Strain measurements on porous concrete samples for triaxial compression and extension tests under very high confinement

This article presents the production of strain measurements on porous concrete samples for use in triaxial compression and extension tests with a very high lateral confining pressure. When a massive concrete structure is subjected to severe loadings (e.g. rock falls, near-field detonations, and ballistic impacts), the material undergoes triaxial loading at a high confining pressure. To reproduce high levels of stress with well-controlled loading paths, static tests are carried out on concrete using a high-capacity triaxial press, called GIGA. This press allows the testing of concrete specimens (7 cm in diameter and 14 or 15.5 cm long) for levels of confining pressure ranging up to 850 MPa and axial stresses of up to 2.35 GPa. The porous characteristic of the material together with the high confining pressure require both developing a material protection device and building strain gauge-based instrumentation of unprecedented design for such confining pressures. In addition, the effect of pressure and other sources of error on strain and stress measurements are identified herein thanks to tests performed on model materials. This study shows that the effect of pressure on strain gauge measurements is negligible, whereas this same effect proves significant in the axial displacement measurement by means of a linear variable differential transformer (LVDT) sensor and must be taken into account therefore during the data processing phase. This article will present the initial results of triaxial compression tests conducted at high confining pressure on both dry and saturated concrete samples instrumented with gauges. It will also provide results of a triaxial extension test conducted at high confinement on dry concrete: a unique step in characterizing the triaxial behaviour of concrete. Moreover, it will be demonstrated that simultaneous axial strain measurements using gauges and the LVDT sensor serve to evaluate strain homogeneity of the sample tested at high confinement.