The noncontact acoustic inspection method focuses on the resonance phenomenon, and the target surface is measured by being vibrated with an airborne sound. It is possible to detect internal defects near the surface layer of a concrete structure from a long distance. However, it requires a fairly long measurement time to achieve the signal-to-noise (S/N) ratio just to find some resonance frequencies. In our method using the conventional waveform “single-tone burst wave”, only one frequency was used for one-sound-wave emission to achieve a high S/N ratio using a laser Doppler vibrometer (LDV) at a safe low power (e.g., He–Ne 1 mW). On the other hand, in terms of the difference in propagation velocity between laser light and sound waves, the waveform that can be used for high-speed measurement was devised using plural frequencies for one-sound-wave emission (“multitone burst wave”). The measurement time at 35 measurement points has been dramatically decreased from 210 to 28 s when using this waveform. Accordingly, 7.5-fold high-speed measurement became possible. By some demonstration experiments, we confirmed the effectiveness of our measurement technique.

The deterioration of concrete structure becomes a social problem in Japan. Social needs increase as for maintenance, check and renewal of concrete structures such as a tunnel or a bridge. The check of concrete structures has been performed conventionally by a hammering test. Not the inspection depended on a human experience and sense, but the inspection by a quantitative measuring system are demanded. We have developed the technique “non-contact nondestructive acoustic inspection method” to measure the internal defects near the concrete surface to a depth of about 10cm using airborne sound and the laser Doppler vibrometer at a distance 5―10 m far from measurement surface. Depending on the surface state (reflectance, dirt, etc.) of concretes, there is a problem to decrease the quantity of the light of the returning laser and there arise optical noise resulting from the leakage of light reception. To remove an abnormal measurement point, we proposed a defect detection algorithm, in which vibrational energy ratio and spectrum entropy are combined. As a result, it enables to distinguish a defect part, healthy part and an abnormal measurement point. However, for a real concrete structure, the gray area exists. We evaluate quantity of acoustic characteristics against a healthy part of concrete statistically. We found the distribution of each acoustic characteristic against a healthy part follow a normal distribution by excluding some outlier. A defective part is separated from a healthy part clearly and visualized vividly.

The non-contact acoustic inspection method using an air-borne sound can detect the cavity defect and crack near the measurement surface by using flexural resonance. By this method using a single tone burst wave, the length of the measurement time of a two-dimensional scan was a practical problem. This is because only one frequency was used for one sound wave emission so that a high signal to noise (S/N) ratio could be obtained with the laser Doppler vibrometer (LDV) of a weak laser power (e.g., He-Ne 1mW). However, two or more frequency can be used at the time of one sound wave emission (multi tone burst wave) by taking into consideration the difference of the propagation velocity of a sound wave and laser light. That is, it becomes possible to perform high-speed measurement as compared with the conventional method, using effectively the time & frequency gate for raising a S/N ratio by terminating measurement before the reflective sound wave reaches a laser head from the measurement surface. The experiment using a concrete test object was carried out and the validity of this multi tone burst wave as high-speed measurement was confirmed from the experimental result.

No abstract

The non-contact acoustic inspection method using an air-borne sound can detect the cavity defect and crack near the measurement surface by using flexural resonance. By this method which uses the conventional single tone burst wave, since only one frequency was used for one sound wave emission, the length of measurement time had become a problem. Therefore, the multi-tone burst wave was devised for high speed improvement. However, measurement time and signal to noise (S/N) ratio change with the parameters in which a multi-tone burst wave is contained at the time of one sound wave emission, such as the number of frequency, the arranging method, pulse length, and the average number of times. Therefore, the experiment using the concrete test object having the styrofoam which imitated the cavity defect was conducted and a multi-tone burst wave was used, it was investigated how the measurement time and the S/N ratio of this method would change. From the experimental result, it became clear that the improvement in the measurement speed of several or more times was realizable maintaining a comparable S/N ratio as compared with the conventional single tone burst wave.