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Sandwich course

Bridges - A new bridge strengthening technology could save German engineers from having to replace hundreds of steel bridge decks. Andrew Mylius reports.

Germany is facing up to a bridge strengthening nightmare as it pays the price for pioneering orthotropic steel box girder bridges in the 1950s and 1960s. Over the last decade, increasing traffic volumes and higher deck loadings have dramatically increased fatigue, and weld cracking is becoming a significant problem.

'There are 3,000 to 4,000 bridges with orthotropic decks in Germany, and most will need attention, ' confirms Johann Matuschek, head of bridges and special engineering at German steel fabricator Krupp Stahlbau Hannover.

The inaccessability of the insidious cracks (see box, page 30) means limited repair options exist, and Germany's transport department faced the heart-sinkingly expensive prospect of replacing hundreds of decks. As the scale of the problem began to emerge over the last decade, Canadian firm Intelligent Engineering began a campaign to establish its steelpolyurethane sandwich - officially the Sandwich Plate System (SPS) - in Europe.

Now Germany's transport department is pioneering SPS to strengthen frail orthotropic steel bridge decks.

Krupp Stahlbau Hannover has just finished strengthening the prosaic, 70m span Schonwasserpark Bridge, which carries the A56 autobahn over a railway at Krefeld, near Dusseldorf, a structure typical of Germany's stock of steel box girder bridges. Schonwasserpark Bridge is the first to have used the technique.

SPS works by creating a relatively slender, very light, but extremely stiff deck plate. An overlay system is being used at Krefeld, in which the polymer is sandwiched between the original deck and a new steel top plate. The sandwich acts as a global beam, in which the old and new deck plates become top and bottom flanges and the poly mer is the web, strong in every direction. Wheel loads from heavy vehicles are spread to the edges of the plate, dramatically reducing local deck and stiffener deformation and adding years to the life expectancy of the deck-stiffener welds. Stress in the connection is reduced from around 42MPa to 16MPa.

Krupp Stahlbau began strengthening the Schonwasser Bridge by stripping off the asphalt road surface and grit blasting the 12mm thick underlying steel deck to a white metal finish. A 2m by 7m grid of 30mm deep beams was then fixed to the deck, onto which new 6mm thick plates were laid.

The longitudinal beams are located above the deck's longitudinal trough stiffeners, 'which means they could be welded into position, as there's no paint within the troughs to damage', says Intelligent Engineering civils director Guy Turner. The transverse perimeter beams were glued to the deck surface to avoid heat damage.

Once connected to the deck, every new plate effectively became a tank which could be pumped full of polyurethane.

Every plate came equipped with an injection hole and a valve at each corner. The valves were used to release air from the tank as the polyurethane was introduced. The polymer itself was a two-part compound, with the constituent parts mixed in the injection nozzle as the operation takes place.

Each panel took less than three minutes to fill.

Hot air was blasted through the cavity before the polyurethane was injected to remove moisture - water reacts to produce foam, 'which isn't much use in a bridge deck', says Roger Dalton, a consultant with Canadian bridge design specialist Buckland & Taylor, and technical advisor to SPS developer and patent owner Intelligent Engineering.

'Injecting the polymer is an experiential skill - you develop a feel for it. What you're trying to achieve is a wave of polyurethane that fills the void from top to bottom, and travels across the whole plate like that. If you fill too slowly, and it spreads across the bottom before filling to the top, you can get air pockets.

Fill too fast and you get air bubbles, ' says Turner. Hammer testing is used to detect voids.

These are fairly rare, but can be easily filled by drilling a pair of small holes and squirting in polyurethane which, because it is a homogeneous material, bonds seamlessly with the rest.

The liquid polyurethane generates 80°C of heat and expands as it hardens. Intelligent Engineering has exploited this to drive the polymer into the pores of the steel: The air vent and injection valves are closed as soon as the void is fi lled, building 0.4bar of pressure. To prevent the upper steel plate bulging upwards during the cure, I-beams are placed across it and anchored down with powerful magnets.

Turner emphasises that, though never before used for bridge construction, SPS polymer injection technology is every day stuff. 'This process has been used for the last four decades in the automotive industry. The Mercedes S class has over 40 bits of moulded polyurethane in it.' He adds that in fatigue tests, the 6mm steel plate gave out before the bond between the steel and polyurethane could be broken.

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