There was an interesting comment during practice this morning from Jeff Hammond (channeling Darryl Waltrip) about dark spots on the track, which indicate (he said) the cars were “knocking off” asphalt.  I received a number of questions about this and whether it might indicate that Atlanta could have the same problems we saw in the season opener at Daytona?

Atlanta was the first race at which I followed around the 19 team for my book.  It’s been about 13 years since the track was last paved.  Josh Browne took me out to the track and showed me how rough the track surface is.  Atlanta is one of the fastest and roughest tracks on the circuit and part of the reason for that is the composition of the track.

Most NASCAR tracks are made from asphalt – only a few are concrete.  Asphalt is a combination of aggregate (small rocks) and bitumen, the tarry black stuff that holds it all together.  Asphalt is type of composite – a material made of two things, but having properties superior to either.    Bitumen comes from the heaviest components of crude oil, with a consistency of molasses (which is why it has to be heated before being applied to anything).  We often refer to the viscous black stuff as ‘asphalt’, which irritates the heck out of the people who deal with roads for a living.

About 95 percent of the paved roads in the US have asphalt surfaces.  Most airports use asphalt for their runways because it stands up to heavy loads well.  Aggregate makes up 80-95% percent of the volume, with the remainder being the binder and air voids.  The picture at right shows voids, which were either there when the surface was originally laid, or represent places where there used to be a piece (or pieces) of aggregate.  I took the pictures, incidentally, at the Smithsonian Museum of Natural History, where they take the difference between asphalt and concrete very seriously.

The size of the aggregate used varies, depending on the requirements for the surface use and when the surface was laid.  As we’ve learned more and more about the long-term behavior of composite materials like asphalt, recipes have evolved.  Binders nowadays add some polymeric molecules that increase the adhesion of the rocks to each other, and the resiliency of the road.

As the track weathers — which means gets hotter and colder, wetter and dryer, loaded by racecars and sitting idle — it changes.

Some liquids have a high vapor pressure, which means they evaporate easily.  Acetone (like in nail polish remover) or toluene (paint thinner) disappear if you leave them out of their container because the molecules of the solvent gradually diffuse into the air.  Water evaporates much more slowly than volatile solvents like the ones I mentioned above.  Believe it or not, some of the oils in the bitumen also evaporate over time.  This happens on a time scale of months to years and it happens faster when the track gets warm.  The surface oils go first and the oils deeper in the surface start making their way up to the surface.  The force of the cars running along the track also wears the asphalt.  Binder, as well as small bits of aggregate, come loose during a race  and form ‘marbles’

The diagram at left illustrates how a track might change over time.  The top picture shows the track as it was laid down, with the second and third pictures showing later times.  The asphalt is gradually worn down.  You can imagine a gentle wear, like on a surface street, where speeds are rarely high and the surface doesn’t change temperature much.   You also can imagine that race cars might be a little tougher on the road.  The wear depends entirely on the type of bitumen (how sticky is it, how does it respond to heat) and the mix of rocks used for the aggregate.  The middle picture shows that some of the binder has worn down, exposing more of the rocks and making a bumpier surface.  The last picture shows the situation after a lot of the bitumen has worn off.  A lot of the rocks have come loose or are ready to do so.  The longer it’s been since a track has been resurfaced, the faster it is likely to wear.  The teams aren’t making it any easier on the track, either.  Aerodynamically, you want the splitter as low to the track as possible.  Every time a splitter bangs on the track, it knocks around the surface.  Cars on low tire pressure are lower to the ground and they can bottom out, scraping metal against the asphalt and essentially shaving the track.  Even the jackposts can bang on the track and knock the asphalt surface.

Another major contributor to wear is freezing and thawing of water.  Water is one of the few liquids that expands when it gets colder.  Since asphalt is porous, water can get down into the voids between the rocks, freeze, and push outward, creating internal stresses.  Even when the water liquefies again, there is some residual damage.

You can see seams on the Atlanta track, almost line line markers.  When the asphalt is laid, it is laid by a machine with a finite width.  The lanes being laid down right now at Daytona are 21 feet wide.  The asphalt isn’t a continuous layer all the way across the track.  One of the reasons asphalt is so strong is that each piece of rock interlocks with the rocks surrounding it.  Not only is the binder keeping the rock together, the rock is interlocked like the old rock walls that dot the Northeastern farms, which makes it much stronger.  The tamping down of the rock (pushing very hard on it while vibrating) is designed to rotate the rocks so that they pack as closely as possible.   That’s not the case at the seams, which is why those areas are the ones where you often see cracks first.  As I’ve shown at right, even though the bitumen is essentially continuous (I left a thin line for emphasis), there is a big difference in how the rocks interlock on either side of the seam.

The Daytona pothole was a big deal because a big ol’ chunk of asphalt came off all at once, leaving a big hole.  It is much easier to patch a small area than a large one. Putting sealer over the seams is a precaution, as those areas are inherently weaker than the others.  There’s a big difference between gradual wearing and catastrophic failure.

The problem, of course, is that asphalt is entirely opaque.  Even though the track sends people out to inspect the surface before the race, after practices, etc., all they can see is the track surface.  They can’t see whether there’s a crack just below the surface waiting for a car to come around, bottom out and take a piece of the road surface with it.  Asphalt surfaces can sustain some amount of cracks, but there’s a point at which the structural integrity is compromised.  Reaching that point is like walking off a cliff you didn’t know was there:  there’s no warning and everyone will ask you afterward why you didn’t anticipate it.

This is one reason drivers are always so tentative about a track when a repaving is announced.  The chances that the track will change are about 100 percent.

The Federal Highway Administration statistics tell us that there are 2,734,102 miles of paved public roads in the U.S.  There are scientists and engineers who study how different types of roads stand up to traffic and weather.  Racing doesn’t have that advantage.  Add up the total miles of NASCAR racetracks in the country and even if you count the local tracks, there are only a few hundred miles of pavement.  Tracks are used irregularly, by very different types of vehicles.  We don’t have a comprehensive database of how different types of asphalt age.  I can’t imagine we even have two tracks in the country with identical banking, identical types of asphalt and identical weather.  The folks who work at racetracks have to be ready for virtually anything.

Aside

Come see me and a bunch of my friends on October 23rd and 24th in Washington DC.  We’re going to be part of the very first USA Science and Engineering Festival.  The National Mall (and surrounding areas) will be invaded by hundreds of scientists, engineers and educators.  You can hear talks from the rock-guitar-playing Director of the National Institutes of Health, astronauts, inventors and more.  Our booth (on the science of motorsports, of course!) will be located at 13th and Pennsylvania Avenue.  The Office Depot show car will be there, along with opportunities for you to learn firsthand how tires can handle tons of force, how a 3,600-lb racecar can put six and a half tons of force on a set of tires, and why physicists don’t believe in centrifugal force.  In addition, you can learn about green racing and how what happens on the track might eventually end up in your own passenger car.  More information can be found at http://www.usasciencefestival.org/.  Stop by and say hello!