Effect of Specimen Sizes on Flexural Compressive Strength of Concrete

It is important to consider the effect of member length when estimating the ultimate strength of a concrete flexural member. It is also essential to evaluate the effect of neutral-axis depth on the flexural compressive strength of a beam. The current experimental data is still insufficient, however, for a proper evaluation. For all types of loading conditions, the trend is that the strength of a member tends to decrease when the member length and depth increase. In this paper, the length and depth variations of a flexural compressive member have been studied experimentally. A series of C-shaped specimens subjected to axial compressive load and bending moment were tested. More specifically, four different length (h = 10, 20, 30, and 40 cm) and three different depth (c = 5, 10, and 20 cm) concrete specimens are tested to investigate the size effect of member length h and neutral axis depth c, respectively. The thickness of the specimens was kept constant where the size effect in out-of-plan direction is not considered. The test results are curve fitted using Levenberg-Marquardt’s least square method (LSM) to obtain parameters for the modified size effect law (MSEL) by Kim, Eo, and Park. The analysis results show that the effect of specimen length and depth on ultimate strength was apparent, but their effect on the ultimate strain was negligible. Finally, more general parameters for MSEL are suggested.