In this paper, isothermal drying process in cementitious materials is described through a thermodynamics-based macroscopic modelling. This modelling involves the transport mechanisms governed by Fick's law and Darcy's law, without adopting the hypothesis of constant gas pressure. One essential input parameter required by the model is the capillary curve of the material, which is obtained through experimental water vapour desorption isotherm, for the high-performance and ordinary cement pastes and concretes studied here. The second required key-parameter is the intrinsic permeability of the material. Given the lack of experimental data, this property is identified here by fitting the relative weight loss plots corresponding to a drying test at RH = 50%, for each studied material. The relevance of the approach is illustrated by the good agreement between the moisture profiles provided by the model and the ones measured by gamma-ray attenuation, as a function of time. The influence of the initial moisture state of the materials, which results from self-desiccation, and the specific behaviour of high-performance materials are in particular pointed out in both cases. The analysis of drying process on the basis of the described modelling leads to new conclusions on the relative contribution of each moisture transport mechanism. Thus, only the Darcian moisture transport in liquid form contributes significantly to the medium and long-term drying of weakly permeable materials. The significance of the identified permeability has been analysed through the modelling, as well as through comparison with the values provided by the Katz-Thompson's relationship and by direct gas permeability measurements. This parameter has been thereby interpreted as water permeability. Finally, a method has been proposed to assess the water permeability of weakly permeable concretes when direct measurement is not possible. This is an inverse method, based upon the precise analysis and the modelling of the drying process of the material.
展开▼
