Specimen shape and the problem of contact in the assessment of concrete compressive strength

This paper proposes a critical analysis of the studies which, since the 1950s, have attempted to quantify the influence of specimen shape on the determination of concrete compressive strength, with special regard to the problem of conversion from cylinder to cube strength and vice versa. From such a retrospective analysis, it emerges that the problem of contact between the platens of the testing machine and the concrete specimen plays a crucial role for the explanation of the variability of the concrete compressive strength as a function of specimen shape. To obtain quantitative predictions and to investigate on the influence of the friction coefficient, uniaxial compressive tests are numerically simulated by using a nonlinear finite element model. Both the constitutive nonlinearity of concrete and the nonlinearity due to contact are taken into account in the formulation. The results of the proposed parametric analysis permit to evaluate the evolution of the conversion ratio between cylinder and cube strength as a function of the friction coefficient. This sheds a new light on the complex nature of this nonlinear relationship, whose value approaches 1 for a friction coefficient close to 0.01, simulating the presence of Teflon, and then approaches asymptotically 0.78 for f = 0.60, as is typical of steel-concrete interfaces