Alkali-Aggregate Reaction (AAR) is a chemical reaction in concrete between alkalis in cement and reactive minerals in aggregates, leading to expansion and cracking.
Explanation
AAR is a broader term that includes different types of reactions, the most common being Alkali-Silica Reaction (ASR). These reactions produce an expansive gel that absorbs moisture and increases in volume.
As expansion occurs within the concrete, it generates internal stress that leads to:
- Map cracking
- Surface distortion
- Progressive deterioration
The reaction requires three conditions:
- Reactive aggregates
- Sufficient alkali content
- Presence of moisture
AAR develops slowly over time and may not be immediately visible in early stages.
Why It Matters
If left unmanaged, AAR can:
- Reduce durability
- Increase permeability
- Contribute to structural weakening
While the reaction cannot be reversed, its effects can be managed through moisture control, protective systems, and targeted repair.
Accurate diagnosis is essential to distinguish AAR from other forms of cracking.