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Cost cuts on a plate Carbon fibre plates are being used to strengthen a three span bridge near Milton Keynes, reports Max Soudain.

Technological developments that reduce the cost of strengthening on the UKs road bridges can make all the difference to bridge owners and in particular local authorities.

Thousands of road bridges are being assessed for their loading capacity to ensure those on trunk roads can carry 40t vehicles from January 1999. Cheaper remedial work could mean that councils can keep more routes fully open.

Near Milton Kenyes, the local council has chosen to use carbon fibre plate bonding, the first UK use of the technique on a multiple span bridge. In 1995, Haversham bridge between Wolverton and Haversham was assessed by Milton Keynes Council as part of the then Department of Transports 15 year assessment programme.

Inspection of the 50m long, three span steel reinforced concrete bridge, built between 1958 and 1960, revealed serious deficiencies in the reinforced anchorage and that the overlap of reinforcement in the tops of the deck beams was inadequate for the new weight limits.

Faced with having to impose a 7.5t weight restriction on the bridge and lose an important local route, the council applied for a transport supplementary grant in 1996, to fund half of the 136,000 needed for the work.

The original strengthening design used steel plates bonded and bolted to the concrete deck slab. But a number of tenderers submitted alternatives, says Milton Keynes Council assistant bridge engineer Simon Griffiths.

The chosen bid came from specialist contractor Concrete Repairs and its designer Mouchel Consulting. They proposed to substitute carbon fibre reinforced polymer plates for the steel. The long term savings in maintenance costs were one of the main factors that swung the decision, explains Griffiths. And although the council had not used the technique before, the extensive research that Mouchel had carried out (see box) allayed any fears we had.

CFRP offers four times the strength of steel for only a fifth of the weight and it requires almost no maintenance during its working life. Nor is it susceptible to corrosion from salt or moisture.

On site handling of CFRP is also easier. The material is light enough to be carried around without lifting equipment, with one man easily carrying a 10m strip, says Mouchel principal engineer Dr Sam Fashole-Luke.

CFRP bonded plates are becoming a popular way of reducing sagging in reinforced concrete beams on single span bridges. But this is the first time in the UK that it is being used to strengthen a multiple span continuous bridge.

There could be sagging in the Haversham bridges central span at the same time as hogging in both side spans. The reinforcement detail in the top of the beams is insufficient for this load case, as the steel does not have enough overlap, explains Fashole-Luke. This meant that strengthening also had to be carried out on the top surface of the bridge deck.

The top plates are laid either side of each of the piers on to the deck directly above the longitudinal beams, three of which run three beneath each carriageway. A 6m length of CFRP made by Fibreforce Composites runs towards the centre of the span from each of the piers, with 10m towards the bridge abutments.

The first stage is to remove the road surface down to the deck slab. The top surface is cleaned with water jets and vacuumed to remove as much dirt as possible. A two part adhesive manufactured by Sika is then mixed and laid directly on to the concrete.

Next, one side of the CFRP strips is coated with adhesive. To ensure an even thickness, a specially made wooden tray is used. It looks crude but it stops adhesive being wasted, says Concrete Repairs managing director Tony Rimoldi. Adhesive is poured into the tray and after the protective tape is peeled off the strip, it is pulled through and out under a scraper that removes any excess. Once in place they are tamped down with a piece of wood and excess adhesive cleaned off.

The whole operation takes a week, with the adhesive fully cured in 24 hours. Adding to this, it takes main contractor Richard Ashford a week to remove the road surface and a few more days to relay it.

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