IS 2386:2021 Part 7 is the Indian Standard (BIS) for methods of test for aggregates for concrete - part 7: alkali-aggregate reactivity. This standard details the testing methods to evaluate the potential alkali-aggregate reactivity (AAR) of aggregates intended for use in concrete. It outlines both the chemical method for rapid screening and the long-term mortar bar method for assessing deleterious expansive potential.
Covers methods for determining potential alkali-aggregate reactivity of aggregates, including the mortar bar method and chemical method.
Key reference values — verify against the current code edition / project specification.
| Reference | Value | Clause |
|---|---|---|
| Test | Potential alkali-aggregate reactivity | Scope |
| Methods | Mortar-bar expansion + chemical method | Method |
| Reactive if | Expansion exceeds the specified limit | Criteria |
| Mitigation | Low-alkali cement / SCM / non-reactive aggregate | Remedy |
| Read with | IS 383 / IS 456 (AAR) / IS 16715 / IS 3812 | Cross-ref |
IS 2386 (Part 7):2021 specifies Methods of Test for Aggregates for Concrete — Part 7: Determination of Alkali Aggregate Reactivity — the most-modern part of the IS 2386 series, updated comprehensively in 2021 from the 1963 original.
Alkali-Aggregate Reactivity (AAR) is one of the most damaging concrete durability issues — a chemical reaction between alkalis from cement and certain reactive minerals in aggregate forms an expansive gel that cracks the concrete from within. Failures take 5-20 years to manifest, then accelerate. By the time visible, repair cost is 10-100× the original cost of avoidance.
Use Part 7 when: - Sourcing aggregate from a new quarry for any large concrete project (dams, bridges, mass concrete, marine structures, high-strength concrete) — mandatory pre-procurement screening - Investigating cracking in old concrete — radial / map cracking patterns, white gel exudation, slow progressive damage suggest AAR - Specifying low-alkali cement for AAR-prone aggregates - Designing mitigations — flyash / silica fume / GGBS replacement levels needed
The 2021 revision introduces modern accelerated tests: - Mortar bar method (Annex A) — fast (16-30 days) - Concrete prism method (Annex B) — slow but more representative of field conditions - Chemical method (alkali content) (Annex C) — quick screening of aggregate chemistry - Petrographic examination (Annex D) — microscope-based identification of reactive minerals
Reaction mechanism: alkali ions (Na⁺, K⁺) and hydroxyl ions (OH⁻) from cement attack reactive silica in aggregate, forming alkali-silica gel:
``` SiO₂ (reactive) + 2 NaOH → Na₂SiO₃ + H₂O ```
The Na₂SiO₃ gel absorbs water and expands, generating internal pressure that cracks the surrounding concrete. The reaction is slow (years to decades) but ultimately catastrophic.
Reactive mineral families:
Slowly reactive (decades to manifest): - Strained / cryptocrystalline quartz — found in deformed metamorphic rocks (quartzites, schists) - Microcrystalline quartz — found in cherts, flints, some sandstones - Granitic gneisses with strained quartz — common in central India, parts of Karnataka, Andhra Pradesh
Rapidly reactive (years to manifest): - Opal, chalcedony — amorphous / cryptocrystalline silica forms - Volcanic glass — andesite, basalt, rhyolite tuffs (relevant for Deccan plateau aggregates) - Cristobalite, tridymite — high-temperature silica polymorphs in volcanic rocks
Slowly reactive carbonates (alkali-carbonate reaction, ACR): - Dolomitic limestones with clay content — different mechanism, equally damaging - Less common in India but worth screening for highway / bridge aggregates
Indian regional risks: - Deccan basalt (Maharashtra, Madhya Pradesh): potential AAR risk from glassy basalt fines - South Indian gneiss / schist: variable; some sources reactive - Aravalli quartzite (Rajasthan, Haryana): generally OK but micro-quartzites suspect - Eastern Ghat charnockites: low risk - Andhra granitic gneiss: variable; testing recommended for major projects
1. Mortar Bar Test (Annex A) — accelerated, fast:
Acceptance: - Expansion < 0.10% at 16 days: aggregate is non-reactive for normal concrete - Expansion 0.10-0.20%: potentially reactive; further testing needed - Expansion > 0.20%: reactive; not suitable without mitigation
Fast (30 days), low cost, but accelerated conditions don't fully represent field. Best as a screening test.
2. Concrete Prism Test (Annex B) — slow, definitive:
Acceptance: - 1-year expansion < 0.04%: aggregate non-reactive - 1-year expansion 0.04-0.06%: mild reactivity - 1-year expansion > 0.06%: reactive; mitigation required
More representative of field conditions than mortar bar; slower; preferred for high-stakes decisions.
3. Chemical method (Annex C) — quick screening:
Fast (couple of days), but accuracy is limited; OK for first-pass screening.
4. Petrographic examination (Annex D) — definitive identification:
Definitive but requires geological expertise; results subjective and depend on petrographer's experience. Always pair with one of Methods 1-3 for actionable results.
Once an aggregate is identified as reactive, project options are:
1. Use low-alkali cement — IS 269 / IS 1489 / IS 12269 OPC with Na₂O equivalent ≤ 0.6% by mass of cement. Indian OPC typically has Na₂O equiv of 0.5-1.2%; specifying low-alkali variants adds cost and may need imported cement.
2. Replace cement with SCM — partial replacement reduces alkali contribution: - Class F fly ash 20-30% (most common Indian SCM) - Class C fly ash 30-40% (higher CaO; less effective than F) - Silica fume 5-10% (very effective; expensive) - GGBS (Ground Granulated Blast-Furnace Slag) 35-50% (excellent AAR mitigation) - Calcined clay (LC³ metakaolin) 15-25% (emerging; growing acceptance)
3. Limit cement alkali content — total alkali content of cement-paste portion of concrete ≤ 3 kg Na₂O equivalent per m³ of concrete. Calculation: (cement content × alkali %) + (SCM × alkali %) ≤ 3 kg/m³
4. Change aggregate source — if mitigation is impractical or costly, switch to a non-reactive aggregate from another quarry. Logistics + cost analysis needed.
5. Use water-reducing admixtures — reduces total water demand, hence concrete porosity, hence available water for AAR reaction. Modest mitigation; works in combination.
6. Limit aggregate proportion — for severely reactive aggregate, dilute with non-reactive coarse aggregate to reduce reactive fraction below threshold. Specialist approach.
For Indian dams, bridges, marine structures: any aggregate source with even 'mild reactivity' rating should be supplemented with 25-40% SCM replacement of cement. The marginal cost is small (₹50-150 per m³ of concrete) compared to lifecycle repair / replacement cost (₹10,000-50,000 per m³ for AAR-damaged concrete).
1. Skipping AAR testing on small / 'short-life' projects — assuming AAR doesn't apply because the project is 'just a regular building'. But buildings last 50-80 years; AAR develops in 5-20 years; the damage manifests during the building's service life. Test on all projects of significant value.
2. Using only one test method — relying on mortar-bar test alone (accelerated, conditions unrepresentative of field) or only chemical test (limited accuracy) gives misleading results. Use at least two methods, ideally mortar-bar + concrete prism + petrographic.
3. Not accounting for moisture in field service — AAR requires moisture to proceed. Indoor sheltered concrete may not develop AAR even with reactive aggregate; outdoor exposed concrete develops AAR rapidly. Risk-tier the application accordingly.
4. Forgetting blended cements have lower alkali contribution — IS 1489 (PPC) has lower effective alkali than IS 8112 (OPC 43) because the fly ash component dilutes alkalis. Project specifications that just say 'M30 concrete' without specifying cement type miss this distinction.
5. Using marginally non-reactive aggregate for premium concrete — AAR threshold is approximate. An aggregate at 0.09% mortar bar expansion (just under 0.10% threshold) may push over the threshold with high-alkali cement or in extreme service conditions. Conservative practice: choose aggregates well below thresholds for premium / long-life concrete.
6. Lab certification stale — aggregate properties vary across a quarry. A test certificate from 2 years ago doesn't represent today's quarry face. Mandate fresh testing for every major procurement (annual minimum for ongoing supply).
7. Mitigation not properly designed — using 15% fly ash 'because it's cheap' doesn't fully mitigate AAR. Mitigation levels depend on aggregate reactivity + cement alkali; should be specified by the design engineer based on actual test results, not nominal percentages.
8. Visible AAR cracks misdiagnosed as shrinkage — AAR cracks have specific patterns: map-like, with whitish gel exuding. Shrinkage cracks are linear, dry. Misdiagnosis leads to wrong repair (e.g., applying surface waterproofing won't stop AAR-driven cracking from within).
IS 2386 Part 7:2021 is the current revision — a major modernization from the 1963 vintage. The 2021 revision: - Aligned with ASTM C1260 / C1293 international methods for mortar bar and concrete prism testing - Added petrographic examination as a formal acceptance method - Introduced the concept of 'potentially reactive' (intermediate) classification - Added explicit mitigation strategies in Annex E - Linked to IS 383:2016 acceptance limits
Indian AAR landscape: - Awareness has grown — Major water-utility / NHAI / dam-authority projects now routinely test for AAR - Building construction lag — Most building projects still don't test; assume aggregate is 'OK' based on regional practice. This creates long-term risk. - Forensic cases — Several documented AAR failures in India: Mettur Dam (Tamil Nadu, 1980s), parts of Bhakra Nangal Dam concrete, some bridges in Maharashtra. Lessons partially learned.
Lab capability: - IIT geotech / materials labs, Central Soil and Materials Research Station (CSMRS), Central Road Research Institute (CRRI), Structural Engineering Research Centre (SERC) Chennai: AAR-capable, internationally recognized. Cost: ₹15,000-40,000 for a complete AAR evaluation (mortar bar + concrete prism + petrography). - Commercial private labs: most don't have full AAR capability; refer specialist work to the above institutes. - University / college labs: variable.
For procurement specifications: - Building projects > ₹50 crore: include AAR testing requirement in tender; conduct on samples from each quarry before bulk procurement - Dam / bridge / marine projects: mandatory; include in Material Quality Plan - Smaller projects: at minimum, request aggregate supplier's recent AAR test certificate; reject suppliers who can't provide one
For premium concrete in unknown AAR-status regions (e.g., Andhra-Karnataka border granite-quarry belts, Maharashtra trap-rock area): default to using 25-30% fly ash replacement; this provides AAR mitigation by default. Modest cost premium; large long-term risk reduction.
Long-term: AAR is going to remain a critical durability issue as Indian infrastructure ages. Specifying engineers should treat AAR mitigation as standard practice on any project with a > 30-year design life. The cost of testing + mitigation is trivial; the cost of an AAR-damaged structure is catastrophic.