AbstractThe primary objective of this study was to investigate the constitutive relations and damage properties of concrete in complex stress states with different stress paths under high pressure. Monotonic hydrostatic pressure, cyclic hydrostatic pressure, uniaxial stress with lateral confinement, and multiaxial proportional stresses were applied in tests. High stresses reaching up to 500 MPa under the aforementioned four loading paths were achieved through a true triaxial testing system. A series of uniaxial tests were carried out after triaxial tests to evaluate the damage degree of concrete. Microscopic observations were also made to visualize the changes of microstructures of the specimen before and after the triaxial compressive tests. The most important finding of this study is that concrete suffers damage even before it is loaded to the ultimate strength under multiaxial compressive stress states, different from the commonly used concrete models that assume the concrete damage only occurs in the strain softening stage. This means the conventional deviatoric strength theory alone may not be sufficient to model concrete material damage under complex stress states, instead the combined effect of hydrostatic and deviatoric stress should be considered together in predicting the concrete material damage. This study also found cyclic hydrostatic loads could induce more severe damage to concrete than monotonic hydrostatic loads with the same stress amplitude.