Severn Bridge suspension cable corrosion is worse than that found on the older Forth Road Bridge and more serious than the Highways Agency expected, it emerged this week.
Initial findings from intrusive investigations on the cables show that the mid-span section has suffered high levels of corrosion, affecting 50% of the cross section. The results confirmed the need to fit a £20M dehumidification system to dry out the cables and halt deterioration.Highways Agency senior bridge engineer Martin Lynch told NCE
: 'The dehumidification system will be installed on the main span cables where corrosion is worst this summer.' This offers the opportunity to monitor the success of the system before installing it on the whole bridge in 2008.The dehumidification system, also known as dry air injection, works by blowing warm dry air though the gaps between the 8,322, 5mm diameter strands that make up each cable. Water ingress through cracks in the zinc-based sealant is believed to be the main cause of corrosion.Lynch said that he was 'comfortable the bridge could carry normal traffic loading'. However, load restrictions on vehicles over 7.5t to a single lane in each direction, as enforced last September (NCE
21 September 2006), will remain in place until the full testing and analysis has been completed to accurately calculate the bridge's resulting loading capacity.