Not a hole lot you can do to keep roadways smooth

By Ivan Amato
Tuesday, March 30, 2010

KaTHUMP-kaTHUMP. That's the sound of rubber meeting potholed road. If you're the one kathumping, you're probably adding a few staccato profanities to that street beat. None of this should blind you to the presence of nature's grandeur in the mix. We're talking here about the magnificent power of water and ice to defeat the pavement's man-made mix of gravel, sand and asphalt.

It may seem dumb as a brick, but pavement is seriously complex stuff, even to materials scientists and civil engineers who get paid to know everything about constructed landscapes, including roadways. That gold in your wedding ring or glass in your window is mostly a single chemical composition in mostly a single molecular architecture. Compared with that simplicity, the asphalt concrete mix that makes up almost 90 percent of the nation's roadways is as complex as human psychology.

About 95 percent of your typical city road surface is an aggregate of sand and stone crushed into pieces of many shapes and sizes. The rest is mostly asphalt, a sticky gunk that gets heated at paving time so it will flow like warmed maple syrup and coat all of those bits and pieces in the aggregate. Asphalt is what remains behind when crude petroleum is distilled and refined into fuels and basic ingredients for making chemicals. It's also cheap, abundant and good at cementing the concrete mix into pavement you can drive on.

At least until water and winter conspire to show the road who really is king.

In the winter, water perpetrates its street violence through a combination of its fluidity when it's a liquid and its odd knack for expanding when it freezes, instead of contracting, as almost every other liquid does. The reason is simple chemistry. If you could get up close to a water molecule, it would have the shape of a Mickey Mouse head, in which two hydrogen atoms (Mickey's ears) bond to a larger central oxygen atom (Mickey's head). Like human partners, these atoms don't share everything equally, including the electrons that are their bonds' chemical glue. The oxygen atom stakes a stronger claim on these electrons, so it basks in a negative charge, while its hydrogen buddies come off on the positive side.

The combo of mouse-head angularity and polarized electrical charge has consequences for the pothole phenomenon: When the temperature is above freezing, water molecules move around so fast that they don't get locked into a specific arrangement by their electrical charge; this allows them to flow and assume the shape of any container, including pores in the pavement.

Here's where things get interesting chemistry-wise (and can lead to overtime for road crews): As water freezes, what was a mosh pit of molecular motion slows down enough that the frenzy no longer overcomes the attraction between positively charged hydrogens and negatively charged oxygens. These opposite charges align with one another in a rigid and, because of the molecule's angularity, less compact way than when water is liquid; as a result, it takes up much more space.

The resulting solid lattice of ice occupies 9 percent more volume than the liquid from which it formed. In the cracks and pores of pavement (or inside your house pipes), freezing water can exert 15 tons -- tons! -- per square inch of pressure on its surroundings. It's like Samson's ruinous pushing ploy on the pillars of the temple of Dagon. Numerous freeze-thaw cycles that come with snowy winters and nightly applications of de-icing salt multiply the damage. Little cracks become bigger ones become potholes become your neighborhood car gulpers.

But that's only part of the story. Below the pavement, a related and often more damaging process can unfold. When water that has percolated all the way to the interface between the soil (or a crushed stone base layer above that) and the overlying pavement freezes there, that 9 percent expansion heaves the pavement upward and upsets the road's basic support. Meanwhile, vehicles rolling on the now weakened roadway impose their own compression and tension that leaves wounds: millions of potholes. Ironically, potholing is less problematic in regions where temperatures remain frigid in the winter rather than hopping over and under 32 degrees. Fewer freeze-thaw cycles mean fewer potholes.

An anti-pothole tactic most often deployed for high-volume roads such as interstates, including the Capital Beltway, is to pay more -- a lot more -- up front for so-called perpetual pavements. These designs include expensive drainage systems, with more layers, better grading and sometimes pipes to carry runoff, but they still require intermittently milling off the top layer and replacing it with a new one, according to Chris Williams, a civil engineer at Iowa State University at Ames. He rates France as tops in the world for road quality because of its rigorous standards for pavement materials and design.

In their own noble quest to help the nation better manage the annual plague of potholes, Williams and his colleagues have been trying to create the perfect patch, using piles of spent asphalt roofing shingles, which they chop into pieces and mix into various blends. Williams claims that the asphalt, granular topping, lime dust and fiber in the shingles improve the patches' toughness and makes them last several times longer than the usual few months. He also is developing what he says should be more-affordable pothole-resistant pavements, using non-petroleum "bio asphalts" derived from waste materials such as corn stalks and switch grass. In tests, these pavement materials have proven to be more pliable and crack-resistant and, he says, better at keeping moisture at bay.

Meanwhile, UCLA materials scientist Jenn-Ming Yang and his partners, including the City of Los Angeles, are also taking on potholes, which in L.A. form less from freeze-thaw cycles than from the city's famously hellish traffic volume. Yang and his fellow pothole heads are banking on dicyclopentadiene, a byproduct of the petrochemical industry that can be turned into synthetic plastic that is tough enough to stop bullets. The researchers are looking into adding these materials into asphalt concrete patching mixes to produce tougher, less porous and much-longer-lasting road Band-Aids.

It could be a while, though, before such materials reach our streets. Until then: kaTHUMP-kaTHUMP.

Amato is a writer and editor based in Silver Spring.


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