Sunday, October 08, 2006

Concrete possibilities

Sep 21st 2006
From The Economist print edition

Materials: It has been in use for centuries. But now, tired of being walked all over, concrete is ready for a high-tech upgrade.

THE Wizard of Menlo Park had a magic touch, but it sometimes failed him. In 1906 Thomas Edison declared that he had hit upon the “salvation of the slum dweller�—cheap concrete houses cast from single, reusable moulds. Though his Edison Portland Cement Company went on to supply concrete for New York's Yankee Stadium and the first concrete highway, the great man's dreams for concrete died amid complex, expensive moulds and 11 unsold demonstration houses.

A century later, materials scientists and their business partners have been picking up where Edison left off. In their search for more high-tech concrete mixtures, they have found a fast, innovative way to make cheap, durable housing for both the developing and the developed world. Other researchers have been extending Edison's asphalt altruism in new directions, trying not only to reduce concrete's environmental impact but also to use concrete to clean up the environment.

The recipe for concrete is simple and has been around, in one form or another, since the days of Ancient Egypt. The bulk of the material consists of aggregate—fine particles such as sand and coarse ones such as gravel or crushed stone. When water and a powdered cement are mixed in, they undergo a chemical reaction that hardens and binds the aggregates into a solid mass. To make the cement, materials such as limestone and clay are heated in large kilns to over 1,000°C. At such high temperatures, water and carbon dioxide are driven off and the limestone and clay begin to fuse to form new compounds. These are then ground into a fine powder that goes by the name of Portland cement. In America alone over 100m tonnes of the stuff are used each year.

But like good chefs, materials scientists have long known that they can tweak the basic concrete recipe to create any number of desired effects. For example, adding chemicals that encourage the trapping of tiny air bubbles makes concrete more durable, because it gives water room to expand into when it freezes, thereby avoiding tiny cracks. In the late 1990s researchers began to experiment with another additive—small amounts of electrically conductive steel or carbon fibres. Even though the fibres make up less than 1% of the concrete by volume, they have a large effect: the resulting concrete gains the ability to conduct electricity.

Electric avenues
Such concrete has a range of interesting properties. If you compress electrically conductive concrete, the fibres get slightly closer together, increasing the concrete's electrical conductivity. So if a road is made from conductive concrete, it will be able to monitor and weigh passing traffic.

That is not all a conductive concrete road can do. Passing an electrical current through a wire causes it to heat up, just like the filament in a light bulb. An electric current will heat a road, a bridge, or a runway made of conductive concrete in just the same way. For the past three winters, the Roca Spur Bridge outside Lincoln, Nebraska, has been warming itself using an electric blanket of conductive concrete. Christopher Tuan of the University of Nebraska-Lincoln and his former student, Sherif Yehia, have been carefully monitoring the bridge. Using electrical heating, they can maintain Roca Spur at a toasty 10°C above the ambient temperature, warm enough to keep it free of snow and ice throughout the winter.

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