Numerical and experimental investigations for the modeling of the time-dependent deformation characteristics of concrete on the mesoscale with coupled models for mechanical and hygric effects

混凝土随时间变化的变形建模通常采用纯现象学方法，其中通常引入由于徐变和收缩引起的附加变形。然而，有实验证据表明，这些影响是以一种非线性的方式耦合的（皮克特效应），并且与机械引起的损伤和试样内的水分分布密切相关。该项目的目的是开发和实施混凝土的数值模型，该模型能够模拟机械载荷和随时间变化的局部水分分布的耦合效应。模拟将以基于作者开发的混凝土几何模型的异质中尺度结构的显式表示进行。混凝土模型是由集料、水泥浆和界面过渡区（ITZ）作为一个额外的薄弱环节组成的三相复合材料。采用损伤-塑性组合模型对水泥浆体包括软化在内的非线性行为进行建模，并用梯度损伤公式对其进行正则化。与以前版本中用于大非局部半径的非局部方法相比，这减少了数值计算的工作量。混凝土的粘性特性是塑性模型的扩展，采用带硬化的perzyna型徐变模型来模拟。这允许机械损伤和粘性蠕变变形之间的直接耦合。在保证满足热力学原理的前提下，采用修正的凝固理论对水泥浆体在硬化过程中随时间变化的宏观特性（强度、刚度）的发展进行了建模。力学模型与含水率之间的相互作用是通过将混凝土建模为多孔介质，分解与骨架和毛细压力相关的宏观应力分量来实现的。此外，含水率对凝固速率也有影响。该模型采用内聚界面方法，扩展到准确地描述水分传输作为裂缝张开的函数。该项目的目标是开发一个基于物理的蠕变和收缩数值模型，该模型能够准确地捕捉复杂的宏观效应。耦合物理效应（黏性水泥基质、水分输运、水泥水化）的建模和细观结构的直接表征是否能够解释复杂的现象学性质和相互作用，值得研究。因此，用对中尺度物理过程的现实理解来解释实验结果是容易的。此外，由于本构参数具有明确的物理含义，简化了模型的标定过程。

The modeling of time-dependent deformations of concrete is commonly performed with pure phenomenological approaches, where usually additional deformations due to creep and shrinkage are introduced. However there is experimental evidence that these effects are coupled in a nonlinear way (Picket effect) and are strongly related to mechanically induced damage and the moisture distribution within the specimen.The purpose of the project is the development and implementation of a numerical model for concrete that is able to simulate the coupled effects of mechanical loading and time-dependent local moisture distribution. The simulation will be performed with an explicit representation of the heterogeneous mesoscale structure based on a geometry model for concrete developed by the author. Concrete is modeled as a three phase composite with aggregates, cement paste and the interfacial transition zone (ITZ) as an additional weak link. The nonlinear behavior of the cement paste including the softening is modeled with a combined damage-plasticity model, which is regularized with a gradient damage formulation. This reduces the numerical effort compared to the nonlocal approach used in previous versions for large nonlocal radii. The viscous character of concrete is modeled with an extension of the plasticity model using a Perzyna-type creep model with hardening. This allows for a direct coupling between the mechanically induced damage and viscous creep deformations. The development of the time-dependent macroscopic properties (strength, stiffness) as a function of the moisture content during hardening of the cement paste is modeled with a modified solidification theory while ensuring to satisfy thermodynamic principles. The interaction between the mechanical model and the moisture content is realized by modeling concrete as a porous medium with a decomposition of the macroscopic stress components related to the skeleton and the capillary pressure. In addition, the moisture content influences the solidification rate. The ITZ is modeled using a cohesive interface approach that is extended to accurately describe the moisture transport as a function of the crack opening.The objective of the project is the development of a physics-based numerical model for creep and shrinkage that is able to accurately capture the complex macroscopic effects. It should be investigated if the modeling of coupled physical effects (viscous cement matrix, moisture transport, cement hydration) and the direct representation of the mesostructure are able to explain the complex phenomenological properties and interactions. As a result, the interpretation of experimental results with a realistic understanding of the physical processes on the mesoscale is facilitated. Additionally, the calibration process of the model is simplified due to clear physical meaning of the constitutive parameters.