Aspects of the impact-echo method for the detection of voids within metal ducted post-tensioned bridge beams
This thesis is concerned with the development of a non-destructive testing technique, namely the impact-echo method, for the evaluation of tendon ducts in post-tensioned prestressed concrete bridges. The impact-echo method has been shown to be useful for damage detection in concrete structures, particularly plate-like structures. However, when it is applied to beam-like rather than plate-like structures the resulting spectra are complex and difficult to interpret. One source of complications is the excitation of cross-sectional modes in the beam. This thesis discusses an investigation into the nature of these modes, and their potential applications for non-destructive testing. Finite element models have been developed to investigate cross-sectional modes where grouted and ungrouted ducts are present and the results from these are described together with an experimental validation. It is concluded that a shift in the frequency of the dominant cross-sectional mode can be used to identify large ungrouted ducts. In addition, the frequencies of the dominant cross-sectional modes can be predicted for both grouted and ungrouted cases using finite element modelling. Further finite element analysis showed that varying the compression wave speed in grout over a realistic range had minimal effect on the amplitude and frequency of the dominant cross-sectional mode.