Coda wave has been demonstrated to be a powerful tool for non-destructive evaluation and test (NDT) since it isvery sensitivity to changes in media. This sensitivity is attributed to the fact that its trailing parts have traveled a largevolume and may have traversed the defect region repeatedly. The diffusion equation, describing the propagation of theaverage energy, is one of the basic theories in current coda wave-based NDT techniques. Diffusion coefficient is usuallyassumed to be independent of stress changes and defect positions in concrete structures; however, the heterogeneity andinhomogeneity inherent of concrete materials may cause this assumption problematic, especially for large-size concretestructures. Here, a typical four-bending test with varied loads is performed on a real-size reinforced concrete beam. Acouple of transducers are installed to cover its top and side surfaces to collect coda waveforms at each loading step. Thendiffusion coefficient values are calculated under varied external loads and at multiple receiver locations by applying thediffusion equation to the associated coda wave measurements. The results show a trend that diffusion coefficient valuesin general increase with loads, but minute cracks break this trend and lead decreases in its values by introducing moretortuous 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 potentialapproach for concrete structural NDT applications.
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