| Concrete behaviour is often difficult to predict for all load scenarios using a single material model (e.g. elasticity, plasticity, fracture mechanics, etc.). This is because concrete nonlinearity is the result of two major energy dissipation mechanisms: the evolution of the internal microcracks, and plastic flow or yielding. Each mechanism may be triggered, depending on the nature and level of loading, the composition of the material, the shape, orientation and distribution of the internal microvoids and defects, as well as the interaction among these parameters. Therefore, a unified theory should appropriately acknowledge the complex heterogeneous nature of concrete material and trace the appropriate active energy mechanism(s) while accounting for all deformation histories. Such a model should also be able to switch from one mechanism to another without interference by the user. When the switch is made, the model should appreciate the history and type of deformations of the material. The new concept of micromechanical damage mechanics, which is further developed in this study, is capable of tracing the behaviour due to crack evolution. Classical theory of plasticity, on the other hand, is well suited and accepted to describe the behaviour of a material undergoing plastic flow.; Accordingly, this thesis combines a process micromechanical damage model and the general theory of anisotropic plasticity in order to obtain a unified plain concrete model. The model is simple and requires few unfitted material parameters. In the proposed model, the cement-paste material is assumed to be an homogeneous and isotropic matrix while the paste-aggregate interfacial properties are indirectly accounted for through the treatment of the mechanical evolution of the pre-existing microcracks. The proposed model is capable of tracing monotonic, cyclic and non-proportional loading. The 2-D version of the model is implemented in the finite element program Nonlacs and subsequently used to determine the full response of concrete elements and structures subjected to various stress regimes. The results of the proposed method of analysis compare well with available experimental data.; When fibres are introduced to strengthen concrete and to enhance its tensile carrying capabilities, they contribute more complexity to the previously mentioned mechanisms. This stems from the deformation, sliding, and/or pull or push-out of the fibres. They usually reinforce the crack and bridge its faces and hence reduce its opening and sliding displacements. Therefore, another elasto-plastic damage based model is formulated in this study to trace these nonlinear effects. The fibre deformation and crack bridging pressure and displacement functions are developed by employing the shear lag theory. The fibres are assumed to be randomly distributed and may experience de-bonding, sliding, fracture, and/or pull or push-out. However, the effect of chemical reaction between the fibres and surrounding concrete are neglected. The overall tangential elasto-plastic damage stiffness of a fibre-reinforced cracked concrete element is developed in a similar fashion to the first model. This model is also implemented in the general purpose finite element program Nonlacs. The results of this model are in good agreement with their experimental counterparts. |
