Acoustical diagnosis of concrete crack depth based on its resonance

Concrete is useful and indispensable material for the construction of modern buildings. However, it contracts after drying and causes cracks on its surface and in structure body. Moreover, long use period after construction make the crack deeper and wider. These bad states will cause peeling off and falling accidents. To prevent the accident, diagnose of the crack is necessary. Famous detection method for crack depth using ultrasonic wave was not used for inspection of concrete wall with wide area because there were faults with high cost and severe test condition as using grease. Therefore, authors tested other detection techniques for the crack depth, and then we found that the air column resonance being caused by a crack is similar to a wind instrument. It can be understood that the length of 1/4 of the wavelength of the first order resonance sound wave corresponds almost to the depth of air column with one end closed and the length of 1/2 of wavelength also to the depth with both ends open. We propose the method of nondestructive detecting crack depth based on the resonance phenomena. It has been understood that it would be able to detect for the width within 1mm on concrete wall in typical building by the spectral analysis peak at frequency of several hundred hertz. Under assumption of expanded use for special concrete constructions with very thick wall, we made some detection experiments by the addition. If depth increased, and width narrows, it became difficult to detect by the first order resonance as at frequency lower than one hundred hertz. Fortunately, higher order resonance phenomena, as at frequency several hundred hertz, were found and were able to be clearly caught in the experiments. It is possible to detect even by deep depth of about 1m with width of about 1 and 2mm. In the paper, principle and measurement ways of diagnosis based on the sound resonance are introduced, and the examination results are described.