This study is intended to characterize the evolution in triaxial behavior of a standard concrete subjected to confining pressures varying from 0 to 600 MPa. Hydrostatic and triaxial tests, with several unloading-reloading cycles, are carried out on concrete samples using a high-capacity triaxial press. These tests serve to identify the evolution of the elastic unloading characteristics of concrete, depending on both confining pressure and axial strain. A number of optical observations are also provided to allow visualizing the evolution in concrete damage mode in the middle of the sample. Experimental results indicate a sizable change in concrete behavior with confining pressure. At low pressure values, Young's modulus decreases and Poisson's ratio rises sharply with axial strain. The concrete exhibits brittle behavior with failure caused by a localized damage mechanism. In contrast, at high confining pressures, the concrete becomes a ductile material, and the evolution in its unloading characteristics is negligible. Failure is thus associated with diffuse material damage. The concrete behaves like a granular material controlled by plasticity, meaning that the damage phenomenon observed at low confinement is completely inhibited. © 2009 RILEM.