Mesoscopic analysis of damage mechanisms in concrete material
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Concrete is a highly non-homogeneous composite with large heterogeneities of quasi-brittle character. Failure of concrete structures is usually accompanied by cracking of concrete, which is strongly affected by the mesoscale structure and the behaviour of the interface between the aggregates and the mortar matrix, especially under complex stress conditions. Analysis of the failure mechanisms of concrete at the mesoscale is therefore crucial for a better understanding of the macroscopic behaviour of the material, which can in turn contribute to improved design of concrete structures and finding new ways to enhance the material properties. This research aims to investigate the intrinsic failure mechanisms of concrete-like materials from a mesoscale point of view. To do this, continued developments from existing work on mesoscale modelling are carried out to cater the needs of realistically simulating the damage process in concrete under complex loading conditions. The new developments focus on two key aspects. Firstly, techniques to realistically simulate the fracture process of concrete are developed and these involve the incorporation of a combined cohesive and contact mechanisms for the interface between aggregates and mortar matrix. Such interface modelling allows the crack initiation and propagation at the mesoscale to be explicitly represented. Secondly, a full 3D mesoscale finite element model for concrete-like materials with random aggregates and the possibility of high packing density is developed. Use is then made of these enhanced mesoscale models to explore the intrinsic mechanism governing the fundamental behaviour of concrete such as fracture propagation in tension and compression, the well-known size effect and the dynamic strain rate effect. The research investigation begins with an analysis of the size effect in plain concrete beams under three-point bending using a generic 2D mesoscale model. The analysis aims to provide preliminary insight into the use of a mesoscopic computational tool for examining the concrete damage mechanisms with the well-known size effect phenomenon as a benchmark scenario. The shapes and the sizes of the fracture process zone (FPZ) during the whole fracture process are captured. The role of detailed FPZ features is discussed accordingly. On the other hand, the results also point out the deficiencies of the continuum-based mesoscale framework at capturing the evolution of the local fracture process, and to resolve this problem requires explicit simulation of the initiation and propagation of the micro-cracks and thus a realistic reproduction of the fracture process zone, and this becomes the subject of research in much of the later chapters of the thesis. To cater to the needs of better representing the fracture process in concrete, a coupled cohesive-contact interface approach is proposed to model the crack initiation, crack propagation and the friction mechanism within the transition zone between the coarse aggregates and the mortar matrix. The cohesive-contact combined model is verified to perform well under simple as well as complex loading conditions. The interface approach in a mesoscale model framework provides a new platform for investigating the failure mechanisms in terms of the cohesive fracture process and the contact friction process. A more comprehensive and robust mesoscale interface modelling approach, in which the cohesive plus contact interface is inserted along all mesh grids, is developed to study the complex dynamic behaviour of concrete with the consideration that fractures can spread in a fine distributed manner within larger damage areas including the strong aggregate, particularly under high loading rate. By allowing local fractures to develop explicitly, the issues with fracture damage description with a continuum material model can be largely resolved. The effectiveness of such an approach is demonstrated and employed in an investigation into the intrinsic mechanisms governing the sensitivity of the dynamic tension resistance with the loading rate. Subsequently, a re-visit of the size effect in terms of the evolution of the fracture process zones using the mesoscale model with cohesive plus contact interface model is conducted and the results are presented. The preliminary observations from using the continuum-based mesoscale model are examined and verified. Additional insight into the fracture processes in the concrete beams with various sizes is obtained and the intrinsic mechanisms of the size effect are further discussed. On the real 3D mesoscale modelling methodology, the new development focuses on achieving a realistic representation of the actual shapes and sizes of aggregate particles and at the same time allowing for high volumetric ratios of aggregates (packing density) to be attained. In addition to specific techniques to enhance the conventional take-and-place procedure, an algorithm to generate supplementary aggregates to allow increased packing density is proposed and implemented. Example 3D mesoscale specimens so created are then verified against standard experimental tests such as uniaxial compression, uniaxial tension and compression with lateral confinements, and applied to examine the dynamic behaviour of concrete under high strain rate compression.