Science paves the roads of the future
Traffic congestion is growing worse nationwide, resulting in higher costs for businesses and residents and declines in local economies. To address those problems, the Federal Highway Administration has recommended increasing highway capacity as well as widening local arterial roads. A variety of new paving materials and methods in asphalt and concrete are available to meet the demand for new road construction.
Two relatively new forms of asphalt, Stone Matrix asphalt (SMA) and rubberized, are becoming more widespread, says Kent Hansen, director of engineering for Lanham, Md.-based National Asphalt Pavement Association. SMA is an open-grade mix that relies on large pieces of stone aggregate combined with sand, asphalt and fiber, to form a tough, durable surface, Hansen says. Valued for its resistance to ruts, SMA has been used on high-volume stretches of interstate in Southeastern states.
SMA was used in Denver’s Transportation Expansion Project, which is scheduled for completion in November. “Although initial costs are more than regular mixes, it does seem to be more durable and rut resistant,” says Tim Aschenbrener, materials branch manager for the Colorado Department of Transportation. He adds that CDOT engineers are expecting a 12 to 15 year overlay cycle, compared to seven or 10 years using other kinds of hot-mix asphalt.
Rubberized asphalt, which was first used in Phoenix in the 1960s, is common in the Southwest and Florida. It incorporates pieces of recycled tire rubber that absorb sound and reduce surface damage caused by extreme heat and cold, according to the Alhambra, Calif.-based Rubberized Asphalt Concrete Technology Center.
Perpetual pavement has been used for nearly five years and involves layering different types of asphalt to strengthen road surfaces. That reduces stress on the bottom layers, Hansen says, limiting maintenance to the top two or three inches.
A few state highway departments are testing warm-mix asphalt, a technique in which asphalt is mixed and poured at temperatures 50 to 100 degrees lower than typical hot-mix asphalt. The technique releases fewer greenhouse gases, such as carbon dioxide, and may extend the road construction season into cooler months.
Concrete, which offers durability and long life, is a common pavement in states with high traffic volume, particularly in the Midwest, says Mike Ayers, director of pavement technology for the Skokie, Ill.-based American Concrete Pavement Association (ACPA). Ayers says the usual life of a concrete road is 30 to 40 years, but there are some that are almost 100 years old. Concrete’s low-maintenance requirements have made it popular for paving local roads.
Stringless technology is one of the latest advances in concrete paving, Ayers says. It involves the use of lasers and a global positioning system to guide a slipform paver when laying a new road, rather than a piece of string tied to stakes planted along a roadway under construction.
Research also is being done on dowel bars, metal bars that distribute the weight load through the slabs on heavily trafficked concrete highways, increasing road life and reducing noise. “If it has a significant amount of truck traffic, dowel bars are pretty much a necessity,” Ayers says. Researchers are studying bars that use less steel or are made from reinforced composites.
Ultra-thin whitetopping, overlays of concrete used to patch asphalt roads, are becoming widespread, says ACPA Vice President Bill Davenport. Fast-track paving, which uses rapid-setting concrete to fix roadways in a few hours, also is becoming more common.
Engineers are going to be looking for new materials and techniques to lower the life cycle costs of paving and user costs such as vehicle wear and tear, says Anthony Kane, director of engineering and technology services for the Washington-based American Association of State Highway and Transportation Officials. “And, innovation isn’t done yet,” Kane says.