Diffusion Coefficient Estimation and Its Application in Interior Change Evaluation of Full-Size Reinforced Concrete Structures

Diffuse waves have been proved to be more sensitive to medium interior changes than direct waves due to much longer traveling path and propagation time in media. This makes diffuse waves a useful tool in nondestructive evaluating and testing (NDT) applications. The diffusion equation is an analytic model for describing diffuse wave propagations, which provides a basis for most diffuse wave–based NDT approaches. When applying the diffusion equation, the diffusion coefficient ($D$) is usually assumed to be independent of medium changes and measurement locations. However, the heterogeneity and inhomogeneity inherent in multiple-composite concrete materials cause the change of diffusion coefficient when subjected to load and interior structural changes, especially for large concrete structures. In this research, a four-bending test on a full-size reinforced concrete beam was conducted, and the $D$ values were evaluated under various load levels and multiple receiver locations. The results show that $D$ in general increases with the increase of load, and the change is affected by microcracks in concrete. This study of the relationship between the diffusion coefficient and stresses/structural changes provides a novel approach for evaluating medium stress and damages. In addition, the work also suggests a potential method to improve current diffuse wave–based NDT techniques by considering $D$ variations caused by stresses and defects.