Engineers and chemists began exploring ways to add rubber to asphalt in the 1920s and came up with several processes to produce what's known generically as rubberized asphalt. In the “wet process,” crumb rubber (shown) is blended at high temperatures with liquid asphalt and other binders. “Asphalt rubber” refers to a specific material containing at least 15% ground rubber as defined in ASTM D6114-97 (2002). Photos: Rubber Pavements Association
Mix temperatures for asphalt rubber are slightly higher than conventional hot mixes, and workability is limited. The material should be at least 350° F for placement and fully finished before the temperature gets below 275° F. Such requirements mean that steel-wheeled rollers must be used for compaction.
In 2002, Arizona DOT placed 1 inch of asphalt rubber on a stretch of concrete pavement that carries 150,000 vehicles/ day. The resulting reduction in traffic noise—averaging 3 to 5 decibels—was so striking that the public clamored for more. By the end of that year, the department announced it would install the overlay on all 115 miles of the state's freeways.