Microscopic damage mechanism has an important impact on the macroscopic mechanical properties of concrete materials. Based on the statistical damage theory and the experimental phenomena,a statistical damage constitutive model for concrete under biaxial compression was proposed, in which two microscopic damage modes, rupture and yield,were considered.The whole damage evolution process was driven by the principal compressive strain. The concept of “equivalent transfer tensile damage strain” was introduced, and the compressive damage in press direction was controlled by the lateral tensile damage strain. By this model, the envelope composed of the stress-strain curves under biaxial compressive proportional load path was predicted. Furthermore, the strength envelope was obtained. The damage mechanism of concrete under biaxial compression was discussed from the view points of biaxial strength, deformation properties and the envelope shape. The results showed that it can accurately reflect and predict the constitutive behavior of concrete under biaxial compression, and reveal the microscopic mechanism of damage evolution. The yield damage mode plays a critical role, by which the whole deformation and failure process can be divided into the statistical uniform damage phase and the local failure phase. The two characteristic states，the peak nominal stress state and the critical state when macro-crack appears,were distinguished,and the latter was proposed as the final failure point in the constitutive model.
