Bridges

This paper discusses the risk management of corroded external post-tensioning bars (Macalloy Bars) fitted to the steel Barron River (Kuranda) Bridge on the Kennedy Highway west of Cairns.

The Macalloy bars were retrofitted to the bridge immediately after the bridge was opened to traffic. After 62 years’ service, there were concerns that the Macalloy bars could spontaneously snap in-service due to corrosion pitting and wear, axial prestress, and stresses from traffic and other loads. The consequences of a bar snapping are high and include the potential for considerable damage to the supporting steel girders leading to bridge closure while unscheduled repairs are undertaken.

Theoretical calculations and corrosion literature indicated the stress below the corrosion pits were likely excessive and the fatigue performance under traffic loading was unproven. A series of laboratory fatigue and ultimate load tests were conducted on bars removed from service. A combination of actual corroded bars and simulated corrosion pits were tested. The results showed that the ultimate tensile strength reduced with increasing pit depths with concerning reductions for larger pits. There was an even larger decrease in the ductility of the bars (extension) with increasing pit depth to levels substantially below the specified minimums for the bars.

The paper summarises the bridge’s operational context, the risk management actions taken to keep the public and the workers safe while maintaining access across the bridge. The methods used to inspect and measure the corrosion pits and to prevent further corrosion are also discussed.

To conclude, the paper discusses the risks posed by the premature failure of Macalloy bars, the steps taken to minimise the duration of exposure to the ongoing risks associated such a premature failure of Macalloy bars. Minimising potential disruption, community and economic impact was a major consideration. The technical decision frameworks applied are also discussed.