Analysis of hydraulic concrete strength test methods

Today, methods of determining the tensile strength of concrete are of considerable interest. These methods using the anisotropy of brittle materials and, in particular, concrete, with respect to compressive and tensile stresses, make it possible, based on the findings of the theory of elasticity, to determine the tensile strength when tested with a compressive load, which can be transferred to the sample much more easily than tensile forces. The results of tests of concrete by the method of compression of cylindrical samples and crushing of cube samples with hewn ribs between two round metal rods lying in the same plane closely match the tensile strength values obtained with direct tension. For all studied compositions, the value of the strength ratio increases as the water-cement ratio increases, both for concrete without additives and with additives of surfactants. Using additives of surface-active organic substances without changing the dependence of the strength ratio on the hardening time makes it smoother. It has been found that the fast decreasing value of the strength ratio, hardening period that corresponds to its minimum value, and further increase or stabilization of Rp / Rc depend on a number of factors of the cement used, storage conditions of surfactant additives, composition, etc. Under the influence of the different factors the position of the characteristic points on the curve of the strength ratio dependence on the hardening time can change and obey the general pattern. For concretes and mortars, the decrease in the strength ratios in the transition from 28 days to the age of 90 days makes up to 10–25% (and in some cases much more), which must be taken into account in appropriate cases when designing structures.