自然潮差下粉煤灰混凝土氯离子扩散性能时变性

Time-dependent chloride diffusivity of fly ash concrete in marine tidal environment

基于自然潮差环境下的现场暴露试验,研究了粉煤灰掺量对混凝土氯离子扩散性能及其时变性的影响。测试了4个暴露时间后,粉煤灰掺量分别为水泥质量的0%、20%、30%、40%和50%的混凝土内的自由氯离子浓度,同时用MIP法测试了试验混凝土的孔隙率;根据Fick第二定律,计算得到粉煤灰混凝土的氯离子表观扩散系数,并基于其表观扩散系数分析了混凝土的氯离子即时扩散系数,建立了粉煤灰掺量与混凝土氯离子扩散系数的时间衰减系数的关系模型;最后以实测试验混凝土氯离子浓度验证了氯离子即时扩散系数的时变模型。结果表明,混凝土的氯离子表观扩散系数及即时扩散系数均随暴露时间延长和粉煤灰掺量的增大而减小;混凝土氯离子即时扩散系数的时变性表明,粉煤灰混凝土的前期抗氯离子能力较强,后期逐渐减弱;混凝土氯离子扩散系数的时间衰减系数均随着粉煤灰掺量的增加而逐渐增大,氯离子即时扩散系数的时间衰减系数在暴露360 d后基本趋于稳定;基于暴露360 d后得到的混凝土氯离子即时扩散系数时变模型能较准确地预测混凝土中自由氯离子浓度。

Through field exposure tests of concrete in marine tidal environment, the influence of fly ash additive on chloride diffusivity and its time dependency is studied. First, free chloride concentration of the concrete is measured at four exposure times for fly ash contents of 0%, 20%, 30%, 40% and 50%, and porosity of the concrete is tested using a mercury intrusion porosimetry(MIP). Second, apparent diffusion coefficient of the concrete is calculated by Fick’s second law and used to determine instantaneous diffusion coefficients. An equation is formulated to describe the relationship of fly ash content versus diffusion coefficient. Finally, a time-dependent model of instantaneous diffusion coefficients is developed and verified against the measurement of concentrations. Calculation results of this model show that both apparent and instantaneous diffusion coefficients decrease with exposure time and fly ash content. And the time dependency of diffusion coefficients indicates that chloride resistance of fly ash concrete is greater in the early stage than that in the late stage. The age reduction factor of diffusion coefficients keeps increasing with fly ash content. However, at the exposure time of 360 d, this factor of instantaneous diffusion tends to be stable. Thus, our time-varying model of instantaneous chloride diffusion coefficients after 360 d exposure is accurate in prediction of free chloride concentration of concrete.