How to Evaluate ASR Risk in Aggregate

HOW-TO · AUG 2026 · 3 MIN READ

Alkali-silica reaction is concrete cancer: reactive silica in the aggregate plus cement alkalis plus moisture makes a gel that swells and cracks structures for decades. Whether a source passes ASR review decides whether it can sell into DOT and structural work — here's the evaluation ladder, bottom to top.

01Start with petrography (ASTM C295)

A petrographer identifies the reactive suspects under the microscope: opal, chert, chalcedony, strained quartz, volcanic glass. Petrography doesn't pass or fail a source — it tells you what you're dealing with and how hard to look. Skipping it to 'just run the bar test' is how sources get mislabeled in both directions.

02Run the 14-day screen: ASTM C1260 (AMBT)

Mortar bars in 1N NaOH at 80°C — brutally accelerated. Common interpretation: expansion ≤0.10% at 14 days suggests innocuous, ≥0.20% indicates potentially reactive, and the 0.10–0.20 zone is the argument zone. Fast and cheap, but famous for false positives on some aggregates — which is why it screens rather than convicts.

03Confirm with the one-year truth: ASTM C1293 (CPT)

Concrete prisms at 38°C for a year; ≤0.04% expansion is the classic pass. Slow and costly, but it's the reference method — when C1260 and C1293 disagree, C1293 wins. A source with a failing C1260 and a passing C1293 plus clean field history is usually a usable source with a paperwork problem.

04If reactive, prove the fix: ASTM C1567

Rerun the accelerated test with the actual proposed mitigation — fly ash, slag, silica fume, or blended cement — and show expansion under 0.10%. This converts 'reactive aggregate' into 'reactive aggregate with a working recipe,' which is how a lot of the country's concrete gets built.

05Weigh the service record like the evidence it is

Twenty years of the same stone in local pavements and structures with no map-cracking is powerful data — many specifications explicitly allow it. But it only counts with the alkali loading and mix context documented; 'we've always used it' without records is folklore, not history.

06Translate to the governing spec

Every owner runs its own framework: AASHTO R 80's risk-based approach, state DOT tables, project specs with their own thresholds. Florida, Texas, and the western states treat identical numbers differently. The evaluation isn't done until it's mapped onto the spec that governs the sale — and any contradiction between old and new test data is resolved in writing, not hoped away.

The TrapTrusting one test in isolation. A lone passing C1260 on a source petrography would flag, or a decades-old passing C1293 contradicted by fresh bar tests, is exactly the kind of gap that surfaces in litigation after the pavement cracks. ASR conclusions are a weight-of-evidence verdict: petrography + accelerated + long-term + field history, all telling one consistent story.

Rule of ThumbC1260: 0.10 good, 0.20 bad, between is a fight. C1293 overrules C1260. Mitigation is proven with C1567, not adjectives — and the service record is evidence only when it's documented.

ASR forensics is a ShotRock specialty — contradiction resolution between test eras, spec mapping by state, and the weight-of-evidence memo that lets a source sell.

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