Computational modelling of concrete structures subjected to high impulsive loading
The behaviour of concrete structures subjected to high impulsive loading such as blast involves complex responses at the constituent material as well as local to global structural levels. To fully describe the processes involved, detailed numerical simulation is generally required and it is in fact commonly employed nowadays in this field of investigations. However, the demands on a rigorous computational model with the capability to represent different regimes of responses throughout the entire process, namely the stress wave stage under the immediate impulsive (blast) loading, the development of local composite mechanism (such as shear), and finally the global bending / residual structural state, have not been established nor thoroughly investigated in the literature. This thesis aims to fill in this gap and develop an effective and efficient modelling framework for reinforced concrete (RC) structures under impulsive loading, with a particular focus on the analysis of complex dynamic shear mechanisms and the residual structural capacities. This thesis uses a benchmark RC slab as a testbed to firstly examine the validity of commonly applied finite element setup and typical material models for the analysis of the structural response into the global deformation phase and the residual state. This is followed by a detailed scrutiny of the demands on the concrete material model in terms of preserving a realistic representation of the tension/shear behaviour and the significance of such features in simulating realistically the structural response in a reinforced concrete environment. Deficiencies of a widely used concrete material model, namely the Karagozian and Case concrete (KCC) model, in this respect are investigated and a modification scheme to the relevant aspects of the material model is proposed. The modification is demonstrated to result in satisfactory improvement in terms of ensuring more robust simulation of reinforced concrete response to blast loading. To deal with the inevitable modelling uncertainties in the part of concrete surrounding reinforcing bars in a numerical model, an equivalent transitional layer model is proposed for use in finite element modelling of RC structures subjected to impulsive loading. The main objectives of the equivalent transitional layer are to achieve a consist transfer of stress between rebar to concrete outside the transitional zone, and to maintain a realistic relative “sliding” displacement between the outer edge of the transitional layer and the rebar, while the inner edge of the transitional layer is perfectly bonded (with node-sharing) to the rebar. With appropriate descriptions of the softening and failure of the material for the transitional layer, the deformation profile and the strength can be reasonably represented in a consistent manner using the perfect-bond scheme which is commonly adopted in this field of applications. The transitional layer also incorporates features to ensure mesh-independent bond strength. Validation of proposed transitional layer model is carried out against results from RC pullout and beam experiments. The above modelling framework is subsequently employed to investigate the dynamic shear resistance of RC beam/slab under impulsive loading, recognising that the information on the dynamic shear strength in very scarce in the literature. The influence of loading rate on the change of shear span, which alters the shear resistance mechanism and generally results in an increase of the shear capacity, is discussed. The influence of the strain rate enhancement of the material strength on the dynamic shear capacity is also evaluated.