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The paper describes the use of the ultrasonic pulse velocity technique to measure two important features of fracture in notched concrete beams: the extent of the zone of microcracking that develops ahead of the notch or crack, and the depth of the main (or stress-free) crack. Beams of 100 mm in width and ranging from 100 to 500 mm in depth were tested, and ultrasonic pulses were transmitted longitudinally through the beams at various elevations. Ultrasonic readings taken at elevations above or below the visible crack tip were found to be sensitive to the development of the microcracked zone or to main crack growth, respectively. Based on pulse transit time measurements, the average depth of the microcracked zone was about 44% of the residual beam depth, taking all the beam sizes together. Crack depths estimated from pulse transit times were found to agree with visible crack depths to less than 10% on average.
Senior lecturer, University of the Witwatersrand, Johannesburg,
Stock #: CCA10025J