The heterogeneities contained in concrete will cause strongly multiple scattering behaviors during the propagation of ultrasonic waves, forming the so-called coda waves. External loads will slightly change concrete structural size and further introduce stretching effects on coda waveforms. In this paper, coda waves are collected from several concrete samples under different loads, and the waveform variations are quantified through a stretching technique. The results show that their stretching ratios are varied according to external load strengths, which implies that stress changes in concrete can be detected by coda wave measurements. The presented study could be very value for nondestructive testing of concrete structures.
Coda waves experiencing multiple scattering behaviors are sensitive to weak changes occurring in media. In this paper, a typical four-point bending test with varied external loads is conducted on a 30-meter T-beam that is removed from a bridge after being in service for 15 years, and the coda wave signals are collected with a couple of sources-receivers pairs. Then the observed coda waves at different loads are compared to calculate their relative velocity variations, which are utilized as the parameter to distinct the compression and tensile zones as well as determine the neutral axis position. Without any prior knowledge of the concrete beam, the estimated axis position agrees well with the associated strain gage measurement results, and the zones bearing stress and tension behaviors are indicated. The presented work offers significant potential for Non-Destructive Testing and Evaluation of full-size concrete structures in future work.
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