Effects of Strain Rate and Moisture on the Tensile Strength of Heterogeneous Materials

Strain rate effects on tensile strength of concrete and mortar were investigated using three different size specimens, two different sized split Hopkinson pressure bars SHPB and a standard material test machine. All combinations of data show the same trend of abrupt increases in tensile strength at a strain rate of about 1.0sec. An analytical expression end an empirical relation for concrete tensile strength are presented as functions of strain rate and crack velocity. Crack velocity of concrete and mortar as a function of strain rate was determined using ultra high-speed photography, crack gages, and time of failure for splitting tensile tests in a SHPB. Crack velocity data shows an asymptotic tendency toward the limiting crack velocity, at a strain rate between 20 - 7Osec. Fracture toughness, at quasistatic strain rates for concrete and mortar are presented. A malfunction on the ultra high-speed camera prevented the completion of fracture toughness data at high strain rates. Dynamic precompression pulses applied to SHPB specimens were used to show that crack damage results in tensile strength reductions in concrete and mortar. Ultrasonic wave velocity measurements of the damaged specimens showed damage is directional and should not be treated as a simple scaler.