The Science of Potholes: A Pittsburgh Story

Braddock Ave has fallen victim to the common woe of potholes

Braddock Ave has fallen victim to the common woe of potholes

Another flat. I pull out the jack, grab the tire iron, and fumble the spare. As I switch out my tire, I curse the drivers, children, and small animals that pass without a sympathetic glance. On my way to the mechanic, I drive like an asshole, braking and swerving around the craters plaguing the streets. My tires receive no relief, as no street remains unscathed. Dropping my car off at the harried mechanic, I ponder how my tires could be so vulnerable to potholes. Why are potholes line up along the tire paths? Why do potholes cluster in packs? Why are most potholes round and cavernous?

Potholes are a public enemy not only in Pittsburgh, but universally, as well.* The commonplace of the pothole plague is due to the ubiquity of the two simple pothole ingredients – water and traffic. Two recipes, dubbed as fatigue and raveling, combine the water and traffic to transform smooth city pavements into a pockmarked battlefield.
 

Fatigue

BTW

* The origin of the name “potholes” is under debate. It may be from its look-a-like eroding riverbed rock, which geologists call potholes. I prefer the theory during the 15th century wagons would frequent dirt roads, leaving rut marks and exposing underlying clay. Pottery makers would dig out this exposed clay for their next pot, and irritated wagon drivers would refer to these holes in the rut paths as “potholes.”1

Driving on paved roads is an everyday luxury of my city life, and I easily forget that the pavement lies mere millimeters above sediment and clay. Rain, sleet, and snow-melt beat exposed sediment and clay into soft mush, quickly washing out unpaved backwood country roads. The sediment under city pavement is no different. During spring thaw or a heavy rainfall, water will trickle through the city pavement, and soak this underlying sediment. The once sturdy sediment bed can now flex and pull like putty, leaving little support for the road. Those pesky early spring months with constant freezing and thawing temperatures also causes the wetted sediment to expand and contract like Kobayashi’s stomach, further decomposing it into a useless base. As cars and trucks continue to drive over this pavement, now held up by our flimsy putty, the road flexes up and down, resulting in bowl-shaped craters.2 This is why potholes are round and cavernous, and inconveniently pockmark pavement where traffic tires commonly hit.

Water submits underlying soil into putty, leaving pavement vulnerable to potholes.  Figure from the superb review by Eaton et al.2

Water submits underlying soil into putty, leaving pavement vulnerable to potholes. Figure from the superb review by Eaton et al.2

Raveling

Rain washes away asphalt adhesive, leaving pesky crocodile cracks, a precursor to potholes.  Figure from http://en.wikipedia.org/wiki/File:Asphalt_deterioration.jpg

Rain washes away asphalt adhesive, leaving pesky crocodile cracks, a precursor to potholes. Figure from http://en.wikipedia.org/wiki/File:Asphalt_deterioration.jpg

Fatigue may be the dramatic recipe for fresh potholes, but the lowkey recipe, raveling, is more common. Asphalt consists of tiny pieces of semi-solid petroleum called aggregates. These aggregates are little sharp black rocks, and are glued together by a black adhesive. Excessive water washes away this glue, and the road starts to crack like a crocodile’s back. Next time you are on the road, keep your eyes open, and you’ll see these crocodile cracks everywhere. Vehicle tires are now free to pick at and carry away the little rock pieces, allowing the potholes to prosper.2 This is why potholes seem to cluster together in devilish pothole communities; no amount of swerving will save your tires from a raveling pavement.

Rally the Troops

“This is an all hands on deck to fill potholes in our city streets,” Pittsburgh Mayor Peduto exclaimed in early January 2014.3 Unfortunately for us cold weather warriors, the permanent repair can only be done by a “hot mix asphalt,” a process that demands warmer weather.4 Particularly troublesome potholes can be temporarily repaired during the cold winter months with a “cold mix asphalt.”4 Amazingly, many cold mix asphalts don’t effectively bind to the road pavement, but merely fill the pothole like dense, heavy packing peanuts; a more permanent hot mix asphalt must be layered over the quick fix during warmer months.**

BTW

** BTW: For frosty pothole repairs, Pittsburgh fills the pothole with Cold Patch, a material that sits in the pothole in the winter, but turns into a permanent patch when heated in the summer months. Since winter driving may carry away a little of the top, this may be why patched potholes appear to have a dip in the center.5

Many exasperated commuters have banded together to demand higher quality roadways; they demand concrete pavement. While concrete is still a type of rock material cemented by an adhesive,*** it lasts over twice as long as asphalt.6 The durability of concrete is the reason why the most traffic-heavy neighborhood streets are spotted with slabs of concrete pavement. Many specialists insist that the increasing material costs of petroleum-based asphalt and the frequent need for asphalt repairs deems concrete the cheaper and superior pavement option. The push for concrete pavement been aggressive; the concrete and asphalt rivalry is one step away from a full on twitter-war.

More Specifically

***Concrete is coarse gravel that is glued together by cement. Cement is a form of calcium carbonate that has reacted with CO2 and water.

But alas, fighting potholes is a tireless battle. Just driving down Penn Ave in Pittsburgh, it is clear that concrete is not immune to potholes. And patching a concrete pothole is a troublesome task. Asphalt boasts the possibility of installation in a couple of hours, while concrete takes days.7 Patching a concrete pothole is equally a sluggish endeavor, leading impatient road specialists (rallied by more impatient commuters) to temporary patch concrete potholes with hot/cold mix asphalt. Asphalt and concrete are like milk and whiskey: they don’t mix. The hastily repaired concrete pothole is therefore prone to wreak havoc once again in just a few commuter days.
 

And the winner is …?

Pothole_decision_tree

Perhaps the best weapon against potholes is a shiny new concrete street surface. Perhaps it’s a watchful eye over all those cracks that allow water to sneak into the soil, turning it into putty. There could be better engineered drainage, improved leveling of streets to prevent water pooling. Perhaps the ideal weapon is merely adjusting the components of typical pavement. Asphalt pavement specialists are playing with peppering asphalt with rubber, which they claim to take some of the edge off of raveling.8 Perhaps it’s futile to prevent potholes, and we should focus on indestructible shocks and tires. The jury may still be out on how to best fight potholes, but we don’t have to live this way.

Whatever tactics we choose, fighting potholes will be an uphill battle that commuters of this country will never stop fighting. In the meantime, just make friends with your prospering car mechanic.

Thoughts?

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