WEBVTT - How does asphalt work, and where does it come from?

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<v Speaker 1>Welcome to brain Stuff from how Stuff Works dot com

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

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<v Speaker 1>how does asphalt work and where does it come from?

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<v Speaker 1>Asphalt is one of those things, like water or electricity,

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<v Speaker 1>that no American could live without. Most American roads, like

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<v Speaker 1>of them, are paved in asphalt, meaning that our commutes

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<v Speaker 1>and our shopping trips depend on the stuff. Everything we

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<v Speaker 1>buy at the store comes in via truck on those

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<v Speaker 1>same roads, so without asphalt there wouldn't be anything to

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<v Speaker 1>buy either. Millions of tons of asphalt are made and

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<v Speaker 1>laid every year in the United States, yet we tend

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<v Speaker 1>to take asphalt completely for granted. Let's take a look

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<v Speaker 1>at how this essential material actually works and asphalt roads.

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<v Speaker 1>Starts off with crude oil pumped out of the ground.

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<v Speaker 1>The crew it oil ends up at an oil refinery,

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<v Speaker 1>and there it gets boiled. The refinery takes the crude

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<v Speaker 1>oil vapor and captures it at different temperatures to separate

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<v Speaker 1>the molecules into different groups. There are very short, lightweight

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<v Speaker 1>molecules like propane and butane, with three or four carbons

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<v Speaker 1>in the carbon chain. Gasoline molecules typically have ten carbons

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<v Speaker 1>motor oil has twenty to fifty carbon atoms in the chain.

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<v Speaker 1>The very longest chains, typically a hundred fifty carbons long,

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<v Speaker 1>are asphalt. These are the heaviest molecules in crude oil.

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<v Speaker 1>Asphalt is black and solid at room temperature. You have

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<v Speaker 1>to apply heat to turn it into a liquid. To

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<v Speaker 1>make the hot mixed asphalt or h m A found

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<v Speaker 1>on most roads, you start with a big, rotating, heated drum.

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<v Speaker 1>Into your drum, you put gravel and sand and raise

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<v Speaker 1>the temperature to three hundred degrees fahrenheit or so. Then

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<v Speaker 1>you add five or six percent asphalt from the refinery

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<v Speaker 1>and mix until all the gravel is thoroughly coated. The

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<v Speaker 1>drum dumps this hot mixture into the back of a

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<v Speaker 1>dump truck and it gets laid by an asphalt spreading

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<v Speaker 1>machine to make a road. After several hours, the mixture

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<v Speaker 1>cools off, the asphalt solidifies, and you have a road.

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<v Speaker 1>An interstate highway that handles thousands of cars and trucks

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<v Speaker 1>a day uses a lot of asphalt. The layer of

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<v Speaker 1>asphalt might be a foot thick sitting on top of

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<v Speaker 1>a gravel base up to two feet thick. A normal

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<v Speaker 1>road through your neighborhood has only a few inches of

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<v Speaker 1>asphalt in two layers. The base layer uses thicker gravel.

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<v Speaker 1>The surface layer uses smaller pieces of gravel to provide

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<v Speaker 1>a smoother surface that cuts down on noise and tends

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<v Speaker 1>to repel water better. One interesting thing about asphalt is

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

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<v Speaker 1>material by weight in the United States. Old asphalt can

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<v Speaker 1>be ground up, reheated, and re mix to make new

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<v Speaker 1>asphalt in a process that's very efficient. So what's not

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<v Speaker 1>to like? Asphalt is relatively inexpensive, easy to make, easy

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<v Speaker 1>to lay, and it lasts a long time. About The

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<v Speaker 1>only problem is the fact that it does wear out eventually.

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<v Speaker 1>One obvious sign of this is the infamous pothole. Potholes

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<v Speaker 1>more often arrived in the winter for two reasons. First,

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<v Speaker 1>cold asphalt is more brittle than warm asphalt, so that's

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<v Speaker 1>more likely to crack. A hot asphalt road in the

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<v Speaker 1>summers has some tendency to be self healing, but that's

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<v Speaker 1>definitely not the case on a cold, dark winter night,

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<v Speaker 1>so a small crack forms and lets water in. If

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<v Speaker 1>that water freezes, it widens the crack. Since the asphalt

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<v Speaker 1>layer is simply sitting on top of the gravel layer below. Eventually,

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<v Speaker 1>a chunk of asphalts several inches thick, pops out when

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<v Speaker 1>a heavy car truck passes by. Immediately you have a

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<v Speaker 1>four or five inch deep hole in the road. Cars

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<v Speaker 1>driving over that hole quickly crushed the edges and make

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<v Speaker 1>the whole bigger, and they scour out the gravel beneath

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<v Speaker 1>the asphalt. Suddenly, in just a couple of days, you

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<v Speaker 1>have a hole that's two ft around and a foot deep.

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<v Speaker 1>It's a full blown pothole that can eat your tires

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<v Speaker 1>and wreck your fancy rims. So what does the future hold?

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<v Speaker 1>There is some discussion of replacing asphalt with glass roads.

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<v Speaker 1>The road would have a glass surface that protects banks

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<v Speaker 1>of solar cells and l E d s. The l

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<v Speaker 1>e ed s could display stripes and messages, and the

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<v Speaker 1>solar cells could generate enough electricity to power the entire country.

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<v Speaker 1>It'll be interesting to see if glass roads ever become

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<v Speaker 1>a reality. For more on this and thousands of other topics.

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<v Speaker 1>Because it has staff works dot com