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.

  One Response to “Daytona Potholes: Avoidable or Not?”

  1. Diandra,

    Thank you for this column. You always make science understandable. I’m telling you, if you were my Science teacher, I would’ve gotten A’s. You need your own TV show. Forget about Bill Nye, you rock!!

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