Numerical investigation of concrete subjected to compressive impact loading. Part 1: A fundamental explanation for the apparent strength gain at high loading rates

This study investigates the response of structural concrete to high rates of loading. The research is based on a finite-element (FE) program capable of carrying out three-dimensional (3D) nonlinear static and dynamic analyses which has been found to be capable of yielding realistic predictions to the response of plain- and reinforced-concrete structures under arbitrary static and dynamic actions. The FE model incorporates a 3D material model of concrete behaviour which is characterised by both its simplicity (fully brittle, with neither strain softening nor load-path dependency) and its attention to the actual physical behaviour of concrete in a structure (unavoidable triaxiality which is described on the basis of experimental data of concrete cylinders under definable boundary conditions). In the present context of impact loads encompassing also very high loading rates, however, the most significant feature of this model is that it is based on the use of static material properties of concrete, in an attempt to elucidate whether or not the effect of loading rate can be attributed primarily to the inertia of the structure’s mass and not, as is at present widely considered, to the loading-rate sensitivity of the material properties of concrete. Thus, this simplified FE model is employed here to investigate the effect of loading rate on the behaviour of prismatic concrete elements under high rates of uniaxial compressive loading in an effort to identify the fundamental causes of the experimentally observed apparent strength increase of specimens when a threshold range of loading rates is exceeded. The ensuing analytical (numerical) results are compared with published experimental data in order to assess how accurately the former can mimic laboratory-based evidence. It will be seen that the findings of this study validate what constitutes a major departure from current thinking as regards material modelling of concrete under high loading rates. Once the fundamentals for such a conclusion have been established, a companion article (Part 2) outlines the results of a parametric investigation into the effects various parameters have on the response of concrete specimens to high rates of impact.