Dr. Diandra: The concrete facts about Nashville Superspeedway
Twenty-seven tracks host points-paying NASCAR Cup Series races in 2022. Only Nashville Superspeedway was designed with a concrete racing surface. Martinsville installed concrete in the turns in 1976, while Bristol and Dover transitioned from asphalt to concrete in 1992 and 1995, respectively.
The 1.33-mile Nashville track is a D-shaped oval and the longest of the four concrete-containing tracks. At 14 degrees of banking, it’s slightly more banked than Martinsville (12 degrees), but much less banked than Dover (24 degrees) or Bristol (24-28 degrees.) If we count half of Martinsville, concrete accounts for 13% of NASCAR Cup Series surfaces. Dirt makes up 3.7%, leaving asphalt to account for the other 83.3%.
Concrete vs. asphalt
Concrete and asphalt are both composites: aggregate (also known as “small rocks”) bonded together by a glue-like material called a binder. Concrete dates to the Roman Empire, while the first asphalt roads weren’t constructed until 1848. The nature of the binders explains the difference in timelines.
Concrete usually uses a portland cement binder, a mix of limestone and clay. Asphalt uses bitumen, a tarry black substance derived from the heaviest components of crude oil, as a binder. The binders determine the application method. While concrete is poured and cured, asphalt must be heated to a high temperature before extruding and being allowed to cool.
Because asphalt is more flexible than concrete, asphalt can be laid in long, continuous swaths. Concrete must be poured in sections to prevent damage from weather-induced expansion and contraction. The lines between concrete sections also assist with water drainage. That’s needed because concrete is less porous than asphalt.
Asphalt’s flexibility means it doesn’t spread out loads. Asphalt experiences larger, more concentrated stresses than concrete. The figure below shows typical stress distributions (in red) for asphalt and concrete.
As you might guess -- from this graphic or your own personal experience with potholes, asphalt is more easily damaged than concrete. Asphalt simply can’t stand up to the high forces of race cars taking tight curves at high speed.
Transportation engineer Van Walling compiled the fascinating (as-of-yet-unpublished) compendium Oval Track Almanac. The three volumes document 45 years of extensive research of more than 1,000 tracks in the United States and abroad.
Martinsville, Walling explained, turned to concrete because race cars damaged the asphalt in the turns. Trucks can damage asphalt in expressway off-ramp loops the same way.
“Between high temperature and the force of the vehicles,” Walling said, “asphalt can be moved, creating a texture like a washboard.”
While “shoving”, as the phenomenon is called, is annoying for an off-ramp, those bumps create real trouble for race cars. Track operators have no option beyond frequent resurfacing or reconstruction — or switching to concrete.
That’s not to say that concrete tracks are impervious. In 2004, Jeff Gordon lost a Martinsville race due to concrete coming off the track. In 2018, a chunk of Dover’s concrete surface loosened and damaged Jaime McMurray’s car. Debris from the impact broke windows in a pedestrian crossover above the racing surface. That episode prompted Dale Earnhardt Jr. to tweet that “Asphalt is for racing. Concrete is for sidewalks.”
Walling, who has studied the original blueprints for Daytona International Speedway, said NASCAR founder Bill France Sr. wouldn’t necessarily agree.
“He intended the corners at Daytona to be concrete,” Walling said. “The problem was cost.”
Concrete requires a much greater up-front investment, and France was already struggling for funding.
“He initially planned a 60-foot racing surface,” Walling said, “but ended up settling for 40 feet.”
If France hadn’t found the money, Walling says, Daytona might have ended up as a much flatter track. Upfront cost is why almost all new tracks are built with asphalt, even though the upkeep is more expensive in the long run.
How concrete changes racing
The primary grip mechanism on any racetrack is the tire deforming around the aggregate. Concrete, by its nature, is smoother than asphalt. When NASCAR measured track surface roughness in 2019, Martinsville, Dover and Bristol were the three smoothest tracks.
The second grip mechanism is the adhesive interaction between rubber molecules on the track and on the tire. Although Goodyear designs their tires to lay down rubber on concrete tracks, the rubber doesn’t stay put.
“At speed,” Greg Stucker, Goodyear’s director of racing, said, “the track will progressively turn black as the cars lay rubber on the concrete surface and then turn back white under the caution flag as the tires pick back much of that rubber. Keeping pace with that transition is an important element of the race strategy.”
A driver loses traction on a concrete track much faster than on asphalt. Drivers have already spun more in 2022 with the Next Gen car than in all of 2021. The accident rate is also up.
Nashville’s concrete surface may pose a real challenge. Dover, the only 2022 race on a full concrete track so far, had 13 cautions. That’s almost double the number of cautions in each of the two races before, and triple each of the two before that.
One positive, through is that concrete doesn’t wear as fast as asphalt. Even though the car is new, the surface won’t have changed much since last year. The tires are also familiar. Teams ran the Nashville left-side tire three times (including at Dover) and the right side six times this year. They’ve even run the same left-right configuration twice: at Charlotte and the Texas All-Star Race.
Black and white
Track color matters.
The sun emits a spectrum of electromagnetic waves. The tiny band we can see is what we call light. But the sun also provides infrared waves, like the heat lamps restaurants use to keep food hot. Its ultraviolet waves are why you should wear lots of sunblock at the track.
Different colored surfaces interact differently with the sun’s waves.
We see objects because they reflect, emit and/or transmit light. A red car absorbs all wavelengths of light except those corresponding to red. Only red wavelengths reach our eyes.
White surfaces reflect most wavelengths of light. That’s why you see concrete as white -- white light is the sum of all colors of light. Black surfaces, on the other hand, absorb a lot of light. Because no light is reflected, you see black. The same thing happens with infrared waves, which cause black surfaces to heat faster than white surfaces.
White tracks also reflect more light into the drivers’ eyes. Drivers will need tinted visor peel-offs for the 4 p.m. local (5 p.m. Eastern) start, which will be broadcast on NBC.
Heat causes the bitumen in asphalt to release oils that make the track more slippery. That doesn’t happen with concrete.
The end result is that a concrete track doesn’t change over the course of a race as much as an asphalt track. Nashville Superspeedway should be easier for crew chiefs to keep up with because temperature changes won’t change the racing surface as much.
On the negative side, if a team misses the setup, there’s much less likelihood that the track will come to them during the race.