Better Roads

October 2013

Better Roads Digital Magazine

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Page 13 of 82

Photo courtesy of Tom Kuennen RoadScience Elements of Gomaco Smoothness Indicator (GSI) the Americas Formula 1 race track just outside Austin where approximately 80,000 tons of HMA was placed on the track. Nearly 10,500 tons of surface mix were placed on the wearing course alone. When the paved run-offs and verges were included, a total of 21,000 tons of critical wearing course were placed. "There are many differences between building a Formula 1 track and a high-level interstate-type highway," Byrum says. "No. 1, the surface mixture, is a high-performance friction course with a high level of skid resistance, and asphalt cement is present at 6.5 percent of the mix. At PG 82-22, the liquid asphalt was highly modified with SBS polymer. It could be considered a Superpave mix, similar to a stone-matrix asphalt (SMA), with high binder content and gap-graded aggregate, providing rock-on-rock contact with minimal fines. It must be durable and skid-resistant." The smoothness spec was the toughest aspect of the job, he adds. "More than anything, the flatness spec is what made this a huge challenge," Byrum says. "The surface course has a tight specification for smoothness, 2 millimeters over 4 meters [0.08 inches over 13 feet]." Meeting those specs on the top layer meant each lift below also had to be super-smooth. The 3.1-in.-deep base course has a PG 64-22 binder with a smoothness spec of 4 mm over 4 meters (0.18 in. over 13 ft.), while the 2-inch-deep intermediate "binder" lift in the middle has PG 76-22 liquid asphalt, with a little SBS modifer in it, and a smoothness spec of 3 Text INFO to 205-289-3789 or visit 12 October 2013 Better Roads RoadScience_BR1013.indd 12 9/30/13 3:51 PM

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