The first two-year cycle of tests at the National Center for Asphalt Technology Test Track, Auburn, Ala., was completed recently. The testing of 46 different sections was conducted by subjecting the pavements to the equivalent of 10 to 12 years of traffic loading in two years. Heavier-than-normal trucks punished the pavements with 10 million equivalent single axle loads (ESALs=one 18,000-lb single axle load).
The results of this first series of tests were presented at the National Transportation Symposium at NCAT. Most of the 250 attendees were representatives of the state departments of transportation that sponsored test sections at the track.
Here are some of the findings:
* A number of mixes with varying aggregate types, asphalt grades and aggregate gradations provided good performance;
* The total average rut depth was very small - 0.12 in. after 10 million ESALs. The worst-performing sections had 0.25 in. of rutting;
* Mixes that were intentionally designed to be more likely to rut did result in the greatest rut depths, but the magnitude of these ruts was still small;
* Fine-graded and coarse-graded mixes provided approximately equal performance;
* The higher PG grades clearly resulted in lower rut depths, and the results indicate that potentially more asphalt binder may be added to mixes using higher PG grades to improve durability without increasing rutting; and
* Several of the tests that were evaluated have been found to be useful in helping to identify mixes with rutting potential.
For more on the story, read the January issue of ROADS & BRIDGES magazine.
