The Arizona project

by Gene Morris


 
The major problem facing highway, road, and street agencies in the mid 1960’s was that the systems that had been improved after World War II were near the end of their design life. The pavements needed rehabilitation and/or structural upgrading.
   Overlays of the pavements, whether rigid or flexible, were the obvious answer. Unfortunately, when overlayed, all the cracks and/or joints in the existing pavement reflected through in a relatively short time. A solution was extremely important.
   It had been recognized since the beginning of asphalt pavements that if an elastic component could be introduced into the asphalt matrix performance of the pavements could be greatly improved. But all efforts to do this resulted in cost increases that were unacceptable.
   Then a materials engineer for the City of Phoenix came up with the answer. Charles 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.
   The use of this material, now called Asphalt-Rubber, was economical since it involved a waste product. In time it was technically defined by the American Society for Testing Materials (ASTM).
   Phoenix immediately began field testing this material by placing small test squares of it over areas of pavement distress. Test sections were also placed on state highways to evaluate cold weather applications.
   From these simple tests it was readily apparent that material held great promise. But there were a lot of questions that had to be answered first. Some of the questions were:
  1. What was the reaction process between the asphalt and the vulcanized rubber? How could it be optimized? How could it be evaluated? What material properties of the asphalt and the rubber were critical to the end product?
  2. How could the materials be tested? Standard asphalt tests were empirical and not applicable.
  3. How were the material’s properties affected by temperature change or aging?
  4. What type of paving systems could be utilized with these materials?
  5. How could it respond to wheel loads?
  6. What equipment modifications or new equipment would be needed?
   Getting answers was a monumental task and involved many factions of the industry
  • a joint effort of civil engineers, chemists, engineering professors, materials suppliers, and contractors. Government agencies included the City of Phoenix, Arizona Dept. of Transportation (ADOT), Federal Highway Administration, and Army Corps of Engineers. Primary material suppliers were Arizona Refining and Sahuaro Petroleum.
   ADOT conducted or sponsored numerous lab research projects and built many field test and full scale projects between 1968 and 1988. This effort resulted in over 75 formal papers and many presentations.

   Here is what we found.
  1. Asphalt-Rubber chip seals (SAM) control fatigue cracking and have an effective life of 15 years.
  2. Asphalt-Rubber interlayers (SAMI) controls all cracking, substantially reduces stresses in overlays and provides membrane control of expansive clay.
  3. Open-Graded surface courses are effective overlays of rigid pavement at a thickness of an inch, and of asphalt pavement at a thickness of a half-inch. They also provide high skid resistance, reduced water splash, cut noise by at least half, and have a high resistance to aging.
  4. Gap-Graded hot mix applications are structurally equal to standard asphalt concrete of over 2-to-1, are effective on asphalt concrete, have a high resistance to rutting, and ideal for urban arterials and residential streets.
   The cost savings to the agencies have been substantial. Two sections of the
concrete pavement on I-40 near Flagstaff needed rehabilitation.
   One five mile section was reconstructed using standard design. It took two years to construct and needed a maintenance overlay in 10 years.
   The other 10-mile section used a 2½- inch gap-graded Asphalt-Rubber mix followed by a ½-inch open-graded Asphalt- Rubber surface course. It took four months to construct and was still in excellent condition with no reflective cracking after 10 years.
   The state estimates that it saved over $30 million on this single project.
   Another example is the one-inch Asphalt-Rubber open-graded overlay placed on I-19 south of Tucson in 1988. It cost $2.45 per square yard. The alternative solution was to grind and groove the pavement at an estimated cost of $5.00 per square yard and a projected life of eight years.
   After 13 years, the Asphalt-Rubber overlay has been maintenance free and is still is excellent condition. Besides its ride quality, the overlay has reduced noise levels by more than 50 percent, which has been very important to nearby residents.
   To date, nearly all of Arizona’s interstate highway system has been rehabilitated with one of the Asphalt-Rubber applications.
   One other application that should be mentioned is maintenance crack sealing. Prior to the development of Asphalt-Rubber there were no economical crack sealing materials nor equipment to routinely seal pavement cracks.
   Now, virtually every state and county have programs for routinely sealing pavement cracks. The development of Asphalt-Rubber also led to a new industry with a worldwide market and an additional industry to grind old tires. Over 85 percent of the rubber from the tires in Arizona is recycled into the roadways.
   The Arizona project has developed a new paving material that has paid major dividends to government agencies, found a positive use for a major waste product, and created new industries.


Gene Morris, P.E., is still active in Asphalt-Rubber research and technology transfer after a long career in the public and private sector that included serving the Arizona Department of Transportation as Director of the Arizona Transportation Research Center. He also served as an asst. district engineer, senior resident engineer, quality control engineer, and research engineer. His agency career spanned from 1962 to 1983. In the private sector, Morris has been affiliated with Western Technologies, Inc., International Surfacing Systems, and was co-owner of Pavement Technologists, Inc. Morris was the ADOT engineer who introduced Charlie McDonald’s work to state agencies throughout the U.S. He has authored over 50 technical publications and conducted field reviews on the performance of the state’s 2,500 miles of Asphalt- Rubber pavements. Morris is an active participant in RPA workshops and conferences and serves as the vice chairman for its Technical Advisory Board.

 
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