Where the Rubber Meets the Road
by K. Robert Wendel

It started out as a quick and temporary fix on a Phoenix street more than 40 years ago.
Now, rubberized asphalt is the toast of Valley commuters with the first major road application in 2002, on the U.S. 60 freeway in the Valley's east side.

Soon, all of the Valley's freeways will be covered in the rubberized asphalt. The Arizona Department of Transportation and the Maricopa Association of Governments are spending $34 million to overlay 115 mi. of roads. Not only does the rubberized asphalt cut road noise levels from 50 to 90 percent, it's an environmentally friendly solution for the millions of discarded car tires normally headed for a landfill.

And it was quite a surprise.

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"The original intention of ADOT's use of rubberized asphalt was to reduce cracking in the hot mix asphalt," said Professor Kamil Kaloush at Arizona State University's Ira A. Fulton School of Engineering. "U.S. 60 was an experimental project and it wasn't until they started driving on it that they noticed the noise reduction."

That noise reduction has led to public clamoring for the asphalt, which contains crumbs of ground-up tires. States across the nation are eagerly watching the asphalt's performance. Scientists and engineers are monitoring other aspects of the paving. On some road sections, researchers are placing heat sensors to determine the asphalt's heat loading characteristics in relation to the Valley's heat bubble.

The material's secret is its porous nature, with tiny voids absorbing the sound pressure. The question, researchers say, is what happens and how long does it take to fill those voids up?

"The thing we are looking at is when these voids fill up with dust and other material, will we still get the benefit of the noise reduction?" Professor Kaloush said. "Our initial indications show a noise reduction of eight decibels, but they can go as low as four or five decibels, so we still have some noise reduction in the long run."

Tests are also underway to determine if the asphalt increases fuel economy, as well as performance and wear testing. ASU researchers are also experimenting with the crumb rubber and concrete combinations.

"Through my own personal observations when I drive on rubberized asphalt I seem to get better gas mileage than on concrete surfaces," said Donna Carlson, director of communications for Tempe-based Rubber Paving Association. "I think with the smoother pavement you get better gas mileage."

Typical mixes generally feature a 20 percent combination of rubber crumb with asphalt. Approximately 1,500 tires are used for every lane-mile of rubberized paving.
The asphalt was originally used with a "chip seal" that employed gravel friction course.
Phoenix used the chip seal asphalt until 1989, when concerns about vehicle damage were mounting. The city then employed the rubberized asphalt as a one-inch overlay to prolong the life of streets. During the 1990s, the city resurfaced more than 200 miles of streets with 450,000 tons of rubberized asphalt, which used about 1.1 million old tires. The city reported that rubberized asphalt placed on Dobbins Road in 1989 has performed without maintenance for 14 years and has an estimated life span of up to 18 years.

"Rubber asphalt was originally promoted as a better mousetrap to make pavement last longer and require less rehabilitation," said Dan Lance, deputy state engineer for the ADOT. "Depending on the product, rubber asphalt can extend the life cycle 30 to 50 percent."

ADOT also is considered a pioneer in the use of rubberized asphalt in paving projects. More than 4.2 million tons of rubberized asphalt has been used on Arizona highways since 1988, at a cost of more than $225 million. Those projects have resulted in the recycling of about 15 million old tires.

"This is generating a lot of interest across the country," Carlson said. "It can get pretty expensive for a state with miles and miles of concrete roads to tear them up.
Rubberized asphalt is a way to get additional service out of concrete pavement while providing a smooth and quiet road."

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