by Douglas Carlson
RPA Director of Government Relations

Ground tire rubber (crumb) has been used successfully as a beneficial modifier to asphalt pavements since the late 1960s when Charles McDonald of the City of Phoenix developed a time-temperature formula that has become known Asphalt-Rubber (A-R).

It was defined by the American Society of Testing Materials (ASTM) in 1988 as: a blend of asphalt cement, reclaimed tire rubber, and certain additives in which the rubber component is at least 15 percent by weight of the total blend and has reacted in the hot asphalt cement sufficiently to cause swelling of the rubber particles (ASTM D8-88).

However, standard practice in most major state users, such as Arizona, California, and Texas, is to use around 18 to 20 percent rubber. Florida, also a major user, utilizes a lower percentage of rubber in some of its mixes but is experiencing good performance.

The research is there

Through many years of federal, state, international and industry research, combined with years of successful field performance, it has been proven that A-R resists cracking, can last up to twice as long, is stronger, and can be used in reduced thickness when compared to conventional asphalt pavements.

Research by the State of California led to development of the widely accepted practice of placing A-R in reduced lifts, one inch of A-R to two of AC. This research has been verified by work at the University of Alaska (fatigue testing) and the University of California, Berkeley (accelerated pavement testing with the HVS) where results indicate the reduction may even be greater than 2: 1.

These characteristics allow for substantial long term cost savings due to longer pavement life and reduced maintenance costs. As an additional benefit, A-R pavements have been proven to reduce road noises by 50 percent or more. (see Sacramento photo & caption on page 7).

While the use of A-R appeals to engineers because it improves the performance of asphalt, a secondary benefit is the removal of millions of tires from the waste stream, preventing the buildup of tire

stockpiles that can become an environmental nightmare.

If it's so good, then why isn't it used routinely and everywhere? There are several obstacles to overcome.

Patents slow progress

The first and most notable obstacle was patents that did not expire until 1992. The royalties required by the patent holder increased the cost to the agency/owner and ultimately to the taxpayer. Due to the high costs of the patented materials, widespread adoption and use of the material was restricted. Also, the use that did occur was primarily in the Southwestern part of the U.S.

Since the patents have expired, any paving contractor is able to use the material. A-R is now in the public domain and major agencies have been able to develop their own specifications instead of relying on the specs recommended by the patent holder.

The number of contractors involved in the industry has increased dramatically since 1992. Through its focus on technology transfer, the RPA is assisting contractors and agencies to shorten the learning curve, which has helped expand the use of A-R throughout the world.

Because of the patents and other factors, like the added expense of tire recycling, shipping costs of crumb rubber, and the blending of liquid asphalt and rubber, the material was notably more expensive than conventional asphalt.

The slow development of peripheral industries, such as tire recycling and A-R blending equipment manufacturing, was another factor. While costs could be recouped through longer pavement life and the ability to use reduced pavement thickness, agencies opted to spread yearly budgets as far as they could and went with less costly paving materials.

It takes time

Specifications for paving materials must be developed by each agency before they can be used. This usually requires laboratory research and field testing and can take a long time. The work on ASTM Spec D6114-97 19984.03, section 3.2.1, adopted in 1998, first began in 1988, for example.

As a "new" product, A-R was usually put into an experimental category, which is seldom used for routine purposes. The late Congressman Sonny Bono, who championed the material as Mayor of Palm Springs, was instrumental in organizing 40 cities to petition California to ask FHWA to remove its experimental status in the state.

Even though Los Angeles County and several cities in Southern California had been using A-R for years, the material languished in the "600" (experimental) section of the Public Works Standards' Greenbook. In Greenbook 2000, the A-R specs will appear in sections 200 and 300 as routine use materials.

The bad years

Another factor inhibiting growth came from the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991. This law included a section that mandated an increasing percentage of highway projects using scrap tire rubber.

The potential market for "rubberized pavements" was established and every
imaginable method of using tire rubber in asphalt was tried. Most didn't enhance pavement performance in any significant way and many failed sooner than conventional asphalt.

This "black gold rush" resulted in a great many failures and destroyed the credibility of established, successful materials like A-R. The confusion that developed nationally led many to close their minds as soon as the word rubber in pavements was mentioned.

Agency engineers tend to remember only the disasters and as a result, some have a prejudice against any material containing tire rubber. In addition, most resist having the federal government dictate what material must be used for jobs that they design.

Fortunately, that ISTEA section was repealed after many tumultuous exchanges in Congress.

Another by-product of the "rubber mandate battle" was a split between AASHTO, NAPA, and the A-R industry, which is slowly being repaired.

Its time has come

As time has passed and research and experience have been combined, A-R specs and use strategies have evolved to a state where proof of the material's success can be measured in pavements around the world.

The results and facts are hard to ignore. By reacting crumb rubber with liquid asphalt, an extremely durable, flexible, crack resistant paving material can be obtained.

A-R can provide a superior driving surface, last twice as long, and use half as much material as conventional asphalt. It saves money and the environment. The added benefit of removing millions of scrap tires from the waste stream has made it especially attractive in recent years.

Waste tires pose a difficult problem. They are costly to recycle and carry a negative value (costs to taxpayers are roughly $2/tire for tire pile remediation). Several states have adopted tire disposal fees to control waste tires and develop recycled tire markets.

Waste tire and highway agencies are partnering to explore ways they can work together to utilize crumb rubber in their pavements. After examining the facts, they are turning to the established public process, Asphalt-Rubber.

In the new millennium, A-R is a product whose time has come.

It takes time! From left, at the first Asphalt-Rubber User Producer Meeting in Scottsdale, AZ in May, 1980: Dr. Jerry Love, chief, FHWA Research; Dr Jon Epps, Texas A&M; Charles McDonald, Phoenix; Dr. Bob Galloway, Texas A&M; and Gene Morris, director, Arizona Transportation Research Center. The group mapped out short and long term research programs including a Life Cycle Cost Analysis, completed 18 years later, and a QC/QA manual that will be completed this year. Dr Epps, now at University of Nevada, Reno, participated in both projects. Morris, retired, is now a technical advisor to RPA.