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March 19, 1993
From tiny molecules do gargantuan potholes grow —
a Purdue scientist studies crumbling highways
WEST LAFAYETTE, IND.–Each spring, the cycle continues. Not birds returning or lawns greening or flowers blooming. The first sign of a returning spring is the appearance of potholes.
Millions–maybe billions–of potholes. Sometimes it seems, more holes than pavement.
We hit 'em. We curse 'em. We patch 'em. Hit 'em, curse 'em, patch 'em. Hit 'em curse'em patch'em hit'em curse'em patch'em. Ad infinitum.
What we can't seem to do is to fix 'em for any length of time. or prevent them. That is partly because, before now, nobody really examined at the molecular level exactly what causes potholes. As with many things at the atomic level, the problem is more complicated than one might guess.
"The problem has been extremely hard to study because of the scale we're dealing with," says John Cushman, professor of agronomy at Purdue University. "But the hope is that once we understand the mechanics of what's going on at the molecular level, we can work in a more rational fashion on preventing the problem of crumbling materials in highways, runways and buildings."
Which might mean that the flashing-arrow signs and miles of orange cones that mark never-ending highway rebuilding projects could become as rare a sight on the highway as "check-your-oil-ma'am" full-service gas stations.
Although the scale of study is minuscule, as any motorist knows, potholes aren't a small problem in any sense of the word. According to Kumares Sinha, professor of civil engineering and head of transportation and urban engineering at Purdue, repairs to potholes cost $5 billion to $6 billion each year in the United States. Total highway maintenance costs in the United States come to roughly $4O billion per year.
Potholes have become enough of a problem that President Bill Clinton won votes, in part, by promising to fill more of them–to repair our crumbling infrastructure, as it was phrased so often on the campaign trail. The enormity of the pothole problem is a big part of the reason why the U.S. Army Research office has helped fund Cushman's research.
A visitor expecting to find Professor Cushman's office to be decorated with chunks of asphalt and pieces of crumbling concrete would be wrong; instead, his office shelves are filled with reams of white paper covered in hieroglyphic-like scientific formulas. "A tremendous amount of math and physics go into understanding this," says Cushman, who has doctoral degrees in both soil physics and mathematics.
Since the movements and interactions between the molecules can't be seen even with the most powerful microscopes, Cushman and his colleagues (physics professor Martin Schoen of Witten, Germany, and agronomy professor Dennis Diesthler of the University of Nebraska) rely on computer models to demonstrate the atomic motions in the system. "We've created a virtual laboratory on the computer," Cushman says.
Under the pavement is a porous soil structure. In addition to the natural minerals and organic matter that make up the soil, there are also spaces, or "pores," which are often filled with water.
In the winter, as the air temperature above the pavement drops, so does the temperature of the soil near the pavement. This often causes the water in the soil pores to freeze, forming a layer of ice parallel to the road surface. Scientists call this layer of ice the "lens," since it is thicker in the middle than at the edges, just like the lens in a pair of eyeglasses.
The ground below the pavement, on the other hand, is warmer since it isn't as affected by the fluctuating above-ground temperatures. Under the pavement, groundwater moves from the warmer region below ground to the colder area near the pavement. "Why the water moves is one of the things we are trying to understand," Cushman says.
The water forms a film between the soil grains and the ice lens. This film is roughly wide enough for 1O layers of water molecules. Scientists call this water "vicinal water" because it is in such confined spaces. As the molecules of the water come into contact with the ice lens, they freeze, thickening the lens as layer upon layer of water molecules are added to it.
As the ice lens thickens, it needs to take up more space, which it can do only by pushing something out of the way. It has only two options. The ice lens may thrust the soil and pavement upward, resulting in immediate road damage known as "frost heaves." or, more commonly, the ice lens may compress the soil below, leaving the pavement above unchanged.
The problem with the latter scenario is that when the warm breezes of spring finally blow, the ice lens melts and flows away. "You're left with a big hole," Cushman says. "The structure above begins to collapse." Ergo, pothole.
"The more cycles of freezing and thawing there are, the more times the pavement moves up and down, causing more potholes or damage," Cushman says. "A mild winter in which the temperature hangs around the freezing point with multiple cycles of freezing and thawing could mean that there is a lot more damage than an extremely cold winter with one hard freeze and thaw.
There are a few practical ways to prevent the ice lens from forming and creating havoc on our pavement. "The simplest is good drainage of the road bed," Cushman says. "Unfortunately, in many cases, the existing engineering approaches aren't feasible because of cost or a lack of materials.
"The key to correcting the problem chemically is understanding the molecular behavior of water under the ice lens. The vicinal water must support the weight of the ice, road and overlying soil. If you destroy the ability of the vicinal water to support this load, you destroy the ability of the ice lens to grow."
The answer to the problem would seem reasonably simple: Inject some sort of environmentally friendly chemical into the vicinal water layer to prevent the ice lens from forming, and bing, bang, boom, no more potholes.
Water, unfortunately, doesn't believe in the bing bang theory; vicinal water molecules behave nothing like the water that comes from the kitchen faucet.
"We're finding that it's a different creature," Cushman says. "It has different viscosity, it diffuses differently, it freezes at different temperatures, chemicals diffuse differently in it–I could go on and on. All of its properties are different."
Experiments performed in Cushman's laboratory have found that this behavior is not unique to water. "If we take a fluid as simple as argon, and confine it to spaces or pores this small, it also behaves unexpectedly," he says.
These properties of water and other fluids in porous structures are a concern of environmentalists as well as civil engineers, since the dispersal of chemicals such as pesticides is also affected by the vicinal water.
"This work has applications in all sorts of fields. People would never believe half the things we do, but research on the movement of fluids in porous media is important to all sorts of problems, not just potholes. We study lubrication on a molecular scale, environmental contamination problems, biological transport in cell membranes. This work even has application to the folding structure of proteins, since the ability to fold is based on the pores the structure of the protein creates." The work on protein structures is important to the biotechnology industry, among others.
"We work on problems from scales of molecules to miles, from atoms to reservoirs," Cushman says. "Our work is constantly evolving as we find more applications for this information."
NOTE: A broadcast-quality video news release on this story is available from the Purdue Agricultural Communication Service. Contact: Chris Sigurdson, 765-494-8396. Also, a color version of the enclosed graphic is available on photographic slide.
Contact Purdue News Service (765) 494-2096 or purduenews@purdue.edu
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