Influence of free water on concrete triaxial behavior: The effect of porosity

This work examines the effect of porosity and free water on the behavior of concrete under high confinement. For this purpose, three types of concretes are designed with either low, medium or high porosities but featuring the same aggregate skeleton, yielding: an ordinary referenced concrete (OC), as already characterized in previous studies; a high performance concrete (HPC) with very low capillary porosity; and a low performance concrete (LPC) with the same composition as OC except for a much higher entrained air porosity. The results of triaxial tests, conducted on these three concretes up to a confinement of 600 MPa at both low and high saturation ratios, are presented. The conclusions from past studies on OC are extended herein for the cases of HPC and LPC: the free water quantity exerts a major influence on the concrete strength capacity as well as the volumetric stiffness for all three types of concretes. These results also show that the free water influence on concrete behavior depends on both the amount and nature of this porosity: the effect of modifying the entrained air porosity is much weaker than that of capillary porosity. To better quantify the effect of concrete saturation ratio, results are then completed by triaxial testing performed on OC with intermediate saturation ratios. Lastly, an empirical triaxial failure criterion of concrete, which takes into account the uniaxial strength, porosity and saturation ratio, is proposed; this new criterion accurately reproduces the results presented, as well as, a large database obtained from previous studies.