The simultaneous detection of multiple defects in concrete structures is a task of pivotal importance for non-destructive testing and evaluation. Diffuse waves experiencing multiple scattering inside media are demonstrated to be sensitive to weak defects. Here, an analytic model is presented for diffuse wave decorrelation associated with sensitivity kernel that describes the time-of-flight distribution in strongly scattering environments. The model is then used for generating three-dimensional images that involve estimating perturbations at each localized position through an iterative, non-linear algorithm. With the consideration of loads and micro-cracks effects on diffuse waves, an application of the approach to a real-size concrete beam shows features that denote the positions and depths of multiple existing cracks. Extension of the approach to other strongly scattering media such as tissues and volcanos is straightforward. This study offers great potential for practical applications such as structural health monitoring, medical image generation, and seismic monitoring.

The preferable way to monitor structural behavior is to measure the actual stress in the structural members. Unfortunately, stress relieving by cutting holes or slots has been the only method applicable to date for measuring the in situ stress in concrete members. Other methods such as strain gauges can only measure the change of stresses and are time consuming. Investigations have been made to find nondestructive methods for stress measurement in concrete. The progress made on stress evaluation through acoustic-wave analyses is reviewed in this paper. While the traditional elastic–acoustic waves are able to detect stress changes in concrete, it is only applicable in the elastic range and is not suitable to most field conditions. Alternatively, the coda-wave interferometry and nonlinear acoustic waves are sensitive to the microstructural changes in concrete and provide potential field applicability for in situ measurements. These coda-wave and nonlinear parameters may be used as a qualitative/quantitative indicator to microscale-damage diagnostics and stress changes.

Climate change could impose great influence on infrastructures. Previous studies have shown that solar radiation is one of the most important factors causing the change in temperature distribution in bridges. The current temperature distribution models developed in the past are mainly based on the meteorological data from the nearest weather station, empirical formulas, or the testing data from model tests. In this study, a five-span continuous Prestressed-concrete box-girder bridge was instrumented with pyranometers, anemometers, strain gauges, displacement gauges, and temperature sensors on the top and bottom slabs and webs to measure the solar radiation, wind speeds, strain, displacement, and surface temperatures, respectively. The continuously monitoring data between May 2019 and May 2020 was used to study the temperature distributions caused by solar radiation. A maximum positive lateral temperature gradient prediction model has been developed based on the solar radiation data analysis. Then, the solar radiation boundary condition obtained from the monitoring data and the lateral temperature gradient prediction model were utilized to compute the tensile stresses in the longitudinal and transverse directions. It was demonstrated in this study that the tensile stress caused by the lateral temperature gradient was so significant that it cannot be ignored in structural design.

This article presents an experimental study of estimating stresses in concrete by applications of coda wave interferometry to establish an acoustoelastic modulus database. Under well-controlled laboratory conditions, uniaxial load cycles were performed on three groups of 15 × 15 × 35-cm concrete prisms, with ultrasonic signals being collected continuously. Then, the coda wave interferometry technique, together with acoustoelastic and Kaiser theories, are utilized to analyze the stress-velocity relations for the distinct ranges before and after historical maximum loads, forming an acoustoelastic modulus database. When applied to different concrete samples, their stresses are estimated with a high degree of accuracy. This study could be used to promote the development of novel nondestructive techniques that aid in structural stress monitoring.

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.