严酷环境混凝土初始缺陷的定量表征及形成机理研究

An important reason for the poor durability and low service life of concrete under severe environmental conditions is the initial defects in concrete caused by insufficient curing. Due to the ignorance of the importance of initial defects and the lack of explicitly quantitative characterizing methods, most current studies focus on standardly-cured concrete without initial defects and little has been known about the generating mechanism, characterization of initial defects in concrete as well as their influences on macroscopic performance under severe environmental conditions. In this project, a large-range panoramic fluorescent microscope as well as an impregnating and dyeing technique with little/no secondary microcracks introduced to the concrete will be developed firstly, then explicit characterizing methods and characterizing index for initial defects in concrete will be developed based on quantitative analysis of panoramic microstructures of concrete. Also the features in the panoramic microcrack structure and pore structure in concrete specimens which are standardly cured for fewer than 28d under severe environmental conditions (different temperature/relative humidity/wind speed) will be investigated thoroughly, and the distribution and structural characteristics of initial defects as well as the their influence on macroscopic properties of concrete will be revealed. The initiation and propagation of initial microcracks under the influences of temperature/relative humidity/wind speed will be simulated by finite element method, and then be compared and verified with the experimental results to know more clearly about the generating mechanism of the initial microcracks. Expected achievements of this project will be a guide for the onsite curing and durability design for concrete under severe environmental conditions.

严酷环境地区混凝土耐久性差、服役寿命远低于设计值的一个重要原因是现场养护时间不足导致混凝土含有一定程度的初始缺陷。由于对初始缺陷重要性认识不足和缺少直接的定量表征方法，目前混凝土耐久性研究大多针对标准养护条件下的混凝土（无初始缺陷），对严酷环境条件下混凝土初始缺陷的量化表征及其对宏观性能的影响规律知之甚少。本项目将开发大范围全景显微成像设备和微裂纹微干扰/无干扰浸染技术，基于全景微裂纹定量分析研究混凝土初始缺陷定量表征方法和表征指标；系统研究严酷环境条件（不同温度/湿度/风速）下标养时间不足28d的混凝土初始缺陷的结构特征，并进一步研究初始缺陷对宏观性能的影响规律；采用有限元模拟混凝土在温度/湿度/风速作用下初始微裂纹的萌生和扩展过程，与试验结果对比揭示混凝土初始微裂纹的形成机理。本项目成果对于严酷环境地区混凝土耐久设计和养护施工措施的制定具有重要的理论指导意义。

初始缺陷的存在是严酷环境混凝土耐久性差、服役寿命远低于设计值的一个重要原因。然而由于缺少初始缺陷直接定量表征方法和对初始缺陷重要性认识不足，已有混凝土耐久性研究大多针对标准养护条件下的无初始缺陷混凝土，导致研究成果无法用于保障严酷环境混凝土结构的耐久性设计。本项目开发了世界首台超大范围混凝土全景微裂纹定量分析技术，建立了混凝土初始缺陷直接定量表征方法；揭示了极端干燥、极端低温、剧烈温变三种典型严酷环境下下混凝土由于养护试件不足造成的初始缺陷的结构特征，及含初始缺陷混凝土的性能演变规律。试验结果表明，早期养护时间越短、暴露于严酷环境的时间越长，混凝土内部初始缺陷程度越高，混凝土的力学性能、耐久性能和传输性能降低越明显；混凝土的初始缺陷可以用初始微裂纹密度和孔结构定量表征，初始缺陷程度越高，初始微裂纹密度越高、毛细孔中无害少害孔比例越低（有害多害孔比例越高）；极端低温和剧烈温变环境下混凝土初始缺陷的分布是随机的、均布的，而极端干燥下初始缺陷主要集中在表层2cm范围。初始缺陷的存在对混凝土的长期力学性能和耐久性能有持续的不利影响，因此在实际施工中应注意养护避免产生初始缺陷。开发了过渡区网格的划分方法-骨料膨胀法，建立了基于CT图像的混凝土三维三相数字模型，模拟了干燥环境中混凝土由于湿度扩散而产生的微裂纹的萌生和扩展过程，发现微裂纹主要集中在表层2cm范围，且以过渡区微裂纹为主，与试验结果吻合良好。本项目成果揭示了微裂纹对混凝土耐久性具有不利影响、不容忽视，初始缺陷特征为混凝土数值模拟提供了重要的原始数据，对于严酷环境地区混凝土耐久设计和运行维护制度的制定具有重要的理论指导意义。成果已应用于三峡、乌东德大坝混凝土的健康状态识别及丰满老坝混凝土冻融损伤程度的定量评估，应用前景广阔，经济社会价值巨大。