| Summary: | 博士 === 國立中興大學 === 土木工程學系 === 91 === The objective of the thesis is to develop a new device for tracing the start time of impact when using the impact-generated stress wave technique to evaluate the quality of concrete structures. The device is based on the simple circuit principle and comprises a conductive impact device, a sensing film, and an auxiliary circuit. With the new device, stress waves are generated and the start time of impact is traced so that the evaluation work only needs a single receiver. Thus, the cost and work of evaluation and signal analysis can be reduced, which largely enhances the evaluation efficiency. The device based on the simple circuit starts the data acquisition system by pressing down the steel sphere to contact the sensing film. However, the sphere can only really contact the surface of concrete when it is continuing to be pressed down to extend through the sensing film, so, the actual time the steel sphere starts to make contact with concrete is later than the time when it sends out the initial voltage signal. And the time difference is called the penetration time. There is a need to perform calibration to obtain the penetration time and, furthermore, to measure the wave speeds. The slope-interception method is adopted to calibrate the penetration time. Furthermore, epoxy smeared on the concrete surface is used to improve the surface condition and then reduce the penetration time. For practical use, the thesis also tries to propose methods for measuring the wave speed in accuracy without calibration. Experimental results show that it is necessary to perform calibration when evaluating relatively thin or shallow cracks due to the short travel time of wave propagation. The use of epoxy as surface coating can reduce the penetration time and then improve the measurement accuracy. If one can increase the distance between the impact point and the receiver long enough, there is no need to calibrate the penetration time and the measurement accuracy can be still maintained. This is because the penetration time can be neglected due to a relatively long travel time of the wave propagation.
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