In recent years, the area of structural engineering has found new interesting prospects in the theory of waves. The acoustoelastic properties of materials are utilized in a new promising structural health monitoring (SHM) technique. The propagation of ultrasonic waves through stressed elements gives the ability to find invaluable information on the stress-state of the concerned element. This information is then analyzed with the use of Coda Wave Interferometry (CWI). This technique explores the late arrivals of the wave field, which is referred to as the coda. Due to scattering of the waves through heterogeneous elements, this part of the wave field holds additional information compared to the early arrivals, which makes it very suitable for application on concrete elements.
The use of ultrasonic waves with CWI in concrete elements has been studied enthusiastically by engineers due to its promising prospects. Nevertheless, this technique is still developing both in theory and in practice. The majority of previous similar studies have been conducted in laboratory settings, and thus conditions are very controlled. In this study, the monitoring technique is applied in conjunction with Smart Aggregate (SA) to a concrete structure for practical use. The SA sensors are positioned in groups at key location in the first concrete cast of a cast-in-situ prestressed concrete bridge, which entails two bridge spans of approximately 30 meters length. Measurements are done at different phases during the construction of the bridge, when the stress-state at the positions shifts due to change in loading and boundary conditions. The resulting data is analyzed with CWI and subsequently evaluated whether it coincides with expectations. Deviations from these expectations are rationalized or an attempt thereof is made.
The results show significant differences between field measurement results and expectations based on laboratory tests. These differences are explained by the presence of a greater amount of parameters, which influence the data gathered from field measurements. With the appropriate measures, the various factors acting on the structure are isolated and subsequently validated with laboratory data. The most prominent factors affecting the acoustoelastic properties of the structure are established to be time related. These factors involve concrete shrinkage, creep deformation and the concrete hydration process. With laboratory testing the effect of these factors on the wave propagation velocity are determined. By taking into account the additional factors, a strong correlation between changes in the stress-state and changes in the wave propagation velocity is observed when the data is read in an adjusted manner.
With a proper protocol, the use of smart aggregates in combination with CWI could be very valuable in accessing concrete structures on-site in both its construction stage and service stage. As it stands, prerequisite knowledge of the monitored structure is necessary to make use of the full potential of the SHM involving CWI and SA. The demand for prerequisite knowledge of the structure increases as the monitored structure becomes more complex, because it increases the amount of additional endeavors required to properly asses the acquired data. As such, the discussed monitoring technique in its current stage of development generally has low accessibility for practical use. But with further research and more understanding of monitoring method, the discussed SHM technique might be applicable for general use in the near future.