Structural health monitoring of large concrete structures

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, provide a challenge when implementing such systems, if minor damage is to be detected. For methods based on propagating waves, the minimum size of a detectable defect is generally in the order of the signal wavelength. However, the attenuation of mechanical waves is frequency dependent, with high frequency signals being attenuated more quickly than low frequency signals. There is thus a trade-off in transmission range and sensitivity when designing structural health monitoring systems, and this is particularly evident in civil structures due to the significant attenuation in concrete. The work presented in this thesis aims to implement and investigate methods for structural health monitoring, with a focus on their practicality on large concrete structures. In the first paper the possibility of using voice coil transducers as reciprocal transmitters and receivers of mechanical waves is investigated. It was shown that such transducers can be more efficient than piezoelectric transducers in applications where signals of frequency below a few kHz are used. In the second and third papers it is shown that single-frequency continuous wave measurements, in the range of ~50 KHz, could be used to track progressing damage in concrete samples with similar sensitivity as comparable transient measurements. The advantage is that the continuous waves can be measured at signal-to-noise ratios well below that of the useful limits for the transient measurements. This indicates the prospect of increasing operational distance between two transducers, for a given signal frequency