A case study of replacement of external post-tensioning cables

This paper presents a case study of an economical intervention on an early prestressed post-tensioned concrete structure. Built in 1960, a 60 m span concrete railway bridge at Clabecq in Belgium had a simply supported two-cell box girder. It was post-tensioned with one of the last applications in Belgium of the Blaton-Magnel 'sandwich' system. The cables were external to the concrete and only protected from corrosion by cement grout. Maintenance inspections in the 1990s revealed that corrosion of the tendons was so severe that the future safety of the bridge was compromised. The owner decided to replace all existing cables with new tendons placed in high-density polyethylene ducts, with each tendon composed of 15 monostrands. This paper describes the assessment of the existing structure, attempts to estimate the residual pre-stressing force and the design and implementation of the repair. It also discusses problems encountered and lessons learned.