Philippe Poullain

Etude du couplage thermochimique au sein de matrices cimentaires en cours d'hydratation

ABSTRACT At very early age, cementitious systems can undergo thermal variations due to the double exothermic and thermoactivated property of cement hydration. The parameters of this thermochemical coupling (thermal conductivity, specific heat capacity, apparent activation energy and chemical affinity) have been identified from the literature and measured on cement pastes during the first 24 hours of hydration. They are then used in a 2D numerical model for the computing of hydration degree and temperature field of hardening cement-based materials. The aim is to quantify the effect of the thermophysical parameter values on the prediction of the materials behaviour at very early age. The sensitivity study shows that the influence of thermal property values is particularly significant in the case of temperature field computation.

Phenomenological interpretation of internal erosion in granular soils from a discrete fluid-solid numerical model

Internal erosion in granular soils may involve different steps: the detachment of solid particles from the granular skeleton under the action of water seepage; the transport of the detached particles carried with the water flow in the pore space; and eventually, for some erosion processes, such as suffusion, the possible reattachment of some transported particles to the solid skeleton of the soil, acting as a filter. The first part of this paper is devoted to the description and interpretation of the first step about the particle detachment. The analysis is mainly based on direct numerical simulations performed with a fully coupled discrete element-lattice Boltzmann method. Dynamics of the solid granular phase is represented thanks to the discrete element method in which each solid particle is explicitly described, whereas dynamics of the interstitial water flow is solved with the lattice Boltzmann method. Interactions between the solid phase and the fluid phase are handled at the particle scale avoiding the introduction in the model of some phenomenological constituents to deal with fluid-solid interactions. Numerical modellings of hole erosion can be interpreted similarly to laboratory hole erosion tests where the erosion rate is linearly related to the hydraulic shear stress. Further investigations from the numerical results suggest that the erosion rate for hole erosion in granular soil, can also be interpreted as a function of the water flow power according to a power law. The latter interpretation is applied to experimental data from suffusion tests on a cohesionless soil and glass bead mixtures. Here again, if change of erosion rate due to filtration is discarded, erosion rate is correctly described by the water seepage power according to a power law. Finally, a simple phenomenological model is suggested to describe the whole suffusion process, based on the previous results, to describe the particle detachment, and completed to take also into account the transport and filtration phases. Predictions of this model are compared with experimental results from suffusion tests on glass bead mixtures.

The effect of particle shape on the marginal rigidity state in 2D granular media

Experimental results from an investigation of the assembly of two-dimensional granular piles are presented. Three different particle shapes were used in the investigation of the evolution of piles towards isostatic and isotropic granular assemblies. Good agreement is found with previous experimental results for all particle shapes. We attempt to classify the divergence of a characteristic length-scale of the system as a power law and provide an estimate of the critical exponent. We find that the results are independent of particle shapes and speculate this may be a general feature of two-dimensional convex particles.

Sensitivity analysis of the transient heat and moisture transfer in a single layer wall

Abstract The determination of the energy consumption of a building requires the computation of the coupled heat and mass transfers inside the wall. The existing models require the determination of the hygrothermal properties of the materials, and of the boundary and initial conditions, prior to computing numerically the temperature and vapour fluxes and the energy and mass balance of the building. However, the experimental measurements do not give accurate values, yielding a wrong estimation of the energy consumption. In order to determine the influence of the uncertainties on the values of the input parameters of the Kunzel model, we carried out a local sensitivity analysis (LSA) by varying by 5% the hygrothermal properties of the materials, the surface transfer coefficients and the initial conditions. The results of the LSA are treated by means of quartile boxes in order to identifiy the most influent parameters. The LSA is applied to two walls made of highly porous and hygroscopic materials: limestone and hemp concrete. The results show that the surface transfer coefficients and the adsorption isotherm are the most influential parameters. However, the conclusions are slightly different between the two materials, because of differences in the relative importance of transfer phenomena.