A concrete surface showing map cracking, expansion and staining supports several explanations that have almost nothing to do with one another. Alkali-silica reaction, sulfate attack and freeze-thaw damage all produce internal expansion and surface distress, and all three can look similar from a distance. They arise from different causes, call for different remedial approaches, and point at different parties — the aggregate source, the mix design, the exposure, the placing contractor. Surface appearance does not distinguish them. Examination of the concrete's own microstructure does.
What petrographic examination involves
Cores are cut, polished and prepared as thin sections, then examined under transmitted and reflected light — the procedure described in ASTM C856. The examination reads the concrete's internal record: aggregate types and their condition, paste microstructure, air-void system, crack patterns and their relationship to aggregate particles, and any reaction products present. Most deterioration mechanisms leave a distinctive and directly observable signature.
Alkali-silica reaction and its signature
ASR occurs when reactive silica in the aggregate reacts with alkalis in the pore solution to form a gel that swells as it takes up water. The petrographic signature is specific: reaction rims on affected aggregate particles, cracks originating within those particles and extending into the surrounding paste, and gel visible in cracks and voids. The pattern of cracks radiating from individual aggregate particles is what distinguishes it, since the expansion originates inside the aggregate rather than in the paste.
Sulfate attack originates in the paste
External sulfate attack proceeds from sulfates in groundwater or soil reacting with hydration products to form expansive phases, degrading the paste and reducing its strength. The signature is paste-centred rather than aggregate-centred: cracking through the paste, characteristic reaction products in voids and cracks, and progressive softening working inward from the exposed surface. Because it comes from outside, it shows a gradient with depth that ASR does not.
Freeze-thaw damage and the air-void system
Freeze-thaw deterioration results from water freezing in the pore structure and generating pressure the paste cannot accommodate. The controlling factor is the air-void system: correctly entrained air provides space that relieves that pressure. Petrography measures air content and, more importantly, the spacing between voids, which is what actually governs protection. Damage manifests as cracking parallel to the exposed surface and progressive scaling, and finding an inadequate void spacing in a structure exposed to freezing is usually the whole finding.
Delayed ettringite formation as a further candidate
Where concrete reached high temperatures at early age — from mass placement or accelerated curing — expansive phases can form later within hardened concrete, producing expansion and cracking without any external sulfate source. It is distinguishable petrographically and is worth considering specifically, since it points at the placing and curing operation rather than at the aggregate or the exposure, and it is easily mistaken for either of the other mechanisms.
More than one mechanism is common
These processes are not mutually exclusive and frequently combine. Cracking from any cause admits water and increases permeability, accelerating everything else. ASR cracking can allow freeze-thaw damage that would not otherwise have occurred; sulfate ingress follows cracks produced by any mechanism. Petrography can usually identify which came first from the relationship between crack sets and reaction products, and that sequence is often more useful than a single-cause label.
Supporting tests once the mechanism is identified
Petrography identifies the mechanism; other work quantifies it. Aggregate reactivity testing establishes whether the source was susceptible. Chemical analysis of the concrete and of the surrounding soil or groundwater establishes sulfate availability. Compressive strength on cores establishes what capacity remains. Expansion testing on cores can indicate whether a reaction is still active, which bears directly on whether repair will hold.
The documentary record that closes the loop
A mechanism identified in the concrete is connected to responsibility through the records: the approved mix design, aggregate source and any reactivity testing performed, cement chemistry, admixtures, batch tickets, placing and curing records, and the specified exposure class. A finding of ASR in a structure where the aggregate was never tested for reactivity is a different matter from the same finding where testing was performed and the aggregate passed.
What to preserve and sample
Full-depth cores from damaged and undamaged areas both, with locations and orientations recorded, since the comparison is what establishes the pattern. Samples of the aggregate source where obtainable. Soil and groundwater samples where sulfate attack is a candidate. The construction record including batch tickets and any early-age temperature data, and the exposure history of the structure including drainage, de-icing and any prior repairs.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.