Highway engineers around the world have tried to incorporate scrap tire rubber in asphalt pavements since the 1950s. (Hanson, 1984) Some of the earliest experiments involved incorporating natural rubber with bitumen in the 1840s. (Heitzman, 1992) It was their hope to capture the flexible nature of rubber in a longer lasting paving surface. The task was difficult and early asphalt-rubber formulas provided little or no benefit, the result was a modified asphalt pavement that cost more and had a shorter service life than conventional asphalt.
It was not until the 1960s that a formulation was discovered that was successful. Charles H. MacDonald worked with the City of Phoenix after retiring from the U.S. Bureau of Public Roads (now FHWA). He first thought of asphalt-rubber while travelling across the country inspecting highway material sources for the Bureau of Roads. His mobile trailer's roof cracked and he used asphalt as a quick patching material. However, after frequent moves and long exposure to the sunlight, the asphalt would oxidize and become brittle. The roof crack "reflected" through to the surface of each successive asphalt patch. He thought he could solve the cracking problem if he incorporated rubber in his next round of patching. (Winters, 1989)
While devising methods to repair potholes on the streets of Phoenix, Arizona, MacDonald experimented with adding ground tire rubber to hot liquid asphalt. He found that after thoroughly mixing crumb rubber with asphalt and allowing it to react for periods of forty-five minutes to an hour, new material properties were obtained. This material captured beneficial engineering characteristics of both base ingredients; he called it asphalt-rubber. (Huffman, 1980) The asphalt was absorbed by the rubber particles, which swelled in size at higher temperatures allowing for greater concentrations of liquid asphalt contents in pavement mixes. He used this material to create "band-aids" for pothole repair. The patches worked so well, that the city eventually tried using asphalt-rubber as the binder for chip seals. A chip seal is a rehabilitation strategy where the hot liquid asphalt-rubber is sprayed by a distributor truck directly on the road surface and aggregate material is then spread and rolled into place.
By 1968, the Arizona Department of Transportation began numerous and diverse research and development projects involving asphalt-rubber under the direction of Gene Morris, the director of the Arizona Transportation Research Center. (Epps et al, 1980) By 1975, crumb rubber was successfully incorporated into hot mix asphalt. Based on the department of transportation's research, agencies in other states were able to follow the progress and development of asphalt rubber. California and Texas placed chip seal test sections in the 1970s and hot mix applications in the 1980s. Florida developed an asphalt rubber binder with lower rubber contents to avoid the patents in the 1980s. In 1988, American Society for Testing and Materials (ASTM) published the definition of asphalt-rubber. ASTM D8-88 read, "a blend of asphalt cement, reclaimed tire rubber and certain additives, in which the rubber component is least 15% by weight of the total blend and has reacted in the hot asphalt cement sufficiently to cause swelling of the rubber particles."
Widespread use of the material was limited based on its experimental status and patent restrictions. However, as many as twenty-three states had placed test sections using A-R by 1990. Extensive research was completed in 1992 through the Construction Productivity Advancement Research Program sponsored by the Army Corps of Engineers and private industry (Anderton, 1992). Additionally, a pooled fund study of crumb rubber modifiers in asphalt pavements sponsored by the FHWA and several states was initiated in 1995. Although the Pooled Fund Study was not completed, a Summary of Practices in Arizona, California and Florida was published by the Transportation Research Institute of Oregon State University (Hicks et al, 1995) as well as an interim report on Construction Guidelines (Hanson, 1996). These reports have been helpful to agencies that wish to develop specifications for crumb rubber modified asphalt.

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Rubber Pavements Association
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