Effect of elevated triaxial cyclic and constant loads on the mechanical properties of plain concrete

An experimental programme was carried out to investigate the effect of elevated cyclic or constant triaxial stresses on the change in mechanical properties of plain concrete. Triaxial cyclic tests on cylinders, with confining pressure and axial load varying in phase opposition at 1 Hz, were performed in three different deviatoric planes. The maximum stress deviator was 90% of the deviator at failure in a given deviatoric plane; the minimum deviator was 80% of the maximum one. In constant-load tests, the applied stresses were equal to the stresses at the maximum deviator applied in cyclic tests. At the cycle amplitudes selected, the creep effects under constant load are usually larger than the dynamic effects. The results obtained show that: the residual strength is increased with respect to the strength of companion virgin specimens, for any test condition; the increase in residual strength is much more important in triaxial tests than in uniaxial tests and increases with the test duration; the decrease in elastic modulus increases with the isotropic pressure at which tests are performed; and the test duration has a less pronounced effect on the decrease in elastic modulus.