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Petrographic analysis is a method of testing hardened concrete to track down problems in the mix. At a minimum, information gleaned from a petrographic analysis will include: an estimated water-to-cement ratio; the degree of cement hydration; the extent of paste carbonation; the presence and estimated content of fly ash or slag estimation of air content; identification of reactive aggregates; identification of harmful alkali-aggregate reaction; and sulfate attack or other chemical attack. Petrographic analysis involves examining the concrete with the unaided eye, and microscopical examination using stereomicroscopic, petrographic, and metallographic microscopes. It is primarily a qualitative, rather than quantitative, analysis, though some modified techniques help determine, on a volume basis, the percentage of paste, fine aggregate, coarse aggregate, or any other constituent. Sampling is carried out on cores taken from the concrete in question. There should be at least two cores taken--one from the afflicted section of concrete, and one from an unaffected part. The tests are carried out according to procedures described in ASTM C 856, "Standard Recommended Practice for Petrographic Examination of Hardened Concrete."