Why Do Asphalt Tracks Change During a Race?
Although people talk about ‘asphalt’ tracks, asphalt is only one component of the track surface. Asphalt itself is a thick, sticky mixture of hydrocarbons – molecules made of hydrogen and carbon atoms. The first asphalt users got their materials from concentrated deposits in the Earth. These natural deposits were created from petroleum crude oil that rose to the surface. Crude oil is a mixture of many different types of hydrocarbon molecules that range from very light molecules like methane (CH4) and propane (C3H8) to much heavier hydrocarbons with 100 or more carbon atoms. Heat from the Sun vaporized the lighter hydrocarbon molecules, leaving a tar-like material composed of the heaviest hydrocarbons. In the early 1900’s, this process was sped up and moved indoors to petroleum distilleries, which separate crude oil into lighter weight fuels like LP gas, liquid fuels like kerosene, gasoline and diesel fuel, and heavier lubricating oils. What is left over – literally the bottom of the barrel – is asphalt.
Asphalt has two dominant characteristics: It is waterproof and it is really, really sticky. Asphalt’s waterproofing ability is why it was used as early as 6000 BC for shipbuilding. The Egyptians used asphalt as a preservative in the mummification process. The Arabic work for asphalt is mûmîa, which is almost mummy. Stickiness is why we have prehistoric animal skeletons in the LaBrea tar pits, and why asphalt was used as mortar in early building projects. Those two qualities together explain why about 80% of the asphalt in the US is used for making roadways. Asphalt is also the dominant material used in NASCAR tracks. Bristol and Dover are concrete and the corners at Martinsville are concrete, but the rest of the tracks are asphalt.
More properly, these track surfaces are asphalt concrete. Asphalt per se is the sticky stuff. Asphalt mixed with aggregate – crushed stone and gravel – is asphalt concrete, which is what is used for track surfaces. The asphalt and aggregate are heated and mixed together. Heating makes the asphalt less viscous so that it can be pumped through tubes and spread, and also evaporates any water left in the mixture. The asphalt concrete mixture is deposited on the road, leveled, and then compacted with a steamroller. If you look at an asphalt roadway, you often can see the individual asphalt-coated rocks.
The composition, condition and temperature of the asphalt play a major role in racing. Aggregate can vary from small pebbles to inch-sized rocks or larger. The type of aggregate impacts the durability and the roughness of the track. Freshly laid asphalt concrete often has sharp edges from the aggregate poking up and these tracks – like Atlanta Motor Speedway – tend to be very fast because they produce a lot of grip. As they wear, the edges of the aggregate become less sharp and some of the asphalt binder wears away, exposing a little more of the aggregate. The tracks usually lose a little grip and the speeds get a little slower over the years.
Indianapolis, which was last repaved in Fall 2004, is an extremely rough track. The television commentator Larry MacReynolds called it as ‘a cheese grater’. A rough track gives you a lot of grip, but takes quite a toll on tires. Every time a car makes a lap, it leaves a little bit of tire on the track. The rougher the track, the more tire lost on each lap. The upside is that the rubber scraped from the tires coats the track surface and fills in spaces between the aggregate. This rubber coating produces additional grip and mitigates the wear on the tires.
The advantages of having some rubber on the track is one reason drivers hate rain. Rain washes rubber from the track, leaving it ‘green’. The two Friday practices at Indy were rained out. You may have heard Jeff Burton talking about the importance of having a long Saturday practice so that they could get some rubber on the track. The Speed Channel showed tires worn down to the cords after only 15 or 16 laps on the first runs, but significantly less tire wear on tires from runs after the cars had been out on the track for awhile. NASCAR decided to call two competition yellows to give teams a chance to check tire wear.
The other way the asphalt surface can change is when the temperature changes, and Indy is one of the most temperature-sensitive tracks. Asphalt is a collection of hydrocarbon molecules that can be classified into three general groups: cyclics, aromatics, and aliphatics. Aromatics and cyclics are molecules in which the carbon atoms form rings. They differ in the type of bonding between the carbon atoms (the electrons in an aromatic are delocalized, which means that all the electrons are shared by all the atoms in the ring) and their structure: Aromatics are flat while cyclic molecules are three-dimensional. The first aromatics discovered had a pleasant smell, which is why they got called aromatics. Aliphatics are molecules in which the carbon atoms are arranged in a linear chain.
These hydrocarbon molecules bind to each other, forming the molecular network around the aggregate that gives asphalt its structure. The bonds between molecules are relatively weak, so bonds are constantly broken and re-formed, which is one reason that asphalt concrete changes its shape with time and usage. Those are long-time effects; however, there are shorter-term effects as well.
Bonding changes with temperature. Paraffin is solid at room temperature, softens and eventually turns into a liquid when heated. The temperatures at which these transitions happen are determined by how the molecule is put together. For example, octane is an aliphatic molecule with eight carbon atoms. Cyclooctane has the same number of carbon atoms, but they are arranged in a ring instead of a straight line as in an octane molecule. The melting point of octane is -57 °C (-70.6 °F) while the melting point of cyclooctane is 14.59 °C (58.2°F) How the atoms that make up the molecule are attached makes a huge difference in their properties.
When the track warms, some of the hydrocarbons change how they are bonded. Aliphatic molecules soften first. The effect on the track should be evident from the fact that ‘aliphatic’ literally means ‘oily’. Drivers talk about the ‘oil in the track coming out’ when the track warms. More oil means less grip. One explanation for why Indy is more temperature sensitive than other tracks is that its asphalt has a larger proportion of aliphatic compounds relative to other tracks.
Most tracks have greater temperature sensitivity right after repaving. Just as Mother Nature creates asphalt from crude oil by removing more volatile (more easily vaporized) molecules, over the course of a year or a few years, a new track will change as the volatile components are removed. This is why drivers usually are skeptical about the quality of racing on freshly paved tracks. Concrete, by the way, has much less temperature sensitivity, so there shouldn’t be as great concern about going to Bristol’s newly re-surfaced track in a few weeks.
The engineers and scientists that study asphalt concrete are trying to make this material more durable and less sensitive to temperature changes. For example, adding rubber or rubber-like molecules to the asphalt binder to make it less sensitive to the freezing and thawing that cause surfaces to crack.
While they’re at it, I wonder if anyone is working on a track material that would dry quickly after a rain. I’m sure Boris Said, who didn’t get to run in the Pepsi 400 at Daytona in 2007 after qualifying was rained out (despite posting a great qualifying time), would be one of the many people who would appreciate that particular innovation.