Alkali-Silica Reaction (ASR) is a chemical reaction within concrete between alkalis in cement and reactive silica in certain aggregates. This reaction forms an expansive gel that absorbs moisture and causes internal cracking.
Explanation
ASR develops when three conditions are present:
- Reactive aggregate
- High alkali content in cement
- Sufficient moisture
As the gel expands, it creates internal pressure that leads to map cracking, surface distortion, and long-term structural weakening. Unlike surface defects, ASR originates inside the concrete mass and can progressively reduce durability and load-bearing performance.
In the UK, ASR is more commonly associated with structures built between the 1960s and 1980s, particularly where concrete is exposed to persistent moisture.
Why It Matters
If left unmanaged, ASR can:
- Increase permeability
- Accelerate reinforcement corrosion
- Lead to spalling or structural damage
Diagnosis typically involves core sampling and petrographic testing. While ASR cannot be reversed, its progression can be controlled through moisture management, protective coatings, and targeted structural repair.