Roadblocks remain for rubber-modified asphalt on roadways

AKRON—Rubber-modified asphalt is a proven technology, yet many barriers remain to its acceptance, according to two speakers at the International Tire Exhibition & Conference in Akron Sept. 13-15.

Recent problems in obtaining infrastructure funding have led to lax maintenance and deterioration of the nation's roads, according to Gary Houston, vice president/general manager of VSS Emultech.

“A road is a load-bearing device for the purpose of allowing the safe and quick conveyance of people, goods and services,” he said.

“You can do everything right when you build a road, but the road will deteriorate. If we don't maintain our nation's roads, they will deteriorate faster and faster.”

The advantages of asphalt as a paving material, Houston said, are manifold. Asphalt offers waterproofing, aggregate durability and dust abatement, he said; it is non-hazardous and viscoelastic, and it offers strength as part of the road's structural system.

Adding crumb rubber to asphalt provides two major advantages, he said: It improves the engineering properties of the asphalt, and it rids the environment of waste.

“Adding GTR (ground tire rubber) to asphalt produces a complex fluid, or a colloidal system,” he said. “Understanding the rheology of the system allows the optimization of the GTR for enhanced performance characteristics.”

Yet despite the usefulness of rubber-modified asphalt in many aspects of roadbuilding, neither road construction companies nor state highway departments have much incentive to use the material, Houston said.

“There's an inadequate understanding of the material,” he said. “It will take a multifaceted approach to move GTR-modified asphalt forward.”

Redmond Clark, president of Asphalt Plus L.L.C., elaborated on the reasons why acceptance of rubber-modified asphalt has been slow in many states.

“State DOTs (Departments of Transportation) control technology access to roads, and their focus is high-quality, low-cost roads,” Clark said.

Rubber-modified asphalt costs more than conventional asphalt, and many state DOTs haven't forgotten their bad experiences with the material in the past, starting with the “Plus Ride” material in the 1980s, he said.

Even worse was the failure of the mandate for rubber-modified asphalt in the Intermodal Surface Transportation Efficiency Act in the 1990s, he said. State DOTs resented the mandate, and lack of knowledge about the material and its processes led to disastrous field trials.

“The DOT Old Guard is skeptical,” Clark said. It doesn't help that crumb rubber quality can be inconsistent, that lab testing can't predict field performance, that rubber content can't be verified in the field or that the rubber-binder interaction is uncertain, he said.

“The younger DOT generation is more receptive, but they understand they have to be careful,” Clark said. “If you create a successful new road design, you get no credit, but if it fails, you get the blame.”

More than 90 percent of U.S. roads are paved with asphalt, and asphalt modified with another material comprises 10 to 20 percent of that, according to Clark.

Potentially, modified asphalt could consume 50 to 75 percent of natural rubber, yet it consumes only about 4 percent, he said.

Rubber-modified asphalt competes with polymer-modified asphalt, he said. Rubber and plastic polymers offer the same benefit of making asphalt more flexible and crack-resistant, and side-by side field tests show them to be comparable in performance, he added.

As of today, however, state DOTs are using polymers much more often than rubber in modifying asphalt, according to Clark. Florida has even dropped rubberized asphalt in favor of polymers, he said.

Polymer asphalt is generally less expensive than the wet-process rubber asphalt commonly used today, he said, costing $6 to $7 per mix ton compared with $6-$10 for rubberized asphalt mixed at the asphalt terminal or $7-$9 for rubberized asphalt mixed on-site.

Fortunately, new processes for dry-process asphalt rubber—in which engineered rubber is injected like aggregate during mix production—offers new hope for expanding the use of rubberized asphalt, Clark said.

Mix lab testing for dry-process asphalt rubber is comparable to that for both wet-process and polymers, he said. After more than 5,000 lane miles in up to 13 years of service, dry-process performs just as well in all weathers as wet-process and polymers, he said.

Best of all, dry-process is cheaper than the competition, at $3.50-$4.50 per mix ton, Clark said.

In trying to persuade the market, there are two gatekeepers asphalt rubber manufacturers must face, according to Clark—state DOTs and contractors.

“DOTs want equivalent or better pavement quality from what they're already using, better quality control methods and lower life-cycle costs,” Clark said. “Contractors want lower costs and minimal change in their production operations.”

A growing body of field comparisons in such states as California, Arizona, Florida, Georgia, Louisiana and Illinois shows that dry-process performance is comparable in the field with both wet-process and polymers, he said.

“Asphalt is a mature industry, and cost changes of less than 50 cents per ton often drive technological change,” Clark said.

Polymer-modified asphalt, wet-process asphalt rubber and dry-process asphalt rubber are equivalent in performance, but dry-process costs as much as $3 per ton less than wet-process or polymerized asphalt, he said.

“In difficult economic times, it is not enough to be environmentally sustainable,” he said. “Technologies have to be economically sustainable too. Market-sensitive technologies will allow deeper asphalt market penetration.”