On August 1, 2007, the bridge carrying Interstate 35W across the Mississippi River at Minneapolis collapsed. Thirteen died, 145 were injured.
On September 18, 2008, the fallen bridge's replacement opened. The new St. Anthony Falls Bridge contains 323 sensors to monitor for structural weaknesses, strained joints and corroded concrete.
It's one way technology can make structures - from bridges to office towers - safer, stronger and better looking. Stronger concrete and steel makes structures more stable and permits innovative designs, while sensors warn before disaster strikes.
Usually, says Catherine French, a structural engineering professor at the University of Minnesota who heads a project to collect and interpret data from the new bridge, sensors suggest issues requiring investigation and possibly repairs. In earthquake zones, though, they might even close gates like those on railway crossings when they detect sudden damage.
Sensor-equipped bridges remain rare, but are growing more common. And while bridges are an obvious place to use sensors, dams and levees are also good candidates, Ms. French says. Even ordinary buildings could use sensors to detect signs of trouble.
Meanwhile, researchers are working on stronger, more durable materials.
A good deal of work has focused on concrete. Concrete isn't completely impervious to water, which dissolves loose lime in the concrete, creating microscopic channels through which water can penetrate farther. In cold climates, the water freezes and expands, enlarging the cracks. The water also rusts reinforcing steel.
New forms of concrete aim to eliminate these problems by making the concrete more waterproof. The St. Anthony Falls Bridge is made of high-performance concrete containing coal-combustion byproducts fly ash and silica fume, making it denser and more waterproof, says Alan Phipps, senior vice-president and director of operations at Tallahassee, Fla.-based FIGG Bridge Engineers, which built it. Materials like this mean bridges built today could last 100 years, versus 40 to 50 for older bridges, he says.
Ultra High Performance Fibre-Reinforced Concrete (UHPFRC) embeds microscopic steel fibres in the concrete, giving it more than 10 times the strength of conventional concrete, says Vic Perry, Lafarge SA's North American vice-president and general manager for Ductal - Lafarge's trade name for UHPFRC. "You can make a bridge deck with no reinforcing steel."
The material also can be poured much thinner than conventional concrete, so architects use it to create unusual-looking structures. A roof panel could be made from Ductal as little as 20 millimetres thick, Mr. Perry says.
UHPFRC also won't break off in chunks as reinforced concrete can. Researchers in Britain have explored its behaviour in explosions and potential for use in security barriers and buildings where terrorist attacks could be a threat. A conventional concrete wall will stop a blast, but the shock wave can propel bits of concrete off the far side, injuring bystanders, says Steve Millard, an engineering professor at University of Liverpool and head of a UHPFRC research project there. UHPFRC won't do that.
A Canadian company, Whitemud Resources Ltd. of Calgary, is promoting water-resistant concrete made with metakaolin. Kaolin, a white claylike material used to make porcelain and fine paper, was historically scarce, but a 160-million-ton deposit was found in Saskatchewan several years ago, says Barry Lester, chairman of Whitemud. Heating kaolin to about 800 degrees Celsius creates metakaolin, a powder that makes a more water-resistant concrete. Metakaolin has been used in several projects in Canada already, Mr. Lester says.
Next could be nanotechnology.
Calcium silicate hydrate, a naturally occurring material in cement, is made up of particles little more than a nanometre in size, says Laila Raki, group leader for concrete materials at the National Research Council's Institute for Research in Construction in Ottawa. Scientists only recently developed the tools to manipulate its behaviour at the nanoscale, she says, but by doing so they can make concrete stronger and denser.
The institute is also experimenting with using other nanomaterials as additives, again with the goal of producing tougher, more durable concrete.
Carbon nanotubes have been incorporated in steel to give it added strength, says David Sykes, managing partner of Remington Partners of Cambridge, Mass., which advises companies on new technologies. Stronger steel permits soaring, airy designs such as those of architect Santiago Calatrava, who designed BCE Place in downtown Toronto. Nanomaterials can also make steel more flexible, he says, so tall structures better withstand wind and other stresses.
And Jerome Lynch, associate professor of civil and environmental engineering at the University of Michigan in Ann Arbor, heads a research group that has developed a mixture of carbon nanotubes and polymer that is very strong and has electrical properties that let it act as a sensor skin. Mr. Lynch says this skin, which he hopes to subject to tests on real-world structures in about a year, "essentially provides a detailed multidimensional view of the behaviour of a structure."
Applied like paint or in prefabricated panels, it would provide both protection and constant updates on a structure's condition from all over its surface rather than just a few spots.