by Mark Belshe

What is and isn't Asphalt-Rubber?

The Rubber Pavements Association is a nonprofit trade group dedicated to encouraging greater usage of high quality, cost effective asphalt pavements containing recycled tire rubber. Our Board of Directors believe we have a responsibility to the agencies and the taxpayers to only promote rubber modified asphalt paving processes that have a long history of research, a proven track record in field performance, and are routinely used by two or more states.

Currently the only process that meets this criteria is Asphalt-Rubber as defined by ASTM D8-88. Under this definition, a minimum of 15 percent crumb rubber by total weight of the blend is required. In actual practice a minimum of 18 percent or more is routinely used. This criteria of proven and tested materials assures agencies that only high quality rubber binders with known characteristics are used.

In recent months we have seen market confusion develop in California because some companies with "imposter" binders are insisting on calling their products "Asphalt Rubber." This is evidenced by an article that was printed in a publication of an asphalt association in Southern California entitled The Next Generation of Asphalt-Rubber Hot Mixes.

In the article, several new products, some proprietary, were described and none of them meet the definition of Asphalt-Rubber referenced above. Why the confusion? Perhaps it is a case of being misinformed or maybe it is a deliberate attempt to ride the coattails of a tested and widely used material without having to wait for research and field performance testing.

In California, Asphalt-Rubber hot mix underwent an extensive 10-year Construction Evaluated Research Program, as defined by FHWA, that substantiated its use in a 2:1 reduction of thickness versus conventional asphalt concrete mixes. To date, the terminally blended binders and other "new" products have not undergone such longterm testing.

Caltrans has recently placed a moratorium on a proposed Modified Binder specification that attempted to co-mingle Asphalt-Rubber with terminal blend binders and is just now undergoing Construction Evaluation Research to ascertain the proper design value of these new materials.

Asphalt-Rubber hot mixes typically require a minimum of 7.5 percent asphalt/ binder content, where terminally blended binder mixes often use an asphalt/binder content of 4.6 to 5.6 percent. This decreased binder content directly correlates to a lesser film thickness on the aggregates. A thinner film thickness has a tremendous impact on the durability and longevity of flexible pavements.

As a completely public process defined by the internationally recognized ASTM, Asphalt-Rubber is subject to open-market competition, has no secret ingredients, or "magic elixirs," and contains a quantifiable amount of crumb rubber from scrap tires. The "new" products contain anywhere from 4 to 10 percent rubber, compared to the 18 to 22 percent found in Asphalt-Rubber.

While not a primary reason to use Asphalt-Rubber, using twice as many tires in roads is an excellent environmental bonus for any public agency, especially in California, which is struggling to deal with 30 million discards annually.

As contractors, material suppliers, and most importantly as taxpayers, the Rubber Pavements Association is concerned with building agencies the longest-lasting, most cost effective roads possible. We become concerned when other products, which purport to provide the same benefits as Asphalt-Rubber, confuse the marketplace.

Should a substantially shortened service life or failure result from one of these unproven products, many will believe that Asphalt-Rubber has failed.

Clearly, we need to do a better job of educating the agencies on what is and what isn't Asphalt-Rubber. In the meantime, we would hope that agencies will ask some hard questions when pressed to use these new products that need a lot of testing and proven performance before they can call themselves equals.