Non Destructive Inspection of Corrosion in Rock Bolts Using an Ultrasonic Waveguide Approach

Abstract

Rock bolts are used frequently to consolidate and stabilize the rock mass in mining and civil engineering structures. Since their installations decades ago, there are concerns that corrosion might have set in, increasing the risk of a lack of holding strength. In this study, preliminary measurements in rock gallery sites have shown that ultrasonic wave guide technic is able to determine the length of rock bolts. Then tests were undertaken on 4 m long bolts with different defects imbedded in a concrete block. They all respectively had different artificial defect to simulate corrosion or fracture in terms of cross-section reduction, length of loss of contact and crack. Besides depth of the defect, the following variables were added: the loss of contact length, the position of the defect and the length of bolt, with measurements taken for different conditions. Based on a physical model for the wave energy propagation and reflection, an inspection method was developed to evaluate the effects of corrosion on section reduction, cracks and loss of contact. Results show a good correlation of the wave energy propagation model to the reflections of the different types of defects. Corrosion type defects (modelled by tapered reduction) were discriminated from crack defects through a signal analysis technic. Also, a reflection from a major defect was differentiated from an end of bolt reflection using the propagation model. Finally, a third approach was developed to identify the presence of a “ghost defect” and to determine its length of loss of contact and its influence on the end of bolt reflection. A thorough inspection method to determine anchor bolt corrosion has been put forth encompassing several inspection parameters.