Asphalt is one of those things, like water and electricity, that no American could live without. Most American roads (96%) are paved in asphalt, meaning that our commutes and our shopping trips depend on the stuff. Everything we buy at the store comes in via truck on those same roads, so without asphalt there would be nothing to buy. Millions of tons of asphalt are made and laid every year in the U.S. Yet we tend take asphalt completely for granted. Let’s take a look at how this essential material works.
An asphalt road starts with crude oil pumped out of the ground. The crude oil ends up at an oil refinery and there it gets boiled. The refinery takes the crude oil vapor and captures it at different temperatures to separate the molecules in the oil into groups. There are very short, lightweight molecules like propane and butane with 3 or 4 carbons in the carbon chain. Gasoline molecules typically have 10 carbons. Motor oil has 20 to 50 carbon atoms. The very longest chains – typically 150 carbon atoms – are asphalt. These are the heaviest molecules in crude oil; The sludge at the bottom of the barrel.
Asphalt is black and solid at room temperature. You have to apply heat to turn it into a liquid. To make the hot mix asphalt (HMA) found on most roads, you start with a big rotating, heated drum. Into it your put gravel and sand and raise the temperature to 300 degrees or so. Then you add 5% asphalt from the refinery and mix until all the gravel is thoroughly coated.
The drum dumps this hot mixture into the back of a dump truck, and it gets laid by an asphalt spreading machine to make a road. After several hours the mixture cools off, the asphalt solidifies and you have a road.
An interstate highway that handles thousands of cars and trucks a day uses a lot of asphalt. The layer of asphalt might be a foot thick, sitting on top of a gravel base up to two feet thick. A normal road through your neighborhood has only a few inches of asphalt in two layers. The base layer uses thicker gravel. The surface layer uses smaller pieces of gravel to provide a smoother surface that cuts down on noise and tends to repel water better.
One interesting thing about asphalt is that it is recyclable. In fact, asphalt is the most recycled material, by weigh, in the United States. Old asphalt can be ground up, reheated and remixed to make new asphalt in a process that is very efficient.
See also in-place recycling using foamed bitumen:
So what’s not to like? Asphalt is relatively inexpensive, easy to make, easy to lay and it lasts a long time. About the only problem is the fact that it does wear out eventually. One obvious sign of this is the infamous pothole. Potholes more often arrive in the winter for two reasons. First, cold asphalt is more brittle than warm asphalt, so more likely to crack. A hot asphalt road in the summer sun has some tendency to be self-healing, but that is definitely not the case on a cold, dark winter night. So a small crack forms and lets water in. If that water freezes, it widens the crack.
Since the asphalt layer is simply sitting on top of the gravel layer below, eventually a chuck of asphalt several inches thick pops out when a heavy car or truck passes by. Immediately you have a four or five inch deep hole in the road. Cars driving over the hole quickly crush the edges and make the hole bigger, and they scour out the gravel beneath the asphalt. Suddenly, in just a couple of days, you have a hole that is two feet around and a foot deep. It is a full blown pothole that can eat your tires and wreck your fancy rims.
So what does the future hold? There is some discussion of replacing asphalt with glass roads. The road would have a glass surface that protects banks of solar cells and LEDs. The LEDs could display stripes and messages, and the solar cells could generate enough electricity to power the entire country. It will be interesting to see if that idea ever appears on a real road.