The transparent bridge – Coda wave analysis
on prestressed concrete structures
A lot of bridges feature age-related damages. However, it is not always possible to detect such damages in inspections of the external building structure. Internal damages such as the fracture of prestressing strands or prestressing wires owing to stress corrosion cracking remain hidden at first. The deficiencies in the load-bearing capacity occurring undetected will become externally visible only after further degradation, leading to extensive rehabilitation measures.
The coda wave analysis is an appropriate tool for non-destructive monitoring of internal structures. As part of an ultrasonic signal, coda waves are able to pass through large areas between ultrasonic transducers embedded in the structural member. In this process, they are affected by changes in strain, temperature and moisture, for example. Two consecutive ultrasonic signals are analyzed with the aid of the coda wave interferometry, which detects changes in the form of correlation coefficients as well as relative velocity changes.
Thermomechanical experiments, carried out on reinforced-concrete beams in a four-point bending test, measured the strain of the reinforcement by means of fiber optic sensors [1] – apart from the ultrasonic measurement. With the aid of the coda wave analysis, it was possible to verify a linear correlation (Fig.) between the mean strain of the structural member and the relative velocity change.
Tentative tests carried out on prestressed beams extended the view from the localization of damages on reinforced-concrete structures [2] to the identification of internal damages on bridge support structures. An artificial failure of a tendon can clearly be discovered by means of the ultrasonic testing results.
Works are part of the FOR 2825 “CoDA” research group funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG), with partners of the Technical University of Munich, Ruhr University Bochum, the German Federal Institute for Material Research and Testing, and Bochum University.
