The California Department of Transportation (Caltrans) has been using scrap tire rubber in asphalt pavements since the 1970s in chip seals and the 1980s in Rubberized Asphalt Concrete (RAC). In 2004, Caltrans and the California Integrated Waste management Board
(CIWMB), now known as CAL Recycling, initiated a joint effort to develop technically sound, cost effective, and environmentally friendly solutions to scrap tire management through the increased use of scrap tire rubber in roadway projects. A full scale field experiment project was constructed for the above purpose. The project is a two-lane overlay project located on Highway 33 near the town of Firebaugh in the central valley of California. Caltrans developed the experimental design and specifications.
The experiment consisted of nine pavement test sections with various rubber-modified asphalt concrete mixtures and a control section of a dense-graded asphalt concrete (DGAC). The rubber-modified asphalt concrete sections included a Gap Graded Rubberized Hot Mix Asphalt (RHMA, field wet process), a Gap Graded Rubber Modified Asphalt Concrete mix (dry process), a Gap-Graded Modified Binder mix (terminal blended), and a Dense-Graded Modified Binder mix (terminal blended). The project specifications required the asphalt rubber binders to have at least 15% Crumb Rubber Modifier (CRM) by weight of asphalt. The nine test sections provided an opportunity to evaluate the constructability and performance of two different layer thicknesses: 45 mm (Half Thickness) versus 90 mm (Full Thickness). The pavement structural sections were designed for 10 years traffic level of approximately 2.5 millions of equivalent single axle loading (ESALs). The paving was completed in June 2004.
Seven years have gone by, how have these test sections been performing? In the summer of 2011, Caltrans engineers revisited the project, conducted another round of field condition survey, and noted significant differences in the surface condition among the various test sections.