Modelling concrete material behaviour under high loading rates

The present work sets out to investigate numerically, by means of dynamic Nonlinear Finite-Element Analysis (NLFEA), the effect of the strain-rate exhibited under high loading rates (associated usually with blast and impact problems) on the material properties of structural concrete and the ensuing cracking process it undergoes. The numerical predictions obtained were initially validated against relevant published data obtained from tests on plain concrete prismatic and cylindrical specimens under increasing loading rates under direct or indirect tension, uniaxial compression and flexure. The numerical studies reveal that the responses under impact loading differ significantly from those under static loading once certain thresholds of loading rates are exceeded. The study builds on previous work [1-3], which has shown that the commonly observed stress-strain relationship of plain concrete specimens under high rates of loading actually describes dynamic structural response rather than material behaviour. A couple of finite element packages were used, in the present study, which adopt different approaches for modelling concrete material behaviour. The comparative analysis of the predictions obtained reveals that when realistically accounting for the brittle nature and the triaxiality which characterise concrete material behavior, the experimentally and numerically observed variation in specimen behaviour (under increasing loading rates) is primarily attributed to parameters associated with structural response and not, as widely considered, to strain-rate sensitivity of the material properties of structural concrete. Furthermore, it is shown that strain-rate sensitivity in concrete is based on an interpretation of the experimental evidence through the use of material models the analytical formulation of which depends heavily on parameters associated with post-failure mechanisms (i.e. strain softening, tension stiffening, shear-retention ability, etc) which attribute ductile characteristics to concrete material behaviour that is not compatible with its brittle nature. The response is also affected by the nature of the problem at hand (a wave propagation problem within a highly nonlinear material). Therefore, it is concluded that at the material level the numerical study shows that the effect of high loading rates on the behaviour of concrete is mainly linked to the development of inertia forces rather than strain-rate sensitivity of its material properties. Overall the aim of the present study is to provide insight into the effect of loading-rate on the mechanics underlying RC structural dynamic response under impact loading.