MOISTURE DIFFUSION IN CEMENTITIOUS MATERIALS - MOISTURE CAPACITY AND DIFFUSIVITY

Based on a model by Bazant and Najjar, and using a new model for adsorption isotherms, moisture capacity and diffusivity of concrete are analyzed. The moisture capacity, obtained as a derivative of the adsorption isotherm, first drops as the humidity increases from zero, then levels off as a constant, and finally again increases when the humidity approaches saturation, regardless of the age, cement type, temperature, and water:cement ratio. The well-known diffusion mechanisms, including the ordinary diffusion, Knudsen diffusion, and surface diffusion, are analyzed and the diffusion in concrete is treated as a combination of these mechanisms. An improved formula for the dependence of diffusivity on pore humidity is proposed. The improved model for moisture diffusion is found to give satisfactory diffusion profiles and long-term drying predictions. The model is suited for incorporation into finite element programs for shrinkage and creep effects in concrete structures.