A Coupled Chemical and Mass Transport Model for Concrete Durability

In this paper a general continuum theory is used to evaluate the service life of cement based materials, in terms of mass transport processes and chemical degradation of the solid matrix. The model established is a reactive mass transport model, based on an extended version of the Poisson-Nernst-Planck equations, which is further developed to take into account sorption hysteresis. The sorption hysteresis is modelled as phase equilibrium between the liquid and vapor phase in terms of third order polynomials. A change in sorption direction (from adsorption to or from desorption) results in inner scanning curves which is established from a set of mathematical criteria. The chemical degradation is modelled with the geochemical code iphreeqc, which provides a general tool for evaluating different paste compositions. The governing system of equations is solved by the finite element method with a Newton-Raphson iteration scheme arising from the non-linearity. The overall model is a transient problem, solved using a single parameter formulation. The sorption hysteresis and chemical equilibrium is included as source or sink terms. The advantages with this formulation is that each node in the discrete system has their individual sorption hysteresis isotherm which is of great importance when describing non fully water saturated system e.g. caused by time depended boundary conditions. Chemical equilibrium is also established in each node of the discrete system, where the rate of chemical degradation is determined by the rate of mass transport only. A consequence of the source or sink term, is the assumption that equilibrium is reached instantaneously in each time step considered. Some numerical problems was found, where the residual requirements for the chemical equilibrium was not reached. Small imbalances, in e.g. charge balance, from the mass transport calculation could cause the above mentioned numerical problems. Two different test cases are studied, the sorption hysteresis in different depth of the sample, caused by time depended boundary condition and the chemical degradation of the solid matrix in a ten year simulation. The relative simple test cases show that sorption hysteresis cannot be neglected in a mass transport model for cement based materials and a description of the chemical degradation is crucial for long term simulation of service life prediction.