干湿交替下表层混凝土中水分与离子传输过程研究
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摘要
干湿交替是对混凝土耐久性最不利的环境之一，而干湿交替下混凝土内水分与离子传输机理比单调干燥或湿润条件下更为复杂。本文对混凝土微结构和水分传输的关系进行了深入讨论，指出毛细孔端部水分供给流量差异导致了液态水在不同尺寸孔隙中流动优先权的不同，并在宏观上表现为干燥、湿润过程中水分传输速率的差异。通过试验，对干湿交替下混凝土内部相对湿度和电导率的变化进行了研究。提出用扩散方程的形式描述水分传输的总效果，并对干燥、湿润过程采用不同的扩散系数。利用“预估－校正”隐式有限差格式求解偏微分方程。提出了干湿平衡时间的概念，并利用该概念将干湿交替下混凝土内水分传输过程分为：均衡、湿润主导和干燥主导过程。计算表明，湿润主导和干燥主导过程的影响深度分别由干燥时间和湿润时间决定。
对混凝土和环境之间的水分交换进行了深入分析，并通过边界层理论进行了近似计算。通过试验，测定了混凝土近表面层内大气的温、湿度变化特征。利用有限元程序，求解了一维“混凝土－环境”传输模型。研究表明，混凝土表面与大气环境之间存在着厚度为几厘米的温、湿度变化明显的过渡区。在确定混凝土传热传湿的边界条件时需考虑该过渡区的影响，而在近表面现象不明显的情况下可采用第一类边界条件。
将干湿交替下混凝土内水分传输模型并入氯离子传输模型，并考虑了离子浓度对水分传输的影响。利用上游加权“预估－校正”有限差分格式求解对流占优的氯离子对流－扩散问题。计算结果和4个月干湿交替试验实测的氯离子含量分布吻合较好，证明了本文模型的正确性。研究表明，由于混凝土对氯离子的强吸附性，氯离子在混凝土中的入侵深度落后于水分传输。
连续2年记录北京典型气候下混凝土内湿度变化，结果表明降雨事件可使混凝土内部孔隙相对湿度迅速恢复至100%，混凝土中水分传输的影响深度约为15mm。最后还就定量计算在实际工程耐久性评估中的应用进行了案例分析。
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.
对混凝土和环境之间的水分交换进行了深入分析，并通过边界层理论进行了近似计算。通过试验，测定了混凝土近表面层内大气的温、湿度变化特征。利用有限元程序，求解了一维“混凝土－环境”传输模型。研究表明，混凝土表面与大气环境之间存在着厚度为几厘米的温、湿度变化明显的过渡区。在确定混凝土传热传湿的边界条件时需考虑该过渡区的影响，而在近表面现象不明显的情况下可采用第一类边界条件。
将干湿交替下混凝土内水分传输模型并入氯离子传输模型，并考虑了离子浓度对水分传输的影响。利用上游加权“预估－校正”有限差分格式求解对流占优的氯离子对流－扩散问题。计算结果和4个月干湿交替试验实测的氯离子含量分布吻合较好，证明了本文模型的正确性。研究表明，由于混凝土对氯离子的强吸附性，氯离子在混凝土中的入侵深度落后于水分传输。
连续2年记录北京典型气候下混凝土内湿度变化，结果表明降雨事件可使混凝土内部孔隙相对湿度迅速恢复至100%，混凝土中水分传输的影响深度约为15mm。最后还就定量计算在实际工程耐久性评估中的应用进行了案例分析。
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.
引文
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