A Damage-Based Approach to Determine the Dynamic Increase Factor for Concrete

The failure strength of cementitious materials is often observed to increase significantly under dynamic loading conditions. A physical mechanism believed to be a significant driver of this rate sensitivity is the cracking and fragmentation process in these brittle or quasi-brittle materials. While high-speed photography can be used to make in-situ observations of specimens under dynamic loading, this optical technique is incapable of identifying the onset of strain localization and major unstable crack growth within opaque materials. In this study, the non-destructive high-resolution 3D imaging capabilities of X-Ray micro-computed tomography are implemented to analyze the state of damage within concrete specimens subjected to controlled high strain-rate loading. A modified Kolsky compression bar coupled with a momentum trapping technique was used to achieve dynamic constant strain-rate deformation before rapid unloading at a predetermined stress level allowing recovery of specimens for internal damage analysis. Results from this novel experimental technique reveal that, for the two materials studied here, a total of 38% to 68% of the dynamic strength increase occurs after the onset of major crack propagation. This result indicates that the dynamic increase factor calculated using the dynamic peak failure strength in these materials may include significant contributions from the fragmentation process, which is not representative of the intrinsic material properties. In light of these experimental results, a new logistic regression-based method is proposed to obtain the dynamic damage initiation stress for high-strength concrete and a more accurate definition of the dynamic increase factor is proposed.