Strain measurements on porous concrete samples for triaxial compression and extension tests under very high confinement X H Vu*, Y Malecot, and L Daudeville Universite ´ Joseph Fourier – Grenoble I, Laboratoire Sols, Solides, Structures – Risques, Grenoble, France The manuscript was received on 31 March 2009 and was accepted after revision for publication on 16 July 2009. DOI: 10.1243/03093247JSA547 Abstract: 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 unprece- dented 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 displace- ment 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. Keywords: concrete, triaxial compression test, triaxial extension test, high confinement, high stress, experimental procedure, strain measurements 1 INTRODUCTION Concrete is the most widely used manufactured material in the world. In particular, it is employed in the building of highly sensitive infrastructure (civil engineering structures, dams, nuclear power plants, etc.). Its mechanical behaviour however is still rather poorly understood, especially under extreme load- ings. When subjected to violent explosion or ballistic impact, concrete undergoes very severe triaxial loadings [1]. In exceptional cases, such an impact may cause complete perforation of the target. The validation of concrete behavioural models, which take the phenomena of fragile damage and irrever- sible strain in compaction into account simulta- neously, thus requires test results capable of repro- ducing complex loading paths. This triaxial behaviour can be identified under quasi-static conditions thanks to a triaxial press with high loading capacity, which allows for a homo- geneous, well-controlled and precisely guided load- *Corresponding author: Laboratoire Sols, Solides, Structures – Risques, Universite ´ Joseph Fourier – Grenoble I, Grenoble, 38041, France. email: vu@hmg.inpg.fr; vuxuanhong2003@yahoo.fr 633 JSA547 J. Strain Analysis Vol. 44