| Concrete, unlike metals, is a non-homogeneous mixture of aggregates and cement gel with complex networks of voids, cracks and other flaws. This complicated structure of concrete makes it very difficult to understand the enigmatic phenomenon of increased resistance of concrete when it is loaded at a very high rate of strain (for example, a magnification factor of 2 of the resistance for strain rates in the range of 10{dollar}sp2{dollar}-10{dollar}sp3/{dollar}sec). There have been several attempts to explain this behavior of concrete, but these are either empirical models or based on questionable hypothetical potential functions. Very recently, application of fracture mechanics principles has found its way in this realm of engineering mechanics and has been proven to be a very useful approach to explain the nonlinear constitutive behavior of concrete. But, a fully comprehensive analytical model taking into account the macroscopic and microscopic structure of concrete was still necessary to explain satisfactorily the apparent strength enhancement under high loading rates. The research presented in this thesis is aimed at obtaining a better insight into this problem and at developing a more rational model for describing such phenomena. An attempt is made here to show how the fracture process of concrete with pre-existing cracks is affected by high loading rates and how it causes an apparent strength enhancement. For very high loading rates, it is shown that the inertial resistances of the material are primary causes of strength enhancement. A self-consistent micromechanical damage model taking into account the modified fracture process and inertial resistances under high loading rates is developed and the capability of the developed model is tested and verified by comparing with experimental data. |
