A Basic Study on Microstructure and Chloride Diffusivity of Deteriorated Concrete due to Construction Defects

本研究揭示了孔隙结构对氯离子向因施工缺陷而变质的混凝土中迁移的影响。硬化混凝土和施工缝的快速干燥可能会给水密性降低带来问题，这是施工违约的因素。为此，采用快速氯离子渗透性试验评价砂浆中氯离子的渗透性，并采用概率密度函数分析砂浆基体的孔径分布。结果的主要发现可以总结如下。(1)干燥混凝土的孔径分布可以用威布尔分布函数进行统计解释。形状参数取决于干燥温度。根据毛细孔体积信息和统计参数，干燥温度的升高导致毛细孔体积在50nm-2μm范围内增大，对氯离子扩散率有较强的控制作用。建立了一种适用于多种干燥条件下氯离子向混凝土迁移的简化预测模型。(2)水灰比越高，施工缝孔径增大幅度在1nm ~ 10μm左右；施工缝早期在节理区与非节理区之间产生氯离子扩散系数的差异。随着时间的延长，差异逐渐减小，但随着时间的延长，氯离子沿节理区的扩散系数略高于非节理区的扩散系数。(3)新鲜水泥浆经冻干回收的未水化水泥砂浆的氯离子扩散系数比普通硅酸盐水泥砂浆的氯离子扩散系数高20%。

This study reveals that the effect of pore structure on chloride-ion migration into the deteriorated concrete due to construction defects. Rapid drying of hardened concrete and construction joints, which may give problems in lowering of water-tightness, are picked up here as factors of construction defaults.For this purpose, the rapid chloride permeability test was employed in the evaluation of chloride-ion penetration into mortar, and the pore size distributions of mortar matrix were analyzed by a probability density function. The main findings from the results can be summarized as follows.(1) The pore size distribution of dried concrete can be statistically explained by Weibull distribution function. The shape parameter depends on drying temperature. The elevation of drying temperature induces a larger volume of capillary pores in the range of 50nm-2μm in diameter, ruling strongly the chloride-ion diffusivity based on the information of the capillary pore volume and the statistical parameters. A simplified prediction model of chloride-ion migration into concretes is established that could have an applicable to a broad range of drying conditions.(2) An increase in pore volume ranging from about 1nm-10μm in diameter of the construction joint is remarkable for the mixture of higher water-cement ratio. The construction joint produces difference of chloride diffusion coefficient between joint zone and non-joint zone at early ages. Although the difference decreases with time, the values of the chloride diffusion coefficient along the joint surfaces are slightly higher than that of non-joint zone even at later ages.(3) The chloride-ion diffusion coefficient of mortar using unhydrated cement, which is recovered from fresh cement paste by freeze-drying, is 20% higher than that of mortar using ordinary portland cement.