Statistical Evaluation of Concrete Strength Tests

Compressive strength data from tests of portland cement mortar cubes and concrete cylinders display central distribution tendencies that render them readily adaptable to evaluation by statistical procedures. Variation in strength, as measured by the statistical parameter coefficient of variation, is as important as average strength in evaluation of strength test data. Using the minimum compressive strength for cement cubes specified in ASTM C-150 as a basis, the optimum choice from among different brands is readily calculated from the average strength and variation among several shipments of the different brands. Statistical evaluation of ninety-two batches of 3,000-psi (nominal strength) concrete reveals that the batching water is the largest contributor to variability of the concrete strength. Using the risk factor given in ASTM C-94 and known variation in concrete strength for a given plant operation, the required average strength can be calculated. It is desirable to keep variation in concrete strength as low as possible, thereby minimizing required average strength and concrete costs.