Ultrasonic transmission to monitor setting and hardening of fly ash concrete

The effect of high replacement levels of cement by fly ash on the microstructural development of concrete was measured through nondestructive ultrasonic transmission measurements. Two different devices, equipped with an ultrasound impactor and ultrasound generator, respectively, were compared. The ultrasonic pulse velocity changed in all mixtures from 200 to 500 m/s (660 to 1640 ft/s) to 4000 to 5000 m/s (13,100 to 16,400 ft/s), but at a different rate. The point where the pulse velocity started to increase sharply, which could correspond to a solid percolation threshold, amounted to 2, 3, 7.5, and 9 hours for the reference mixture and the mixtures with 10%, 40%, and 60% fly ash, respectively. The age at which the asymptotic maximum velocity value was approached varied between 20 hours for the reference and 56 hours for a 60% fly ash mixture. Also, the change in pulse energy depended on the mixture composition. The semi-adiabatic temperature change was at its maximum when the final pulse velocity value was reached. The compressive strength remained significantly lower for 40 and 60% fly ash mixtures than for the 10% fly ash and reference mixture.