What Causes Concrete to Deteriorate?

Concrete can deteriorate because of reactions within its ingredients, attack from its surroundings, construction defects or the loads and movements it experiences. Several processes can act together.

If cracking or movement is affecting your property, explore our investigation of cracking and building movement. An article cannot determine the cause or appropriate solution for an individual property.

A crack, stain or missing piece of cover describes the result; it does not identify which process produced it.

It helps to separate damage to the concrete itself from corrosion of the steel inside it. Some reactions expand or weaken the cement-and-aggregate body. Others undermine the protective environment around reinforcement, allowing corrosion to damage both the steel and its surrounding concrete. An investigation therefore needs material and exposure history as well as visible symptoms.

Potential sources of concrete deterioration; several can act together.

These potential sources can act together. The framework is not a diagnosis chart.

Reactions that start within the concrete

Alkali-silica reaction involves certain constituents of aggregate reacting with alkalis in the concrete. The resulting gel can take up water and expand. Whether that expansion damages the concrete depends on the aggregate, available alkalis, moisture and other conditions. Map-like cracking can be relevant evidence, but the professional guidance also explains that reinforcement can influence the pattern and that reaction products are not equally expansive in every situation.

Alkali-carbonate reaction is a different aggregate reaction involving certain dolomitic limestones. It does not produce the silica-gel exudation described for alkali-silica reaction. Professional guidance associates damaging expansion with particular aggregate constituents and sustained moisture. Similar-looking surface cracking therefore need not represent the same internal reaction.

Delayed ettringite formation adds a construction-history question. Ettringite formed normally at an early stage is not the damaging process described here. Professional guidance discusses a different sequence following elevated temperatures during early curing, with later ettringite formation and expansion where water is available. Early thermal history can therefore matter to a later investigation, even though the damage is being noticed in hardened concrete.

Internal contamination also matters. Chlorides may have entered with mixing water, admixtures or aggregates rather than arriving from the environment later. Dividing causes into internal and external groups is useful for organising enquiries, but it should not hide a material's actual exposure history.

Attack from the surroundings

Sulfate-related deterioration attacks the concrete differently from the loss of protection around steel. Sulfates from soil, groundwater or industrial environments can react with the cement constituents and produce expansion and damage.

In wastewater environments, bacterial processes can also create sulfuric acid. Professional guidance's sewage-pumping-station example belongs to that exposure setting, not to every building with a damp concrete surface.

Acid-sulfate soils are another specific exposure discussed in the book. Oxidation of pyritic material can generate acid, with consequences for both the binder and sulfate reactions. Soil or groundwater information is relevant where that exposure is plausible; the colour of a stain does not establish it.

Carbonation and chlorides affect steel differently

Concrete normally provides reinforcement with an alkaline environment that supports a protective passive film. Carbonation involves carbon dioxide reacting with the cement paste and reducing its alkalinity. If that change reaches the steel, the protection can be lost and corrosion can develop where the necessary moisture and oxygen conditions exist.

Carbonation depth, cover and their variation across the structure are consequently different questions from the size of a visible spall.

Chlorides can disrupt the steel's protection even while the surrounding concrete remains alkaline. Relevant sources include saline water, salt spray and contaminated original ingredients. Their movement and effect depend on the concrete, binder, moisture and exposure. A single chloride number cannot be transferred between different reporting bases or treated as a universal safe level.

Once steel corrodes, expanding corrosion products can crack, separate and dislodge the cover. Corrosion can also remove metal, sometimes in concentrated pits rather than evenly along a bar. The visible concrete damage and the remaining steel section are related but separate parts of the assessment.

Why the exposure record matters

Moisture can carry aggressive substances and support corrosion reactions, but water alone is not a complete explanation for deterioration. Nor does a dry-looking or saturated surface establish that corrosion is negligible. Moisture, oxygen and wetting-and-drying conditions need to be considered together. This does not provide a timetable from the first crack to spalling at a particular building.

Cracks can provide easier routes for aggressive substances to reach reinforcement. Their position relative to steel, the concrete quality and the exposure matter. Deterioration may also extend beyond what is visible, so drawing a line around a stain is not the same as establishing the extent of the process.

For a useful investigation brief, connect the affected element to available construction and material records, its wetting or chemical exposure, and the timing of cracks, stains or previous repairs.

Ask the investigator to distinguish observed damage, evidence for each plausible mechanism and what remains uncertain.