“For the fourth one, we are going to do a sprinkle treatment mix,” says Zehr. “We will use a 9.5-mm mix with local aggregates, but then we will sprinkle quartzite aggregates on top of that, directly behind the paver. The roller will follow and seat those particles, which have been pre-coated with about 1.5 percent of PG 64-22 binder. We just milled up a sprinkle treatment project in southern Illinois that has been in use for 25 or 30 years. Sprinkle treatment has also been done successfully in the past in Georgia, Louisiana, and Iowa. We are going to try it again, and I’m very optimistic that it will be a success.”

Every overlay in the project will be a total of 2 inches thick. Each of the four experimental mixes will be constructed at both 1 inch and 1.25 inches thick and the 4.75-mm SMA will also be constructed at 0.75 inch thick. The remainder of each 2-inch overlay will be a 9.5-mm finegraded level binder course.

The researchers at the University of Illinois will run an extensive battery of both laboratory and field tests. The pavement will be tested for rutting, and for smoothness using the International Roughness Index. Laboratory tests at the University of Illinois will be conducted on both the field-produced mix and the same mixes produced in the lab. These tests will include the modified Lottman test, the dynamic modulus test, the Hamburg Wheel test, the fracture test, the Cantabro loss test, and friction tests. The component aggregate and asphalt materials have already been on hand at the University of Illinois for nearly a year and specimen preparation and testing is ongoing. The results of the lab tests on the specimens fabricated in the lab from the component materials will be compared to same tests on the plant-produced pavement materials. “We are hopeful that the results of this study will help us specify and economically build better overlays in the future,” says Zehr.

Conclusion

In short, thin overlays offer a cost-effective way to extend the life of a good pavement. They have a low life-cycle cost and can improve rideability. Many states use thin asphalt overlays successfully. v

by Daniel C. Brown,Contributing Editor

In association with the National Asphalt Pavement Association

NCAT Pavement Test Track –

A Record of Proven Success

New research shows that asphalt pavement sections can be designed thinner.

The NCAT Pavement Test Track has racked up an impressive record of success since it was built in the summer of 2000. The 1.7-mile oval track in Lee County, Alabama, is managed by the National Center for Asphalt Technology (NCAT).

The test track – the largest asphalt pavement testing facility in the Western Hemisphere – is a pooled-fund project funded through sponsored test sections by highway agencies and industry businesses.

“We are proud to have worked with about 20 different sponsors since the track first started operating,” says Randy West, director of NCAT. “The track has been vital to answering a range of key questions that highway agencies have asked over the past 10 years.”

More efficient designs

Now, significant new research from the test track strongly suggests that certain assumptions which pavement designers have been using for years may be overly conservative, say David H. Timm, PhD, P.E., and Kendra P. Davis. “While everyone agrees that using proven designs is desirable, there are opportunities to save materials and costs and to better utilize materials,” write Timm and Davis in a paper entitled “Recalibration of the Asphalt Layer Coefficient.”

The Alabama Department of Transportation (ALDOT) sponsored research at the test track showing that the state’s asphalt pavement designers can now use a structural coefficient equaling 0.54 rather than 0.44 – and that yields a pavement cross section that is 18.5-percent thinner. As a result, either a substantial cost saving is possible over the same miles of pavement, or more miles of road can be paved with the same resources.

ALDOT has recently implemented the recalibrated structural coefficient for resurfacing and new construction projects. The change was implemented simply by entering 0.54 into the AASHTO Design Guide software instead of 0.44. Timm and Davis caution that other states should apply this change after a careful review of the conditions of this study to be sure that conditions are similar.

How the track works

At the NCAT Pavement Test Track, a fleet of five Class 8 tractors pulling heavy triple trailers runs on the track over 750,000 miles per year. All non-steering axles of the trucks apply a load of 20,000 pounds, the federal legal bridge limit. The fleet, averaging a gross vehicle weight of 155,000 pounds, runs five days a week, 16 hours per day. A two-year trucking cycle on the track applies 10 million equivalent single axle loadings (ESALs) – the equivalent of about 20 years of normal traffic.

In 2000, a three-year research cycle at the test track cost approximately the same as one hour of paving in the United States. The annual savings from improvements in pavement design, however, are estimated at $1 billion. State DOTs invest in accelerated performance testing on the NCAT Test Track because they would have to wait 10 to 15 years on their own field projects to obtain less reliable information.

First cycle results

Results from the first research cycle, begun in 2000, were presented at the National Transportation Symposium in the fall of 2002. It was found that fine-graded mixes could perform just as well as coarse-graded mixes. It was also found that polymer-modified binders cut rutting rates in half, which meant that polymer-modified mixes could be designed with higher asphalt contents to optimize durability.