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Edition:July 2015 – October 2017
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Abstract:This report describes research work to develop an automated acoustic scanning system for rapid concrete bridge deck evaluation. The system consists of multiple channels of ball-chain impact sources and MEMs microphones for continuous acoustic wave excitation and non-contact acoustic sensing. A high precision RTK GPS with centimeter resolution provides real time positions of the scanning system. The lateral resolution is 6 inches, determined by the spacing between adjacent microphones; while the longitudinal resolution is about 1~2 inches depending on the testing speed. With a 6 feet wide testing frame (12 channels) and at a normal walking speed, it will take about 3 minutes to scan one lane of a 300 feet long bridge. Compared to link-chains used in regular chain drag test, the ball-chain impactor developed in this research show much higher sign-to-noise ratio and give very clean acoustic signals. The ball-chain gives similar impact signals as in the impact-echo test, but it can operate in a continuous excitation manner. A signal processing algorithm was then developed to map delaminations in the local coordinates of a bridge. Short time Fourier Transform (SFTF) analysis was used to extract the delamination response, which is identified as high acoustic wave amplitude in the frequency range of 0.5~5 kHz. The area and percentage of delaminated bridge decks are calculated based on acoustic scanning images. The system has been validated in field tests on five bridges in Nebraska. Compared to the traditional chain drag test, the automated acoustic system significantly reduces test tine and improves accuracy. The system is ready to be deployed for routine bridge inspection.
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