For rational and accurate analysis of concrete structures subjected to impact or impulsive loads the knowledge of crack propagation under such dynamic loads is essential. Cracks frequently propagate under mixed-mode (opening and sliding: mode I and mode II) conditions. The rate of loading effects on the mechanical properties have been attributed to the rate sensitivity of crack propagation. The strain-rate effects may be different in mode I and mode II fracture of concrete. Single edge notched beams were subjected to varying rates of loading, to establish the mode I Stress Intensity Factor, KI vs. Crack Velocity, V, relationship for mortar and concrete. Impact tests were conducted using a Modified Instrumented Charpy Impact Test System. The rate of crack growth was obtained using brittle ‘Krak Gages'.
Mode I test results lead to the following conclusions. (1) Slow (prepeak) crack growth for concrete is larger than that of mortar, at a given strain rate. (2) Pre-peak crack growth decreases with increase in strain-rate. This could be the reason for decrease in pre-peak non-linearity at higher strain rates. Hence LEFM approach may be valid at high rates of loading. (3) log KI - log V relationship is non-linear especially at the higher rates of loading.