Dynamic strength increase of plain concrete from high strain rate plasticity with shear dilation

Abstract

An increase in the strength of concrete when loaded dynamically has been noted in the testing literature since the early twentieth century. The origins or mechanisms leading to this increase, despite having been observed in a variety of tests, are not satisfactorily established. Aspects of test setup, specimen design, etc., have been shown to influence the outcome of any given test. More recently, computer representations of concrete have been tasked with analyzing or predicting the dynamic behavior of structures. Computers have also enabled an inward look at the same empirical tests, showing that some strength increase in compression can be captured by implementation of the proper plasticity model. The major factor touted for strength increase is the well known pressure sensitivity of concrete and a mechanism known as ‘inertial confinement’. The present work proposes a new mechanism for dynamic strength increase, focusing on the failure mechanism of concrete in compression known as shear faulting. The faulting process and its associated plastic deformation mode is compared using several material models. Adjustments are made to some parameters within these models to study their effect on dynamic and inertial plastic response. Shear dilation, which does little to increase dynamic strength at moderate strain rates, is identified as a key component of a concrete material model subject to high strain rates. Shear dilation’s effects can be seen in the range of strain rates that are practically attainable in a laboratory by using the split Hopkinson pressure bar apparatus. They may also have an increasingly important effect on problems featuring even higher strain rates, such as blast, impact, and penetration through concrete slabs.