Coda wave has been demonstrated to be a powerful tool for non-destructive evaluation and test (NDT) since it is very sensitivity to changes in media. This sensitivity is attributed to the fact that its trailing parts have traveled a large volume and may have traversed the defect region repeatedly. The diffusion equation, describing the propagation of the average energy, is one of the basic theories in current coda wave-based NDT techniques. Diffusion coefficient is usually assumed to be independent of stress changes and defect positions in concrete structures; however, the heterogeneity and inhomogeneity inherent of concrete materials may cause this assumption problematic, especially for large-size concrete structures. Here, a typical four-bending test with varied loads is performed on a real-size reinforced concrete beam. A couple of transducers are installed to cover its top and side surfaces to collect coda waveforms at each loading step. Then diffusion coefficient values are calculated under varied external loads and at multiple receiver locations by applying the diffusion equation to the associated coda wave measurements. The results show a trend that diffusion coefficient values in general increase with loads, but minute cracks break this trend and lead decreases in its values by introducing more tortuous propagation paths. These results are also consistent with the trend in our direct wave velocity measurements. Diffusion coefficient complementing other wave attributes such as direct wave velocity may offer a novel potential approach for concrete structural NDT applications.
Hanyu Zhan, Hanwan Jiang, Ruinian Jiang, and Jiquan Zhang, "Diffusion coefficients estimated from coda wave measurements for nondestructive evaluation of real-size concrete structures," Proc. SPIE 10972, Health Monitoring of Structural and Biological Systems XIII, 109721G (Presented at SPIE Smart Structures + Nondestructive Evaluation: March 07, 2019; Published: 1 April 2019); https://doi.org/10.1117/12.2514373.
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