Inspection of rock-concrete interface in underground support systems using a Non-destructive Testing technique

Concrete and shotcrete linings are widely used for temporary or permanent support in underground excavations. if applied properly, shotcrete is a durable material with excellent bonding characteristics. A fragmented, damaged or poorly installed shotcrete support system, however, can create a less stable environment than the unsupported rock mass. It is therefore necessary to inspect periodically the integrity of shotcrete lining. At present, a non-destructive testing technique is being developed by the authors to examine the condition of the interface between the rock and the lining. This paper presents the results of the research carried out to reconstruct a detailed 3D image of the rock-concrete interface from ultrasonic data. To obtain high resolution images of the interior of concrete, incident ultrasonic waves could be focused in the test medium using a 3D SAFT (Synthetic Aperture Focusing Technique) algorithm. To test the accuracy and sensitivity of SAFT for profiling the rough rock-concrete interface, a thin concrete slab with an undulating bottom surface was constructed. Ultrasonic data were acquired on a rectangular grid. It is shown that the resolving power of the 3D SAFT algorithm is sufficient to provide an accurate profile of the rock-concrete interface and to locate voids within the medium.