De-icing: There's a science to this, folks
When temperatures drop to 32 degrees or lower and there is precipitation, there's a good chance drivers will encounter slippery patches on metro roads.
When that happens, city, county and state road crews typically begin the process of de-icing. The places most likely to develop ice are shaded areas, bridges and overpasses because of circulating air both above and below the surface, which can cause the temperature to drop even more.
In Atlanta, for instance, streets are canvassed and icy spots are sanded and salted. In most cases the sand and salt mixture, once applied, is adequate, said Valerie Bell-Smith, a spokeswoman for the city's Department of Public Works.
The salt melts the ice and the sand serves as an aggregate to create traction.
But the salt also alters the chemistry of the ice, actually lowering its melting point.
Dr. E. Kirsten Peters, director of communications for the College of Sciences at Washington State University, said “pure” water freezes at 32 degrees. But water that isn’t pure -- like saltwater or rusty water -- has a lower freezing temperature.
“If you add salt to the ice on the sidewalk, you’re making salty water and it will have a lower freezing temperature,” she said. “If you get the melting temperature below the temperature outdoors, then you end up with salty water -- or slushy water -- on the sidewalk instead of solid ice.”
Some localities opt for something other than standard salt.
In addition to sodium chloride (salt), the Virginia Department of Transportation uses liquid magnesium chloride, calcium chloride and other chemicals for anti-icing and de-icing. According to the VDOT, anti-icing is the application of chemicals before a bond occurs between the snow and pavement. It's a preventive tool.
If the temperature drops below 20 degrees, salt may not be enough to melt snow and ice. If temperatures hold there or continue to drop, refreezing could occur. At that point, according to the VDOT, sand or another abrasive is put down to break up ice and increase traction.
Georgia's Department of Transportation uses a 3-to-1 mixture of stone or gravel and sodium chloride. At 23 degrees, the GDOT will add calcium chloride to the mix. But it also has begun testing liquid magnesium chloride in parts of the metro area.
