Propagation of microcracks in concrete under uniaxial compression

This Paper reports the findings of a series of short-term sustained loading tests on concrete cylinders in which the extent of microcracking in concrete was continuously monitored using a novel non-destructive method. The approximate crack areas within the cross-sections of these concrete cylinders were estimated from the measured transverse and axial strain readings obtained with electrical resistance strain gauges attached to the surface of the cylinders. The experimental results showed that microcracks were initiated at a stress-strength ratio ranging between 0·15 and 0·44 (rather than at a specific value), but they did not propagate under sustained loads unless the stress–strength ratio exceeded a threshold value of about 0·5. Beyond this threshold level, the rate of microcrack propagation generally increased with increasing stress but, up to a certain stress–strength ratio (ranging between 0·8 and 0·9), decreased with time during sustained loading. It was also observed that this particular stress–strength ratio had the same range of values as the stress at which the volume of concrete became a minimum. Beyond this critical stress–strength ratio, the rate of microcracking increased rapidly with stress and increased with time under sustained loading, indicating that failure of concrete was imminent.