Alkali-Silica Reaction (ASR)

Alkali-silica reaction (ASR) is a chemical reaction between alkalis in cement and reactive forms of silica in aggregate that causes concrete expansion, cracking, and deterioration. The reaction produces a gel that absorbs water and expands, generating internal pressures that crack concrete from the inside out. ASR develops over years, creating characteristic map cracking and progressive deterioration.

Why It Matters

ASR can destroy concrete structures over time. Once initiated, the reaction continues as long as moisture and reactive materials remain. Cracks widen, concrete expands, and structural capacity diminishes. Repair is difficult and expensive—prevention through proper materials selection is essential.

Testing aggregates before use prevents ASR. Using low-alkali cement, supplementary materials like fly ash, or proven non-reactive aggregates eliminates risk. The cost of prevention is minimal compared to the cost of replacement or extensive repair of ASR-damaged structures.

Technical Details

ASR mechanism:

• Alkalis (sodium and potassium oxides) from cement
• React with reactive silica in certain aggregates
• Form alkali-silica gel
• Gel absorbs water and expands
• Expansion creates internal pressure and cracking

Reactive aggregate forms:

• Opaline or chalcedonic silica
• Volcanic glass
• Strained quartz
• Certain cherts and quartzites
• Regional variation in aggregate reactivity

Conditions required for ASR:

1. Reactive silica in aggregate
2. Sufficient alkali content in cement
3. Adequate moisture (greater than 80% RH internally)

All three must be present—eliminating any one prevents ASR.

Visual symptoms:

• Map cracking (random pattern)
• Surface pop-outs and exudation
• Expansion and displacement
• Gel seepage from cracks
• Progressive deterioration over years

Time frame:

• Typically appears 5-15 years after construction
• Continues progressively once initiated
• Moisture availability affects rate
• Can be slower or faster depending on conditions

Prevention strategies:

Aggregate testing:

• ASTM C1260 rapid test
• ASTM C1293 long-term test
• Identify reactive aggregates before use
• Use proven non-reactive sources

Cement selection:

• Low-alkali cement (less than 0.60% Na2O equivalent)
• Reduces alkali availability for reaction
• May not eliminate risk with highly reactive aggregates

Supplementary materials:

• Fly ash (15-30% replacement)
• Slag cement (35-50% replacement)
• Silica fume (5-10% replacement)
• Dilutes alkalis and consumes free hydroxides

Lithium compounds:

• Lithium salts added to mix
• Alter reaction products to non-expansive forms
• Effective but expensive

Mitigation for existing ASR:

Monitor and document:

• Track crack progression
• Measure expansion
• Assess structural impact
• Determine intervention timing

Surface treatments:

• Sealers reduce moisture ingress
• Slows but doesn't stop reaction
• Temporary measure

Structural reinforcement:

• Confine expansion with external restraint
• Post-tensioning or wrapping
• Maintains serviceability despite expansion

Replacement:

• For severe cases threatening safety or function
• Ensure proper materials in replacement
• Most costly but complete solution

Testing and diagnosis:

• Petrographic examination identifies gel
• Chemical analysis measures alkali content
• Core expansion tests predict future expansion
• Differential diagnosis from other cracking causes

ASR is a durability problem specific to certain aggregate-cement combinations. Most aggregates are non-reactive. Regional experience guides material selection—local concrete suppliers know which aggregates are safe in their area.

Related Terms

• Aggregate - Must be tested for ASR reactivity
• Durability - ASR is a major durability concern
• Cracking - ASR causes characteristic map cracking

Learn More

• Diagnose alkali-silica reaction from a photo →
• Concrete Basics - Understanding concrete materials
• Types of Concrete - Different mix options to prevent ASR
• Concrete Calculator - Calculate project volume