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Potholes revisited

I hope this doesn’t come as a shock to anyone, but Regina has a seasonal problem with potholes. But there may be hope for our pothole problem, and similar problems all over the world, thanks to the work of two University of Washington State University civil engineering professors.

Dr. Thomas Papgiannakis and Dr. Eyad Masad are carefully studying how and why potholes form. Their goal is to help engineers custom-design asphalt to suit the particular ground and climate conditions where a road is built.

In Regina, roads are built on a base 50 to 80 centimetres thick, depending on the traffic load. The bottom layer is clean sand, which aids drainage. Over that goes a layer of dirty sand, then a layer of coarser aggregate, and finally the asphalt. (The city has only been using this “deep granular base” for 15 years or so; older streets were generally built on a concrete base only about 30 centimetres thick, which makes them more susceptible to potholes.)

Regina has a problem with potholes because our roads are built on thick, gooey clay, which traps moisture–and moisture under the road surface is what leads to potholes.

Every pothole begins as a crack. Old pavement cracks because it’s dry and brittle; younger pavement cracks because of the expansion and contraction of the asphalt as the temperature changes.

The first cracks appear in new pavement after about two years. Some run parallel to the roadway, but others, about every 30 metres, run across it. After another couple of years, a second set of cracks shows up, every 15 metres. This process, called “fissuring” or “crocodiling,” continues until somebody seals the cracks–or the pavement breaks up completely.

Crack sealing keeps out moisture, extending the life of the pavement about five years. (New pavement is good for about 18 to 20 years, if properly maintained.) If you can’t keep water out–and sometimes you can’t, depending on when the crack occurs and how successful the seal is–it builds up under the pavement, where the underlying clay helps trap it. If this happens when it’s freezing at night and thawing during the day, it’s bad news. The water expands as it freezes, heaving the pavement up and widening the crack; then thaws, letting in more water, then freezes, pushing everything apart again, then thaws, etc. Water from the underlying clay is also drawn up into the area where freezing is occurring, adding to the problem.

Sometimes there’s more moisture on one side of a crack than the other, which raises one side higher and leaves it less supported underneath. Drive a few cars over it, and eventually a chunk will break way: presto, a pothole! More freezing and thawing and a few more cars, and it can become a veritable canyon.

(By the way, there are technical terms for what cars do to pavement as they drive over it. “Shoving” is the pushing forward of pavement by the wheels of vehicles that brake at traffic lights and stop signs. “Rutting” is the formation of parallel grooves in road lanes, caused by the weight of vehicles.)

We could prevent many potholes by banning cars from the roads during spring thaw, but that’s not very practical in the city. Instead, Dr. Papagiannakis and Dr. Masad suggest, engineers need to tweak their asphalt mix. For instance, they say, you can prevent “shoving” by making sure the rocks in the mix are angular rather than round, and are tightly bound with liquid asphalt. Similarly, extra-strong asphalt can help keep pavement from cracking in freeze-thaw conditions.

The two engineers are promoters of a new approach to designing asphalt mixes called Superpave, which they consider to be the biggest advance in the field since the late 1940s. They’re now heading up the Washington Center for Asphalt Technology, which opened February 1. Its equipment allows them to test asphalt core samples from roads that developed potholes and other problems for microscopic clues to what went wrong, to simulate 10 years of road wear in 24 hours, and even to test asphalt in temperature extremes from 180 degrees Fahrenheit to -30.

Superpave is becoming more and more popular in the States, and while costs for the first Superpave projects were higher than those of traditional methods, the cost difference shrinks to nearly nothing with experience.

Here’s hoping the work of Papagiannakis and Masad, and other brave asphalt researchers, eventually leads to smoother streets right here in Regina.

In the meantime–new shock absorbers, anyone?

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