Filling Potholes: It Boils Down to ‘Gut Feel’

Edmontonians are cursed with hundreds of thousands of potholes every year, and to the folks who fix them, it seems like there are the same number of salespeople hawking the perfect repair product.

“We’re constantly approached by salespeople from companies from all over the world, and every one of them claims their company makes the last word in pothole repair compound,” says Engineering Services’ research engineer Faizal Kanji.

“They claim their product works well in an Edmonton-like climate, even if it’s made in Texas where temperatures are quite warmer than in Edmonton, or in Scandinavia, where humidity’s high and temperatures are moderated by the ocean effect.

“Everyone makes claims, but no one offers performance proofs and solid guarantees,” he says. “What does it really mean when they claim their product is a ‘guaranteed temporary patch’?”

Speak with Faizal and his boss, Hugh Donovan, and you soon lose your preconception that pothole-filling is simple, and you’ll see why Engineering Services does its own studies on pothole compounds.

There are three ways City crews approach a pothole. They can ‘throw-and-go’, which means they fill it with asphalt then drive to the next hole and let traffic compact it. They can ‘roll-and-go’, meaning the crew does the compaction. Or they can give it the ultimate treatment by cutting straight edges around the hole, ‘painting in’ a kind of asphalt cement as a “glue”, then filling and compacting it.

“Each technique takes progressively more time and that’s a huge consideration given we have limited staff and hundreds of thousands of holes,” says Hugh. He says crews have only about eight minutes to spend on each hole.

As to the filling compound that they use, that’s even more complicated than the physical application method.

Typical hot mix asphalt is composed of gravel, sand, oil, and an ‘antistrip’ which prevents the oil from peeling off the gravel. Pothole-filling asphalts have all of the above but also have an additional additive, some kind of an emulsifying agent that keeps the material pliable until it’s packed into the pothole, and which then evaporates over time, leaving the asphalt much more rigid.

Engineers use different combinations of each component for different conditions – warm weather, cold weather, dry or wet pothole – and on different kinds of roads, varying from residential to major arterials. Asphalt can also be applied cold, or heated…again depending on various factors.

“Lots of companies approach us with their ‘magic answer’, which requires use of their proprietary product to mix with gravel, sand and oil,” says Faizal.

Many of these products are ‘epic fails’ in Edmonton’s climate; so Engineering Services has created a test strip along 111 Avenue between the Henday and 184 Street, where various proprietary products are being tested on 36 holes cut in the pavement.

Some of them suffer what’s called ‘pushing-and-shoving’, which means they ‘squish’ a little under vehicle weight, slowly compressing the material or pushing it out of the hole it’s supposed to fill. Others have shrunk and become rigid in our winter cold, causing the fill to literally pop out of the hole when a heavy truck drives over it.

Faizal says there are so many differing factors that go into making asphalt fill that it’s virtually impossible to create a true, objective scientific comparison of their performance against each other.

“What we’re doing with the pothole test strip is developing a ‘gut feel’ for the best products. We’ll test each product for two winters and two summers, then determine which products will give the taxpayer the best cost-effectiveness.”

Edmonton’s unfortunate pothole reality is generated by the more than 120 freeze-thaw cycles we experience during our winters and the spring. Melt water fills cracks in the asphalt, then freezes and expands, widening the crack. Once the crack is big enough, vehicles passing over it break off pieces of the road, and, voila! a pothole!

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