The non-contact acoustic inspection method for concrete structures using the defect detection algorithm based on the statistic evaluation for a healthy part of concrete

Abstract

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.