CORROSION ON the UK's Severn Bridge suspension cables is worse than that found on the older Forth Road Bridge and more serious than the Highways Agency expected, it has emerged.
Initial findings from intrusive investigations on the cables show the mid-span section has suffered high levels of corrosion, which is affecting 50% of the cross section.
The results confirmed the need to fit a $39.4M dehumidification system to dry out the cables and halt deterioration.
Highways Agency senior bridge engineer Martin Lynch told NCEI: 'The dehumidification system will be installed on the main span cables where corrosion is worst this summer.' He added: 'It offers an opportunity to monitor the success of the system before installing it on the whole bridge next year.' 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 (see diagram).
Water ingress through cracks in the zinc-based sealant is believed to be the main cause of corrosion.
Lynch said he was 'comfortable the bridge could carry normal traffic loading'.
But load restrictions on vehicles over 7.5t to a single lane in each direction, as enforced last September will remain in place until the full testing and analysis has been completed to accurately calculate the bridge's resulting loading capacity.
The bridge is currently operated by the Severn River Crossing Group comprising John Laing, Vinci, Barclays and Bank of America UK.
The concessionaire will hand the bridge back to the Highways Agency between 2016 and 2020, depending on traffic flow.
Responsibility for major works rests with the Highways Agency.
Dehumidifi ation systems are still regarded as experimental.
Their use has been limited to structures in Japan and recently on older bridges in Denmark and Sweden (see table). Despite this, Lynch is confident the system will halt corrosion on the Severn Bridge.
In the US, bridges suffering this type of corrosion have had cables resealed and the voids filled with oil. But this is not considered a long-term solution as the oil will eventually leak to expose the strands.
Lynch and his team is now trying to find a long-term solution for the Severn Bridge.
'It will be an immense challenge to strengthen the bridge or replace the cables for a justifi able cost and without causing disruption to traffic, ' said Lynch. 'Cable replacement is a possible option and the cost will be looked at.' The Highways Agency said that no major structural work will be carried out until Severn River Crossing hands the bridge back.
Dehumidified bridges Location Length Year opened
Akashi-Kaikyo Bridge Japan 3911m 1998
Ohnaruto Bridge Japan 1629m 1985
Shimotsui-Seto Bridge Japan 1400m 1988
Kita Bisan-Seto Bridge Japan 1538m 1985
Minami Bisan-Seto Bridge Japan 1648 1988
Innoshima Bridge Japan 1339 1983
Hakata-Ohshima Bridge Japan 840m 1988
Kurushima-Kaikyo Bridge (first, Japan 4015m 1999
H÷ga Kusten Bridge Sweden 1800m 1997 Aquitaine Bridge France 1767m 1967
Ny Lillebµltsbro Denmark 1700m 1970