As the story goes, Charles McDonald towed a small camper-like trailer as he made cross-country trips investigating highway-material sources for his employer, the U.S Bureau of Public Roads. The trailer's roof, apparently made of a fibrous material, often developed cracks, which McDonald patched with asphalt cement, only to have the cracks reflect back through the patch as the asphalt aged. What was needed, he finally concluded, was a material, like rubber, that could seal the crack, yet remain flexible enough over time to resist being affected by the crack beneath.
Perhaps McDonald remembered the trailer's roof when later, in the mid-1960s, he was working as an engineer for the City of Phoenix, Ariz., and looking for an affordable material that could be used to overlay distressed pavement, but without itself being ruined in a relatively short time by reflective cracking. McDonald discovered his material, according to a remembrance written by a contemporary, Gene Morris, then an engineer with the Arizona Department of Transportation, when he (McDonald) "found that ground, vulcanized rubber from old tires, when combined with asphalt at a high temperature for a specific time, would react to form an elastomeric material."
Thus was created "asphalt-rubber." And what, exactly, is this material?
The most informative definition we encountered appeared in a recent technical paper by Doug Carlson, executive director of the Rubber Pavements Association; Cliff Ashcroft, vice president of California Operations for FNF Construction; and H. Barry Takallou, Ph.D.,P.E., president of the Crumb Rubber Manufacturers:
"Primarily, asphalt-rubber is a combination of liquid bitumen (asphalt) and granulated scrap-tire rubber (crumb-rubber modifier — or CRM) in an approximate 4:1 ratio (80 percent bitumen and 20 percent rubber) that is held in agitation at elevated temperatures for 45 to 60 minutes. The CRM is used in 2-millimeter-minus particle sizes, and it does not dissolve during the process. The material can be used in spray-applied applications [chip seals and stress-absorbing membranes] and in hot mixes."
Think of rubber-modified hot mixes as a composite paving system with rubber, asphalt and rocks, says Carlson, but with the rubber added to the liquid for ease of handling and manufacturing.
"The rubber has an affinity for the light oils in asphalt — it soaks them up," says Carlson. "That's the first step in the process, and it's accelerated at elevated temperatures — usually 325 F to 350 F minimum for the 45- to 60-minute reaction time. As the CRM is saturated, the outer surface of the rubber particle gels and becomes very sticky."
Asphalt-rubber does seem to allow design latitude, however, in regard to CRM particle size and the percentage of CRM used by weight of liquid asphalt. For instance, Asphalt Rubber Technology Service (ARTS), a partnership among South Carolina's Department of Health and Environmental Control, Clemson University, and the City of Clemson, formulates asphalt-rubber binders (the mixture of liquid asphalt and modifiers) only with minus #40 mesh CRM, that is, particles passing through a sieve with 40 openings per linear inch.
"Different rubber particle sizes and different amounts of rubber can create vastly different CRM binders," says Serji Amirkhanian, Ph.D., ARTS director. "ARTS projects specify the use of minus #40 mesh crumb-rubber particles, because CRM binders made with this size of rubber can flow through the same pipes, valves and pumps that are used with conventional asphalt binders. Also, these binders could be graded, with some minor modification to the asphalt-testing equipment."
In addition, different asphalt-rubber applications may use different amounts of rubber. A dense-graded surface-course mix that ARTS developed for South Carolina, for instance, uses 10 percent rubber by weight of liquid asphalt, while an open-graded friction course uses 12 percent, and a spray-on binder for stress-absorbing membranes has 20 percent or more.
In California's Department of Transportation (Caltrans), Chief of the Office of Flexible Pavement Materials, Terrie Bressette, says that asphalt-rubber binders typically use 20 percent (± 2 percent) of CRM by weight of binder. Caltrans specifies that CRM content be a combination of 75 percent tire rubber (which may contain a large amount of synthetic rubber) passing a #8 mesh, and 25 percent natural rubber (which must contain at least 44 to 48 percent non-synthetic rubber) passing a #10 mesh.
According to Ali Zareh, Senior Pavement Design Engineer with Arizona's Department of Transportation (ADOT), the state's asphalt-rubber binders typically use 18 to 20 percent CRM by weight of binder, and rubber particles must pass a #10 mesh. The rubber has a texture similar to ground coffee, he says.
According to the material's proponents, the rubber in asphalt-rubber paving materials — that is, hot mixes and spray-on binders —provides long-term elasticity or resiliency by means of its capacity to retard the effects of binder oxidation (aging). The benefit, they say, is a substantially reduced incidence of reflective cracking. In addition, because the rubber stiffens the binder and helps the pavement resist deformation, similar to a polymer-modified binder, reduced rutting (wheel-path depression) is a further benefit claimed. The potential payoff is longer-lasting pavements that require less maintenance and yield lower lifecycle costs than alternatives.
For Caltrans, says Bressette, any pavement-rehabilitation project involves three major issues: load-carrying capacity; crack repair and mitigation; and ride quality. Crack mitigation, says Bressette, is among the primary reasons for the state's using asphalt-rubber. Also, a large portion of the state's highway system is topped with an open-graded friction course — 3/4 inch to 1 inch thick — and much of this surfacing material is produced with asphalt-rubber binder. According to Caltrans, benefits of these thin surface courses include enhanced skid-resistance and reduced wet-weather splash and spray. Carlson adds, too, that asphalt-rubber binders help open-graded friction courses stick tenaciously to the pavement beneath.
To the list of asphalt-rubber benefits, ADOT's Zareh adds that of noise mitigation. When the state, about five years ago, installed a 1-inch lift of an asphalt-rubber open-graded friction course on a concrete pavement to restore the riding surface, an added — but unexpected — benefit was a significant reduction in tire/pavement noise. Nearby residents and motorists alike were so complimentary about the lowered noise, says Zareh, that ADOT developed a "Quiet Pavement" program, under which more than 1,000 lane miles of major freeways in the Phoenix metro area have been so resurfaced.
Both Arizona and California also use asphalt-rubber gap-graded mixes for mill-and-replace applications. California has determined that these surface lifts can be as much as 50 percent thinner than conventional-mix lifts, yet yield equivalent service life. Bressette calls the practice the "half-thickness" design, because "if you typically would use, say, 3 inches of conventional mix in this application, then you might place only 1-1/2 or 2 inches of rubber." And when thinner lifts are used, says Carlson, carbon dioxide emissions from the overall process are reduced.
And, of course, say fans of asphalt-rubber, using the material is good for the environment. An estimated 1,000 scrap tires can be used per lane mile per inch of lift thickness. In Arizona, where asphalt-rubber has been used actively since 1990, says Zareh, the state calculates that it has placed more than 23 million scrap tires into pavement.
According to the Rubber Pavements Association (RPA), only a few states — among them Arizona, California, Florida, Texas and South Carolina — are routinely using asphalt rubber. For these users, the potential benefits apparently outweigh the material's initial highercost — perhaps 25 to 30 percent more, based on the cost of a ton of conventional mix. RPA's Carlson does say, though, that asphalt-rubber projects are gaining ground in a number of other states, including New Jersey, Nevada, Washington, Nebraska and Colorado.
But that said, much of the country's road-building community remains cautious about asphalt rubber. Part of that caution results from a grudge in some state departments of transportation (DOTs) against the 1991 Intermodial Surface Transportation Efficiency Act (ISTEA), which mandated the use of CRM.
"Memories of ISTEA still linger in some DOTs," says Carlson. "The biggest problem with the legislation was the lack of quality control through a standard specification."
But the use of asphalt-rubber has been slowed, too, says Carlson, by other impediments. Because aspects of the process were patented (the last patent expired in 1992), investment and development were discouraged. Added to that, he says, is the perception in some DOTs that asphalt-rubber remains an "experimental" material, and that view is reinforced with a reluctance to deviate from materials and methods that are working effectively.
Yet another deterrent to asphalt-rubber's use — by some DOTs, at least — is the inability of the Superpave system to test a high-viscosity asphalt-rubber binder (more about "high-viscosity" shortly) in order to determine its performance grade (PG). Binder-testing procedures were developed in such a way, says Carlson, that the presence of 2-millimeter rubber particles interferes with the tests. To avoid this problem, many DOTs have opted not to experiment with asphalt-rubber. Those that do use the material, he says, may employ previously used mix-design systems, such as Marshall or Hveem, when formulating binders with CRM. (To Amirkhanian's point, however, the use of finely ground CRM may offer the prospect of performance grading asphalt-rubber binders.)
Aware of the general problem of testing asphalt-rubber binders, the Federal Highway Administration (FHWA) instituted a field experiment in 2000 to compare the performance of asphalt pavements built with binder modifiers that do not fit the PG system with those that do. Using an accelerated-loading facility at the Turner Fairbanks Highway Research center near Washington, D.C., researchers want to determine the value added by the modifier through extended pavement life (if any), then to perhaps establish mathematical models for evaluating binder characteristics. So far, says Carlson, the high-viscosity asphalt-rubber pavement is performing well.
So far, the discussion has focused on the "wet process" for producing asphalt-rubber products — that is, the CRM is added to the liquid (wet) asphalt to create a binder that can be used in hot mixes and spray applications. By contrast, a "dry process" for creating hot mixes involves directly substituting CRM for up to 2 percent of the aggregate. Dry-process mixes are little used, however, mainly because uniformly dispersing the rubber particles throughout the mix is difficult.
Wet-process binders typically fall into two categories. Those in the first category are variously identified as "wet-process/high-viscosity" types, or just "high-viscosity" types or, more commonly, as "field blends." Binders in the second category are identified as "wet-process/no-agitation" types, or "low-viscosity" types or, more often than not, as "terminal blends." The basic distinction between the two binder types is the state of the rubber particles in the liquid asphalt.
Field blends are binders produced on site, at the asphalt plant, with a mobile blending unit that combines the plant's liquid asphalt with CRM. In these binders, most of the rubber particles, although having reacted with the hot liquid asphalt, remain distinct, physical and visible ingredients in the binder. These binders, which must be continually agitated to keep the particles suspended, provide a thick coat of adhesive on the aggregate in hot mixes, and provide a thick coat of adhesive when used in spray applications.
Terminal blends are so called because they are most often produced by suppliers, such as refiners, at their locations (terminals), and then shipped to the asphalt plant. These binders are characterized by having no visible rubber particles. Some definitions of terminal blends say that the rubber particles "dissolve" in the liquid asphalt. The more precise definition, however, says that the rubber particles are "homogenized" in the binder. These binders typically do not require agitation to remain in a usable state, but some manufacturers do add stabilizing ingredients to resist possible settlement.
Although some terminal blends may be produced with rubber particles passing an extremely fine sieve, many are formulated with #30 mesh or #40 mesh particles. The size of particles is not a significant issue, says Donald Goss, Manager of Product Technical Services for Valero Energy Corp., "because the rubber is normally totally homogenized into the asphalt binder." These binders typically have a CRM content of around 10 percent.
When formulating terminal blends, according to Jean Azoury, Group Manager, Technical Services and Marketing Support for Paramount Petroleum Corp., "rubber is introduced into the hot asphalt and mixed and processed into a homogeneous rubber concentrate, which will then be used to manufacture different PG-grades of finished product."
Although opinions vary about which type of wet-process binder works best in a given application, the experts do seem to agree that the two binders should not be used interchangeably. Choosing the correct wet-process binder seems to be a matter of assessing the binder's characteristics in a specific application. For example, a high-viscosity field blend might be better suited for use in open-graded mixes, while a low-viscosity terminal blend might best be employed in dense-graded mixes.
Even states that are convinced of asphalt-rubber's benefits seem careful about using the material — as says Jorge Sousa, vice president of RPA — only where it is "an appropriate and advantageous engineering solution to a pavement problem." But that said, the Association's Carlson notes a steady 10-percent annual growth in asphalt-rubber's use for the last several years.
"Safety enhancements to existing roadways with high incidences of wet-weather accidents will prompt the use of asphalt-rubber friction courses," he says. "And when the FHWA's noise policy is modernized to include the use of road surfaces as a noise-mitigation tool, then the use of rubber will be standard and routine throughout the United States."