As one way to alleviate my otherwise simmering rage while crawling along the Eisenhower Expressway these days, I've been trying to figure out why they can't make a highway that lasts forever. Aside from the need to preserve triple-overtime jobs for road construction workers, is there some other (possibly physical) reason why this can't be done?
Dan Witte, Forest Park, Illinois
Uh, yeah. It’s called reality. Eliminate that and you’ve got the problem licked. Since you seem like the can-do type, Dan, we’ll put you in charge of highway maintenance and see how well you manage. Here’s a rundown on the challenges you’ll face.
Weather. I won’t dwell on this, since you’re from the Chicago area and thus presumably familiar with the concept. The principal phenomenon of interest is the infamous freeze-thaw cycle: snow falls, melts, seeps into cracks, and freezes again. The pressure of the expanding ice inexorably breaks up the pavement. Another factor is road salt, which can filter down into concrete and corrode the steel rebar within. So if you can do something about winter, half your problems disappear.
Traffic. Roads would last a lot longer if it weren’t for all the vehicles driving on them. It’s not uncommon to hear of highways in U.S. urban areas carrying double or more the traffic they were designed for.
Trucks are particularly problematic. The rule of thumb among highway engineers is that road deterioration is roughly proportional to vehicle axle weight to the fourth power. In other words, doubling the weight on an axle increases the wear and tear on the roads by 24, or 16 times. Roads are usually designed assuming that a single axle on a big truck carries a maximum of 18,000 pounds. Compared to a typical car carrying 2,000 pounds per axle, a fully loaded truck stresses the road surface 6,561 times as much. Minor overloading can make a big difference. Exceeding the maximum load by just 10 percent increases road stress by 46 percent — that’s why you see all those weigh stations on highways. So the next job on your list, Dan, is dealing with the damn trucks.
Money. Or more precisely, lack of money. Generally speaking, U.S. highways were built on the cheap, meant to last just 20 years. Unfortunately, some parts of the Interstate Highway System are now 50 or more years old. Highways in Europe are built to endure much longer than those in the U.S. For example, the Netherlands expects its roads to last 40 years.
How do they manage it? Although European highway designers use a variety of advanced techniques, two things stand out: thicker, more durable roadbeds and greater reliance on concrete.
This is something that as highway czar you’re going to need to know about, Dan, so pay attention. The two main paving materials are concrete and asphalt. Concrete is strong and durable, but building roads out of it is complex, expensive, and slow — you need a lot of rebar, the concrete has to cure, etc. Concrete roads also tend to be noisy, and slick when wet (although that can be remedied), and when they do eventually fail, they’re a pain to repair.
Asphalt, by comparison, is cheap, forgiving, and fast. True, it tends to fall apart quickly, but you can easily patch it till things have really gone to the dogs, at which point you just resurface the whole road. You can spread and roll the paving in the morning and drive on it in the afternoon, minimizing complaints by impatient motorists. The drawback is that you have to do this every few years, leaving everybody cumulatively more pissed off.
Don’t get me wrong. You can make long-lasting roads using asphalt; in fact some modern highways use a combination of asphalt and concrete to get the best of both worlds. But there’s no simple way to do this. You have to rip out the original excuse for a roadway and redo it from scratch.
That’s what they did in Chicago a few years ago when rebuilding the Dan Ryan Expressway, a perpetually clogged truck route carrying 300,000 vehicles per day on a road designed for 150,000. The original road typically consisted of 12 inches of aggregate (basically crushed rock), ten inches of concrete, and five inches of asphalt, for a total depth of 27 inches. The new highway has a 24-inch-deep aggregate sub-base, six inches of asphalt, then 14 inches of concrete, for a total of 44 inches.
Good news: it’s supposed to last 30 years — some say 40. Bad news: rebuilding 10 miles cost close to $1 billion. The National Highway System, consisting of all critical U.S. roads, is 160,000 miles long. The American Society of Civil Engineers estimates that the country’s roads and bridges will need $930 billion worth of work over the next five years, less than half of which is likely to happen. So that’s your final challenge, Dan. Once you’ve got the weather and the trucks under control, you’ll have to see if you can make money grow on trees.
Send questions to Cecil via email@example.com.