## 1 Introduction  
The purpose of this study was to design an efficient sensor to measure the concrete interstitial pore pressure at high confining pressure. When a massive concrete structure, such as nuclear power plant, is subjected to rigid missile impact at a velocity of several hundred m/s, the material in the vicinity of the impact zone undergoes high stress levels with a mean stress level up to 1 GPa [1,2]. Furthermore, since protective concrete structures are massive, they have a very long drying process, which induces a non-homogeneous saturation ratio. They retain a quasi-saturated core throughout their lifetime, even though their skin dries very quickly [3]. [PERSON] et al. [4] compared the dynamic and the quasi-static behaviors of concrete under confined compression; the dynamic strength of concrete is slightly greater than the quasi-static strength. Thus, in a first approximation, the dynamic and the quasi-static behaviors of concrete under confined compression can be considered as identical. A research program was launched in 2004 aiming at characterizing concrete triaxial behavior under high confinement pressure in quasi-static conditions with the Giga press (Section 2.1). The overall program was financed by CEA-DAM (French Alternative Energies and Atomic Energy Commission--Direction of Military Applications). Several studies have been performed on samples issued from the same reference concrete called R30A7 where its compressive strength is equal to 30 MPa at 28-days. The maximum aggregate size (8 mm) has been chosen with respect to specimen diameter (70 mm). The experimental campaign has deeply investigated the influence of the water/cement ratio [5], the saturation degree [6], the effect of coarse aggregate shape [7] and the initial matrix porosity for both dried and saturated samples [8,9] on the concrete behavior under high confinement. The obtained results have pointed out that the volumetric behavior of saturated concrete is stiffer than the dry one due to a pore closure effect, as modeled by [PERSON] [10]. The shear strength of dry concrete increases linearly with the confining pressure, while the shear strength of saturated concrete exhibits a plateau. These studies have also shown that the presence of free water inside concrete pores has a preponderant role on its behavior under high confinement. The influence of free water was also noticed under dynamical loading by the authors of [11; 12]. Pore pressure of concrete subjected to high temperature has already been measured [13; 14] but, to the best of our knowledge, the pore pressure due to high mechanical loading has never been measured. The literature reports a considerable number of drained tests on rock and sand for which the pore pressure is well controlled in the samples. The drain condition is achieved by means of pipes connecting the sample upper and lower faces with a hydraulic pump and valves [15; 16; 17; 18]. In addition, several techniques have been devoted to develop reliable and efficient systems to control the interstitial pore pressure during triaxial tests. A fluid pump equipped with valves and pressure transducer was used by the authors of [19; 20], who estimated the permeability and the Biot's coefficients for tight and Bakken rocks. Such systems concern moderate stress levels and cannot be adapted to the Giga press since it would require considerable modifications. [PERSON] [21] used the idea of a water collecting cap while performing triaxial undrained tests on rocks where the specimen was sandwiched between two caps. The pore pressure measurements were made with an endcap containing a pressure transducer (Kulite HKM-375) inserted into a Tygon sleeve at the sample bottom as reported by the author. The study presented herein aimed at completing these previous studies by focusing on the measuring of the interstitial pore pressure into concrete at high confining pressure.