Recycled tires may pave the way for better roads Rubber adds bounce to asphalt on stretch of Route 543

February 08, 1993|By Peter Jensen | Peter Jensen,Staff Writer

The rubber not only meets the road on Route 543. It is the road.

You can't tell it by driving on this Harford County thoroughfare about three miles east of Bel Air. But drop an object anywhere on the stretch of two-lane highway and the blacktop indents ever so slightly and then bounces back. The effect is almost imperceptible to the naked eye, but engineers will tell you that ordinary pavement is rarely so springy.

But then again, this is not your average road: it's kind of a white-wall, all-weather highway.

Nearly one mile of Route 543 has been paved with an asphalt recipe that includes crumb-size bits of worn-out tires as a key ingredient. More precisely, there are between 3,600 and 4,000 ground-up tires incorporated in 4,800 feet of road south of Route 22.

"It looks a little blacker than regular asphalt, but other than that, you can't tell the difference by looking at it," said Brian Dolan, an engineer with the State Highway Administration. "You can't even see a little bit of tire in there. It's all homogenized."

The stretch of road is part of an experiment to find out how best to recycle scrap tires into highway pavement, something the federal government has mandated to help reduce the billions of old tires that are filling landfills nationwide.

About 250 million tires are scrapped annually, adding to an estimated 3 billion tires that have piled up over the years.

Between 10 million and 15 million old tires are stockpiled in Maryland, state officials calculate, and the number is growing as more than 4 1/2 million more tires are tossed each year.

The tire piles are not only an eye sore, but they also take up valuable landfill space, and they pose a fire and health hazard. The state is studying methods of recycling -- converting them to scrap rubber or fuel. And since last year the state has collected a $1 tax on the sales of tires to pay for government efforts to promote tire recycling.

But the lure of what engineers refer to as asphalt rubber may be that it can actually enhance a road's performance. The makers of the product contend that the rubber makes roads smoother, safer (because it gives better traction), and generally more durable than a road made with conventional asphalt.

"You name a desirable property in a road, and rubber-asphalt companies will claim it," said Dr. Mathew W. Witczak, a civil engineering professor at the University of Maryland. "The rubber seems to give it a little more flexibility and, hopefully, you'll have less cracking and better aging."

Federally funded road projects must use at least 5 percent asphalt rubber by 1994. In 1997, states must achieve a 20 percent use.

The idea of adding rubber to an asphalt mix is several decades old. In fact, it joins a host of other materials from chopped glass to melted plastic that scientists have tried adding to pavement over the years -- either to improve the quality of roads or recycle overabundant materials.

Recently, engineers have been conducting experiments that add polymers to roads to lessen rutting. Part of Interstate 70 near Frederick was resurfaced last year with an asphalt that had tiny fibers in it.

Since Route 543 became Maryland's first "rubber road" in the summer of 1991, highway engineers have paid special attention to it. So far, the results of their tests have been generally favorable. But it may be too early to tell. The average life span of a resurfacing project is 15 years.

The bounce in the road, which engineers refer to as the "deflection" effect, is something of a curiosity. In the past, a road surface that dents easily was considered to be in bad shape. But that is not so with a rubber road, which seems to spring back, said Samuel R. Miller Jr., the SHA's deputy chief engineer for materials and research.

Asphalt rubber is generally twice as costly as the conventional asphalt mix, which is 95 percent aggregate -- primarily sand and crushed stone -- and 5 percent liquid asphalt.

Experts estimate that even if asphalt rubber were in widespread use, it would still be 40 percent to 80 percent more expensive than other mixes. Whether rubber roads will be 40 percent to 80 percent better remains to be seen.

Researchers need to determine the best asphalt rubber formula. The mix used in Harford County was a "wet" mix, meaning that the rubber was melted into the road. It contains the equivalent of two tires for every ton of material.

Diane H. Dixon, president of Dixie Construction Co. Inc., the Churchville contractor that paved Route 543 for the State Highway Administration, said workers found the asphalt rubber difficult to use but were impressed with the results.

"It's very gooey and sticky compared to regular asphalt," Mrs. Dixon said. "It rolled out very well though. I guess it would be good for straight, constant widths."

The Maryland Environmental Service, the agency that is spearheading the statewide tire-recycling effort, has awarded the University of Maryland a $410,000 grant to study the various asphalt rubber mixes. Plans call for incorporating asphalt rubber in 24 surfacing projects in the next 30 months.

One of the things researchers want to look at is the feasibility of using shredded tires as part of the stone base of the road and not just as part of the asphalt.

Using recycled tires with asphalt may have some drawbacks. For one thing, Maryland does not have a facility producing the asphalt rubber mix or even one that is grinding tires fine enough for the process.

In addition, there are concerns that the asphalt rubber may give off more toxic fumes than other types of asphalt and that it can't be recycled as readily.

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