Modeling Creep of Short Fiber Reinforced Concrete

短纤维增强混凝土（FRC）作为一种现代通用材料，在隧道衬砌、海洋结构等领域得到广泛应用。为了提高脆性混凝土的力学性能，如延性和减少裂纹扩展，脆性混凝土中掺入短纤维，从而形成复杂的微观结构，包括混凝土基体、粗集料和纤维。在长期恒定荷载作用下，纯混凝土和FRC均发生徐变，变形随时间增加。尽管FRC的蠕变是一个高度非线性的过程，但到目前为止，对这种材料的粘弹性只提出了线性方法。此外，这些模型仅限于单轴应力和变形状态。此外，所提出的FRC蠕变模型理想化了这种材料的微观结构特征，如纤维和骨料的形状和分布。总之，到目前为止，还没有可行的方法来考虑复杂的微观结构。到目前为止，数值模型是基于简化的假设，导致描述物理过程的能力有限。因此，本项目旨在建立一种考虑实际微结构构件的FRC蠕变数值框架。该程序是基于计算机断层扫描（CT）对FRC样本进行扫描，这些样本是从大型楼板上切割下来的。在数值模拟中，将尺度边界有限元法与八叉树算法相结合，实现了基于CT数据的自动高效网格生成。因此，本文提出的数值模型能较准确地反映FRC的真实微观结构。在数值框架的基础上，建立了新的FRC非线性蠕变本构模型。因此，为混凝土基体、骨料和纤维制定了单独的本构方程。虽然骨料和纤维的力学行为可以在弹性框架内建模，但建立了一个新的非线性模型来解释混凝土在多轴应力和变形状态下的非弹性变形。基于纯混凝土蠕变试验确定材料参数，并将本构模型实现到数值框架中。为了验证所提出的数值工具，对FRC试件进行了蠕变试验，并将试验数据与数值模拟结果进行了对比。因此，本项目提出了一种新的模拟FRC蠕变的方法，同时考虑了这种材料复杂的微观结构，精度高。此外，在该框架中引入了一种新的非线性本构模型，以考虑FRC在多轴应力和变形状态下的蠕变。这为长期荷载对FRC构件影响的现实估计提供了基础。

As a modern and versatile material, short fiber reinforced concrete (FRC) is nowadays used in many applications, such as tunnel linings or marine structures. To increase the mechanical properties such as the ductility and to reduce crack propagation, the brittle concrete is mixed with short fibers, which results in a complex microstructure, including the concrete matrix, coarse aggregates, and fibers. Under constant long-term loads, creep occurs both in pure concrete as well as in FRC, thus the deformation increases with time.Although creep in FRC is a highly nonlinear process, up to now, only linear approaches for viscoelasticity have been proposed for this material. Furthermore, these models are restricted to uniaxial stress and deformation states. Additionally, the presented models for creep in FRC idealize microstructural features of this material, such as the shape and distribution of fibers and aggregates. To conclude, to date there is no viable approach which takes the complex microstructure into account. So far, numerical models are based on simplifying assumptions resulting in a limited ability to describe physical processes. Therefore, this project aims at developing a new numerical framework for creep of FRC under consideration of the real microstructural components. The procedure is based on computed tomography (CT) scans of FRC specimens, which have been cut from large-scale floor slabs. For the numerical simulation, the scaled boundary finite element method is used in conjunction with the octree algorithm, which allows for an automatized and efficient mesh generation based on the CT data. Thus, the proposed numerical model reflects precisely the real microstructure of FRC.In addition to the numerical framework, a new constitutive model for nonlinear creep of FRC is developed. Thereby, separate constitutive equations are formulated for the concrete matrix, the aggregates, and the fibers. While the mechanical behavior of the aggregates and the fibers can be modelled within the framework of elasticity, a new nonlinear model is developed to account for the inelastic deformations of concrete subjected to multiaxial stress and deformation states. The material parameters are determined based on creep tests on pure concrete, and the constitutive model is implemented into the numerical framework. In order to validate the proposed numerical tool, creep tests are conducted on FRC specimens, such that the resulting experimental data can be compared to the numerical simulation results.Consequently, this project presents a new simulation methodology for creep of FRC, while considering the complex microstructure of this material with high precision. Furthermore, a new nonlinear constitutive model is included into this framework, in order to account for creep deformations of FRC under multiaxial stress and deformation states. This provides the basis for a realistic estimation of the effect of long-term loading on FRC components.