Experimental and numerical study of fatigue behaviour at the microscale of cementitious materials

Ageing is an inherent feature of cementitious materials. Ageing of material leads to gradual loss of the function of a concrete structure with increasing likelihood of failure. As one of the typical ageing phenomena, fatigue of concrete, has recently received considerable research attention. The phenomenon of concrete fatigue is complicated as it inherently involves multiple spatial scales owing to the multiscale heterogeneous nature of concrete. Over the past century, tremendous efforts have been devoted to concrete fatigue. However, many important scientific problems still remain unsolved. These problems include at least: how does the multiscale heterogeneous material structure of concrete affect the fatigue behaviour; how does fatigue damage evolve inside the concrete; how to properly simulate and predict the fatigue behaviour. The main o bjective of this thesis is to improve the knowledge of fatigue behaviour of cementitious materials at the microscale and to develop a multiscale modelling scheme from micro- to meso-scale to estimate the fatigue properties. Firstly, experimental techniques for characterization of mechanica! and fatigue properties of cementitious materials at the microscale are developed. These techniques include the preparation and testing of micrometre sized sample. For sample preparation, a precision micro-dicing machine is used. With the help of a nanoindentation measurement device, micro-bending tests are performed to study the flexural fatigue behaviour of two major components in the mortar, i.e. the cement paste and the interfacial transition zone (ITZ). The fatigue fracture surface and damage evolution are assessed using an Environmental Scanning Electron Microscope (ESEM) and X-ray computed tomography (XCT). The mechanica! properties, including the strength and elastic modulus, as well as the fatigue properties including the relationship between fatigue life and stress level, the stiffness degradation and residual deformation evolution, can be obtained using the developed microscale testing approach. Secondly, a numerical model using a 2D lattice network is developed to simulate the fatigue behaviour of cementitious material at the microscale. Images of 2D microstructures of cement pastes and ITZ obtained from XCT tests are used as inputs and mapped to the lattice model. Different local mechanica! and fatigue properties are assigned to different phases of the cement paste and interfacial transition zone. A constitutive law for cyclic loading is proposed to consider the fatigue damage evolution. Experimental results obtained at the microscale are used to calibrate and validate the model. The developed model is able to investigate the effect of microstructure heterogeneities on fatigue damage evolution in a very efficient way and can establish a quantitative relationship between the material structure and the global fatigue performance. Moreover, it can also provide valuable insight into the fatigue damage evolution and fatigue fracture phenomena under different stress levels. Finally, a parameter-passing scheme is adopted for upscaling of the mechanica! and fatigue properties. Via this approach, the global fatigue fracture behaviour (i.e. stress-strain response and S-N relation) simulated at smaller scale can be used as input for the fatigue fracture modelling at the larger scale. The model can satisfactorily predict crack patterns, mechanica! and fatigue properties under fatigue loading. The model has fully predictive capabilities at the mesoscale. Hence, the model offers an opportunity to investigate in more detail the influence of material structures at different scales on the macroscopie fatigue performance.