January 22, 2008
USE OF RE-WETTED SALT ON ROADS AND BRIDGES
By: Kevin E. McCarthy, Principal Analyst
You asked for a description of the use of “re-wetted salt” for snow and ice removal in other New England states. You also asked whether any of these states have studied the effect of rewetted salt on roads and bridges.
Re-wetted salt is a mixture of rock salt (sodium chloride) and an additive, most commonly calcium chloride. The calcium chloride can be added in liquid or solid form. The solid form of calcium chloride is hygroscopic, i.e., it can absorb and retain water. It also produces an exothermic (heat-producing) reaction when it absorbs water. As a result, a mixture of rock salt and calcium chloride melts up to eight times as much ice as using rock salt alone at 20 degrees. The calcium chloride breaks down the rock salt cubes more quickly so that they adhere better to road surfaces and also prevents caking of salt piles.
All of the New England states use rewetted salt to some extent, but they use calcium chloride and other alternatives to rock salt sparingly, since they are much more expensive than rock salt.
None of the New England states have studied the effect or rewetted salt on roads and bridges. Sodium chloride, calcium chloride, and magnesium chloride (another deicing agent) are all salts that corrode steel and other metals. A 1993 study prepared for the Michigan Department of Transportation found that calcium chloride is 36% less corrosive of bridge decks and other highway structures than rock salt. The study, which is available online at http://www.michigan.gov/documents/ch3-deice_51440_7.pdf, also reviews the environmental effects of various deicing agents.
Maine primarily uses rock salt, salt brine, and sand for snow and ice removal. It has used liquid calcium chloride in limited quantities to accelerate the melting process for about the past ten years. In recent years, it has used magnesium chloride more often than calcium chloride.
Massachusetts primarily uses re-wetted salt near bodies of water and other “reduced salt areas” and when the temperature falls below 20 degrees. The Massachusetts Highways Department has a website describing the various materials it uses to clear snow and ice off of highways, http://www.mhd.state.ma.us.
New Hampshire has used re-wetted salt for at least ten years, but uses it very sparingly. This year, for example, it has used 11 tons of rock salt per lane mile, but only 12 gallons (19 pounds) of liquid calcium chloride per lane mile. The state tracks the use of salt by Transportation Department district. It also monitors salt levels in wells located near state highways.
The Rhode Island Department of Transportation has used re-wetted salt for approximately two years. It does not have information on the effect of the salt on roads and bridges. However, it has found that the salt has accelerated corrosion of the department's snow plows, even though the salt has an anti-corrosion additive.
The Vermont Agency of Transportation primarily uses rock salt, but does use re-wetted salt at times, particularly when the temperature falls below 20 degrees. It routinely washes its bridges to minimize corrosion.