Petrographic examination of concrete aggregates proposed for use on projects of the Bureau of Reclamation has assumed great importance as a supplement to standard, empirical, acceptance tests which are almost universally applied by large and progressive engineering organizations. Petrographic examinations entail visual inspection and lithologic segregation of various size-fractions of the aggregate, and they may be rapidly made by petrographers experienced in problems of concrete. Physical and chemical properties which are not determinable by any standard tests are estimated and qualitatively evaluated in terms of their effects upon concrete.

Methods employed and the point of view adopted in performance of the petrographic examination are discussed in this paper. Physical and chemical properties of aggregates which significantly affect concrete are outlined.

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