State departments of transportation use rock salt, or sodium chloride, as a deicer to place on highways during winter snow and ice storms. The rock salt provides a positive effect for motorists by melting the ice and snow on the road. Before using rock salt, these departments must weigh its benefits against its long-term effects on the concrete roadway.
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Rock salt is a combination of sodium and chlorine. It possesses the ability to melt snow and ice at temperatures -6.67 degrees Celsius and above. Below that temperature, its effects diminish. Rock salt is easy to acquire and apply with an inexpensive cost. It has been in use extensively on highways in North Carolina since the mid-20th century.
Rock salt is a deicer --- a product that aids in melting snow and ice. Traditionally, highway departments remove the accumulated snow on top of the roadway and then apply rock salt. As traffic drives over the ice and snow, the rock salt aids in the melting process. It produces a clean and safe driving surface on concrete roadways with nothing to clean up. To counteract its problems at lower temperatures, highway departments add calcium chloride --- a substance that works much better below -6.67 degrees C.
Observations of highways where rock salt has been heavily applied have noted more deterioration than on roads with little rock salt use. As pointed out by the Iowa State University Institute for Transportation, too many environmental variables exist to make any scientific conclusions about the effects of rock salt on concrete. Roadways have different usage patterns, different amounts of rock salt applications, different temperatures and different types of concrete. To negate these factors, the Institute took concrete samples from different roadways and applied different deicer products, including rock salt, in a controlled manner. The experiment concluded that other substances, including calcium chloride, cause more concrete deterioration than rock salt.
The Iowa State experiment tested slabs of concrete under six different conditions, applying magnesium chloride, calcium chloride and rock to the slabs to determine what, if any, deterioration would occur. The study also used distilled water as a control. No change occurred from any chemical when the concrete was frozen at 21.1 degrees C below 0 degrees Celsius and then thawed. Each of the other five experiments found deterioration after the application of magnesium and calcium chloride. Rock salt only affected the concrete adversely in two of the five experiments; the study concluded that rock salt was the superior product in terms of preventing concrete deterioration.
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- Iowa State University: Concrete Deterioration By Deicing Salts: An Experimental Study; RD Cody, AM Cody, PG Spry, G Gan, May 1996
- Montana Department of Transportation: Brochures, Reports And Studies-Winter Maintenance White Paper (De-Icer Information); Dan Williams, Dec 31 2003
- Stormwater: Environmental Impacts Of Road Salts And Alternatives In The New York City Watershed; Janis Keating, May/June 2001