ANTI-ICING
A relatively new weapon in the snowfighter’s arsenal in North America is anti-icing. But it has a long history of keeping European roads safe and passable.
Anti-icing measures take place before a precipitation event to prevent the formation or development of bonded snow and ice on the road surface. Research has shown that timely applications of anti-icing materials can cut the cost of maintaining a safe road surface by 90% over the cost of deicing. Anti-icing chemicals are applied in liquid form (brine) to road surfaces just before a snow or ice storm. Liquid sodium chloride (NaCl) is the most effective choice for anti-icing above 15°F.
Anti-icing has many advantagesProducts available for use in an anti-icing program are sodium chloride, calcium chloride, magnesium chloride, potassium acetate, and calcium magnesium acetate. Each product has its own advantages and disadvantages. The most common material in use is sodium chloride (salt) in the form of a brine made from a mixture of rock salt and water. Salt brine is effective to -6° F and is a proven anti-icing agent in use throughout the snowbelt.
Some agencies use calcium or magnesium chloride in a brine solution which is effective down to -60°F, but is more than six times as expensive than salt, and is more difficult to handle. Also, calcium and magnesium chloride residue on road surfaces can attract moisture at lower relative humidity than salt resulting in dangerous, slippery conditions under certain circumstances.
Salt Brine Manufacture
Salt brine is made by mixing rock salt or solar salt with water. The process is simple: the resulting brine should be approximately 23% NaCl.
The proportion of salt to water is critical to the effectiveness of the brine. Too much or too little salt affects the freeze point depressing qualities of the brine. The proper brine mixture is 23.3% salt content by weight. This is the concentration at which salt brine has the lowest freezing point, -6°F. It is known as the eutectic point. This percentage is measured with a salometer, a specialized hydrometer. Salt is added to the water until a 88.3% measurement on the salometer is obtained. This results in the proper 23.3% salt content. Temperature is also a factor, so the Salt Institute has developed a table showing the solubility of salt at various temperatures for your reference.
Commercial brine makers are available at a cost of approximately $4500. Many agencies have made their own brine makers using water tanks and PVC pipe for substantially lower cost. Brine is usually made at the local maintenance facility sites and stored in large tanks in locations convenient for loading into saddle tanks on the sides of the V-box or anti-icing equipment.
Application Equipment
Brine applicators are commercially available for about $1,000. Some agencies have manufactured their own application equipment using large tanks and PVC piping. Some equipment is designed to be loaded onto the bed of spreading trucks, towed behind maintenance equipment or permanently mounted on truck beds. It can be as simple as a gravity fed spraying system with a operator controlled cut-off valve or a more complex (and more controllable)
pump driven sprayer system. Control should be available to vary spreading rates from 25 to 60 gallons per lane mile.
If large, horizontal tanks are used in the design, consider installing baffles inside the tanks to help prevent the liquid from suddenly shifting in the tank, creating a hazardous control situation for the operator.
Application
Accurate
weather and road surface information are critical for the efficient use of anti-icing
chemicals. Road surface temperatures, precipitation amounts and form, wind conditions, and
road environment (sunlight exposure, surface condition, bridges, etc.) all affect the use
and application of anti-icing measures.
Understanding the freeze point depressing qualities of brine is important to its use and application as an anti-icing agent. (See the Phase diagram to the left). As you can see from the chart, the minimum freeze point of salt brine is -6 degrees F. at a concentration of 23.3%. Road surface temperatures are indicated on the side of the chart, solution concentrations along the bottom. The line represents the freeze point of the solution at a given temperature. The darker blue-colored portion in the center of the chart shows the melting range of brine solutions. The area to the left shows the results of a solution with too little salt; the road surface will refreeze unless more salt brine or deicing salt is applied. The area to the right shows the result of too high a salt concentration which, seemingly-paradoxically, will allow the road to refreeze. Weather information is getting better with the introduction of doppler radar reports, often distributed over the Internet or to subscribers of weather service providers. RWIS costs continue to drop as the technology becomes more frequently deployed. Everything from air temperature, dew point, optical weather identifiers, to pavement temperature, surface status, and chemical information is available. Some agencies utilize remote television cameras to monitor traffic and bridge conditions. This information will help agencies accurately determine the appropriate application of anti-icers.
Summary
Anti-icing measures are an important weapon in the snowfighter’s arsenal. The appropriate use of anti-icing techniques results in:
Returning to bare pavement conditions more quickly, saving lives and reducing property damage due to fewer accidents, as well as the reduction of traffic delays and the resulting reduction of losses to local economies;
Reduction in the quantity of deicer use, resulting in cost savings and less environmental concerns; and
Reduction in the manpower necessary to maintain safe road conditions, resulting in less overtime costs, less operator fatigue and safer working conditions.
For a good summary, see Wisconsin Transportation Bulletin No. 22.
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