Concrete strength depends on the amount, composition and quality of the hydrated binder to the fine and coarse aggregate. This paper reviews the way in which concretes can be examined with the petrographic microscope using both thin sections and polished plates. It reports observations made on actual samples, noting the similarity between the densities obtained by petrographic assessment and those established in laboratory tests.

The paper highlights the value of carrying out a petrographic investigation to obtain quantitative information in support of and prior to the conventional chemical investigation, including aggregate/cement paste ratio and void content, assessment of the weight fractions of water, cement and aggregate; assessment of the composition of the binder including the slag to Portland cement ratio, assessment of the PFA to Portland cement ratio and possibly the assessment of the type of Portland cement employed in the mixture. Attention is also drawn to the implications of aggregate shape and grading.

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