Non-destructive ultrasonic method of testing the strength of backfill concrete at deep Talnakh mines

In underground mining with backfilling of mined-out areas, it is necessary to analyze the filling mass quality. Since coring and testing would make a complex process, the method of express analysis comes in handy. As the mechanical methods of express quality analysis are not very accurate, the best option would be to use an acoustic method. But before the latter can be used, it is necessary to conduct a series of laboratory tests in order to under stand the empirical correlation between strength and acoustic pro perties. This paper describes the results of laboratory tests that were carried out for samples of backfill concrete after 7, 28, 45, 70, 90 days of curing to test their strength and acoustic properties. A correlation has been found between the uniaxial compressive strength of the fill and the ultrasonic wave propagation rates. Some field measurements have been taken to determine the propagation rates of longitudinal and transverse waves in the filling mass. Calibration curves have been built for both longitudinal and transverse waves. It was established that for the filling mass that hasn’t cured for 90 days it is the propagation rate of transverse waves that correlates most accurately with the strength; whereas for the filling mass that is older than 90 days it is the propagation rate of longitudinal waves.