Potholes in Atlanta?

 Asphalt, Atlanta Motor Speedway, Materials, Tracks, Uncategorized  2 Responses »
Sep 042010
 

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!

 Posted by diandra at 5:23 pm

Daytona Potholes: Avoidable or Not?

 Asphalt, Daytona, Materials, Tracks  1 Response »
Feb 182010
 

I wake up in the morning listening to our local NPR station. A couple weeks ago, they said that the George Bush Turnpike was closed due to “a buckle in the road”. My husband commented that he knew Texans had big belt buckles, but he didn’t think they were big enough to shut a whole side of the tollway.

Well, the buckle they were talking about was actually three feet high and spanned two lanes. Apparently, the heavy rains we had received created a lot of pressure in the adjoining retaining wall and that pressure pushed the pavement until it buckled and formed our own little miniature mountain range right there in Carrollton.

The problems at Daytona last Sunday weren’t quite of that magnitude (the pothole was about 9″ x 15″ and only 2″ high, but that tiny pothole impacted a lot more people. Including me, who had assured my husband that the race certainly would be over by five as he planned Valentine’s dinner. What happened and how could it have been prevented?

(photo Bill Friel)

Let’s start with thermal expansion. If you’ve ever had a lid stuck on a jar, or a ring stuck on your finger, you may have tried running the jar or the ring under hot water. The metal jar lid would expand faster than the glass jar, thus loosening the seal and allowing you to remove the stubborn lid. That’s because different materials expand at different rates. Metals expand faster than glass and fingers. (The water also provides some lubrication and in the case of jars, may dissolve anything sticky that might be inbetween the threads.)

Most things expand when heated and contract when cooled. Not water. This is good and bad. On the good side, ice is less dense than water, which means that ice can float on top of a pond while warmer, denser water goes to the bottom. The fish and anything else that wants to survive also goes to the bottom. On the bad side — as you know if you’ve ever left a bottle of soda or juice in your car overnight when it got really cold — water expanding at the wrong time can be a mess.

Water freezing and thawing can wreak havoc in other places. Putting in lawn edging in the North is an exercise in futility because the freeze/thaw cycles push the edging up so that, by April, it’s lying on the ground.

The word ‘cycles’ here is important. Most materials are designed to handle constant loads. A car rolling along a flat surface exerts about the same force everywhere along the surface. When you subject a material to repeated cycles of pulling and pushing on it, eventually, it breaks. You can bend a paper clip back and forth a couple of times, but it gets harder and harder to do, and then finally breaks. Each time you bend the paper clip, you make a little change in its microstructure. It’s like a game of pick-up sticks (or Kerplunk). Everything is fine up to a point, but when you push just a little too far, the whole thing comes down.

Normal temperature changes outside make most things expand and contact. There are joints in concrete sidewalks, for example, to allow for this expansion. Otherwise, two slabs of concrete would start pushing against each other and you’d have your own miniature version of plate tectonics.

Asphalt is made up of two components: aggregate (small pieces of rocks) and binder. Go get a bunch of rocks roughly 1/2 inch in diameter and put them in a jar. Try to pack them as closely as possible. It’s not easy to do, and if you don’t believe me, fill the jar up with water, then measure how much water you got in there.

The rocks are mixed with a liquid binder to hold it together, but in the end, asphalt looks like a sponge: rocks held together by binder, with a little bit of air space inbetween. A typical composition for asphalt might be 80% rock, 15% binder and 5% air voids. Here’s a picture from “The Idiot’s Guide to Highway Paving” showing some asphalt close up.

porous asphalt

You want some porosity in the asphalt. Porosity helps asphalt absorb water. A completely smooth, impervious surface would take a very long time to dry and would be more prone to hydroplaning than a rough surface.

The pores, however, cause problems, too. When water gets between stones and freezes, it exerts stress on the asphalt. Not a lot of stress, but enough cycles of stress will eventually produce weak spots and finally cracks. Once a crack is started, it’s very hard to stop (just like runs in nylons) and everytime a car goes over it, the crack gets wet. The weather in Florida was abnormally wet and cold the last few months. Don’t forget that Daytona was literally underwater last summer.

“Well, why didn’t they take that possibility into account?”, some of you are asking. If there is one thing we ought to be teaching in school science, it is that science never has absolute solutions. You can only increase downforce if you’re willing to pay a price in terms of drag or engine heating.

Likewise, if you engineered a track that was totally impervious to freezing and thawing, it wouldn’t drain well and would take a long time to dry when wet. Florida is much more likely to have rain and a need for lots of track drying than it is to have freezing. No track design is perfect. Although asphalt has been in use for many years (the Sumerians used it way back in 3000 B.C. as an adhesive on statues), we don’t have a lot of data on how highly banked asphalt racetracks that see speeds of 200 mph behave. There are really only two superspeedways, both constructed 1959-1960 and you can tell from the racing that they have very different characteristics, despite their apparent similarities.

Asphalt is not an easy material to work with, either. You start with crude oil, remove everything that seems useful (gasoline, diesel, oil, paraffin, etc.) and the sticky, goopy mess left over is used to make binder. You’ve probably seen (and/or smelled) asphalt machines puffing smoke near highway construction sites. The binder softens when it is warm and hardens when cool. Asphalt is usually laid down around 275-300 degrees Fahrenheit and gradually cools to a solid.

Liquid asphalt patches often consists of asphalt binder in a solvent — the same way pigment molecules are suspended in a solvent to make paint. You apply the liquid and wait for the solvent to evaporate, leaving behind a solid. The problem is that evaporation usually takes a long time. A re-surfaced asphalt driveway usually needs a day or two before it’s ready to be used. Heating will quicken the process, which is why the track workers were using a blowtorch on the patched area. Of course, the area that had the problem was the one part of the track that wasn’t in the Sun and thus was colder than everywhere else!

Eventually, they literally turned to Bondo. (My first car was a ’69 Buick LeSabre, so I know all about Bondo!) Bondo is a two-part putty that cures via a chemical reaction that is significantly less sensitive to temperature than asphalt patches. Of course, Bondo won’t stick as well to asphalt as asphalt sticks to asphalt, so Bondo is not the ideal solution. There’s that tradeoff again: you can make a fast repair that doesn’t last very long, or a slow repair that lasts longer. With a race in progress and FOX rapidly reaching the point where they were ready to interview drivers’ dogs because everyone else had already been interviewed, any repair that would get us to the end of the race was the right one.

Repaving is estimated at about $20 million dollars, and there’s no guarantee that (if it had been done between February and July ’09), the torrential rains of summer ’09 and the cool weather wouldn’t have caused problems. The next repave is tentatively scheduled for February 2012. Repaving can totally change the character of a track and not always for the better. They have plenty of time to patch the track between now and July (although there are other events scheduled for the track). An in-depth evaluation by an engineering company is in process. Whether patching will be sufficient or a total re-paving is necessary will be determined by the results of that evaluation. And while the folks doing the evaluation are some of the best in the business, the nature of the world is that there are no guarantees. The only Law of Nature that is certain is Murphy’s Law.

 Posted by diandra at 1:28 am
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