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The Road to Greener Blacktop.

What's black and white and green all over? Asphalt pavement! Covering 94% of paved U.S. roads, asphalt pavement is completely recyclable, and research in recent years has yielded many ways to enhance asphalt mixes and save landfill space by incorporating both the pavement itself and waste materials that otherwise would be landfill-bound. About 18 million tons of asphalt pavement occupying some 10 million cubic yards of space is sent to landfills each year. Without the recycling already being done today, that volume would climb to nearly 52 million cubic yards.

Asphalt pavement is composed of 95% crushed rock aggregate and 5% asphalt cement, a sticky petroleum refining by-product that holds it all together. According to the National Asphalt Pavement Association, about 80% of the asphalt pavement taken up each year--some 73 million tons--is recycled and reused in highway applications. Asphalt pavement can be stockpiled and recycled later by milling into gravel-size chunks, heating, and adding more liquid asphalt cement and aggregate as needed. Or it can be recycled on the spot using techniques known as hot inplace and cold in-place recycling, which basically involve digging up and crushing the pavement, mixing it with fresh asphalt emulsion, and laying it back down, all in one pass.

Among the many recyclables that can be added to asphalt pavements are rubber tires and waste toner. According to the Rubber Pavements Association, asphalt pavement supplemented with ground rubber tires provides a quieter ride and a strong surface with less material. Rubber-modified asphalt pavement currently eats up about 10 million scrap tires per year, says Doug Carlson, director of government relations for the association. Industry figures indicate that 273 million scrap tires were being generated annually as of 2000. Carlson says that if 25% of total lane miles of urban U.S. roads were surfaced with rubber asphalt pavement annually, nearly all the nation's discarded tires could be removed from the waste stream.

Studies at the Center for Transportation Research at the University at Texas at Austin show that toner from spent cartridges and manufacturing waste can fortify asphalt pavements, making a "stiffer" mix that holds up well in hot areas. Toner, made of styrene polymers and carbon, is considered environmentally benign. In September 2000 the Texas Department of Transportation, which sponsored the studies, patented preferred methods for creating toner-modified asphalt.

The 1998 Transportation Equity Act for the 21st Century promotes recycling in U.S. road construction. Under the act, the Recycled Materials Resource Center was established at the University of New Hampshire in Durham. Director T. Taylor Eighmy says the center is developing standards and procedures for optimizing various recycling techniques. For more guidance, the United States is turning to several European countries that already have experience with sustainable road building. In 1999 a delegate of U.S. engineers, scientists, and paving specialists met with representatives from Sweden, Denmark, Germany, The Netherlands, and France to learn more about the policies, programs, and techniques being used successfully in those countries.
COPYRIGHT 2001 National Institute of Environmental Health Sciences
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Author:Booker, Susan M.
Publication:Environmental Health Perspectives
Date:Apr 1, 2001
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