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.
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.
* 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
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.
** 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
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.**
***Concrete is coarse gravel that is glued together by cement. Cement is a form of calcium carbonate that has reacted with CO2 and water.
And the winner is …?
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.
2. Eaton, R. A.; Joubert, R. H.; Wright, E. A., Pothole primer-A public administrator’s guide to understanding and managing potholes. US Army Corps of Engineers – Cold Regions Research and Engineering Laboratory 1989
3. http://www.post-gazette.com/news/transportation/2014/01/15/Bring-on-the-blitz-Pittsburgh-clamps-down-on-potholes/stories/201401150064 ↩
4. http://www.pavementinteractive.org/article/hma-patching/ ↩
5. https://www.city.pittsburgh.pa.us/pw/html/paving_processes.html ↩
6. http://www.wrmeadows.com/press/PCA_pavingreport.pdf ↩
7. http://www.cityofchicago.org/dam/city/depts/cdot/PotholeFAQ_winter1011.pdf ↩
8. http://www.clemson.edu/ces/arts/benefitsofRA.html ↩