Nondestructive Inspection of Concrete Structures by Using Sound Wave

We developed a nondestructive-inspection method for concrete structures by using sound wave. Irradiating sound waves with one frequency produced by a speaker to a concrete wall including a defect, the region between the defect and the concrete surface resonates. Then by irradiating the sound wave while changing its frequency, we can detect defects with arbitrary natural frequency. The vibration of the surface is measured with laser Doppler vibrometer and internal defects are detected based on the vibration. This method has the following advantages, compared with the former method by a shock tube; (1) the sound wave irradiation device is simple, (2) the influence of the sound wave to the laser measurement can be neglected because the sound pressure level is low, (3) scanning inspection along concrete walls is possible, (4) the present method is safe. We inspected concrete specimens containing an artificial defect by the present method and confirmed its effectiveness. As a result, a disk-shaped defect of 200 mm in diameter and 100 mm in depth could be inspected successfully, where the frequency was continuously changed from 1 kHz to 10 kHz while 10 seconds.