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

Asphalt is an extremely common, oil-derived material that's used to pave a majority of roads in the United States. Tune in to learn more about asphalt.

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Episode Transcript

Speaker 1 (00:00):
Welcome to brain Stuff from how Stuff Works dot com
where smart Happens. Hi. I'm Marshall Brain with today's question,
how does asphalt work and where does it come from?
Asphalt is one of those things, like water or electricity,

(00:22):
that no American could live without. Most American roads, like
of them, 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 wouldn't be anything to
buy either. Millions of tons of asphalt are made and

(00:45):
laid every year in the United States, yet we tend
to take asphalt completely for granted. Let's take a look
at how this essential material actually works and asphalt roads.
Starts off with crude oil pumped out of the ground.
The crew it oil ends up at an oil refinery,
and there it gets boiled. The refinery takes the crude

(01:05):
oil vapor and captures it at different temperatures to separate
the molecules into different groups. There are very short, lightweight
molecules like propane and butane, with three or four carbons
in the carbon chain. Gasoline molecules typically have ten carbons
motor oil has twenty to fifty carbon atoms in the chain.

(01:26):
The very longest chains, typically a hundred fifty carbons long,
are asphalt. These are the heaviest molecules in crude oil.
Asphalt is black and solid at room temperature. You have
to apply heat to turn it into a liquid. To
make the hot mixed asphalt or h m A found
on most roads, you start with a big, rotating, heated drum.

(01:50):
Into your drum, you put gravel and sand and raise
the temperature to three hundred degrees fahrenheit or so. Then
you add five or six percent 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

(02:14):
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

(02:34):
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's recyclable. In fact, asphalt is the most recycled
material by weight in the United States. Old asphalt can

(02:57):
be ground up, reheated, and re mix to make new
asphalt in a process that's very efficient. 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

(03:19):
more often arrived in the winter for two reasons. First,
cold asphalt is more brittle than warm asphalt, so that's
more likely to crack. A hot asphalt road in the
summers has some tendency to be self healing, but that's
definitely not the case on a cold, dark winter night,
so a small crack forms and lets water in. If

(03:40):
that water freezes, it widens the crack. Since the asphalt
layer is simply sitting on top of the gravel layer below. Eventually,
a chunk of asphalts several inches thick, pops out when
a heavy car truck passes by. Immediately you have a
four or five inch deep hole in the road. Cars
driving over that hole quickly crushed the edges and make

(04:02):
the whole bigger, and they scour out the gravel beneath
the asphalt. Suddenly, in just a couple of days, you
have a hole that's two ft around and a foot deep.
It's 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.

(04:23):
The road would have a glass surface that protects banks
of solar cells and l E d s. The l
e ed s could display stripes and messages, and the
solar cells could generate enough electricity to power the entire country.
It'll be interesting to see if glass roads ever become
a reality. For more on this and thousands of other topics.

(04:45):
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