Polymerized Crumb Rubber Modified Mixtures in Minnesota - Final Report Author(s): Newcomb, David; Stroup-Gardiner, Mary; Kim, Jong, Allen, Bruce & Wattenhoffer-Spry, Jeff, University of Minnesota Origin: Minnesota Date: January, 1994 Categories: Performance, Noise Key Words: Crumb rubber, CRM, polymerized Summary: This research program was based on laboratory and field studies. All work was limited to the use of a 10-mesh crumb rubber in the dry process. An evaluation of the asphalt-rubber interactions indicated that there should be a reasonable level of interaction between the crumb rubber and the asphalt cement selected for the project. A suggested criterion for defining an acceptable level of interaction would be to establish a minimum viscosity of 15 poise for a neat asphalt cement modified with 20 percent crumb rubber. When designing a crumb rubber modified mixture, the aggregate gradation should be substantially gapped. The target gradation used in the construction of the Babbit, Minnesota test sections should be considered as a guideline for an acceptable gapped gradation. Stockpile gradations should be adjusted for crumb rubber gradations volumetrically; generally, 1 gram of crumb rubber occupies the same volume as 3 grams of aggregate for a given sieve size. The optimum asphalt content for CRM mixtures should be based on air voids form 1.5 to 3 percent.