基于粘弹性模量的水泥石内应力计算及收缩开裂机制与调控技术

Hardened cement paste is a very intricate multi-scale porous material and its microstructure evolution is accompanied with volumetric shrinkage, which, if restrained by the presence of aggregate and other restraints, results in the non-uniform distribution of internal stresses and increases the risk of micro-crack generation and ultimately bulk cracking. Internal stress calculation is the key technology in the prediction and mitigation of self-desiccation induced cracking problem in concrete, of which the accurate characterization in the modulus of hardened cement paste is an essential component. The research proposal puts forward a new methodology to investigate the internal stress development by the introduction of continuous restraint into concrete and the concurrent measurement on restrained and free autogenous shrinkage strains. A viscoelastic modulus associated with the porous hydration product is computed by the force equilibrium and strain compatibility principles, which serves as the key input in deciphering the cracking mechanism. The viscoelastic modulus is examined in various cementitious systems with the presence of different restraints. This facilitates the incorporation of the viscoelastic modulus into the framework of micromechanics, which leads to the construction of a numerical model for stress field calculation and the characterization of micro-crack generation and propagation in the hardened cement paste. This paves the way for the prediction model and the mitigation techniques, which is essential in improving the understanding of viscoelasticity in cementitious materials and the design and preparation of modern concrete of high durability and longevity.

水泥石是一种典型的多尺度脆性介质，其微结构演变伴随着不同程度的体积自收缩。骨料等约束介质的存在导致水泥石不均衡内应力的产生进而增加微裂纹发生与混凝土开裂风险。水泥石内应力分布状态是混凝土自收缩开裂预测与调控的重要依据，而其粘弹性模量的准确表征则是其中的关键要素。项目拟利用混凝土在连续约束介质限制下的自收缩特性，结合其自由收缩研究水泥石内应力发展历程，采用力平衡及本征应变相容性原理计算水泥石的粘弹性模量，为混凝土开裂机制的解析提供关键技术支持；引入不同约束介质系统研究多元胶凝材料体系的粘弹性模量变化规律，以此构建基于粘弹性模量的细观力学体系，建立水泥石不均匀内应力场数值计算模型，定量表征水泥石微裂纹的产生、扩展过程，并提出基于粘弹性模量的收缩开裂机理，为水泥石非均衡内应力的控制技术提供技术指导。本项目对水泥基材料粘弹性特性的完善、高耐久长寿命现代混凝土设计与制备技术具有重要意义。

项目针对现代混凝土在复杂服役环境下容易出现体积收缩开裂进而耐久性下降等问题，研究混凝土在复合约束介质下的收缩特性与机理，提出基于粘弹性模量的混凝土收缩开裂预测模型。项目的主要研究内容包括：研究并探明影响水泥石自收缩的关键因素，基于Kelvin和Laplace定律揭示内部内部相对湿度与水泥石孔结构的孔径大小对混凝土收缩有重要影响，水灰比的降低与矿物掺合料的使用均会降低水泥石内部相对湿度，同时二次火山灰效应细化孔结构，因而会增加混凝土的自收缩；揭示了超高性能混凝土的自收缩特性与机理，表明超高性能混凝土的自收缩主要集中在早期，其发展规律呈现出明显的阶段性，与水泥水化进程以及水泥石基体的刚度密切相关，通过建立热膨胀系数的预测模型计算其热变形，进而实现超高性能混凝土的自收缩与热变形的有效分离；研究不同约束介质（离散相骨料与钢纤维、连续相钢筋）对水泥石自收缩的影响规律，分别建立数值计算模型，最终建立基于粘弹性模量的水泥石开裂预测模型。项目关于自收缩调控的研究成果用于低收缩超高性能混凝土的制备，并成功应用于一项生活垃圾预处理车间的改建工程。项目整体成果为高抗裂现代混凝土的制备与应用提供重要技术支撑。