Wang Xinmiao, Xue Feng, Gu Xin, Xia Xiaozhou
College of Mechanics and Engineering Science, Hohai University, Nanjing 211100, China.
Materials (Basel). 2024 Jul 27;17(15):3727. doi: 10.3390/ma17153727.
The internal pore structural characteristics and microbubble distribution features of concrete have a significant impact on its frost resistance, but their size is relatively small compared to aggregates, making them difficult to visually represent in the mesoscopic numerical model of concrete. Therefore, based on the ice-crystal phase transition mechanism of pore water and the theory of fine-scale inclusions, this paper establishes an estimation model for effective thermal conductivity and permeability coefficients that can reflect the distribution characteristics of the internal pore size and the content of microbubbles in porous media and explores the evolution mechanism of effective thermal conductivity and permeability coefficients during the freezing process. The segmented Gaussian integration method is adopted for the calculation of integrals involving pore size distribution curves. In addition, based on the concept that the fracture phase represents continuous damage, a switching model for the permeability coefficient is proposed to address the fundamental impact of frost cracking on permeability. Finally, the proposed estimation models for thermal conductivity and permeability are applied to the cement mortar and the interface transition zone (ITZ), and a thermal-hydraulic-mechanical coupling finite element model of concrete specimens at the mesoscale based on the fracture phase-field method is established. After that, the frost-cracking mechanism in ordinary concrete samples during the freezing process is explored, as well as the mechanism of microbubbles in relieving pore pressure and the adverse effect of accelerated cooling on frost cracking. The results show that the cracks first occurred near the aggregate on the concrete sample surface and then extended inward along the interface transition zone, which is consistent with the frost-cracking scenario of concrete structures in cold regions.
混凝土内部孔隙结构特征和微气泡分布特性对其抗冻性有显著影响，但与骨料相比其尺寸相对较小，使得它们在混凝土细观数值模型中难以直观呈现。因此，基于孔隙水的冰晶相变机理和细观夹杂理论，本文建立了能反映多孔介质内部孔径分布特征和微气泡含量的有效导热系数和渗透系数估算模型，并探究了冻结过程中有效导热系数和渗透系数的演化机理。对于涉及孔径分布曲线的积分计算采用分段高斯积分法。此外，基于断裂相代表连续损伤的概念，提出了渗透系数的切换模型以解决冻融开裂对渗透性的根本影响。最后，将所提出的导热系数和渗透系数估算模型应用于水泥砂浆和界面过渡区（ITZ），并基于断裂相场法建立了混凝土试件细观尺度的热-水-力学耦合有限元模型。在此之后，探究了普通混凝土试件在冻结过程中的冻融开裂机理，以及微气泡缓解孔隙压力的机理和加速冷却对冻融开裂的不利影响。结果表明，裂缝首先出现在混凝土试件表面的骨料附近，然后沿界面过渡区向内扩展，这与寒冷地区混凝土结构的冻融开裂情况一致。
