Publication Number: FHWA-SA-96-019
Date: August 1996

Superpave System Puts More Durable Pavement Within Reach for Minnesota County

When Blue Earth County, Minnesota, set out to try the new Superpave system on its roads, highway engineers didn't know if the system would prove economically feasible or practical to construct. "I was reasonably certain Superpave would perform better than our conventional mix," says Alan Forsberg of the Blue Earth County Highway Department. "But I wanted to determine if Superpave was constructable and what the costs were."

To answer those questions, Blue Earth County last year constructed one of the first pavements to use the Superpave volumetric mix design system in Minnesota. The project, completed last August, is a 50-mm (2-in) overlay over existing asphalt pavement on a 5.7-km (3.5-mi) stretch of County State Aid Highway 30.

The overlay is split into three 1.9-km (1.2-mi) sections. One section was paved with the county's conventional mix to serve as a control section. The other two sections were constructed with Superpave mixes using two different performance-graded (PG) asphalt binders: a PG52-22 and a PG52-34. This project design allows the county and the State to observe how the performance of Superpave mixes constructed with different PG binders compares with Minnesota's conventional mix under identical conditions.

Neither the county highway department nor the job's contractor was equipped to design the Superpave mix, so the mix was developed by the Minnesota Department of Transportation (DOT) in its mobile pavement laboratory. The laboratory is equipped with the Superpave gyratory compactor, ovens, standard aggregate test devices, and other equipment needed to select binders and aggregates and use the Superpave volumetric mix design procedures.

After its first year of service, the overlay has proven to be superior to Minnesota's conventional mix. "We had no problems at all during construction despite very high temperatures," Forsberg says. "It's a very stable mix."

Moreover, the Superpave pavement is up to the challenge of Minnesota's fierce winters. Because of climatic conditions, the site ideally needed a PG58-40 binder to achieve 98 percent reliability. But the cost for a polymer-modified asphalt that would meet those specifications was prohibitive, so the highway agency used two asphalt binders not specifically suited to such low pavement temperatures (PG52-22 and PG52-34). While the PG52-22 binder has not proven effective, the section constructed using the PG52-34 binder is performing well under less-than-ideal circumstances.

"The mix resisted low-temperature cracking quite well, especially considering that it was just a thin overlay on a badly cracked surface and that we had an exceptionally cold winter," Forsberg says. "The Superpave pavement is doing much better than our own design."

As Forsberg hoped, the Superpave system was within the county's abilities and budget. "The Superpave pavement was easy to construct, and very economical-only $1,000 to $2,000 more per mile than our conventional mix," Forsberg says. The Superpave mixes used more expensive, higher quality aggregate than the usual Minnesota mix, but the higher cost for aggregate was offset by savings on asphalt. The project would have, however, been more expensive if the agency had used the PG58-40 asphalt binder ideally called for.

Forsberg is very pleased with the Superpave overlay. "It's been a very interesting and successful project," says Forsberg. "I'm convinced that Superpave is a much higher quality and more durable mix than what Minnesota has been using." Forsberg and Blue Earth County Highway Department engineers Larry Lapoint and Jack Hermer are planning to use a Superpave mix in a new section of road on County State Aid Highway 8 south of Mankato.

Minnesota DOT is also convinced. Jim Schmidt of the Minnesota DOT materials lab reports that the State will adopt the PG binder specification in January 1997, and in January 1998 will make the Superpave volumetric mix design standard for roads designed to carry 3 million equivalent single-axle loads or more.

The Blue Earth County project is representative of the cooperation between Minnesota DOT and county and local transportation agencies. The Superpave overlay project was funded by the Minnesota Local Road Research Board (LRRB), an advocacy group composed of State and local agencies dedicated to improving pavements throughout the State. "In the past few years, pavement performance in Minnesota has not been the best," says Schmidt. "Local agencies are looking to us for leadership." The LRRB's support of local Superpave construction allows the DOT to help provide local governments with longer-lasting roads, while everyone involved learns more about the benefits of the Superpave system.

For more information, contact Jim Schmidt, Minnesota DOT, 612-779-5619 (fax: 612-779-5580), or Stan Graczyk, FHWA, 612-290-3245 (fax: 612-290-3256).

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