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

混凝土时间相关变形的模拟通常采用纯唯象方法，其中通常会引入由于徐变和收缩而产生的附加变形。然而，有实验证据表明，这些效应是以一种非线性的方式耦合的(Pickket效应)，并且与机械损伤和试件内的水分分布密切相关。本项目的目的是开发和实现一个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的混凝土数值模型。模拟将使用基于作者开发的混凝土几何模型的非均匀中尺度结构的显式表示来执行。混凝土被模拟为三相复合材料，骨料、水泥浆体和界面过渡区(ITZ)作为附加的薄弱环节。考虑软化的水泥浆体的非线性行为用损伤塑性组合模型来模拟，该模型用梯度损伤公式正则化。与以前版本中使用的大非局部半径的非局部方法相比，这减少了数值工作量。采用带硬化的Perzyna型蠕变模型对塑性模型进行了扩展，模拟了混凝土的粘性。这允许机械引起的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，用改进的固化理论模拟了水泥浆体硬化过程中宏观性质(强度、刚度)随含水率的变化规律。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，实现了力学模型与含水率之间的相互作用。此外，水分含量对凝固速度也有影响。ITZ采用内聚界面法进行建模，该方法被扩展为准确地描述作为裂纹张开的函数的水分传输。该项目的目标是开发一个基于物理的蠕变和收缩的数值模型，能够准确地捕捉复杂的宏观效应。耦合物理效应(粘性水泥基质、水分传输、水泥水化)的模拟和细观结构的直接表示是否能够解释复杂的唯象性质和相互作用，这是值得研究的。因此，便于解释实验结果，并对中尺度上的物理过程有一个真实的理解。此外，由于本构参数的物理意义明确，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.