Guided Ultrasonic Waves for the Inspection of Post-Tensioned Bridges

Abstract

This paper investigates the use of guided waves to inspect the embedded tendons in post- tensioned bridges. The unexpected collapse of the Ynys-y-Gwas bridge in South Wales[1] has highlighted the need for an inexpensive, reliable inspection method for testing the thousands of existing post-tensioned bridges. Post-tensioned construction allows large spans to be constructed with a light, inexpensive design. Figure 1 summarizes the technique, which involves constructing the framework of the bridge out of concrete, leaving metal or plastic tubes called ducts at predetermined locations. Steel bars or multi-wire strands (collectively referred to as tendons) are threaded through the ducts once the concrete has hardened. The steel tendons are tensioned and anchored by small collets at their ends. The tensioned steel forces the concrete into compression so that is better able to support the required loads. Once the steel is tensioned, the ducts are filled with grout to provide corrosion protection. However, large air voids can be trapped in the grout, providing areas for corrosion to occur, which could lead to catastrophic failure such as the Ynys-y- Gwas collapse. The current inspection techniques have not proven to be entirely satisfactory [2], especially for inspecting the portion of the tendon near the anchorages, which is a location where the tendon is particularly prone to corrosion.