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The Sound of Science - 'Potholes'

K: Welcome to the Sound of Science on WNIJ. I'm Kate Powers.

J: And I'm Jeremy Benson.

K: Spencer asked why we have so many new pot holes each spring. Jeremy, what's going on with the roads in the spring?

J: It all starts long before spring, Kate. Water is great at getting into every crack or crevice in anything, right?

K: Yes…

J: Even when a road is brand new, there are tiny crevices in the pavement. Most of the year, it doesn't matter when water gets in; it either flows out or evaporates. But when winter comes, the water in those cracks freezes. When water molecules arrange themselves into their crystal structure they take up more room. The water expands.

K: And the water closest to the cold air freezes first! 

J: …trapping the rest of the water below it.  That forming ice exerts a great deal of pressure on the surrounding road, cracking it a bit to make room for the ice crystals to form. Every time the ice thaws, the water seeps further into these new cracks. Then when it freezes, the cycle repeats. Soon you have visible cracks and then a few that go all the way through the pavement.

K: If the ground below isn't frozen, the water can drain away. But if the ground is frozen like we often experience in winter, the water is trapped between the pavement and frozen ground. This time, when the water freezes, those forming ice crystals push up on the pavement, eventually popping it right out of place!

J: Isn't the power of freezing water amazing!

K: I wouldn't sound so positive about it.

J: This cycle of freezing and thawing is part of the weathering process which breaks down bedrock into boulders, gravel, and sand, and is necessary to form mineral-rich soil for growing food. The freeze/thaw cycle can account for many potholes in the northern U.S., but we can get potholes anywhere.

K: Hot temperatures and water also play a role in potholes. All paving materials expand as they get hotter and contract as they get colder. We can see the extreme result of that on hot summer days when the pavement buckles. Most of the damage is not as visible as a buckle, but when water seeps through the cracks formed this way, it softens the ground below the pavement, allowing it to deform or break. We see the fast version of this when a road is washed away by flood waters. 

J: Sounds like road repair people have job security throughout the U.S. If any of you have questions about the science in everyday life, shoot us an email at STEMOutreach@niu.edu

K: You've been listening to The Sound of Science on WNIJ.

J. Where you learn something new every day.

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