The North Lincolnshire firm is expected to start its work later this year. It has already carried out secondary inspections and dehumidification of the suspension cables on the Severn Road Bridge and the Forth Road Bridge.
A C Spencer spokesman said: “It’s likely the Humber Bridge will face the same problems as the others. What we’ll do is split the strands, dry them and cover the cables in a waterproof surface. This prevents any further deterioration.”
The work will officially be awarded by midsummer and will then start immediately.
Last year Faber Maunsell carried out the surface inspections of the cables to identify any visible erosion.
Bridge master and engineer Peter Hill has not released any details of the job but said dehumidification work on the cables is likely to be needed.
He said: “About 15 years ago we installed dehumidification devices on the saddles and anchorage. The deck is also fully dehumidified. But there’s no other foreseeable latent defects.”
The cost of dehumidifying the cables will depend on the state of corrosion but Mr Hill said it will be significantly more than the £4.5 million inspection costs.
He added: “A tender will be going out for that work once the investigation is complete.”
The Humber Bridge was built in the 1970s by the British Bridge Builders consortium comprising Clarke Chapman - John Thompson, Cleveland Bridge, the Cementation Group and Redpath Dorman Long.
It was the world’s longest suspension bridge until 1998 when it was superseded by the Akashi-Kaikyo Bridge in Japan.
A total of 480,000 tonnes of concrete was used to build the bridge when it opened in 1981.
The bridge boasts 32.4 ha of painted steelwork with 8 ha exposed to the elements.
The bridge towers are 36 mm further apart at the top than the bottom to take account of the curvature of the earth.
The main cable contains 11,000 tonnes of steel wire.
The bridge is constantly moving. It bends more than 3 m in the middle in winds of 80 mph.
Analysis: Dehumidifying challenges coating
By Philip Tindall
The methods used for detailed inspection of suspension bridge cables originated in the USA.
Some of the bridges there are older than similar ones in the UK. The extent of corrosion and a number of broken wires found on some bridges highlighted the need for an internal inspection of the cables as the only way to determine the true condition.
Protection of steelwork has traditionally been carried out by metallic coatings and paint systems. An alternative approach, used on several overseas bridges, is to use a dehumidification system to reduce the moisture content of the air around the steelwork to levels where corrosion will not occur.
This technique has been used successfully on the inside of several steel box girder bridges, including the deck of the Humber Bridge. Application of this technology to cables is comparatively new and its long term performance is yet to be proven.
Philip Tindall is the UK technical director for bridges at Hyder Consulting and worked on the original project