November/December 2009
Asphalt rubber creates quieter, safer, more durable, and longer-lasting roads—and could consume up to half of the nation's scrap tires annually—but the drive to get states to accept this technology has been anything but smooth.
By Chelan David
In the mid-1960s, Charles McDonald, an engineering supervisor for the city of Phoenix, began looking for a way to maintain cracked pavement. McDonald mixed crumb rubber with asphalt, and the city used the mixture to fill cracks and potholes in city streets. Pleased with the material's performance, Phoenix started using asphalt rubber as a stress-absorbing membrane on its airport runways and area roads.
In 1975, the state of Arizona began applying the product to rural highways, citing benefits such as durability—especially crack resistance—and a smooth riding surface. By the late 1990s, when the state began applying a 1-inch top layer of rubberized asphalt to concrete freeway expansion projects, it discovered an unexpected benefit: The highways were noticeably quieter.
In 2002, Arizona launched the Quiet Pavement Pilot Program, a three-year, $34 million project to cover 115 miles of Phoenix-area highways with asphalt rubber to create "a smoother ride for motorists and quieter neighborhoods for those who live adjacent to the roads," according to the program's Web site. All told, the state has used more than 15 million tires in highway paving projects since 1988, it reports. A few other states—most notably, California and Texas—have jumped on the asphalt rubber bandwagon, but most states have just a few pilot projects using small amounts of crumb.
Asphalt rubber seems like a textbook win-win product: It creates roads that are superior to conventional asphalt roads on several performance measures, and it has the potential to absorb millions of scrap tires, keeping them out of landfills or tire piles. "With the ever-increasing pressure to reduce carbon footprints and emissions, as well as [for] waste stream reduction, we feel that the production of asphalt rubber is the future of the scrap tire industry," says Jerry Woosley of State Rubber & Environmental Solutions (Denver City, Texas). But it still faces many obstacles on the road to realizing its full potential.
Creating Asphalt Rubber
Conventional asphalt road surfaces are actually asphalt concrete, a combination of asphalt, a thick, sticky petroleum byproduct, and various types of aggregate. Asphalt rubber incorporates at least 15 percent crumb rubber (by weight) into that mix. Most asphalt rubber specifications call for a crumb that's 99.99 percent steel-free with no more than 0.1 percent to 0.5 percent fiber, according to Doug Carlson of the Rubber Pavements Association (Tempe, Ariz.). The crumb size can vary depending on the production process.
There are two primary processes—wet and dry—for making rubberized asphalt. In the wet process, which is the most common, asphalt producers attach mobile mixing equipment to the "hot plant" where they make regular asphalt hot mix. They add crumb rubber to the liquid asphalt in a ratio of 20 percent rubber to 80 percent asphalt, then they allow the mixture to soak while stirring it for about one hour at 350 degrees F. The result is asphalt rubber binder, a fluid that's even thicker and more viscous than straight asphalt. Contractors apply wet-process asphalt rubber hot mix in the same manner and with the same equipment they use to lay conventional asphalt hot mix—they mix it with the aggregate and apply it as a new road surface. With slight equipment modifications, contractors also can use asphalt rubber as a chip seal, in which they spray-apply the asphalt rubber binder to an existing road and roll a thin layer of aggregate over it. Asphalt rubber can be used to create a dense-graded surface that's impervious to water, a water-permeable open-graded surface, or a gap-graded surface, one with drainage characteristics in between the other two.
The asphalt rubber binder's higher viscosity results in the final pavement containing more binder: Regular asphalt hot mix uses about 5 percent binder, whereas an asphalt rubber hot mix will use 8 percent to 10 percent binder. "In a chip seal...the asphalt rubber material stays in a thick membrane [that's] about 6/10 of a gallon per square yard compared to about 2/10 of a gallon per square yard for regular asphalt," Carlson says.
In the dry process, a contractor adds crumb rubber (typically one or more coarse grinds) to the aggregate before mixing it with the asphalt binder. This method does not require special mixing equipment at the hot plant.
Pavement performance
The primary benefit of using asphalt rubber, Carlson says, is that the pavement takes on some of the engineering properties of tire rubber. "The road will perform more like a tire with respect to crack resistance, flexibility, and durability, and over a wider range of temperatures than conventional asphalt roads," he explains. "The asphalt binder will stay more solid when it's hot out, [be] less likely to rut due to relative softness, and [remain] more flexible in the cold—[making it] less likely to crack due to brittleness." Lab and field tests have demonstrated such performance advantages.
Crack resistance. Researchers at the Federal Highway Administration's Accelerated Loading Facility (McLean, Va.) subjected several test sections of road to 100,000 passes of simulated truck traffic and monitored the performance of the control section and each test lane. The asphalt rubber section did not crack, even after more than 300,000 passes. After about 10 years of service, Carlson says, an asphalt rubber road will have 60 percent less cracking than regular asphalt.
Lifespan. Asphalt rubber roads last longer, tests indicate. This is in large part due to their crack resistance, but the rubber also "helps delay oxidation, keeping the binder much less brittle over time," Carlson says. "The rubber particles that soaked up the light oils in the asphalt when it was blended will release those light oils back into the mix … keeping it softer and more flexible. When [conventional] asphalt ages, it loses the aromatic hydrocarbons to evaporation. In A-R, the effect of evaporation or oxidation is delayed." The result, he says, is that "regular asphalt may last seven years, and rubber pavement could last 12 to 15 years."
Noise reduction. Studies in Arizona have documented noise reduction ranging from 6.7 dB on an interstate highway to 10 dB on a Phoenix city street with the application of asphalt rubber hot mix. The Quiet Roads Web site says the project averages a noise reduction of 4 dB or better, which amounts to 50 percent less noise. A Texas study found an average 14 dB reduction on a test segment of interstate highway in that state. The Western Pavement Maintenance Association (Fair Oaks, Calif.) notes that a 6 dB noise reduction is the equivalent of constructing a sound wall.
Smoothness. Researchers measure road smoothness according to the International Roughness Index. The abovementioned Texas study found the test stretch of highway had an average IRI reading of 209 (poor) before the addition of an asphalt rubber overlay and 81 (good) afterward. Similarly, Arizona measured the IRI of a road that had been surfaced with asphalt rubber 13 years earlier and found it was still very good (74).
Water dispersion. Asphalt rubber also does a good job dispersing water and cuts down on the amount of spray kicked up by traffic, says Doug Nintzel, a spokesperson for the Arizona Department of Transportation (Phoenix). The Texas study confirms that performance improvement. The test highway section experienced fewer major accidents on rainy days after the application of an asphalt rubber permeable friction course. Asphalt rubber surfaces "should be strongly considered in areas prone to wet weather accidents," concluded Dale Rand of the Texas Department of Transportation (Austin) at a 2007 conference in San Antonio.
It's worth noting that these results are almost always from projects using wet-process asphalt rubber, which constitute the "overwhelming majority" of asphalt rubber applications, according to the Recycled Materials Resource Center (Durham, N.H.). RMRC's review of dry-process rubber projects revealed mixed results: Some projects have performed as well as or better than conventional asphalt; others have not. "The big challenge to a dry process is that the rubber continues to absorb the asphalt even after it has been placed," Carlson notes. "Dry-process pavements will exhibit the same distress as a conventional pavement" if the aggregate absorbs too much binder. "They tend to crack quickly or lose aggregate," an effect called raveling.
A Bevy of Barriers
With those proven benefits, why aren't more states covering their roads with asphalt rubber? Serji Amirkhanian, a professor of civil engineering and director of the Asphalt Rubber Technology Service at Clemson University (Clemson, S.C.), lists several reasons, including misconceptions, cost, resistance to change, a lack of qualified contractors, and continuing bad feeling over a 1990s-era program that gave asphalt rubber a bad name.
That bad feeling came from the U.S. Intermodal Surface Transportation and Efficiency Act of 1991, dubbed ISTEA. Section 1038 of the law basically ordered state transportation departments to spend a portion of their federal highway funds on asphalt rubber. Some state highway officials, as well as traditional paving interests, put up fierce resistance, however, and in 1995 Congress repealed the ISTEA mandates regarding asphalt rubber. The resentment over ISTEA still lingers to this day.
Some of the opposition to ISTEA came from states that objected to the principle of the federal government dictating road construction, but poorly executed projects that were carried out before proper mix formulas were established also tarnished the product's reputation. "The rush to use asphalt rubber nearly killed it because the technology had not matured," says Larry Galehouse of the National Center for Pavement Preservation (Okemos, Mich.).
More recent projects have demonstrated the improved performance described above, but asphalt rubber binder currently can't get one specific seal of approval, a performance grade from the American Association of State Highway Transportation Officials' (Washington, D.C.) Superpave system. "The asphalt rubber binder does not fit into the national performance grade system because it has tire rubber particles in it that are 1 mm in size or larger," Carlson explains. "The testing apparatus used to grade the liquid asphalt samples is too small for the asphalt rubber to fit into." His association does not have "the millions of dollars needed to reconfigure the apparatus and conduct the widescale verification testing," he says.
Another significant barrier is asphalt rubber's higher upfront costs. Asphalt rubber hot mix can cost substantially more per ton than ordinary asphalt hot mix, taking into consideration the extra asphalt and rubber in the mix. Estimates are anywhere from 10 percent more to 160 percent more, depending on the application. But asphalt rubber proponents point out that the material's longer lifecycle and other benefits more than compensate for those higher up-front costs. First, asphalt rubber can be applied at half the thickness of traditional asphalt—½-inch to 2-inch layers, compared with 2-inch to 4-inch layers—which translates to savings on material and installation costs as well as construction time. For example, a conventional highway resurfacing project would remove 2 inches of the old surface and put back 2 inches of new material; with rubber, only 1 inch of material would be necessary, resulting in half the overall tonnage.
Further, "the higher cost is offset by its longer-lasting characteristic," Nintzel says. Arizona reports that its asphalt rubber roads have one-third the maintenance costs of conventional roads. If the road lasts twice as long—or longer—before needing repair or replacement, that can cancel out the higher initial expenses. And asphalt rubber can save money for drivers, too—what researchers call user costs. Smoother roads reduce vehicle operating costs and improve fuel efficiency, according to the Rubber Pavements Association. All told, over a 25-year period, asphalt rubber pavement is more cost-effective than conventional pavement considering installation, maintenance, and user costs, Arizona State University researchers concluded in a 2002 analysis. And that study did not factor in noise reduction. If the use of asphalt rubber precludes the need for sound barriers, the lifecycle cost would be even lower.
Unfortunately, in this economic downturn, when state transportation departments have resorted to closing highway rest areas to balance their budgets, anything that costs more is hard to justify, regardless of its long-term value. "I don't think there is still a debate over the fact that rubber in asphalt [increases] performance and longevity of highways. However, it is still difficult to convince states to spend the extra funds for rubberized asphalt," State Rubber's Woosley says.
Eileen Sheehy, manager of the Bureau of Materials for the New Jersey Department of Transportation (Trenton, N.J.), concurs that tight budgets are an issue for many states. Obtaining blending equipment can be a challenge as well, she says. Some traditional pavers are resisting asphalt rubber because they don't want to hire another company to blend the rubber into the asphalt binder, nor do they want to buy "prohibitively expensive" new mixing equipment. "Currently, we have a project that's on hold because there are no blending units," she says. "It's a chicken-and-egg kind of thing: Which comes first, more companies doing blending, or more projects requiring that kind of equipment?"
Then there's just resistance to change. "Many engineers don't want to try something different and new," Carlson says. For optimal performance, asphalt rubber requires slightly different application techniques and aggregate mixes than conventional asphalt, so engineers have to think differently, he explains. "The asphalt market in some states has just one decision-maker when it comes to materials selection for highway construction. It can take anywhere from three to five years of technology transfer and demonstrations to change materials or a pavement engineer's mind."
With those barriers to contend with, the Rubber Manufacturers Association (Washington, D.C.) expects the asphalt rubber pavement market—currently consuming about 30 million tires a year—to remain relatively stable, growing an average of 10 percent a year, says RMA vice president Michael Blumenthal. The states currently using asphalt rubber regularly—Arizona, California, Florida, and Texas—will most likely maintain their current levels. Texas used about 2.2 million tires for paving applications in 2006, whereas California reported using 3.5 million tires for asphalt rubber paving projects in 2008. Several other states—including Nevada, New Jersey, New Mexico, and South Carolina—are using relatively small amounts of asphalt rubber, but not enough to have a major impact on the overall market demand.
Building the Market
Asphalt rubber is still just a small portion of the overall market for tire-derived products, but from a tire recycler's perspective it's an attractive market, says Jana Nairn, CEO of Golden By-Products (Ballico, Calif.). It consumes a large volume, it provides a consistent demand, and the consuming companies tend to be stable and pay their bills. Each year Golden By-Products sells more than 10 million pounds of crumb rubber for asphalt rubber pavement. Nairn considers the market "healthy and growing," noting that "as demand grows, so will supply."
Woosley is equally optimistic about its prospects. "An increasing number of state DOTs are either currently using recycled rubber in their highways, or they are developing state specs and installing test sections," he says. Processors that can't currently meet the desired crumb specifications will add equipment to meet the increased demand, he predicts.
If every state used asphalt rubber, the market could "easily" use about half of the scrap tires the United States discards annually in thin surfacing for new road construction and highway maintenance and rehabilitation programs, the RPA's Carlson says. Tire recyclers, state and federal agencies, and industry groups are all working in a variety of ways to stimulate this market growth.
The best way to encourage the use of asphalt rubber is to educate the states that don't use it of its advantages by showing successful completed projects, says Jonathan Levy, ReMA's director of state and local programs. Seminars at the 2010 tire recycling summit, held in conjunction with the ReMA convention, hope to do just that.
"Working directly with state DOTs is vital to the process," Woosley says. "Especially for those states that are not currently using rubberized asphalt or are developing a paving program that does." Providing education outreach to state transportation personnel, contractors, and design engineers is essential, Blumenthal adds.
At the federal level, the RMRC, which is a partnership of FHWA and the University of New Hampshire (Durham, N.H.), promotes the appropriate use of recycled materials in the highway environment. FHWA also works with the U.S. Environmental Protection Agency to help break down barriers and provide technical guidance, and it has initiated a joint research project with the Modified Asphalt Research Center at the University of Wisconsin-Madison to look at states where asphalt rubber is in use and compare specifications to determine common practices and define variables that could alter performance.
And, though federal mandates are clearly a non-starter, federal incentives have their supporters—and their skeptics. ReMA is currently working with Congress to have legislation introduced which will encourage states to use asphalt rubber, Levy says. Federal incentives could help offset some of the upfront costs associated with the product. The RMA is not convinced of incentives' value, however. Blumenthal points out that such incentives only work as long as they are in effect: Once the incentives run out, the use of asphalt rubber is likely to stop. Not only have incentives in the past failed to develop self-sustaining markets, he says, they also allow opponents to state that asphalt rubber is not economically viable because it is only used when there are economic incentives.
Although asphalt rubber still has many hurdles to clear, the continued slow-but-steady growth is encouraging to those in the industry. "Agencies that have implemented the use of asphalt rubber see the extended performance, durability, crack resistance, and related maintenance cost savings," Carlson says. "For those that look at AR to provide an extra benefit or improved characteristic, the proof is in the pavement." •
Chelan David is a Seattle-based writer.
Asphalt rubber creates quieter, safer, more durable, and longer-lasting roads—and could consume up to half of the nation's scrap tires annually—but the drive to get states to accept this technology has been anything but smooth.