Equivalent gas permeability of concrete samples subjected to drying

Abstract

The aim of this work is to give an equivalent gas permeability in order to estimate the associated flow through a concrete specimen exposed to drying, for example with a non-uniform moisture distribution inside (equivalent to a saturation profile), and related to the apparent permeability value, calculated for the same dry sample. The chain of reasoning to obtain this permeability for a given drying situation requires estimation of the water content profile via a model of drying, and of the relative permeability (determined for a given constant moisture content in concrete) over the entire saturation field. The method is based on a finite element idealisation of the sample thickness by a discrete approach in one dimension. It is the analogy between Darcy's law and Ohm's law that allows the local relative permeability to be likened to a local conductivity. Then the existing relation between conductivity and conductance leads to the determination of an equivalent conductance that is transformed into an equivalent conductivity. Thus the equivalent permeability that is calculated on the basis of this modelling is representative of a real in-situ phenomenon. This approach allows a more precise estimation of the flow rate variation in structures subjected to pressure increase as, for instance, in security tests of nuclear containment vessels.