Characterizing stiffness development in hydrating mine backfill using ultrasonic wave measurements

Ultrasonic velocity measurements have previously proven valuable in measuring the development of stiffness of cement mixtures. We have successfully applied this technique to “cemented paste backfill” (CPB), an engineered mine backfill material having significantly higher void ratio and lower binder content than conventional mortars and concretes. The development of P- and S-waves was measured for samples containing 3% and 5% Portland cement, for a 1 week period after mixing. Two sets of samples were tested; one stored in air and one submerged in water. For the air-dried samples, a declination in P-wave velocity was seen due to moisture loss, followed by a small increase in velocity due to continued cement hydration. The submerged samples showed constant P-wave velocity. The S-wave velocity increased for both sets of samples, showing greater increase for the air-dried samples, which reflects the effect of the suction developed on stiffness. However, the effect of binder content variation on stiffness is less significant in air-drying CPB than in submerged CPB. The results compare well with the trends reported for hydrating materials as well as natural soils, suggesting that ultrasonic wave measurements can be used as a nondestructive test to be correlated with other forms of laboratory testing.