Compaction at the screed using a European-sourced paver, and use of new oscillation vibration for compaction, were key to successful placement of a new hot mix asphalt (HMA) pavement at Indianapolis Motor Speedway this summer.
Vigorous compaction at the screed via tamper and pressure bars was paramount for application of the stone matrix asphalt (SMA) pavement, selected by the track and its consultant for its durability, strength and impermeability under the punishment of professional race cars and tires, and weathering.
The screed was achieving 89 to 90 percent compaction as the material exited, followed by breakdown rolling in static mode, then unique oscillation compaction to 95.5 percent density, and finish rolling in static mode.
SMA is a gap-graded (low medium-sized aggregate and fines) hot mix asphalt design which brings together robust, coarse aggregate and as much as 6 to 8 percent liquid asphalt. Its lack of medium-sized aggregate — and fines percentage less than 15 percent of the aggregate weight — results in a strong mix with a rut-resistant, stone-on-stone structure that develops internal friction and resistance to shear.
But because the gap-graded SMA emphasizes large aggregate, the low-penetration grade asphalt conventionally used can drain out of the coarse aggregate structure. To keep the asphalt in place, cellulose fibers or other asphalt modifiers are added at the plant to keep the binder in place.
Approximately 19,000 tons of SMA were being placed in two lifts, each lift approximately 1.6 million square feet in size.
Time to repave track
Track management did not wait until pavement failure before launching this reconstruction. "Racetracks are probably the most critical placements of asphalt that you can put down," says Kevin Forbes, P.E., director of engineering, Indianapolis Motor Speedway. "We look at an aging racetrack as a potential problem, and we don't wait for it to become a problem. From all points of view, there probably was nothing wrong conventionally with this racetrack. But it was getting to the point at which it was not worthy to run race cars on."
Heritage Research Group of Indianapolis designed and produced the new mix, and is overseeing placement and production operations.
Ultrasmooth surface required
For a high-profile track like Indy, smoothness considerations are elevated above standard highway benchmarks. "Our smoothness is hard to define, using the standard California Profilograph, even with zero blanking band," Forbes says. "Our issue is with deviations which occur over 100 feet, which are significant to race car drivers. The typical Profilograph measures every 30 feet, so it won't even pick our standard up."
Because the mat had cracked, though, discontinuities were present in the pavement. "The cracked sections started to curl up between sections, and they would create a very high frequency, but low amplitude, shudder to the drivers, as opposed to definable dips or bumps," Forbes says. "And that cracking allowed so much water into the mat that we decided it was time to start over."
Reconstruction of the track began as contractors took from August 9 to 12 to meticulously remove the 36-inch strip of 1909-vintage bricks, known as the "Yard of Bricks," at the Speedway's start-finish line. The bricks eventually will be cut into smaller pieces and sold to the general public.
On August 16, a ceremonial "first cold milling" took place, with renowned professional racer Al Unser Sr., riding a Wirtgen W 2200 with 14-foot drum, owned by Javelina Construction, Fishers, IN, as subcontractor to McCrite Milling & Construction Co., Inc., New Albany, IN.
The 14-foot-wide mill wasn't used just for speed of removal, but to help maintain smoothness in the final product. "As we try to maintain a very uniform plane on this racetrack, we felt that averaging across a 50-foot-diameter surface with a 14-foot head is better than averaging with a 6-foot-8 or 7-foot head," says Forbes. The track had a very smooth surface to begin with, and little correction to be done after the milling machine removed the 2 1/2 inches.