Alkali-silica reaction (ASR), sometimes called concrete disease, is a damaging chemical reaction that occurs inside concrete between the alkali hydroxides in cement paste and reactive forms of silica present in certain aggregates. The reaction produces a hygroscopic gel that absorbs water and expands, generating internal pressure that leads to cracking, spalling, and loss of structural integrity over time.
How it works
ASR requires three conditions to occur simultaneously:
- Reactive silica — Certain types of aggregate (such as chert, flint, opal, or some volcanic rocks) contain amorphous or poorly crystalline silica that is susceptible to chemical attack.
- Sufficient alkali — The cement paste must contain enough sodium and potassium hydroxides (alkalis) to initiate the reaction.
- Moisture — Water is essential because the ASR gel absorbs moisture and swells. Without adequate moisture, the reaction slows or stops.
When all three conditions are met, the alkalis dissolve the reactive silica, forming a gel that expands as it absorbs water. This expansion creates internal tensile stresses that exceed the concrete’s strength, resulting in a characteristic pattern of map cracking (also called pattern cracking) on the surface.
Application
Understanding ASR is critical in several construction contexts:
- Concrete mix design — Engineers select non-reactive aggregates and use low-alkali cement to prevent ASR from occurring.
- Supplementary cementitious materials — Fly ash, ground granulated blast furnace slag (GGBS), and silica fume are added to concrete mixes to bind alkalis and reduce the risk of ASR.
- Inspection and monitoring — Existing structures in humid environments (bridges, dams, pavements) are regularly inspected for signs of ASR-related cracking.
- Repair and remediation — Affected structures may require crack injection, surface sealing to limit moisture ingress, or in severe cases, partial or full replacement of damaged elements.
Prevention
The most effective strategy is prevention at the design stage: use non-reactive aggregates, limit cement alkali content, and incorporate supplementary cementitious materials.
Related terms
- Concrete durability
- Aggregate
- Supplementary cementitious materials
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