Structural health monitoring of concrete structures using diffuse waves

Abstract: The work presented in this thesis has aimed to investigate and implement techniques for ultrasonic measurements in structural health monitoring applications for civil structures. The focus of the work has been to make these systems practical in real applications, where the large size of the structures, and the changing environments they are exposed to, pose problems for many methods which otherwise fare well in laboratory settings.There is an increasing demand on the safety and reliability of the civil structures that make up our cities and infrastructure. The field of structural health monitoring aims to provide continuous non-destructive evaluation of such structures. Large concrete structures, such as nuclear power plants or bridges, provide a challenge when implementing such systems. Especially if minor damage is to be detected and even located. Methods based on propagating mechanical waves are known to be useful for detecting structural changes, due to the coupling between the properties of such waves and the mechanical properties of the material. The sensitivity of such measurements generally increase with higher frequencies, and ultrasonic waves can be used to detect minor cracks and early signs of damage. Unfortunately, concrete is a complex material, with aggregates and reinforcement bars on the same order of size as the wavelengths of ultrasonic waves. Ultrasonic waves are quickly scattered and attenuated, which makes traditional pitch-catch measurements difficult over long distances. However, multiply scattered waves contain much information on the material in the structure, and have been shown to be very sensitive to material changes.In this project continuous wave excitation has been used when creating the multiply scattered wave fields. This enables narrow-band detection, which is shown to enable the detection of significantly weaker signals, and thus increase the maximum distance between transducers. Techniques for localizing damage using such continuous wave fields, as well as methods for compensating for effects of changing environmental conditions, are demonstrated. Recommendations are also given for future designers of structural health monitoring systems, as to the choice of frequency, when using multiply scattered wave fields.