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Potholes: How They Happen, How We Try to Fix Them

By May 24, 2011July 8th, 2014No Comments

A pothole by definition is a problem. No one intends for a pothole to form and exist – they are not here by intent. Unfortunately, the technology does not yet exist to build impenetrable pavement because all roads are subject to inevitable atrophy. The best we can hope is to minimize pothole formation for as long as possible.

To know the best ways to fix potholes – and prevent them from forming in the first place – it helps to understand how pavement is constructed and what it is about the road surface that typically fails. Second, it helps to think of pavement as somewhat like human skin: both will eventually age, but a little help – fixing small cracks before they get bigger, for example – can go a long way toward keeping things looking and working better for a longer period of time.

How pavement is constructed – and breaks down

The basic Macadam method of road building was invented in the 1800s. It is a technique that is fundamentally the same as how we continue to construct pavement today (albeit, with many improvements along the way). It involves a layer of large gravel laid over dirt or bedrock, which is then topped by smaller stones (angular shapes, not rounded) that are bonded by petroleum-derived products. The emulsification – the stone-oil mix, what we call asphalt – is compressed and it becomes largely impermeable to moisture.

Keeping out water is pretty important, as we explain in the next paragraph. This water-proofing is aided by constructing roads with a slight grade that crowns in the center of the road. Gravity leads rainwater and melting snow and ice to the sides of the roads, which allows the pavement to remain stable. Over millions of miles of pavement, this method has obviously proven to be a great system that works.

Except until it doesn’t. The clearest example of pavement failure is in underpasses, where the road dips below a railroad or cross street. Water, doing what water does, accumulates in such areas. That water in semitropical or tropical climates, such as you find in Los Angeles or Honolulu, can cause damage easily enough by finding its way into the sub-layers of the road. As water erodes the dirt below, the top layer of pavement collapses, causing a pothole to open up.

This process is exacerbated in places such as Buffalo, Boston, Chicago and Minneapolis, where frequent freeze-thaw cycles lead to a frenzy of expansion-contraction. Particularly in the late winter and early spring, this happens when temperatures fluctuate above and below 32 degrees (Fahrenheit) several times each day for weeks at a time.

But water isn’t the only cause of potholes. Other factors include time and traffic. And layered on top of these – or perhaps, underneath – is neglect. That is when the authorities in charge of maintaining and preserving roads, highways, streets and bridges fail to seal cracks and fill small potholes in timely and effective ways. Allow a small crack to develop in pavement and the water will get in. Seal that crack shortly after it forms and you might be able stop that small break from turning into a big pothole.

How potholes can be prevented

So it stands to reason that intervening on these causative factors – water, time, traffic and neglect – could prevent pothole formation and support the overall integrity of the pavement. But a breakdown of each shows the challenges present in trying to do that:

Water: This is from nature, but man manages the drainage. You can build more roads with crowning centers, and you could create better drainage at the low spot on underpasses. Most cost-effectively, this is in the original design of the road.

Time: We cannot bend the time-space continuum outside of science fiction. But we can track which roads are aging and prioritize them for preventive maintenance.

Traffic: There is little chance that any road in America will be less traveled in the future (excepting, of course, where some roads will be allowed to deteriorate past usability, as is happening in some rural areas). With a population expected to grow from the present 305 million people to 420 million in the next forty years, that means more cars going to work and more goods being transported to homes, stores, factories and distribution centers.

Neglect: This is the most controllable factor. And perhaps the one most lacking on our aging network of streets and highways. Federal funds for highway maintenance are around $90 billion per year, when estimates for the real need is closer to $225 billion to $340 billion per year. State, county and city budgets are not exactly taking up the slack, as many of those government bodies are facing the same budget shortfalls as seen on the federal level. And as transportation officials note, a dollar not spent on maintenance today multiplies several times over in road rebuilding a few years down the road. Better and more cost-effective repair methods and materials are in development, but no one speaks of a “silver bullet” solution short of having more money to work with.

So while the science of roads and potholes is relatively simple, the answer for keeping our roads intact is far from our grasp.

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