Determining Crack Depth and Measurement Errors Using Time-of-Flight Diffraction Techniques

Abstract

The objectives of this study were to demonstrate the capability of the time-of-flight diffraction technique for measuring the depth of surface-opening cracks in reinforced concrete structures and to determine errors in measurement. In this technique, two receivers were placed on opposite sides of a surface crack to monitor surface disturbances caused by the arrivals of stress waves generated by impact. One receiver and the impact generator were located on the same side of the crack. This receiver was used to trace the start time of the impact. The second receiver, placed on the opposite side of the crack, was used to find the arrival time of the P-wave diffracted from the bottom edge of the crack. Thus, the travel time of the P-wave from the impact point through the crack tip to the second receiver could be obtained, which can help determine the crack depth. Two reinforced concrete beams were constructed as experimental specimens. Cracks were created by applying loads to these specimens. The depth of the cracks was measured by the time-of-flight diffraction technique first. Subsequently, verification of the measured results was performed by drilling cores. Experimental results showed excellent agreement between the measured depth and the true depth. Because this technique is based on using digital signal measurement and analysis, there exist errors inherent in the digital measurement, as well as in the method of signal analysis. Factors that affect the accuracy of crack depth measurement are discussed. It was shown that the maximum value of error in crack depth measurement depends on the errors in determination of wave speeds, the errors in positioning the impactor and the receiver, the sampling interval of data, and the depth of the crack itself.