strategy changes radically. For localised carbonation, traditional restoration — removing the degraded cover, cleaning and repassivating the reinforcement, rebuilding with cementitious or resin-based mortar — is effective and relatively cheap if the affected area is limited. When carbonation is spread across large surfaces, electrochemical realkalisation treatments are preferred, which regenerate the alkaline environment around the reinforcement without breaking out the cover.
For chloride attack, localised restoration often isn't enough: if the chloride concentration in the surrounding concrete stays above threshold, corrosion will resume at the edges of the repair (the "galvanic ring" effect). The most effective solution on major structures (bridges, piers, coastal car parks) is cathodic protection: a system of anodes (impressed-current or sacrificial zinc) that electrochemically reverses the corrosion process across the whole surface, extending the structure's service life by decades without extensive demolition.
For ASR there's no definitive cure: once triggered, the reaction continues as long as reactive aggregate, alkalis and moisture are available. The interventions available are mitigation measures — surface waterproofing to reduce moisture ingress, monitoring expansion with strain gauges, and in severe cases external structural strengthening (FRP carbon-fibre plating) to compensate for the loss of load-bearing capacity, rather than repair of the material itself.