Specimen and aggregate size effect on concrete compressive strength

Test results obtained on the effect of specimen size and aggregate size on concrete compressive strength and modulus of elasticity are presented. The concrete was prepared using 4.75, 9.5, 19.0, 37.5, and 75-mm nominal maximum size aggregate. Over 600 cylinders were cast and tested for concrete compressive strength and modulus of elasticity. Four different sizes of plastic cylinder molds were used: 150 by 300 mm, 100 by 200 mm, 75 by 150 mm, and 50 by 100 mm. The testing was carried out in accordance with ASTM standards for concrete compressive strength (ASTM 39) and modulus of elasticity (ASTM C 469). The test results were presented in terms of the concrete modulus of elasticity-to-compressive strength ratio (E/f(c)') versus cylinder diameter, maximum aggregate size, age at testing, among other factors. The results revealed that a size effect exists. The strength ratio was higher for the larger nominal maximum aggregate size concrete at each testing date. Hence, as the nominal maximum aggregate size decreased, the strength ratio decreased. The strength ratio followed a dished trend with respect to specimen size. For each cylinder size, as the nominal maximum aggregate size increased, the coefficient of variation increased. It was also found that for the same nominal maximum aggregate size specimens, the coefficient of variation increased as the cylinder size decreased.