Get a grip

The newly repaved Indianapolis Motor Speedway gives tires something

Asphalt Article June 20, 2005
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Visitors to the Indianapolis Motor Speedway last August would have heard a roar different from the one made by racing cars and cheering crowds. They would have heard the sounds of milling machines grinding off the top 2.5 in. of asphalt and pavers laying a new racing surface around the 2.5-mile track.

When the asphalt paving work was finished, the result was a good, smooth track surface. The problem was that the drivers were used to the previous surface, which gave them more grip because it had been diamond ground in 2001. The drivers had been driving on a diamond-ground surface for four years and they liked it.
“There’s no way that we could have put any type of new hot-mix back that would have had the same grip as the existing diamond-ground surface,” Bill Pine, P.E., a research engineer at Heritage Research Group, Indianapolis, told ROADS & BRIDGES. Heritage Research provided the mix designs, quality assurance testing and overall project guidance for the speedway repaving.

The speedway management wanted to give the drivers the increased traction they were accustomed to. They also wanted to give the drivers a uniform surface texture, but that was altered by diamond grinding that had been done in February of this year to create extra smoothness at the bottom of the turns.
The speedway management decided to diamond grind the whole track surface. They hired Penhall Corp., Anaheim, Calif., a specialist in diamond grinding racetracks.

“We did not diamond grind the entire track to make it smoother,” Pine stated. “Smoothness was a nonissue at that point. They were very happy with the smoothness; they simply wanted the degree of grip that they had before, and they wanted a consistent amount of grip everywhere.” This increases the drivers’ control on the track and allows for more competitive racing on all parts of the track.
Diamond grinding essentially cuts shallow grooves in the pavement surface with a series of what look like saw blades with diamond cutting edges. The diamond grinding cut longitudinal grooves 1?8 in. wide with 1?8 in. between them and 1?16 in. deep all the way around the 2.5-mile Indianapolis oval, which is 50 ft wide in the straights and has about 78,000 sq yd of surface area. The ridges between grooves are not stable, so they need to be worn off a little.

After the grinding, workers performed the standard practice of dragging weighted metal grates around the track. “That knocked the majority of those ridges down to just a very small ridge between the grooves,” said Pine.

In the end, the speedway had grooves about 1?32 in. deep—just enough to provide a good grip for Indy cars riding on “slick” tires, which have no tread.

The Indianapolis Motor Speedway hosts races in a variety of car classifications, including NASCAR and the Indy Racing League, which runs the Indianapolis 500-Mile Race every Memorial Day. Formula 1 cars run at Indy as well, but they use only 1 mile of the oval track, spending most of their time (1.5 miles) on a road course constructed inside the oval track.

The Formula 1 road course was constructed in 1999 preparation for the return to the U.S. of the Formula 1 U.S. Grand Prix in 2000. A peculiar trait of the Formula 1 course is that the cars travel in a clockwise direction, which is consistent with other Formula 1 races but the opposite of most American auto races, including the Indy 500.

Indianapolis was originally built in 1909 with a brick pavement, hence the nickname the “brickyard.” The majority of the original bricks remain in place but are now covered by layers of asphalt. The exception is at the start-finish line, where the famous yard of bricks is still exposed to remind everyone of the history of this track. The venue became famous for hosting what became one of the biggest spectator events in the world, the Indianapolis 500. Stock cars did not make their appearance at Indy until the first Brickyard 400, part of the Winston Cup series, in 1994.

Asphalt pavement designers usually have to choose between strength and durability. Racetracks lean toward strength, but that usually costs them in durability.

Full tilt

“The real problem with the existing mix was durability—cracking,” said Pine. “Through our investigation last spring, by far the majority of the cracks that were there were top-down cracks.” Such cracks are caused by thermal contraction of the asphalt to the point where the stress at the surface exceeds the tensile strength of the mix at the cold temperature. Cracks form at the top and work their way down. They found very little reflective cracking.

“That’s what led us towards an SMA mix,” said Pine. “With a racetrack, the Indianapolis Motor Speedway for example, strength is important three days out of the year. That’s when they have their three major races. They simply cannot afford to have a strength failure during a race. But other than that time—the other 362 days—it simply has to be durable.”

A stone-matrix asphalt (SMA) mix, with lots of coarse aggregate and high stone-on-stone contact, provides high strength. “But it also has a rich mastic surrounding those particles,” added Pine, “that provides long-term durability.”
Two lifts of SMA were chosen to give the racecourse a strong, rut-resistant and impermeable surface. The SMA mix design for the bottom lift called for dolomite coarse aggregate with a nominal maximum particle size (NMPS) of 9.5 mm. Prime contractor Grady Brothers Inc., Indianapolis, used a PG 76-28 polymer-modified asphalt binder at about 6.5% by weight of the mix and 0.3% cellulose fibers to make sure the liquid asphalt stuck to the aggregate instead of running out.
The bottom lift was 1.5 in. thick after compaction. Grady used a Vogele AG Super 2100 paver to lay both SMA lifts.

The surface SMA lift utilized two different sizes of steel-slag coarse aggregate, produced by U.S. Aggregates and Heritage Slag Products, two Indianapolis companies owned by the Heritage Group. The NMPS was 4.75 mm, and the mix contained about 6.8% PG 76-28 and 0.1% cellulose fibers. The two specially sized aggregates were made specifically for this mix. They are not typical sizes. Steel slag is a hard byproduct of the steel-making process and provides the high strength needed for a racecourse.

U.S. Aggregates also produced the aggregate for the dolomite SMA. The performance-graded and polymer-modified asphalt binders and emulsions were produced by Asphalt Materials Inc., Indianapolis. Milestone Contractors LLP, Indianapolis, produced the hot-mix and performed quality control.
The surface lift was 1 in. thick after compaction.

Grady Bros. used the Vogele paver’s high-density screed to get fast initial compaction. With higher initial density, the mat retained heat longer and gave the crew more time to hit the target final density.

The need for speed

Grady was shooting for 94% in-place density (6% air voids) and found that the screed gave them almost 90% of maximum theoretical density to start.

“An SMA in general is very sensitive to temperature as far as achieving compaction of the mix in the field,” noted Pine. “It needs to be rolled very quickly and while it’s still hot.” Grady had to use soapy water with all of the compaction rollers to prevent the mix from being picked up by the drums, but some of that water is going to mix with the SMA and cool it.

“That increase in density from the Vogele screed really decreased the amount of water going into the mix, and that gave us more compaction time with a mix that normally doesn’t have much compaction time.”

Pine said Grady achieved an average of about 94.5% density on both lifts as measured in cores taken from the finished pavement. The team did quality control with a nondestructive, non-nuclear density gauge; they also took 5-7 cores per day randomly around the track for quality assurance.

Five rollers compacted the fresh SMA. They all had steel drums, and they all ran in static mode. Two 84-in.-drum Hamm HD 130 units were used for breakdown compaction. Two Hamm HD 90 units, with oscillatory capability, performed intermediate compaction. And another HD 130 did finish rolling.
The parties involved in the paving job were interested in using the oscillatory rollers because of their ability to knead the hot mat to the desired density by moving forward and backward instead of the usual up-and-down action of a vibratory roller, which may damage the aggregate or flush asphalt to the surface.

“When we laid the first lift in the first lane we did try to oscillate. . . . The problem we had was the rollers wanting to actually slide downhill in the turns,” Pine commented. “The banking at IMS is basically 9.25°, and we did have some problems with keeping the rollers on the mat. Because of that, we ran them static and still achieved what we thought was very good density.”

The high density achieved with both SMA lifts will provide low permeability and increased durability.

Pieces of oval

Since the cracks were predominantly at the surface, the Indianapolis Motor Speedway decided to mill off the top 2.5 in. of old asphalt and replace it. The milling contractor, McCrite Milling and Construction Co. Inc., New Albany, Ind., used a 14-ft-wide Wirtgen W 2200 cold milling machine to remove the old asphalt pavement down to old hot-mix asphalt that had been laid in the past. The 14-ft-wide drum helped maintain a smooth surface across the width of the track.

Milling started on Aug. 16, 2004, after the NASCAR Brickyard 400 race. Under the first lift of fresh SMA went a stress-absorbing membrane interlayer, basically a modified liquid asphalt that forms an impermeable barrier. After the 1.5-in. (6,900-ton) dolomite SMA lift, subcontractor Safety Grooving and Grinding Inc., Napoleon, Ohio, did a minor amount of diamond grinding to make the base course as smooth as possible.

The dolomite SMA went down in four passes each about 13.5 ft wide starting from the outside so the construction vehicles would not have to drive across the new mat to get to the next pass. The crew used a Roadtec SB 2500 Shuttle Buggy material transfer vehicle throughout the asphalt placement.

The speedway management did the typical smoothness measurements with a California profilograph, but they also made measurements with a high-speed profilometer. Finally, they brought in an Indy Racing League car and driver to do a few laps and see how the surface felt at Indy 500 speed. The driver then reported back to Kevin Forbes, director of construction and engineering for the Indianapolis Motor Speedway. Forbes was the speedway’s point person for the paving project. Greg Raymond was the project superintendent for Grady Brothers.

Next the 1-in. (5,250-ton), steel-slag SMA was laid, again in four passes, but this time 12.5, 12.5, 14.5 and 14.5 ft wide. They wanted the bottom lane in the surface to be a little wider so the cars in the main racing line would have their right-side wheels below the longitudinal joint.

The new Indianapolis pavement got its first big test this Memorial Day Sunday in the Indianapolis 500. The big story of the race was Danica Patrick, the first woman ever to lead a lap in the race’s history. She had a chance at winning until the last few laps. Speeds on the track were as fast as ever. The pavement was not an issue. Time will tell whether the track’s new SMA pavement has the strength and durability to stand up to the pounding of cars that, at racing speed, exert more than twice as much force as their standing weight.

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