Deicing with salt
Plowing is the best way to remove snow and slush from pavements. However, clearing winter roads to bare pavement usually requires deicing chemicals, The most common chemical is salt (sodium chloride), usually mined rock salt that has been crushed, screened, and treated with an anti-caking agent.
Deicing chemicals work by lowering the freezing point of water. A 23.3% concentration of salt water will remain liquid to -6°F. Before a dry deicing chemical can act it must dissolve into a brine solution. It uses moisture from water, snow or slush on the road surface.
Changing ice or snow into water also requires heat from the air, the sun, or the pavement. Chemicals only lower the melting temperature; it takes heat to change ice to water. Even when pavement is below freezing, it holds some heat which can help melt snow and ice.
Factors affecting deicing action
Many factors affect the process of melting snow and ice. Decisions on how and when to apply materials are best left to field supervisors and operators who can assess conditions and adjust to changes.
Concentration If too much chemical is used, not all of it will dissolve into solution, and some will be wasted. Too little chemical may not sufficiently lower the solution’s freezing point. When salt is dissolved into brine on the road it is near 23% concentration and freezes at -6°F.
As snow and ice melt, the extra water dilutes the solution, raising its freezing point. For example, a 10% salt brine solution will stop melting ice and can refreeze at about 20°F, which may require more salt. The ice will not melt or melted snow may refreeze, wasting the chemical.
Temperature Salt’s effectiveness is directly related to the surface temperature of a snow- or ice covered road. As temperatures go down, the amount of deicer needed to melt a given quantity of ice increases significantly.
Figure 1 shows that salt can melt five times as much ice at 30°F as at 20°F. Small differences in pavement temperatures have a noticeable effect. Truck mounted temperature sensors give operators information to make better application decisions.
Time The longer a deicing chemical has to react, the greater the amount of melting. At temperatures above 20°F both salt and calcium chloride can melt ice in a reasonable time. However, at 10°F it takes an hour for salt to melt 1⁄8” of ice.
Amount of ice melted by one pound of salt Temperature °F Time
Weather When sun warms the pavement, the heat speeds up melting. Radiant heat may raise pavement temperature 10°F or more above the air temperature. On clear nights, pavements will be colder than the air. Use less chemical when temperatures are rising and more when they are falling.
Applying chemicals during blowing snow and cold temperatures will cause drifting snow to stick to the pavement. If chemicals are not used, the dry snow is more likely to blow off the cold road surface.
Surface type Snow and ice may melt more rapidly on a concrete surface because it gives up heat more rapidly. Because asphalt absorbs more solar radiation it may have more heat available for melting snow. This is why snow melts rapidly next to bare asphalt pavement areas.