Cold, warm, then cold again: How Montreal's unseasonable weather is taking a toll on its infrastructure

On Feb. 7, frozen streets thawed and icicles began to drip as temperatures climbed from –15 C to 2 C.

On Feb. 10, slush-covered roads hardened to ice as the temperature dove from 5 C to –8 C. 

And on Monday, rain turned to sleet then to snow as temperatures plunged more than 10 degrees, from 4 C to –14 C in less than 24 hours.

Temperatures in Montreal have oscillated on either side of the freezing mark at least a dozen times so far this winter, according to a CBC review of weather records. 

The dipping and rising of the mercury is bringing mild weather to the city followed by bitter cold spells, but rapid cooling and warming cycles also stress concrete and asphalt infrastructure, leading to more cracks in overpasses and potholes on roads. 

"This year the number of freeze-thaw cycles is more frequent, particularly in the [Montreal] metropolitan area, with temperatures rising more often above the freezing mark," said André Cantin, a meteorologist with Environment and Climate Change Canada (ECCC). 

January was a particularly mild month, Cantin said, with the average temperature five degrees higher than normal. Even February has so far been mild, Cantin said, aside from a two-day stretch of near record-breaking cold.

When temperatures rise in winter, ice and snow melt, sending water seeping into crevices in asphalt roads, concrete overpasses and conduits. When temperatures then fall below freezing, some of that water freezes — expanding as it changes state, creating cracks and fissures. 

Engineers have long known that freeze-thaw cycles degrade concrete and lead to potholes in roads. It's partly why infrastructure has a service life that takes into account the number of freeze-thaw cycles the material can endure. 

Cantin said that the fluctuations between warm and cold, mild and frigid, cannot all be blamed on climate change, but winters like this one are likely to become more common as global average temperatures climb.

Montreal researchers recently used data from satellites to help them track when the soil in Quebec is frozen and when it isn't so they can understand how climate change is affecting freeze-thaw cycles. They predict that as global temperatures rise, the ground will thaw and refreeze more frequently in the Montreal area — possibly even twice as often. 

This will disturb soil conditions and stress infrastructure, leading to earlier failures in water mains and roads, according to Ali Nazemi, an associate professor in Concordia's department of building, civil, and environmental engineering and one of the study's co-authors. 

"If it is frozen in the night, and is unfrozen in the morning, that actually creates pressure in the soil that can be devastating," Nazemi said. 

Nazemi said older infrastructure that is nearing the end of its service life is particularly susceptible to the wear of the freeze-thaw cycle.