Modelling the drying shrinkage of porous materials by considering both capillary and adsorption effects

Abstract

This paper presents a poromechanical model for drying of unsaturated porous media valid for a large range of relative humidity. Using the proper laws of thermodynamics, this model is derived and permits to account for different effects that contribute to the effective stress development: the average pore pressure effect, the energy of the interfaces effect, the surface adsorption effect and the Shuttleworth effect. The majority of the input parameters of this model are simply assessed by using two commonly known techniques for the characterization of pores structures applied on experimental desorption isotherms: the B.E.T theory (Brunauer et al., 1938) and the BJH technique (Barrett et al., 1951). Another input parameter (linked to the Shuttleworth effect) is fitted on experimental drying shrinkage strains. This model is tested and validated with experimental data for different porous materials - hardened ordinary cement paste, high- performance concrete and Vycor glass - found in the literature. The obtained results show a satisfactory evaluation of the drying shrinkage strains for all three tested materials, with the possibility of considering zero fitting parameter. Compared to other poromechanical models found in the literature such as the classical Biot-Bishop (Biot, 1941) model and the (Coussy et al., 2003) model, our model appears to be capable of displaying the transition at a certain relative humidity between the capillary pressure effects and the surface adsorption effect, which manifests itself by a plateau in the drying shrinkage strains curve at this value of relative humidity.