The inspection of tendons in post-tensioned concrete using guided ultrasonic waves

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