What is Alkali-Silica Reaction (ASR)?

Alkali-silica reaction (ASR) is a reaction between reactive silica in aggregate and alkalis in concrete that forms a gel, which swells and cracks the slab.

How it works

The gel absorbs water and swells. Three things must be present: alkali hydroxides in the pore solution, reactive minerals in the aggregate, and moisture. Most alkalis come from the portland cement. ASR stops below 80 percent relative humidity inside the concrete (FHWA).

What it looks like on a pavement

The FAA PAVER manual lists these visual indicators:

• Cracking, often in a map pattern.
• White, brown, gray or other colored gel or staining at the cracks.
• Aggregate popouts.
• Expansion, such as slab faulting, joint misalignment and extruded joint seals.

Joint closure from ASR expansion can end in a blowup, which is a buckling or shattering of the slab edges. Cracks in a pavement tend to align longitudinally, because the slab is restrained in that direction. ASR usually appears after several years, not in the first few years. Plastic shrinkage cracking appears within the first year.

How it is rated and confirmed

PAVER rates ASR as distress 76. Severity is low, medium or high, based on foreign object debris (FOD) potential and on crack width and slab movement. A tight crack is 1 mm or less. If high-severity ASR is recorded, no other distress is recorded.

Visual inspection cannot confirm ASR. Coring and petrographic analysis is the only definitive method.

How it is prevented

FHWA names three routes: avoid reactive aggregate, use low-alkali cement, and use suitable supplementary cementing materials. FAA Item P-501 requires aggregate testing to ASTM C1260 and ASTM C1567, and low alkali cement of less than 0.6 percent equivalent alkali.