Prewetting is the process of spraying deicing salt with a solution of liquid chemical before spreading the salt on the roadway. Prewetting the salt helps it work more effectively as a deicing agent for two reasons: First, wet salt clings to the road instead of bouncing off or being swept off by traffic. The result is that less salt is spread, saving money and minimizing the threat to the environment. Second, to be effective as a deicing agent, salt requires moisture. Moisture dissolves the salt, releasing heat and thereby melting the ice and snow, as well as breaking the ice-road bond. When temperatures drop below freezing there is no moisture on the road, and salt alone is ineffective. Prewetting the salt, however, ensures that there will be enough moisture to facilitate the melting process. Then, prewetted salt works faster and at lower temperature than does dry salt, with less waste. Sodium chloride (salt) brine is a low-cost, effective alternative to liquid calcium chloride as a prewetting agent.
Is prewetting the wave of the future or just another passing fancy? In Iowa we are willing to bet on prewetting with salt brine as the method of our future for removing snowpack and ice on our roadways. For years maintenance supervisors at a few garages were experimenting with ways to keep more of the salt and sand mixture on the roadway. Several modifications were made to the spreader equipment, and a special chute was developed to drop the salt/sand mixture closer to the pavement to reduce the amount of materials lost through bounce, wind, and traffic. A few supervisors had begun wetting their loads with water hoping that this might help keep the materials on the roadway but were quiet about any successes for fear the water would freeze the load or add additional ice to the existing poor road conditions. Many of the modifications over the years proved to be great successes, but we knew there had to be a better way to keep the material on the roadway surface to speed the removal of snowpack and ice.
Despite attempts with bigger and better equipment to improve the way we removed snowpack and ice, our operation changed very little over the past 30 years. Our policy stated that 50/50 (half salt and half sand) would be applied at the rate of 300 pounds per lane mile to snowpack and ice. Basic snowpack and ice removal operations had changed little during this time despite the rapid improvements in other equipment.
SHRP publications (1) provided several studies on prewetting applications that had been conducted in Europe in the 1960s and early 1970s that appeared to be working to keep materials on the roadway surfaces. Further study of the properties of salt gave us an even better picture of how and why prewetting works and why prewetting with liquid brine can work even better. Prewetting is the process of spraying salt or other deicing chemicals with a solution right before spreading on the roadway.
WHY PREWET WITH SALT BRINE?
Sodium chloride has been selected as the prewetting agent of choice in Iowa primarily because of its low cost and ready availability, but the real benefits from prewetting occur when the brine is mixed with the dry materials. Sodium chloride must find moisture from some source before it can generate the heat needed to begin the melting process. Under past operations dry sodium chloride could not generate heat until it came into direct contact with the snow or ice on the surface. As the sodium chloride found the moisture in the snow or ice it began to generate heat which melted more snow and ice, accelerating the melting process. During snow storms with cold air temperatures and low humidity, sodium chloride had difficulty finding available moisture, which slowed the melting process considerably. Adding sodium chloride brine to dry sodium chloride at the spinner before it struck the road surface provided extra moisture to quicken the melting process.
Calcium chloride, on the other hand, is hygroscopic, which means that it can take moisture freely from the air so that even before the material strikes the snow or ice covered roadway it is generating heat and ready to melt on contact with the surface. Since calcium chloride is generating heat before contact with the surface, it is capable of melting snow and ice at a faster rate than dry sodium chloride. Calcium chloride brine would work very similar to sodium chloride brine used in Iowa, but because of the higher cost of calcium chloride and the residue that is left on the pavement, it has not been considered for mass use in Iowa for several years. The eutectic point of calcium chloride is much lower than sodium chloride, but in the past we have found that calcium chloride leaves the pavement wet for several days, where the pavement de-iced with sodium chloride is dry shortly after the storm. Under blowing snow conditions we have found that snow will stick to the wet surface, quickly refreezing the roadway surface.
Studies conducted in Germany and Finland used a brine solution to prewet their sodium chloride and experienced success keeping more of the materials on the road surface than with dry materials. One study in Germany measured the amount of materials left on the road surface after 100 vehicles passed the test site and found that nearly 30 percent of the prewetted materials stayed on the road surface, while only five percent of the dry material could be found. The studies done in other countries also confirmed that prewetted materials adhere better to the roadway and the added moisture causes the sodium chloride to begin working immediately on contact with the surface.
Word was spreading in the state that prewetting seemed to be a method worth trying. The garages in Council Bluffs, Adair, and Des Moines found plans on how to make brine-making equipment from common materials found at a local hardware or farm equipment store. Cattle water tanks were used to make the brine along with lots of PVC pipe, pumps, and a large enough tank to store brine for an operation (Figure 1). In one garage the manufactured brine was pumped into an eight-foot by 16-inch drainage pipe that had been capped at both ends and mounted directly behind the cab. A pump was then used to pump the brine through hoses to the rear of the truck where it was then directed onto the material at the spreader. At the other garages similar brine manufacturing equipment was made and special containers were mounted on the tailgate where the brine was fed by gravity onto the dry materials at the spinner.
ELECTRIC LIQUID SPRAY SYSTEM
In the spring of 1995, Swenson Spreader of Lindenwood, Illinois, demonstrated an electric sprayer system with either 60- or 100-gallon polyethylene tanks that could be mounted to our trucks either on the tailgate or along the V-box. The sprayer system is equipped with a variable speed control system that allows the operator control of the amount of brine directed onto the dry materials at the spinner. The electric motor/pump combination is operated from standard 12-volt D.C. system with a rating of 2.8 gallons per minute. Brine is pumped from the polyethylene tank through hoses capped with rubber fittings that discharge the brine onto the spinner at controlled speeds. The original sprayer design called for brass nozzles in place of the rubber fittings but experiences in other states with clogging moved us toward the rubber fittings.
At the same time we met with VariTech Industries of Garfield, Minnesota, who designed a brine-making system that produces 600 gallons of liquid brine at 23.3 percent solution in less than an hour (Figure 2). Dry salt is loaded into the top of the tank and water is pumped through the salt from below. Once the loading area has filled, water will begin overflowing the tank through four-inch diameter screened holes into the main storage tank below. At this point the brine is tested with a hydrometer to determine the salinity level of the solution. If the brine needs adjustment, a separate fresh water valve can be opened to mix fresh water with the brine bringing the solution to the 23.3 percent optimum. The system is also equipped with an internal float that shuts off incoming water once the lower tank is full. At this point the brine is ready to be pumped to the tailgate spreaders through a 55-gallon-per-minute pump system and 1H inch discharge hose equipped with a quick disconnect coupling. The system also comes equipped with a remote control for pumping the brine into the tanks which makes filling quick and easy. The minimum purity of rock salt is 95 percent; therefore, the brine makers have to be cleaned out four or five times each year. Another important part is to make sure all screens are in place as the impurities will plug up the system.
Leland Smithson had returned from a European and Asian tour of snow and ice operations and was convinced that prewetting was a proven method to remove snow and ice. He and I were convinced that prewetting was the way to go and recommended to management that the Maintenance Division purchase brine-making and spreader equipment for use on all Interstate highways in the state. The order was placed to equip the 30 Interstate garages with one brine making plant and liquid applicators for the Interstate trucks.
Since this equipment is not mass produced in great numbers the equipment arrived at many Interstate garages after the first storms of winter had begun. The spreader equipment also arrived later than we had hoped but by mid winter all Interstate garages had their brine making and spreader equipment in place and actively in use.
Though the equipment has been in place less than one winter supervisors and equipment operators are reporting good results from the use of materials prewetted with brine. The success at Interstate garages has spilled over to other garages that are now manufacturing their own brine plants and creating sprayer equipment from available parts. There have been 15 additional shop-made brine plants and 105 shop-made liquid applicators added to the initial Interstate buy this past winter.
We are pleased with the results this winter and have found the following benefits to using prewetted materials:
Materials readily adhere to the roadway better than with dry materials.
Prewetted materials begin dissolving snowpack and ice faster than dry materials.
The quantity of materials can be reduced when prewetted, since less materials leave the roadway during spreading.
Corrosion of equipment is minimized by spraying the liquid salt at the spinner.
Operators report quicker melting with the prewetting process, which means better service to the traveling public.
FUTURE OF PREWETTING IN IOWA
The Iowa Department of Transportation plans to expand prewetting to other garages in the future but recognizes that there are several lingering questions that need to be answered before we declare prewetting successful in Iowa. The use of finer salt works faster on the roadway, but is it porous enough to work in the present brine makers? Are equipment operators and supervisors aware of conditions that prewetted salt should not be used? Can we make brine and store brine in large quantities before the winter season? Should brine be stored indoors or underground? Can we reduce the manpower and time needed to make brine? Can we transition to anti-icing with this system smoothly?
Though we face challenges in the future with prewetting, we feel that this method works in Iowa and will become a standard operating practice as we continue our expansion and education.
Thomas J. Donahey and Dennis Burkheimer
Iowa Department of Transportation, Maintenance Programs Office, 800 Lincoln Way, Ames, Iowa 50010.
1. R.R. Blackburn, E.J. McGrane, C.C. Chappelow, D.W. Harwood, E.J. Fleege. Development of Anti-Icing Technology. SHRP-H-385. Strategic Highway Research Program, National Research Council, 1994.