The task of removing 4,399 square yards of asphalt from the 2.5-mile track required equal care, but the milling process was ahead of schedule only a few hours into the project, with the machine chewing up asphalt at the rate of 45 feet per minute.
That's approximately half the speed at which construction crews perform the same task on a highway project. The slower speeds simply reflect the need to create a surface capable of safely accommodating 220-mph race cars upon the project's conclusion.
"We don't want to take anything away from state highways because they are very well constructed," Forbes says. "But it's a different application of forces, and it's different speeds. It becomes very, very critical that we give the drivers the smoothest, flattest, most predictable surface we can. If the drivers know the surface underneath them is very uniform, very reliable and very consistent, that will allow them to pull out all the stops in terms of their ability to drive their race car."
Some 19,000 tons of asphalt was milled from the track, 1 in. surface, and 1 1/2 inches of intermediate course.
SMA spec'd for track
Heritage recommended SMA for this critical application. "A lot of people look at SMA for its stability and strength, but it's also the most durable mix that we can build," says Bill Pine, P.E. of Heritage. "We chose to go to two SMA lifts, for strength, durability and impermeability. We're putting a very dense mix in place of the open-graded mix to keep water out of the pavement entirely. Our goal is to get 95.5 percent density with 4 1/2 percent in-place air voids."
The two lifts are slightly different in composition. "The bottom lift is 9.5 nominal size dolomite SMA with a dolomite coarse aggregate, with dolomite aglime and mineral filler," Pine says. "We're using 0.3 percent cellulose fibers and about 6.5 percent PG 76-28 polymer-modified asphalt binder."
"The 76 side will give us good stability, while the -28 side will do well for us in the wintertime," Forbes says. "We are using the SMA principally for its high film thickness around the individual aggregate, which will give us a 'rubber-band' effect that SMAs are so good for, that will help us fight that brittle cracking stress that occurs in winter."
The aglime and mineral filler together work to control the density of the mastic [glue] between the aggregate particles. Aglime is 100 percent passing the 475 sieve, and 17 percent passing the 075, whereas the mineral filler is 100 percent passing the 150 sieve, and around 75 percent passing the 075. Mineral filler is basically dust, and aglime is a stone sand with a fair amount of dust in it.
A tack coat of AE90S emulsion was being placed on the milled surface by an Etnyre distributor truck, prior to placement of the intermediate base course, and a fog coat of lightly modified polymer tack will be placed between the intermediate and friction courses.
Steel slag in friction course
The friction or riding course will be a 4.75 nominal size steel slag SMA with the same aglime and mineral filler as the intermediate course, but with a blend of two different sizes of steel slag coarse aggregates, developed by Heritage Slag Company, Pine says.
The demands placed on racetrack asphalt are profoundly different from that of an interstate highway. "Compared to a conventional interstate mix, we're looking for more durability," Pine says. "We'll need strength in the apex of the turns, where the cars are focused in a line; outside of that, durability is by far the main characteristic that we must meet."
So on the banked curves, where specified, the asphalt binder will be switched from a PG 76-28 to a PG 82-22 to increase the stiffness of the mix in these areas.
On the first day of paving, SMA was being placed hotter than usual, at 365 degrees F, to make sure there was enough heat in the SMA to get compaction despite cooling with ambient temperatures in the low 60 degrees F.
"On a cloudy, windy day, if we're going to err on temperature, we want to err a little hot," Pine says. "We can always hold the rollers back, but if we get it too cold we'll have density problems. You've got to lean one way or the other, and we've chosen to lean toward the conservative side."