Performance results from the track were correlated with results from the laboratory, providing mix designers with some assurance of quality in their future work. Critics of the project pointed out that rutting overall was very low. However, statistically-significant relationships provided research sponsors with enough confidence to implement the findings.
The research generated by the 2000 project also produced useful tools for mix design and performance prediction. Rut depths were measured weekly on the surface of all 46 experimental sections. As pavement temperatures changed with the seasons, the relationship between the rate of rutting, temperature and age was identified.
It was found that truck axle passes could be grouped into temperature bands, with progressively greater weight factors applied to axle passes at higher temperatures. A model was developed to predict rutting performance as a function of temperature-banded ESALs, laboratory performance and age. This methodology was later used successfully to compare rutting performance from the NCAT Test Track to rutting performance under a Heavy Vehicle Simulator (HVS).
The follow-up cycles
In the second loading cycle of the track, West says NCAT and track sponsors continued to evaluate the rutting performance and durability of surface mixes in the majority of sections. Researchers also began building and analyzing structural test sections to validate the newly-developed AASHTO Mechanistic-Empirical Pavement Design Guide (M-E PDG) and other mechanistic-based design methods.
“This work has been key to validating engineering response models and calibrating transfer functions to better predict pavement performance,” says West.
The third cycle continued to advance pavement design methods including proving the perpetual pavement design approach and establishing field fatigue endurance limits. Sponsors also built sections to define better critical limits for air voids, understand top-down cracking, explore dual-layer open-graded friction course surfaces, and evaluate asphalt mixes with 45 percent RAP.
“We are currently at about the halfway point of the fourth cycle,” says West. “This cycle continues to build on the knowledge of previous cycles.” Although many of the sections built in previous cycles continue to be trafficked and closely evaluated, researchers added a group of full-depth sections with high RAP contents, warm-mix asphalt, and other sections with uniquely modified mixtures.
New sections for the next cycle will be built in 2012. “We hope to continue to evaluate new warm-mix asphalt technologies, push the limits higher on recycling, and test pavement reinforcing systems and methods to mitigate reflection cracking,” says West.
“And who knows what someone else may want to evaluate at the track?” West asks. “It is the best place on the planet to test new pavement ideas.”
Since the test track opened, agencies have used it to prove what works and what doesn’t. “When you build sections with different mix designs and/or different pavement cross sections and traffic them like we do, it is evident what works best,” says West.
“We at NCAT are pleased to operate one of the premier asphalt research facilities in the world,” West concludes. “We enjoy working with sponsors to address research needs that result in real improvements in materials, design and construction of longer-lasting pavements. We look forward to many more years of continued success in the effort to make asphalt the pavement material of choice.”
For more information, you can go to www.pavetrack.com for a schematic of the NCAT Pavement Test Track. By clicking on a test section, you can obtain results of rutting, IRI in inches per mile, and a crack map. You can also access detailed construction data by section.v
by Daniel C. Brown, Contributing Editor
In association with the National Asphalt Pavement Association
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