SUDBURY -- A spike in freeze-thaw cycles is what led to the surge in pothole complaints in 2019, politicians in Greater Sudbury were told this week.

City crews filled about 99,275 potholes last year, members of the operations committee heard, compared with 55,000 they normally fill in an average year. Pothole complaints jumped by 204 per cent in 2019, also compared to the five-year average.

March 2019 was particularly difficult, with almost double the normal freeze-thaw cycles, which were also substantially higher in April 2019.

Miranda Edwards, who is the project manager for the pothole repair study Sudbury is currently working on, said when spring temperatures warm up, melting snow gets into cracks in the pavement. When it freezes again, it expands the cracks.

"When water enters the cracks in the pavement, it's trapped in between the asphalt and the frozen sub-base. So as the temperatures go below freezing overnight, this water freezes and expands and expands the asphalt," said Edwards.

Potholes form as this process is repeated, getting worse each time. Such was the case in spring of last year.

"In 2019, we used 37 per cent more material compared to the average of the material used in the previous two years," Edwards said.

"The average amount of calls in that five-year period March of 2013 to 2018 was 475 calls regarding potholes. In March of 2019, we received 1,445 pothole-related calls."

That corresponds with data from Environment Canada, which said there was a large increase in freeze-thaw cycles in March and April last year. In 2020, when those numbers dropped, so too did the number of pothole complaints.

"So while there are many factors that lead to asphalt cracking and the formation of potholes … one of the most common you'll see is during freeze-thaw cycles," said Edwards.

When repairing potholes, crews in Sudbury use two methods: one is known as "throw and roll," and is used for emergency repairs during bad weather. The other is "detailed patching," which is done in good weather and are designed to be semi-permanent.

As part of the pothole repair study, which began in April 2019, city staff are evaluating the performance of different repair materials and techniques to see which work best under various conditions.

"It's a multi-phase study with the intent of assessing performance of pothole repair for material performance, as well as preparation method in different weather conditions," Edwards said. "We compared seven different material types."

Each type of pothole repair material was evaluated in terms of how long the repair lasts, how much it costs to buy, and how labour intensive it is to actually fill the pothole.

"So what we're looking for is the lowest cost with the best performance," she said.

As part of the study, they aim to determine which repair mixes are best for different conditions – wet or cold weather, hot weather, condition of the asphalt, etc.

"Moving forward, we can begin to work on the next phase of the study. Part of that will be to identify the parameters of our test area, as well as identify the different types of pothole repair techniques and pothole repair equipment that we would like to evaluate to the next phase," she said.

Local colleges have contacted them to participate in the study, Edwards said, and they are exploring that possibility now.