Assessing the effects of insufficient rebar and missing grout in grouted rock bolts using guided ultrasonic waves

One of the challenges in field monitoring of grouted rock bolts, which normally have a short exposed end, is to detect the defects of the bolt or grout. In this paper, grouted rock bolts are studied using guided ultrasonic waves. Numerical modeling for grouted rock bolts is performed to assess the effects of insufficient rebar and missing grout. The numerical results are verified with laboratory tests on rock bolt samples. With introduction of correction factors at the reflection end, the results indicate that it is practically possible to identify insufficient rebar and grout defects with guided ultrasonic signals received at the exposed end. It also indicates that with the attenuation and wave velocity of guided waves, defective rock bolts with insufficient rebar length or missing grout in the ground can be detected with reasonable accuracy.

► Echo in rock bolts with insufficient rebar was reflected from the end of the rebar.
► Echo in rock bolts with missing grout was reflected from the end of the grouting.
► Grouted rebar length determined with reasonable accuracy using travel time.
► Correction factors used to represent the wave energy loss for the defects.
► Correction factors provided a means of identifying the type of bolt defects.