Salt, extreme cold don't mix on roads
CHAMPAIGN — With extremely low temperatures immediately following this past weekend's snowstorm, it'll take a little longer this time to clear the roads.
Why? The answer is in the chemistry of road salt.
"It's just too cold," said Walt Kelly, the head of the groundwater section for the Illinois State Water Survey.
OK, so there's a little more to it than that, but that's the general idea. City snow crews in Champaign and Urbana only started laying down salt on roads Tuesday afternoon. Before that, it would have been inefficient and, in some cases, could have make the roads even slicker. It's all because of the subzero temperatures.
Kelly said common salt, sodium chloride, is effective to temperatures around 20 degrees above zero. The salt dissolves with the snow, adding impurities to the water. When you do that, it lowers the freezing point of the water on the road.
"It's increasing the dissolved materials that are in the water," Kelly said. "Like if you took sea water into the lab and put it in the freezer, it would be harder to freeze that than pure water."
But given the windy conditions this week and the ineffectiveness of salt in extreme cold, it would have been a waste to lay it down.
"If we were using salt, because of the heavy plowing, we would be scooping it off the road almost as quickly as we would be laying it down," said public works spokesman Kris Koester.
Colder than 20 degrees, and you'll need more serious chemicals because sodium chloride is not effective. The city of Champaign uses calcium chloride, another kind of salt that will melt snow in temperatures as low as 10 below zero, Koester said.
But just because it can melt the snow doesn't mean it's safe to do so. Koester said that, in the case of this storm, the calcium chloride could have melted the top layer of snow, but would have been diluted by the layer underneath. All that would have happened is that the melted water would have re-frozen when the water-salt mixture diluted — leaving a clean sheet of ice on the road.
And when blowing, dry snow hits the salt, it can glaze over and freeze very quickly.
But enough with the chemistry — what drivers really want to know is when road conditions will get better. That's still kind of hard to say — temperatures rose to a blistering 9 degrees late Tuesday afternoon, and bare pavement started becoming visible in some spots where the salt started working.
Urbana started laying down salt on intersections on Tuesday afternoon, said public works operations manager John Collins.
But "we're not going to get too carried away," Collins said. Another inch or two of snow is expected Wednesday night. By Friday, temperatures are forecast to be above freezing. The roads could be down to bare pavement by then, but no guarantees.
"I'm hoping the sun will continue to shine and temperatures will continue to rise," Collins said.
This winter has been somewhat taxing on Champaign and Urbana's salt reserves.
Champaign public works spokesman Kris Koester said the city has used 2,040 tons of salt through Tuesday. The city has another 1,000 tons on the way and plans to order 1,000 more tons this week.
Urbana public works operations manager John Collins estimates that his city has used more than 600 tons of salt so far. It had 1,200 tons on hand at the beginning of the year and has another 1,200 on the way.
Both cities are paying $57.41 per ton for the salt.