Modeling Creep of Short Fiber Reinforced Concrete

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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.