by Kevin Stuart
The National Center for Asphalt Technology (NCAT) operates a 2.74-km (1.7-mile) fullscale oval outdoor pavement test track near Auburn, Alabama that is being used to
measure under accelerated loadings the performances of asphalt binders and mixtures
used in highway pavements. It consists of 46 pavement test sections, each having a
length of 61 m (200 feet).
In 2009 the Missouri Department of Transportation (MoDOT) sponsored the
construction of two test sections to determine if ground tire rubber (GTR) can be
substituted for SBS in asphalt surface mixtures. Both mixtures were placed on identical
perpetual pavement foundations to ensure that any differences in rutting would be
indicative of the performances of the two mixtures and not that of the underlying base
and sub-grade layers. The pavement layer thickness for both mixtures is 46 mm
(1.8 inches).
Both mixtures are 100-gyration dense-graded Superpave mixtures having the same
aggregate (stone) gradation with a maximum size of 12.5 mm (0.5 inch). Both modified
asphalt binders met the requirements for a PG 76-22. To obtain this PG using GTR, 11
percent 30 to 40 mesh rubber was used.
More than 10 million equivalent single-axle loads (ESAL’s) through two years of truck
trafficking have been applied to these two test sections. Each section is monitored to
measure rutting, surface cracking, surface texture, and smoothness. These
measurements provided no differences in performance at 10 million ESAL’s.
Furthermore, neither test section showed any surface cracking while the rut depths in
both sections were less than 5 mm (0.2 in). The pavements are still being loaded since
performance has been excellent.
The two mixtures were also tested in the laboratory to compare their resistances to
rutting, surface cracking, low-temperature cracking, and moisture damage. In each
test, the performance of mixture with GTR was at least equivalent to the performance of
the mixture with SBS.
Based on this study and other work, MoDOT modified their specifications to allow the
use of GTR in their pavements, thus, providing a choice in the selection of asphalt
binders. It is currently planned to apply another 10 million ESAL’s to the GTR test
section.