Opened in 1964, the Forth Road Bridge forms a strategic link in Scotland’s road network, carrying 24 million vehicles annually. The main cables each comprise 11,618 high tensile galvanized steel wires of about 5mm diameter. Completed bundles were coated in red lead paste, wrapped with soft galvanized wire and painted.
The Forth Estuary Transport Authority were aware of the potential for cable deterioration at Forth and so appointed AECOM as consultants.
The inspection revealed extensive corrosion on surface wires and right to the centre of the cable. An evaluation showed an 8-10% loss of strength to date. If deterioration was not halted traffic restrictions could be necessary in the future.
After a worldwide review for a solution, dehumidification was chosen, involving wrapping the cables in an airtight sheath and injecting very dry air into the voids between the wires, thereby halting the corrosion process by maintaining the relative humidity below the threshold at which corrosion happens. This is first time the technique had been used on a bridge as old as Forth with aerially spun cables.
AECOM designed the dehumidification system after visiting systems in Japan and Europe, provided the site supervision and monitored and evaluated the system after installation. They also developed the access platforms with the contractor and the mechanical and electrical design to permit an initial drying period followed by maintenance of the required relative humidity. The process of drying the cables has been underway since December 2009 and demonstrates a sustainable solution for preserving the life of the bridge.
"This emphasises that sustainability is as much about preserving our ageing infrastructure as it is about new buildings."