Yielding to Change

It’s a familiar old saw: People are creatures of habit. We resist the uneasiness and uncertainty of change in favor of the comfort and security of familiarity. Many of us also have good memories. We are unwilling or unable to forget the past, particularly the events and experiences that caused unpleasantness.

These all-too-human inclinations may be the primary reasons behind the slow acceptance of tire-modified asphalt, despite its proven benefits.

"Is it something new? Yes. And I think that’s the problem with it," Dr. Serji Amirkhanian says. Amirkhanian is the program director of Clemson University’s ARTS (Asphalt-Rubber Technology Service), Clemson, S.C. "People are not willing to try it because it’s something new."

"For all practical purposes, people don’t like to be mandated to, especially state DOTs," Amirkhanian adds. The mandate was repealed in 1995, but he says it left a bad taste in many mouths that is difficult to remove.

Kent Hansen, director of engineering for NAPA (National Asphalt Pavement Association), Lanham, Md., agrees that the mandate was a significant setback for rubberized asphalt. Hansen says the move away from mandates was a positive step. "Working it into a system and showing that it’s just one of the tools available in your toolbox...works much better," he says.

Doug Carlson of the Rubber Pavements Association (RPA), Tempe, Ariz., says that during the mandate period, more than 35 states had demonstration projects using rubber-modified asphalt with plans for additional projects in the future. "Following the mandate era, only four states continued with routine use of the material. Now, about 10 states use it routinely and many more are planning new demonstrations," Carlson says.

CATCHING ON. Arizona, California, Florida and Texas have used rubberized asphalt consistently for a significant period of time. Encouragingly, Carlson says that usage is increasing in Nebraska, South Carolina and Nevada.

"Arizona has the longest use. The City of Phoenix actually is where the material was created and first used in regular application in 1966," he says.

"Tennessee is in a kind of evaluation mode," Carlson says, adding that the northern states of Massachusetts, New York, Vermont, New Hampshire and Connecticut are also increasing their usage.

George Way, chief pavement design engineer for the Arizona DOT says, "Nebraska has certainly begun to use more of it. There’s a fair amount used in Canada, and there’s a good deal of work being done in Europe." He adds, "The states are a little slow for a number of reasons, although there have been many states that have expressed some interest in it."

"Some exciting things are taking place on the scene today that will help states interested in giving rubber-modified asphalt a second try," Carlson says. The American Society for Testing and Materials (ASTM) is developing guidelines for rubberized-asphalt use, while the American Association of State Highway and Transportation Officials (AASHTO) has developed a task force for rubber-modified asphalt. "If something is put into AASHTO language, [then] it’s something these state engineers speak already. It would be an easy thing for them to adopt at that point."

Getting a DOT engineer to approve the use of rubber-modified asphalt is a primary factor in increasing the use of this material. "If he specifies it, the guys will build it," Carlson says.

"Mixing rubber and asphalt is technically not a difficult thing to do. It’s not technically difficult to make a pavement out of it," Way says. "It’s not that hard to do, but you have to have people who are willing and who are interested."

The difficulty can lie in forging a coalition of necessary suppliers and knowledgeable government contacts. "I think you have to have the people who are grinding the tires, you have to have contractors who are interested in this, and you have to have an agency interested in doing this," says Way. "It takes a real joint effort."

The rubber-modified asphalt is a premium product with many benefits, Hansen says. "It has to be cost-effective," he cautions. "That gets tough when you have to haul equipment in to blend it from 1,000 miles away."

"It’s an expensive product to use," Way says. "The actual product can cost anywhere from 50 [percent] to 100 percent more than a conventional product. But you have to remember that you are using it in very thin layers, so as far as project costs are concerned…they sort of balance each other out," he says.

Dale Rand of the Texas DOT says that the higher additional cost associated with rubber-modified asphalt remains an obstacle. He attributes the higher cost in part to the limited number of suppliers who do on-site blending. Specialized equipment, such as a blender and mixing chamber, is necessary for on-site blending of rubber-modified asphalt.

The contractor’s hardware investment can run anywhere from $500,000 to $1 million, Way says.

"The first time I heard that, I thought it was an awful lot of money," Way says. However, his conversations with contractors throughout the years have led him to conclude that they are purchasing expensive equipment on a fairly regular basis. "That big a number, which sounds large to me, is not quite that frightening to them, provided they have enough work. That’s the main thing," Way says. "There has to be enough work in the future to make it worth their while, so they know that they can eventually pay it off."

However, if a contractor is using terminally blended rubber-modified asphalt, Amirkhanian says he will not need additional or specialized equipment.

Rand adds that contractors seem to prefer the terminally blended product, because it doesn’t require additional work on their part. However, Texas has only one supplier that blends the product, affecting price and availability.

Despite the initial increased cost, the performance benefits of rubber-modified asphalt are gaining attention.

The benefits associated with rubber-modified asphalt have to do with the elasticity the rubber imparts to the asphalt. This quality increases the pavement’s ability to resist reflective cracking—when a crack in the underlying layer migrates to the surface layer.

"I personally haven’t seen anything that works as well as far as keeping the cracks from coming back through the pavement," Rand says.

Additionally, Rand says the rubber-modified asphalt has good adhesive properties. "It will stick to the underlying pavement better than other asphalts, typically." In a recent project in San Antonio, Rand says asphalt rubber was used because there were concerns about being able to get bonding to the existing "slick" concrete pavement.

"The mixture has greater film thickness, which means it’s more durable, it lasts longer, it doesn’t age as fast and it stays flexible longer," Way says. "That generally means you have to do less maintenance to it over a 10-year period."

The rubber-modified asphalt is also applied more thinly, ranging from 0.5 inches to 2 inches, Way says, enabling coverage of more lane miles than with the same amount of traditional asphalt.

In terms of miles covered, the Arizona DOT uses rubber-asphalt primarily as a final wearing course, Way says. "It gives a very smooth ride. It also gives a quiet ride. It generates less tire-pavement interface noise than other mixes. In urban areas, that has become a more recognizable feature of it," he says.

The RPA’s Carlson is also familiar with the noise-reduction benefit of rubber-modified asphalt. "Recent studies have shown a six-to 14- decibel reduction in traffic noise when comparing the portland cement concrete pavements before and the asphalt-rubber pavement after in the same locations," he says.

Amirkhanian says the product also has de-icing qualities "because it holds heat for a longer period of time, especially if you have bigger chunks of rubber. It depends on which process you use and the size of the rubber," he says.

Carlson remarks, "Asphalt makes a good road, tire rubber can make it better."

He adds that approximately 550 million tons of asphalt is used yearly in the U.S. "One hundred million tons of hot mix could consume all of the annual [scrap tire] discards within the U.S.," Carlson says. "It is conceivable that 18 percent of the hot mix jobs in the U.S. could easily be designed to use asphalt rubber, and the agencies would save money in the increased performance and take care of a heck of a lot of tires."