The effect of moiture transport and sorption hystersis on ionic multispecies diffusion in concrete

Concrete durability is very much dependent on the moisture and ionic species concentration in the pore solution. Therefore it is of interest to find physically based models for predicting the evolution and variations of these properties for different kinds of relevant boundary conditions. A porous media technique based on the general mixture theory continuum approached is used to establish a set of governing coupled equation describing the process of interest.

In this model the equations are actually derived from examine the entropy inequality of the system. Lagrange multipliers are used to identify properties such as definitions of the chemical potentials of constituents. The non-equilibrium results from such evaluations is subjected to linearization in order to obtain a generalized Darcy flow equation and a set of generalized Fickian equations including for electrical fields induced by the charge character of the mixture of ionic constituents dissolved in pore solution.

The hysteresis in sorption is modelled by an explicit ‘history’ dependent assumption. The key issue in this context is to divide the moisture transport into two parts, vapour and water transport, and describing the mass exchange between them with guidance from the hysteresis equilibrium model. The coupled systems of equations are rewritten in the weak form suitable for development of finite element formulations.

A Taylor expansion is performed in order to reach a Newton-Raphson iteration scheme. The tangential stiffness and tangential damping of the global system is ignorer in the equilibrium iteration obtaining a more computational economic modified Newton-Raphson scheme with good convergence properties. Numerical examples of the performance of the model are presented.

The effect of hystersis in the sorption is shown to affect the diffusion of ions in the pore system. Mainly this is due to the moisture content, that is, an increased diffusion resistance at low moisture contents (and the other way around) as predicted by the hysteresis model during cases with variation of the ambient relative humidity.

Further, discussions of the important influence of electrical double layers at pore walls on the global model response are performed.