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EARLY-AGE EVOLUTION OF THE MASS TRANSFER PROPERTIES IN MORTAR AND ITS INFLUENCE UPON ULTIMATE SHRINKAGE

In order to predict accurately the local shrinkage strains and eventually the shrinkageinduced stresses in concrete, it is necessary to determine in the first place the moisture distribution within the material. The knowledge of the two basic relationships that rule the mass transfer behavior of cementitious materials is required for that matter, namely the sorption isotherms and the diffusion coefficient as a function of the water content. While both of these relationships have been studied rather extensively for well hydrated materials, their evolution during the first few days after casting, which might be quite significant in practice, is poorly documented. This paper presents test results yielded in a research program undertaken to fill this gap. The moisture transfer properties and ultimate shrinkage values of a Portland cement mortar having a w/c ratio of 0.40 were characterized starting from 1 day, 3 days, 7 days and 28 days. The series of experiments performed include the monitoring of weight change during desorption above saturated salt solutions and ultimate length change measurements at different R.H. levels between 50% and saturation, pressure membrane desorption tests, dynamic weight change measurements of specimens having different thicknesses (50% R.H.), mercury intrusion porosimetry and degree of hydration assessment. Using classical assumptions, the coefficient of diffusion for the different levels of maturity is determined as a function of moisture content. The results are discussed in view of their dependency upon maturity, especially the extent to which drying shrinkage is affected by the curing regime duration.