Hugh Donovan’s professional passion is to extend the lifespan of Edmonton’s roadways by one heck of a lot before he retires.
And he and his staff at the City of Edmonton’s Engineering Services branch in the city’s west end appear well on the way to doing just that.
They deserve to take a bow – a really big one – because it looks like their work will save hundreds of millions of taxpayer dollars over coming decades…all because they’re finding out how to make pavement last longer.
Roads have a limited lifespan. They are designed to perform for a minimum of 20 years before rehabilitation, however performance can vary between 10 and maybe 20 years depending on how they’re built. Their relatively short lifespan makes them by far society’s most expensive infrastructure.
So it’s a big win when roads can be made stronger – Hugh has a million ways to describe ‘stronger’ but in this article we’ll just stick to stronger – and less susceptible to water absorption and the 120+ freeze-thaw cycles that Edmonton endures in an average year.
Hugh, the City’s general supervisor of construction services, explains that until about seven years ago, the City used something called the Marshall protocol in its mix design and testing lab, a specific kind of procedure to determine the best mix of asphalt ingredients for the intended use. The protocol involved hammering core samples 75 times at each end, followed by various performance tests.
“We also used a computer program that allowed us to input just five variables – intended maximum load, subgrade soil strength, material structural layer coefficients, and a drainage coefficient – to take a rough shot at predicting the lifespan of a roadway,” says Hugh.
The combination of the lab protocol and the predictive application resulted in the development of a specific asphalt mix of aggregate (sand and stones) and oil.
“But there was a problem. Core sample hammering didn’t duplicate what actually happens when pavement is laid down. In real life, the paver’s rollers vibrate as they roll. There’s no hammering.
“So we were producing mixes that weren’t designed based on reality. And we were using a computer life-projection application that was limited.”
The advent of new laboratory equipment that mimicked the rolling-vibration of real-life paving, and an incredibly powerful new analytical computer application, gave Hugh and his staff a huge boost in their ability to refine asphalt mix to be more appropriate for soil type, climate and many other factors.
The lab equipment enabled real-life tests on core samples of differing mixes. The new, amazing!, super-powerful computer application was capable of crunching more than 100 variables – ranging from the temperature every 15 minutes over a whole year, to the subsoil’s water-absorption factor, to number of hours of sunlight per day over the year and the expected types and weights of vehicles.
“With these added testing capabilities, we’ve field tested various mixes over a five-year period,” says Hugh. “We’ve developed new mixes using what’s called the SuperPave protocols. These mixes involve the use of additives that coat the aggregate, resisting the stripping away of the pavement’s oil binder by absorbed water.
“Washing away the oil weakens the asphalt, so the road breaks faster.”
When the City started more sophisticated testing several years ago, it discovered the Marshall mix lost as much as 58% of its strength after just one freeze-thaw cycle.
“That freaked us out,” says Hugh.
Superpave mixes lose less than 20% strength in one freeze-thaw cycle, and thus weaken far more slowly than the old pavement.
“There’s no doubt that the new mixes will extend roadway life,” says Hugh,” but we’re not yet able to predict exactly how much longer they’ll last.”
A side benefit of the new computer application is that the City can now more accurately determine how many years a substandard paving job has stolen from a road’s predicted lifespan.
“Now we can more effectively pursue the contractor for a partial rebate, or for a complete rebuild,” says Hugh.
Edmonton is the only municipality in Canada, and one of only nine jurisdictions in the country, using the application.