Some Factors Influencing Effect of Core Diameter on Measured Concrete Compressive Strength

Abstract

Various concrete mixtures were produced using crushed limestone and river gravels with four different maximum sizes of 10, 15, 22, and 30 mm (0.39, 0.59, 0.87, and 1.18 in.). The 28-day cube strength of concrete mixtures ranged between 28 and 43 MPa (4061 and 6236.6 psi). Beam specimens were cast by these concrete mixtures and cores were drilled from the beams. Compressive strength tests were performed at the ages of 7, 28, and 90 days on a total of 2268 core and cube specimens and the effect of core diameter on concrete core strength was examined. The strength correction factors were determined to convert the strength of a core with a diameter of 144, 69, or 46 mm (5.67, 2.72, or 1.81 in.) to that of a core having a diameter of 94 mm (3.70 in.). As the maximum aggregate size increased, the strength of the core decreased and, consequently, the correction factors increased. The effect was more pronounced for smaller diameter cores. The correction factors were somewhat higher for cores drilled from river gravel concrete. The age of the concrete was found to be an important factor in the strength correction of different diameter cores, that is, the older the concrete, the lower the correction factor. It should be noted, however, that it is very difficult to use age-versus-correction factor relation in practical terms. Additionally, test results showed that the length-to-diameter ratio of the specimen is more significant for small-diameter cores.