Alkali-silica reaction is a slow chemical reaction inside concrete between reactive silica in the aggregate and the alkaline pore solution of the cement paste. It produces a gel that swells as it takes on water, and that swelling cracks the concrete from within. Because the damage comes from inside the material rather than from an external load, ASR is sometimes called concrete cancer.
The reaction develops over years, so it usually surfaces in older structures as a distinctive map-like cracking pattern, gel seeping from cracks, and in some cases joint closure or misalignment. Confirming it, and separating it from other causes of cracking, is a materials and forensic question rather than something a visual inspection alone can settle.
What Causes ASR
ASR needs three things present at the same time. Remove any one and the reaction cannot progress, which is the basis for both prevention and management.
| Ingredient | Role in the reaction |
|---|---|
| Reactive silica | Certain aggregates contain unstable forms of silica, such as opal, chert, and some cherty limestones, that react with alkalis. |
| Alkalis | Sodium and potassium hydroxides, mostly from the portland cement, raise the pore solution's alkalinity enough to attack the silica. |
| Moisture | Water is needed for the reaction to proceed and for the resulting gel to swell. Below roughly 80 percent internal humidity, expansion largely stops. |
The reaction produces a hygroscopic gel in and around the reactive aggregate particles. As the gel draws in moisture it expands, placing the surrounding paste in tension until it cracks. Over time those internal cracks link up and reach the surface as the characteristic map or pattern cracking.
How ASR Damages a Structure
The visible signs of ASR often resemble other problems, which is why it is misdiagnosed. Common indications include map cracking on surfaces, cracks lined with a clear or whitish gel, surface pop-outs over reactive particles, and, in restrained members, expansion that closes joints or displaces elements.
ASR rarely causes sudden collapse, but it is progressive. Left unmanaged, the cracking admits water and chlorides that accelerate other deterioration, including reinforcement corrosion and concrete spalling. In critical structures such as dams, bridges, and pavements, the expansion and loss of stiffness can become a serious serviceability and safety concern.
How ASR Is Diagnosed and Tested
Confirming ASR starts with cores taken from the affected concrete and examined in a laboratory. Petrographic examination is the definitive step, and it is supported by aggregate reactivity tests when the question is whether a material should be used in new work.
| Method | Name | What it does |
|---|---|---|
| ASTM C856 | Petrographic examination | A petrographer examines a core under magnification for the reaction gel, cracking pattern, and reacted aggregate. The primary way ASR is confirmed in an existing structure. |
| ASTM C1260 | Accelerated mortar bar | A fast screening test of an aggregate's reactivity, run over 16 days before the aggregate is used in a mix. |
| ASTM C1293 | Concrete prism | A longer, more definitive test of aggregate reactivity, measuring expansion over a year or more. |
Petrographic analysis under ASTM C856 is what distinguishes ASR from other cracking mechanisms, because it identifies the reaction gel and the reacted aggregate directly. That laboratory confirmation is the foundation of any sound diagnosis, and it is a core capability of construction materials testing and forensic investigation. Field review of the cracking pattern and moisture condition, paired with the lab work, builds the full picture.
Preventing and Managing ASR
In new construction, ASR is prevented at the mix-design stage. Aggregates are screened for reactivity, supplementary cementitious materials such as fly ash or slag are used to bind alkalis, and low-alkali cement or lithium admixtures are specified where reactive aggregates cannot be avoided.
In an existing structure, the reaction cannot be reversed, so management focuses on slowing it and handling its effects. Reducing moisture intrusion is the most practical lever, since the gel needs water to keep expanding. A structural evaluation determines whether the member's capacity is affected, and monitoring tracks whether expansion is still active. Because the right response depends on the structure and how far the reaction has progressed, a forensic engineering assessment is what sets the course.
