Concrete Drying Modelling in a Variable Temperature Environment

EDF studies the behavior of double walls Concrete Containment Buildings (CCB) in nuclear power plant, for which the inner wall is prestressed and has no metallic liner. The inner wall of CCB is subjected to delayed strains of concrete (which are highly related to concrete drying) causing prestress loss and increased leakage with time. Moreover, the permeability of concrete itself is vastly dependent on its water saturation ratio. For these reasons, a good prediction of strains and leak-tightness of CCB must start with a good estimate of the moisture profile on concrete. On the EDF Lab Les Renardieres site in France, the VERCORS mock-up, a 1/3 scale CCB without a liner, has been built to improve the understanding and the modelling capabilities of ageing and leakage of double walls CCB. The inner wall of the VERCORS mock-up is subjected to a variable temperature drying environment, both in space and in time. In this work, a simplified drying model which take into account effects of temperature on concrete desorption isotherm and drying kinetics is proposed. In order to define a strategy for fitting model parameters, a sensitivity analysis based on Sobol' indices is performed. Next, the model is calibrated against independent sets of experimental data available on VERCORS concrete specimens up to temperatures of 70 degrees C. The calibrated model is validated by predicting the mass loss kinetics of several specimens which experience variable temperature drying conditions. Then, the fitted model is compared to direct RH profiles measurements of 40 cm thick blocks installed at different locations in the VERCORS mock-up. Finally, the prediction of the calibrated model is compared to the saturation profile of VERCORS mock-up measured by Time Domain Reflectometry (TDR) method.