Every driver in Chicago knows the sound and sensation of driving over a pothole. There’s a two-fold thump — “D-DUNK!” — as your front and rear tires go in, and if you’re extra unlucky, the accompanying “HISSSS!” of a flat tire.
Chicago’s pothole-ridden streets are topped by blacktop asphalt, but highways, like the Dan Ryan, are paved with smooth cement concrete. As a Chicagoan, it’s always made me wonder why we use asphalt in the first place.
So when Curious City got this question from Max Melgarejo of the Portage Park neighborhood, I jumped at the opportunity to answer it.
Is asphalt the best choice for Chicago’s streets? It’s so porous that I feel like water and freezing temps would easily destroy the roads.
People who drive in Chicago come into contact with asphalt every day — it’s the skin of our city. But over the past 10 years, the city of Chicago has paid out nearly $3 million to drivers whose cars have been damaged by the poor condition of our roads. Since the beginning of 2017, there have already been more than 22,000 pothole service requests, according to city records.
So why would the city choose to use a material like asphalt rather than something more durable? It turns out there’s more to the streets than meets the eye — asphalt is just one of several different types of paving materials used to construct our roads.
And those infamous Chicago potholes? They’re probably here to stay. Potholes are created when water seeps into cracks, freezes and expands, and then melts, causing crumbling. They’re driven by freeze-thaw cycles — those drastic swings in temperature that are common in northern cities like Chicago.
More than meets the eye
When you peer under the pavement, you’ll find that most high volume — or “arterial” — streets and all new roads use what’s called a “composite pavement,” which combines asphalt and concrete.
Chicago’s composite pavements are like a layer cake, with two to three inches of asphalt on top, five to seven inches of concrete, and then whatever else is under there. In the case of the photo below, that includes bricks, stone, and wooden ballasts, all left over from the prior century of civic evolution.
The top layer of asphalt essentially acts as a wearing layer and protects the concrete. Asphalt is also easier to repair than concrete. While concrete takes several days to “cure” before it can be driven over, asphalt can handle traffic almost immediately. Composite roads benefit from the flexibility of asphalt and the strength and solidity of concrete. Plus, since potholes are mostly confined to that top layer of asphalt, they’re easy to patch.
There’s another big advantage to asphalt, says John Sadler, an assistant chief highway engineer at the Chicago Department of Transportation, or CDOT.
“When we talk pavement, we’re not just talking concrete [and] asphalt. … It is a cover, essentially, for a lot of other things that are underneath our right of way,” he says.
Specifically, he means utilities.
“All told, I think there’s upwards of 30 agencies that have utilities underneath our roadways,” Sadler says. “We have everything from telecommunication lines to sewer, water, gas, ComEd.”
Those utilities frequently need repair. Asphalt is easier to peel up, which allows crews to easily access the infrastructure underneath. In a composite road, when crews need to cut through the asphalt and concrete layers, they cover up the site with an asphalt patch that will match the rest of the street level, keeping the surface relatively smooth. In the winter, when it’s too cold to use asphalt, the city will often use concrete patches, and you can see them jutting out from the surrounding pavement — sometimes giving you the same kind of “D-DUNK!” sound you get when you drive over a pothole.
But what about concrete?
While Chicago’s arterial roads and all new road installations are composite, many residential streets are asphalt through and through with no concrete layer for support.
For Randy Riley from the Illinois Chapter of the American Concrete Pavement Association, all-asphalt roads don’t pass muster.
“We tend to build asphalt pavements, and they have a 15-year service life,” he says, pointing out that concrete lasts longer than asphalt and doesn’t develop potholes or other wear and tear as easily.
“Concrete has a 25-, 30-, 40-year service life before you have to do anything to it,” Riley says. He can point to several concrete roads that have outlasted all competition. Riley says there’s a concrete road in Delavan, Illinois that’s been around for 103 years, and certain streets in the Chicago suburb of Glenview that have been in service for around 80 or 90 years.
Even though it doesn’t need to be repaired as often and doesn’t develop potholes as quickly, concrete is more expensive up front.
Riley admits concrete roads cost 12 to 15 percent more up front than asphalt roads. But he argues that concrete is a more economical choice because it requires less overall maintenance.
“Concrete, if it was built in equivalent design and equivalent carrying capacity, is cheaper than asphalt … period,” he says.
Other experts I talked to stood by composite pavements as a good solution for an urban street grid with underground utilities. “By using the composite pavement, it really allows us to use the best material properties of both concrete and asphalt,” CDOT’s Sadler says.
The roads of the future?
Chicago and most other major American cities use a composite method for paving city roads. But is this the most sustainable way to pave streets?
There are a few notable environmental pluses to concrete: It doesn’t trap as much heat from the sun as blacktop asphalt does, and cars get better mileage on smoother, sturdier road surfaces.
But the two pavements aren’t so different, according to Scott Bernstein at the Center for Neighborhood Technology, a nonprofit that studies sustainability in urban environments.
“It’s sort of a dead heat as to which one uses more energy in its formation and its paving,” Bernstein says.
Instead, Bernstein says that a great way to make Chicago’s streets more environmentally sustainable would be to focus on drainage.
“The two biggest uses of energy in the municipal budget are pumping water and pumping sewer,” Bernstein explains. Improve drainage, and you can improve your energy efficiency.
This is where “permeable pavements” come in. Permeable pavements are made so that there are gaps for water to flow through. For example, “open-graded asphalt” uses a different combination of rocks atop a stone aggregate, so rain can go through to ground aquifers.
Using pavement that absorbs rainwater where it falls helps prevent urban flooding and sewer back-up. It also has the added bonus of cooling down the surrounding area. That’s especially helpful during the hot, sticky Chicago summers.
The city of Chicago has installed permeable asphalt on several streets, including the curb lanes of 33rd Street and Claremont Avenue on the South Side. Permeable concrete is used as part of the city’s “Green Alleys” program, and permeable brick pavers are used in the Argyle “Shared Street” initiative, pictured below.
Ultimately, though, Bernstein says the biggest environmental problem with roads is still the obvious one: cars.
“Something like 98 percent of road-related energy use is in the vehicles, not the road,” he says.
But while we still use cars to get from point A to point B, asphalt allows us to repair our roads quickly and access the utilities underneath them. By utilizing composite pavements for our high-volume roads, our streets have a strong, concrete base, protected by asphalt.
Asphalt’s affordability, and the speed at which it can be repaired, also allows the city to repave more road surface in a given year. In fact, in the last five years alone, about 1,600 of Chicago’s 4,000 miles of streets have been resurfaced.
So, asphalt haters, put that on your street and pave it.
More about our questioner
Max Melgarejo teaches seventh and eighth grades at the Latin School of Chicago. He lives in the Portage Park neighborhood, but got the idea for his question while visiting Lima, Peru.
“In Lima, the streets are made out of cement,” he says. He noticed the main boulevards were in better shape than Chicago’s streets, and he thought about potholes, which is something only a Chicagoan would find themselves thinking about while on a trip in South America.
“Eight or nine times a day, I hit a pothole,” he says. “I wonder why we use asphalt and here they use concrete.”
This wasn’t just a casual trip to Peru — Max was there to track down long-lost family members. His dad died before he was born, and he had uncles and cousins who had no idea he existed. It took him several trips, and when he finally found them, he had to show them his passport.
“It was an amazing experience,” he says. “But that’s a story for another time.”
Max came along with us to Ogden Avenue Materials to help report this story. Get a peek into how asphalt gets made at the facility in the photo slideshow below:
Thanks to William Vavrik of Applied Research Associates and Kevin Burke of the Illinois Asphalt Pavement Association for their help with this story.