ABSTRACT
The Ile de Ré viaduct is a cantilever bridge made of 6 independent viaducts to link the Ré island to the city of La Rochelle in France. Built in 1988, it is 2926m long. Unfortunately, the viaduct experienced a failure of one of its external post tensioning tendon in 2018. The Department de la Charente-Maritime, owner of the bridge asked Sixense and Freyssinet to secure the structure for the replacement of the broken tendon. This securing operation consisted in 3 phases: ultrasonic inspection of the cable anchorage zone, installation of acoustic monitoring and finally, strapping of the group of cables together. The objective of the ultrasonic inspection was to qualify the current state of degradation of the anchorage. Indeed, the cause and location of cable failure was known to be within a short distance (within 2 m at the most) behind the anchorage. Failure was attributed to a corrosion phenomenon associated with a poor injection of cement grout inside the cable duct. Once the cable condition was qualified at the anchorage, the acoustic monitoring allowed to remotely detect any subsequent failure of wire inside the tendons. Any overpassing of the maximum allowed number of broken wires or any acceleration of failing wires would trigger an alert, and put the work in progress on hold. Finally, as an additional safety measure to secure the works within the bridge, the external post tensioning cables were strapped by group in order to prevent any whipping effect that would hurt the operator in case of cables failure. The quantity, strength, and location of straps has been carefully calculated with dynamic load cases. Thanks to this monitoring, the broken cable, 6 cables in poor state and more cable at risks were replaced between 2019 and 2023, allowing to secure the whole viaduct.
