Experimental study of fatigue behavior of concrete under constant-amplitude biaxial cyclic tension-compression with constant confinement

The fatigue tests on biaxial tension-compression under constant-amplitude cyclic loading with constantly confined stress were carried out on plain concrete,simulating the fatigue state subjected frequently to pre-stressed concrete bridges with vehicle loads.A biaxial fatigue test equipment based on the stress-controller close-loop system of MTS is reconstructed.Then,the relationship of S-N and the fatigue strength envelopes of concrete under tension-compression loads with constant confinement are presented based on the analysis of the results of present experiments and fatigue tests under uniaxial tension-compression cyclic loading reported in the available literature.According to the data of stress and strain recorded in the process of fatigue tests,the deformation characteristics of concrete under the state of stress in present study are also investigated,which includes the development of longitudinal and transverse strains,the secondary creep rate and the deformation modulus of concrete in terms of cyclic number.The experimental results indicate that the lateral confinement stress level has little influnce on failure mode,but significant influence on fatigue strength of concrete.