Matthieu Briffaut

Influence of the Saturation Ratio on Concrete Behavior under Triaxial Compressive Loading

When a concrete structure is subjected to an impact, the material is subjected to high triaxial compressive stresses. Furthermore, the water saturation ratio in massive concrete structures may reach nearly 100% at the core, whereas the material dries quickly on the skin. The impact response of a massive concrete wall may thus depend on the state of water saturation in the material. This paper presents some triaxial tests performed at a maximum confining pressure of 600 MPa on concrete representative of a nuclear power plant containment building. Experimental results show the concrete constitutive behavior and its dependence on the water saturation ratio. It is observed that as the degree of saturation increases, a decrease in the volumetric strains as well as in the shear strength is observed. The coupled PRM constitutive model does not accurately reproduce the response of concrete specimens observed during the test. The differences between experimental and numerical results can be explained by both the influence of the saturation state of concrete and the effect of deviatoric stresses, which are not accurately taken into account. The PRM model was modified in order to improve the numerical prediction of concrete behavior under high stresses at various saturation states.

Induced Anisotropic Gas Permeability of Concrete due to Coupled Effect of Drying and Temperature

An experimental campaign is carried out to study the effect of drying shrinkage and temperature on multi-directional gas permeability of dry concrete. Thermal loadings up to 250°C are applied on concrete samples in cylinder (11×22) and dog-bone forms (total length of 61 cm). Samples are sliced for permeability measurements. Permeabilities in longitudinal and radial directions are addressed. The cylinder samples are first sliced then dried or heated whilst the dog-bone samples are first dried or heated then sliced. The average of initial intrinsic permeability for the slices (5 cm height, 11 cm diameter) obtained from the (11×22) samples is found isotropic and equal to 2.93×10-17 m2. In this case, drying shrinkage is isotropic. Furthermore, it is shown that for the dog-bone samples, drying shrinkage may induce micro-cracks preferentially in a certain direction which induces permeability anisotropy. Finally, the evolution of the normalized intrinsic permeability with respect to initial permeability versus temperature is found isotropic. An exponential fitting of intrinsic permeability versus temperature is found based on experimental measurements.

Massive Structure Monitoring: Relevance of Surface Strain Measurement

Most of large civil engineering concrete structures have been instrumented for decades with embedded sensors. To prevent the eventual loss of data, complementary instrumentation of external surface has recently been deployed. This new instrumentation can take different forms but in all cases, to avoid damaging the structure, it will be only superficially anchored. Near the outer surfaces, thermo-hydromechanical concrete behaviour is more sensitive to varying environmental conditions than in the centre of the structures. Therefore, the strain measured near the outer surfaces is not identical to the strain measured by embedded sensors. Consequently the methods of classical physical-statistical analysis must be reviewed. Using a thermo-hydro-mechanical finite element modeling calibrated on a representative concrete and applied on a current part of a thick structure, this work confirms a dependence of strain on the depth. First results show that the depth impact affects both kinetic and amplitude strain.