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Acid tongue

Up in the Highlands of Scotland, work on the Kyle of Tongue bridge refurbishment is well underway. Declan Lynch finds out more.

The Kyle of Tongue Bridge is a vital link carrying traffic in the Scottish Highlands. But harsh conditions on Scotland’s north coast have taken their toll on the 40-year old structure and this summer it underwent a major refurbishment.

“Conditions are challenging up here,” says Aecom site supervisor and corrosion specialist Christian Christodoulou.

“There really can be four seasons in one day.”

Aecom has designed the refurbishment of the Kyle of Tongue bridge on behalf of client the Highland Council. The consultant appointed contractor Concrete Repairs Limited (CRL) to carry out the work.

Tongue estuary

The Kyle of Tongue bridge carries the A838 road between Thurso and Durness across the Tongue estuary. Opened in 1972, it is a 184m long structure consisting of 18 equal length spans comprising precast and prestressed concrete beams supporting a reinforced concrete deck. The deck rests on concrete piers supported by pairs of hexagonal steel piles.

Corrosion had begun to affect the prestressed tendons in the bridge beams.

Routine hammer tests - where engineers tap the concrete surface of the bridge to gauge the surface hardness and thus the level of corrosion - revealed that there was chlorite-induced corrosion on the prestressed tendons.
Inspections from 1999 onwards revealed the problems within the structure and in 2011 the Highland Council let a £1.17M refurbishment contract.

The contract involved concrete repairs to the prestressed ­concrete beams, pile caps, installing new parapet systems, new waterproofing, deck surfacing and painting the steel piles.

Three types of work

Repairs split into three types of work - protecting the steel piles below water, prolonging the life of the repairs to the concrete beams as well as arresting and preventing corrosion in the prestressing tendons. “With prestressed beams you must be delicate,” adds Christodoulou.

The most common form of prestressed concrete bridge remediation would be to install a permanent cathodic protection. But Christodoulou said this would be expensive, requiring mains power - to which there is no access in this remote location - and needs more monitoring.

So Aecom decided to install a new generation of 25mm diameter hybrid anodes, which only needs a 10V power supply, while repairs are carried out.

“It was a much cheaper ­option,” adds Christodoulou.

The refurbishment began in May but the bridge had to remain open throughout the programme albeit with a lane restriction.

Temporary gantry

Contractor CRL fitted a temporary gantry to the bridge to give engineers access to the underside of the deck. The platform was installed on three spans at a time, moving a along the bridge when the repairs were completed.

Engineers detected the corroded areas of the bridge deck and piers and then broke off any delaminated or damaged areas of concrete. “But we were careful not break off any more concrete than necessary and didn’t expose any prestressed reinforcement strands,” says Christodoulou.

Engineers then installed several anodes around a corroded section of reinforcement, link with a wire and connected to a 10V power supply. Power is supplied for about seven days. In that time the corrosion moves to the anodes, changing the environment around the steel to passive conditions - preventing further corrosion. The power supply is then disconnected, anodes are left in place, and dry-sprayed concrete is then applied the section.

The bridge deck was refurbished in a similar manner. Again, engineers identified the corroded areas of the deck using hammer test, broke out the concrete around the affected areas and then inserted anodes.

Once completed, dry concrete was sprayed on, and the engineers installed a waterproof membrane.

A total of 600 anodes were installed in the decks and a further 800 in the piles caps during the refurbishment.

Bridge refurbishment is due for completion next week and the bridge will then be returned to full use.

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