Alkali-silica reaction (ASR) is the durability mechanism people find hardest to believe: the rock inside your concrete is slowly reacting with the glue around it. It is also one of the most over-self-diagnosed problems in the field, because its signature crack pattern can be mimicked by other mechanisms. Both facts make ASR worth understanding carefully.
The three ingredients
ASR occurs when three conditions coexist:
- Reactive silica in the aggregate. Certain minerals (some opaline or chalcedonic cherts, strained quartz, volcanic glasses, and others) react with high-pH pore solution.
- Alkalis in the pore solution, contributed by cement and, sometimes, external sources such as deicing chemicals or seawater.
- Moisture, without which the reaction product cannot swell.
Remove any one ingredient and the reaction does not proceed. This is why mix design (low-alkali cement, supplementary cementitious materials, non-reactive aggregate sources) and moisture management are the classic mitigation levers.
The mechanism
Pore solution in concrete is highly alkaline. In the presence of reactive silica, it dissolves silica from the aggregate surface, forming a gel. The gel itself is not the problem; it becomes a problem when it absorbs water and swells. The swelling pressure acts on the surrounding paste, and over time the resulting stress cracks the concrete from the inside out.
What it looks like: map cracking
ASR's signature visible symptom is map cracking: a network of fine, irregular, interconnected cracks, often with gel weeping at cracks or edges (gel can appear as a white, moist, or glassy deposit). Expansion can also displace elements, closing joints and displacing adjacent construction.
But map cracking is a clue, not a diagnosis. Several conditions produce similar surface patterns, including drying shrinkage over restrained areas, freeze-thaw distress, and overloading. The FHWA's lithium-mitigation guidance makes the same point: diagnosis of ASR requires examination of the concrete, not just its appearance.
How ASR is actually diagnosed
Petrographic examination (ASTM C856) is the accepted diagnostic method. Under the microscope, a petrographer looks for:
- Reaction rims around aggregate particles.
- Gel in pores, cracks, and at aggregate interfaces.
- Cracking patterns that trace through aggregates and paste.
- The aggregate types actually present, and whether they include known reactive minerals.
Combined with the structure's history and exposure, petrography distinguishes ASR from its look-alikes and characterizes how far the reaction has progressed. See our petrography article for what the examination covers.
Why moisture management is part of every ASR conversation
Because the gel only swells in moisture, drying or shielding affected elements slows expansion, and wetting (poor drainage, ponding, leaking joints) accelerates it. Moisture management never cures ASR (the reactive ingredients remain), but it is the main lever for controlling an existing case, alongside engineering assessment of structural effects.
A note on treatments and ASR
Claims that a product "prevents ASR" deserve specific scrutiny: prevention means interrupting one of the three ingredients over the long term, which is a claim requiring product-specific, long-duration evidence. General literature about alkali binding or pore chemistry in cementitious systems does not by itself establish that a particular treatment prevents ASR in field concrete. We hold all product content on this site to that standard; see our PRO Tek evidence overview for how product-specific claims are handled and sourced.
What ASR looks like over time
ASR is slow, which is part of what makes it manageable and also part of what makes it misjudged. The typical progression on an affected element:
- Early: fine map cracking appears, often on the most exposed surfaces. Gel weeping may or may not be visible yet. Expansion, if measurable, is small.
- Progressing: cracks widen, gel staining becomes more visible at cracks and edges, and joints may begin closing. Elements can expand enough to displace adjacent construction.
- Advanced: structural effects become possible: displacement, pressure on adjacent elements, and stiffness loss in the affected members. Corrosion can become a secondary problem as cracks admit chlorides and moisture.
The timeframe is typically years to decades, not seasons. That pace is useful diagnostically: map cracking that appears suddenly on a young slab is more likely drying shrinkage or finishing-related, while slowly developing map cracking over years on older construction fits the ASR profile.
The comparison points that matter for diagnosis
When a petrographer (or an engineer reviewing the situation) distinguishes ASR from its look-alikes, the questions are:
- Distribution. ASR tends to affect whichever elements contain the reactive aggregates, not uniformly across all concrete on a site.
- Crack pattern at depth. ASR cracking commonly passes through aggregates, not only around them; drying-shrinkage cracking lives in the paste.
- Reaction products. Gel in pores and at aggregate interfaces, visible microscopically, is the physical evidence.
- Aggregate identity. Whether the aggregates present include known reactive minerals, confirmed under the microscope (the petrography article covers the method).
- Expansion evidence. Joint closure, displacement, or monitoring data showing movement over time.
No single item is conclusive; the pattern of several together is what makes the diagnosis defensible.
Moisture management in practice
Because the gel only swells with moisture, the practical levers on an existing case are the ones that change moisture:
- Fix ponding and drainage that keep water against the element.
- Repair leaking joints, which deliver water to the interior.
- Shield or cover exposed elements where feasible, allowing them to dry.
- Monitor expansion over time (crack widths and joint gaps, documented annually) so the effectiveness of moisture management is visible in data rather than impressions.
None of this reverses the reaction; all of it can slow expansion meaningfully. FHWA's guidance on mitigation chemistry addresses the material-side options (lithium treatments, mix redesign for replacement concrete) for cases where engineering judgment calls for them.
What this article does not tell you
It does not tell you whether any specific structure has ASR (only petrography and engineering assessment can), and it does not cover mitigation chemistry (lithium treatments and mix-design approaches) in depth. FHWA's guideline document in the references is the primary source for that.
Where to go next
What a Petrographic Examination Can Reveal About Concrete shows how the diagnosis is actually made, and Why Concrete Deteriorates places ASR among the other mechanisms. How Water Moves Through Concrete explains why moisture management keeps appearing in these conversations.