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Since concrete paving units have many shapes and thicknesses, their compressive strengths vary, and these strengths decrease with increasing thickness (like all concrete, steel and wood materials). To better address these differences, ASTM recently approved standardized compression test specimen sizes that normalize compressive strengths to that for a 2 3/8 in. (60 mm thick) rectangular (98 x 198 mm) paver.
Standardizing the specimen size allows for more consistent and comparable strength test results. Research by the Interlocking Concrete Pavement Institute resulted in a uniform dimensional range for test specimens and compressive strength test methods soon to be included in ASTM C140 Standard Test Methods for Sampling and Testing Concrete Masonry Units and Related Units.