AT FIRST glance the 46-metre road bridge into Asturias airport in northern Spain looks just like any other. Concrete columns support its four spans and it is topped with a standard concrete deck. But hidden beneath the deck, and supporting it, is a revolutionary feature: instead of sitting on traditional steel-reinforced concrete beams, the deck rests on beams made of lightweight carbon-fibre composite.
For Necso, the company that built the bridge, and the regional government of Asturias, which paid for it, the economics of carbon composites are compelling. Although the composite beams cost several times as much as the conventional steel-reinforced concrete equivalent, they are little more than 15 per cent of the weight. This made assembling the bridge a breeze, easily making up for the extra materials cost. In March, it took Necso just three days to put the composite beams in place using a light crane. And the entire bridge was finished in weeks.
鈥淎n equivalent concrete bridge would take months,鈥� says Ebby Shahidi of the Advanced Composites Group in Derby, UK, which supplied the composite materials for the beams.
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Already widely used in airframes and motor sport, where strength combined with lightness is critical, composites are becoming an increasingly common choice for civil engineers (see 鈥淧unching above its weight鈥�). The world鈥檚 first all-composite footbridge, a 64-metre span made of a glass-fibre-reinforced polymer and supported by composite cables, opened in 1992 at Aberfeldy in Perthshire, Scotland. Now composites are being used to build and repair bridges designed for the heaviest of traffic.
Low weight is not their only attraction. Unlike steel and concrete composites do not corrode. In cold climates, the use of de-icing salt on roads accelerates the corrosion of steel girders and the steel rods that give reinforced concrete its strength. 鈥淭he initial cost may be higher but the subsequent maintenance is trivial,鈥� says John Busel of the American Composite Manufacturers Association in New York city.
Renovating old bridges has become one of the most important applications for composites in civil engineering. The increasing weight of trucks has meant that many older bridges have to be strengthened, Busel says, and composites also allow historic structures to be strengthened almost invisibly. Take the 79-metre Broadway Bridge across the Willamette river in Portland, Oregon, which was built in 1912. Its centre span lifts to allow ships to pass beneath, and to keep the weight down its road surface is an open steel grid. After more than 90 years of use, the grid had become unsafe, and a lightweight replacement was needed to avoid putting an extra strain on its ageing lifting gear.
In August, the grid was replaced by a glass-fibre-reinforced polymer deck. The composite panels were small and light enough to be carried by fork-lift trucks, rather than needing cranes to lift them into place. So Martin Marietta Composites of Raleigh, North Carolina, completed the job in just four days, minimising disruption to road and river traffic.
The big impetus for the use of composites in civil engineering came in 1995, with the rebuild programme after the Kobe earthquake in Japan. The quake caused massive damage to the city鈥檚 elevated motorway. Many of the concrete columns supporting it had been weakened and could not withstand another earthquake. While some had to be demolished and rebuilt, others were strengthened by wrapping them with layer upon layer of carbon fibre. 鈥淭he columns have now been retrofitted to their original strength and stiffness,鈥� says Len Hollaway, a composites expert at the University of Surrey in Guildford, UK.
The traditional way to repair weak bridges has been to bolt or bond steel plates onto them, but now composites are increasingly being used to do the job. Engineers can stiffen bridges by bonding carbon-fibre and polymer plates onto the weakened section. 鈥淎 composite repair is now more likely than the conventional one,鈥� Hollaway says.
Composite panels are also being used to blast-proof buildings believed to be vulnerable to terrorist bombs. One of the five facades of the Pentagon in Washington DC had been reinforced with carbon-fibre composites by September 2001. It so happened that when a hijacked plane was flown into the building on 11 September, it hit the side that had been reinforced. Though 125 people died, experts say the plane could have done more damage and killed more people in the building if it had not been reinforced. 鈥淭he Pentagon鈥檚 other facets now have carbon-fibre reinforcement, too鈥� Busel says.
Punching above its weight
Composites are made of two or more distinct materials, usually tough fibres buried in a polymer resin that sets hard. In the construction industry, the fibres are either made of expensive carbon, a mid-priced aromatic polyamide (known as aramid), or cheap glass. The fibres give the material strength while the resin 鈥渕atrix鈥� provides bulk and helps to protect the fibres.
Large structural components, such as beams, are often prefabricated off-site. The fibres are embedded in the matrix and cured by heating in a vacuum to avoid oxidising the resin.
Composites can also be made on-site. In the 鈥渨et lay-up鈥� technique used on the concrete columns in Kobe the fibres are laid in place and a resin and catalyst are painted on with a brush or roller. The catalyst cures the resin without the need for heating.
