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Speaker 1: Welcome to brain Stuff from how Stuff Works dot com

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Speaker 1: where smart Happens. Hi. I'm Marshall Brain with today's question,

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Speaker 1: how does asphalt work and where does it come from?

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Speaker 1: Asphalt is one of those things, like water or electricity,

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Speaker 1: that no American could live without. Most American roads, like

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Speaker 1: of them, are paved in asphalt, meaning that our commutes

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Speaker 1: and our shopping trips depend on the stuff. Everything we

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Speaker 1: buy at the store comes in via truck on those

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Speaker 1: same roads, so without asphalt there wouldn't be anything to

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Speaker 1: buy either. Millions of tons of asphalt are made and

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Speaker 1: laid every year in the United States, yet we tend

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Speaker 1: to take asphalt completely for granted. Let's take a look

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Speaker 1: at how this essential material actually works and asphalt roads.

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Speaker 1: Starts off with crude oil pumped out of the ground.

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Speaker 1: The crew it oil ends up at an oil refinery,

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Speaker 1: and there it gets boiled. The refinery takes the crude

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Speaker 1: oil vapor and captures it at different temperatures to separate

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Speaker 1: the molecules into different groups. There are very short, lightweight

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Speaker 1: molecules like propane and butane, with three or four carbons

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Speaker 1: in the carbon chain. Gasoline molecules typically have ten carbons

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Speaker 1: motor oil has twenty to fifty carbon atoms in the chain.

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Speaker 1: The very longest chains, typically a hundred fifty carbons long,

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Speaker 1: are asphalt. These are the heaviest molecules in crude oil.

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Speaker 1: Asphalt is black and solid at room temperature. You have

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Speaker 1: to apply heat to turn it into a liquid. To

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Speaker 1: make the hot mixed asphalt or h m A found

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Speaker 1: on most roads, you start with a big, rotating, heated drum.

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Speaker 1: Into your drum, you put gravel and sand and raise

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Speaker 1: the temperature to three hundred degrees fahrenheit or so. Then

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Speaker 1: you add five or six percent asphalt from the refinery

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Speaker 1: and mix until all the gravel is thoroughly coated. The

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Speaker 1: drum dumps this hot mixture into the back of a

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Speaker 1: dump truck and it gets laid by an asphalt spreading

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Speaker 1: machine to make a road. After several hours, the mixture

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Speaker 1: cools off, the asphalt solidifies, and you have a road.

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Speaker 1: An interstate highway that handles thousands of cars and trucks

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Speaker 1: a day uses a lot of asphalt. The layer of

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Speaker 1: asphalt might be a foot thick sitting on top of

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Speaker 1: a gravel base up to two feet thick. A normal

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Speaker 1: road through your neighborhood has only a few inches of

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Speaker 1: asphalt in two layers. The base layer uses thicker gravel.

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Speaker 1: The surface layer uses smaller pieces of gravel to provide

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Speaker 1: a smoother surface that cuts down on noise and tends

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Speaker 1: to repel water better. One interesting thing about asphalt is

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Speaker 1: that it's recyclable. In fact, asphalt is the most recycled

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Speaker 1: material by weight in the United States. Old asphalt can

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Speaker 1: be ground up, reheated, and re mix to make new

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Speaker 1: asphalt in a process that's very efficient. So what's not

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Speaker 1: to like? Asphalt is relatively inexpensive, easy to make, easy

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Speaker 1: to lay, and it lasts a long time. About The

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Speaker 1: only problem is the fact that it does wear out eventually.

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Speaker 1: One obvious sign of this is the infamous pothole. Potholes

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Speaker 1: more often arrived in the winter for two reasons. First,

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Speaker 1: cold asphalt is more brittle than warm asphalt, so that's

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Speaker 1: more likely to crack. A hot asphalt road in the

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Speaker 1: summers has some tendency to be self healing, but that's

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Speaker 1: definitely not the case on a cold, dark winter night,

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Speaker 1: so a small crack forms and lets water in. If

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Speaker 1: that water freezes, it widens the crack. Since the asphalt

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Speaker 1: layer is simply sitting on top of the gravel layer below. Eventually,

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Speaker 1: a chunk of asphalts several inches thick, pops out when

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Speaker 1: a heavy car truck passes by. Immediately you have a

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Speaker 1: four or five inch deep hole in the road. Cars

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Speaker 1: driving over that hole quickly crushed the edges and make

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Speaker 1: the whole bigger, and they scour out the gravel beneath

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Speaker 1: the asphalt. Suddenly, in just a couple of days, you

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Speaker 1: have a hole that's two ft around and a foot deep.

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Speaker 1: It's a full blown pothole that can eat your tires

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Speaker 1: and wreck your fancy rims. So what does the future hold?

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Speaker 1: There is some discussion of replacing asphalt with glass roads.

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Speaker 1: The road would have a glass surface that protects banks

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Speaker 1: of solar cells and l E d s. The l

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Speaker 1: e ed s could display stripes and messages, and the

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Speaker 1: solar cells could generate enough electricity to power the entire country.

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Speaker 1: It'll be interesting to see if glass roads ever become

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Speaker 1: a reality. For more on this and thousands of other topics.

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Speaker 1: Because it has staff works dot com