Concrete corrosion is the gradual deterioration and degradation of concrete caused by chemical, physical, or biological processes that compromise the material’s structure and strength. Although concrete is often perceived as virtually indestructible, it is in reality susceptible to several forms of attack. The consequences range from superficial discolouration to severe structural damage that can threaten the safety of an entire building or infrastructure asset.
The most common cause of concrete corrosion is not the deterioration of the concrete itself, but the corrosion of the steel reinforcement embedded within it. When the protective alkaline environment surrounding the steel is compromised, the steel begins to rust. Rust occupies a greater volume than the original steel, generating internal pressure that pushes the surrounding concrete outward, leading to cracking and spalling.
Applications
The principal mechanisms of concrete corrosion include:
- Carbonation — Carbon dioxide from the atmosphere penetrates the concrete and reacts with calcium hydroxide, lowering the pH of the concrete. When the carbonation front reaches the reinforcement (typically after several decades), the steel loses its passive protective layer and begins to corrode.
- Chloride attack — Chlorides from de-icing salts or seawater penetrate the concrete and break down the passive layer around the reinforcement, even when the pH remains high. This is the most aggressive form of reinforcement corrosion.
- Sulfate attack — Sulfates from soil or groundwater react with cement compounds to form expansive reaction products (ettringite), which cause the concrete to crack from within.
- Alkali-silica reaction (ASR) — A reaction between alkalis in the cement and reactive silica in certain aggregates. It produces a gel that absorbs water and swells, resulting in a characteristic map-cracking pattern on the concrete surface.
- Physical deterioration — Freeze-thaw cycles, abrasion from traffic, and erosion by flowing water are physical processes that damage the concrete surface over time.
Prevention and repair
Concrete corrosion can be limited through:
- Adequate concrete cover over the reinforcement (minimum 25-50 mm depending on the exposure class)
- Using a low water-cement ratio to produce dense, low-permeability concrete
- Applying protective coatings or impregnations to the concrete surface
- Cathodic protection of the reinforcement in existing structures
- Repair mortars and concrete rehabilitation where damage has already occurred
Related terms
- Carbonation
- Reinforcement corrosion
- Chloride ingress
- Concrete repair
- Concrete cover
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