Georgia’s Experience With Crumb Rubber in Hot-Mix Asphalt

In 1991, the Georgia Department of Transportation (GDOT) began to evaluate the production and placement of crumb rubber hot-mix asphalt. The crumb rubber mix (CRM) used by GDOT was produced by adding ground tire rubber to hot-mix asphalt using the wet process. A test section of CRM was placed on I-75 in Henry County, just south of Atlanta, consisting of a surface mix containing 6 percent crumb rubber by weight of asphalt cement (AC). The test section was evaluated from 1991 to 1995. The test section indicated that the CRM became very brittle over time, as indicated by a large increase in viscosity and decrease in penetration, and by a large amount of transverse reflective cracking. Compared with the control mix, the CRM did not reduce rutting and was more than twice as expensive to place. In addition to the test section, two contract projects were initiated using CRM. These two projects indicated that CRM could be produced and placed using conventional equipment requiring only a few modifications. On-site blending units were used to combine the crumb rubber at a dosage rate of 16 percent by weight of AC. Pump and metering equipment was modified to accurately meter the stiff asphalt material, and correction factors were established for determining the AC content by vacuum extraction, since some of the rubber particles were retained in the aggregate portion of the sample.