基于离散元模型的非饱和混凝土中水分传输机制研究

The ingress of aggressive agent with water into concrete and the resulting detrimental reactions are the main causes for the degradation and failure of material, water permeability is thus taken as an important indicator for durability assessment of concrete. However, it is usually time-consuming and laborious to measure the value of water permeability, the reported experimental results are in a very wide range and show a large scatter. Moreover, the understanding on the influence of external environments and loading effect on water transport in concrete is insufficient, so its water transport mechanism in service is still not clear yet. In this project, computer simulation technique based on discrete element method is utilized to respectively study the packing characteristics of aggregates, cement paste and calcium silicate hydrate on meso-micro-nano scale, such that a multi-scale model of concrete can be built. On this basis, the water transport process in concrete is simulated and the relationship between microstructural changes and water permeability is analyzed. A quantitative relationship among the composition, structure and water permeability of concrete is determined by studying the effects of material composition, water saturation degree and cracks on its water permeability. At last, the mechanism of water transport in unsaturated concrete can be better understood. This project will provide a scientific basis and technical support for the mixture design and durability assessment of concrete in practice.

侵蚀性介质随水分进入混凝土内部所引发的不利反应，是导致材料性能退化及失效的主要原因，水分渗透系数也因此被视作混凝土耐久性评估的重要指标。然而，水分渗透系数的测定通常耗时费力，试验结果呈现很大的离散性。此外，外界环境与荷载作用对混凝土中水分传输的影响规律认识不足，所以服役状态下混凝土中水分传输机理仍不清楚。本项目拟采用基于离散元方法的计算机模拟手段，从细观、微观、纳米三个不同尺度分别研究骨料、水泥浆体和水化硅酸钙凝胶的堆积特性，建立混凝土的多尺度结构模型，模拟水分在混凝土内部的传输过程，分析材料微观结构变化与水分渗透系数的关系。通过研究材料组成、饱水度、裂纹对混凝土水分渗透系数的影响，进一步确定混凝土组成-结构-水分渗透系数之间的量化关系，最终阐明非饱和条件下混凝土中水分的传输机理。本项目研究将为实际工程的混凝土配合比设计和耐久性评估提供科学依据与技术支持。

严酷环境下混凝土材料的失效破坏往往与水分侵入过程密切相关，由此引发的氯离子侵蚀、冻融、开裂、碱骨料反应更是耐久性问题的核心。环境温度和湿度的变化会使材料内部水分含量发生改变并使其处于部分饱水状态。因此，理解非饱和条件下水泥基材料的水分传输过程对于揭示服役过程中混凝土的劣化失效机制具有重要意义。基于离散元方法, 本项目成功构建了水泥基材料的多尺度结构模型，探究了骨料尺寸和形状、水泥熟料组成、水灰比对混凝土、砂浆和水泥浆体细微观结构的影响，阐明了材料组成与结构之间的关联；开发了晶格玻尔兹曼多组分多相模型，实现了水分和气体在不同饱水度下水泥浆体内部的平衡分布，明确了水分分布状态与孔隙结构的关系；提出了晶格玻尔兹曼部分反弹算法，揭示了界面过渡区对砂浆水渗透性的作用机制，厘清了界面过渡区与骨料稀释作用以及传输路径迂曲度效应之间的竞争作用；对混凝土裂缝形貌进行了量化表征并开展了开裂混凝土的水渗透性测试，建立了裂缝宽度、表面粗糙度和迂曲度与水分渗透系数之间的数学表达式。本项目所取得的研究成果可为基于现实工程需求的高性能长寿命水泥基材料组成设计与性能调控提供理论依据和技术支持。