The effects of corrosion on 45-year-old pre-stressed concrete bridge beams

There have been few reports of structures that could be examined in detail for long-term behaviour and performance. One opportunity arose in 2002 when the Sorell Causeway bridge, a 45-year-old, 457 metres long, 34 span, pre-stressed concrete bridge in Tasmania, Australia, was demolished. This followed concerns about its safety in the light of the increasing size and frequency of occurrence of longitudinal cracks in the webs of the beams, the cracks appearing to follow the trajectory of the post-tensioning tendons. Corrosion was suspected. Pre-demolition testing had not provided sufficient information for estimates to be made of the remaining capacity of the bridge. After demolition, three bridge beams were selected for very detailed evaluation. The load capacity of the most severely damaged beam was only half that of the apparently sound beam. It failed catastrophically. The more seriously damaged beams showed severe loss of ductility. Progressive failure of the strands in the tendons was recorded in each case. The corrosion of the conventional reinforcement was consistent with expectations. However, some of the pre-stressing strands showed severe localised corrosion with cross-section losses between 75% and 100% but with little evidence of conventional oxidation rust products. Green rusts and ferrous chlorides were observed. The apparently sound beam also showed severe localised corrosion of strands.