Study On Water And Ionic Transport Processes In Cover Concrete Under Drying-wetting Cycles

Drying-wetting cycles are identified as one of the most serious environmentalconditions for the durability of concrete structures. The involved moisture and ionictransport processes are more complicated than those in monotonic drying or wetting.The in?uence of the morphology of material pore structure on the moisture transportwas investigated in depth and it was pointed out that di?erent water supply rates atcapillary openings cause the di?erence of transport priorities of liquid water in capil-laries of di?erent sizes thus result in di?erent macroscopic transport rates during dryingand wetting. The evolution of relative humidity and electrical conductivity was inves-tigated experimentally for concrete specimens subjected to drying-wetting cycles. Itwas proposed that the di?usion equation can be adopted to describe the total moisturetransport in concrete subjected to drying-wetting cycles but di?erent di?usion coef-ficients were assigned to drying and wetting phases. The partial deferential equationswere solved by finite di?erence method with the"predictor-corrector"implicit scheme.The concept of equilibrium time ratio between drying and wetting was introduced tocharacterize the moisture transport, by which drying-wetting cycles can be classifiedinto drying-dominated, wetting-dominated as well as equilibrium ones. It was foundthat for drying-dominated cases the moisture in?uential depth is determined by wettingtime and by drying time for wetting-dominated cases.The moisture exchange between concrete surface and environment was investi-gated through experiments, boundary layer theory as well as numerical simulations.Experiments were performed to study the relative humidity and temperature distribu-tion and evolution in the near surface region of concrete. The involved"concrete-environment"moisture transport problem was solved by finite element method for onedimension case. A transition layer with a thickness of several centimeters was identi-fied between concrete and air where the relative humidity and temperature vary consid-erably. The existence of this layer necessitates the consideration of near surface e?ect in defining boundary conditions for hydrothermal transport in concrete. The Dirichletcondition can be used as the layer becomes thin enough.The model of moisture transport during drying-wetting cycles was integrated intothe ionic transport model in concrete, in which the in?uence of ion concentration onmoisture transport was taken into account. This convection-dominated convection-di?usion problem was solved by finite di?erence method using the"predictor-corrector"implicit upwind scheme. The calculated chloride distributions in concretewere in good agreement with the chloride profiles from 4-months drying-wetting ex-periments. Due to the chloride binding capacity of concrete the penetration process ofchloride is considerably retarded compared to the moisture transport.Relative humidity in concrete specimen situated in typical climate of Beijing areawas recorded for 2 years. From the recording, it was found that by natural precipi-tation concrete internal humidity resumed to 100% in several hours and the moisturein?uential depth into concrete is about 15mm.At the end, the above transport modelsare applied to the durability design and assessment of a large-scale transport hub and asea-crossing bridge.