Ultrasonic investigation of concrete with distributed damage

An experimental program was conducted to evaluate ultrasonic techniques for measuring distributed cracking in concrete structures. Distributed cracking here refers primarily to microcracking and other high porosity regions which generally precede large cracks. An investigation of distributed cracking yields information on weaknesses in the material which may ultimately lead to major cracking and failure, but also can be used to evaluate distress mechanisms which do not necessarily result inlarge cracks. Distributed cracking in concrete was induced by freeze-thaw cycling and salt-scaling. Ultrasonic tests were used to measure changes in attenuation, pulse velocity, and peak frequency of the ultrasonic waves due to the distributed damage. The ultrasonic measurements were correlated with damage observed using optical microscopy. It was found that ultrasonic pulse velocity was not very sensitive to changes caused by distributed microcracking. The change in signal amplitude (a measure of ultrasonic attenuation) on the other hand was quite sensitive to changes caused by microcracking, although the measurements showed considerable scatter. The peak frequency of the ultrasonic signal was also quite sensitive to the condition of the concrete. These results must be considered in the development field tests for evaluation of concrete structures.