Effects of curing time and frequency on ultrasonic wave velocity in grouted rock bolts

Grouted rock bolts are widely used to reinforce excavated ground in mining and civil engineering structures. To date, opportunities for testing the quality of the grout in grouted rock bolts have been limited to the pull-out tests and the over-coring methods. Both these methods are destructive, time-consuming and costly. These deficiencies have fueled research into the use of ultrasonic methods for testing the quality of the grout in rock bolts. However, only partial success has been achieved in these efforts chiefly due to inadequate knowledge of the ultrasonic wave characteristics such as wave velocity in grouted rock bolts. This paper presents results of an experimental study into the effects of curing time and testing frequency on the velocity of ultrasonic waves propagating along rock bolts grouted in concrete. A substantial wave velocity decrease, as much as 47.7% at certain frequencies, was recorded in rock bolts grouted in fully cured concrete in comparison to non-grouted bolts. The results demonstrate the importance of optimizing the selection of test frequencies as well as suggesting the possibility of a new approach based on wave velocity decrease for testing the grout quality of rock bolts.