Non contact acoustic exploration method for concrete using SLDV and LRAD

The hammering test is a representative method in inspection for cavities and delaminations at shallow area of concrete surface. Although this method is used widely because it is not expensive, efficiency of the defect-judging largely depends on the tester’s experience and long measurement time is necessary for wide area inspection. Other methods have been developed, however, it is necessary to contact or approach to the inspection object during a measurement. Therefore, we propose a new non-contact acoustic imaging method for nondestructive inspection using scanning laser Doppler vibrometer (SLDV) and long range acoustic device (LRAD). In this method, Surface vibration, which is generated by air borne sound, is measured using SLDV. This time, the styrofoam board was buried at shallow depth in the concrete are used as a substitute of a cavity in the concrete. As an experimental result, a styrofoam board is clearly imaged by the vibration velocity of the concrete surface. Furthermore, we confirmed that our proposed method can apply even 10 m away distance, and the measurement distance is about within 20 m under the present conditions. It means that the non destructive inspection for concrete from a long distance is possible.