Seven Year Itch – Evaluation of Caltrans Full Scale Experiment on Asphalt Rubber Modified Pavements

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.
KEY WORDS: Rubberized asphalt concrete, rubberized hot mix asphalt, asphalt rubber
modified pavements, modified binder, asphalt concrete, rubber-modified mixes.