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Petrographic examination of aggregate and hardened concrete has been conducted by the Ministry of Transportation since the late 1940's. There were problems in communicating results of petrographic examination of aggregates to engineers. This resulted in the development of the “Petrographic Number” (P.N.) as a way of expressing the quality of coarse aggregate. Four quality categories are recognized: good aggregate (factor 1), fair (factor 3), poor (factor 6), and deleterious (factor 10). The Petrographic Number is calculated by multiplying the percentages of each rock type by the appropriate factor. The products are then added up to arrive at the P.N. The higher the P.N., the poorer the quality of the aggregate. A perfect aggregate would have a P.N. of 100. Petrographic examination is conducted on sand using automated equipment. The percent of deleterious (mica, shale) or alkali-reactive components (chert, glass) is reported. Petrographic examination of concrete is also conducted. Over the years, many problems have been investigated, such as: firedamaged concrete, frozen concrete, concrete damaged by alkali-aggregate reactions of various types, damage by freezing and thawing, causes of low strength and retarded concrete. In addition, petrographic examination has been used to detect additions of non-standard components to Portland cement.
aggregates, performance, petrographic examination, concrete, classification
Petrographer, Engineering Materials Office, Ministry of Transportation, Downsview, Ontario