Every December I try to remember why I ever left California. Last week Minnesota had a series of snowstorms, followed by subzero (Fahrenheit) temperatures. When it’s really cold, the snow doesn’t crunch underfoot; it squeaks. My cats compete for space in front of their favorite heating vent. And state, county, and city road crews plow the roads clear and spread them with deicing salts.
Multiple snowplows work in tandem to clear a Minnesota highway.
What do deicing salts do?
Deicing salts work by lowering the freezing temperature, melting snow and ice. Each salt has a temperature range over which it’s effective. The Minnesota Department of Transportation (MnDOT) uses mainly sodium chloride and potassium acetate. When the pavement temperature drops below 15 °F (-9 °C), sodium chloride becomes much less effective, so MnDOT will add other chloride salts or use potassium acetate. When it’s really cold, they use sand for traction instead of salt.
For concrete subject to cycles of freezing and thawing, deicing salts make the exposure class more severe. The severity of freeze-thaw exposure ranges from F0 (no exposure) through F1 (freezing and thawing with limited exposure to water) and F2 (frequent exposure to water) to F3 (freezing and thawing with exposure to water and deicing chemicals). The salt may increase the number of freeze-thaw cycles the concrete experiences each winter. Liquid water can penetrate into the concrete, and salt that migrates with it will attract and hold water. These effects will increase the saturation of the concrete.
If it’s cold enough for the salty water to freeze, the thermal shock is more damaging to the concrete than the effects of freezing plain water. For this reason, concrete with too much supplementary cementitious material may be susceptible to salt scaling. So for F3 exposure, ACI 318 imposes maximum limits on how much you can use.
Chlorides act as catalysts for the corrosion of steel. The corrosion exposure class is C3, and you need to design reinforced concrete to resist both freeze-thaw cycling and corrosion.