Road Management Journal
December 1, 1997
Fax: (360) 335-6402
|(This information is reproduced with permission from "Wisconsin Transportation Bulletin No. 6: Using Salt and Sand for Winter Road Maintenance" (revised March 1996 © 1996, Wisconsin Transportation Center). The Wisconsin Transportation Bulletins are a series of fact sheets on street and highway design, construction, maintenance, and management.)|
To make winter roads passable, highway personnel usually must either apply chemical de-icers to melt ice and snow or spread sand to provide traction. Since chemicals and sand are costly and may have negative environmental impacts, you need to know how they work. This publication gives you basic information and practical tips on using de- icing chemicals and sand.
DE-ICING WITH CHEMICALS
Clearing winter roads to the bare pavement usually requires de-icing chemicals. In Wisconsin the most common chemical is salt (sodium chloride) which usually comes from mined rock salt that has been crushed, screened, and treated with an anti-caking agent. De-icing salt is relatively light--just over one ton per cubic yard--and comes as a mixture of three-eighths inch granules to fine crystals.
Another commonly used chemical, calcium chloride, comes from natural brines. It comes dry in pellets or flakes, or in solutions of various concentrations.
Research continues on alternative de-icing chemicals. Calcium magnesium acetate (CMA) is being produced and has few of the negative environmental impacts associated with salt and calcium chloride. Additives to reduce chemicals' corrosive properties are also being used. Currently these alternative materials are more expensive, but can be useful in special situations.
De-icing chemicals work by lowering the freezing point of water. A 23.3% concentration of salt water freezes at -6o F and a 29.8% solution of calcium chloride freezes at -67o F. These low freezing points are what makes salt and calcium chloride useful.
Before a dry de-icing chemical can act it must dissolve into a brine solution. The necessary moisture can come from snow on the road surface or from water vapor in the air (humidity). Calcium chloride has the ability to attract moisture directly from the air.
Changing ice or snow into water requires heat from the air, the sun, the pavement, or traffic friction. Even when the pavement is below freezing, it holds some heat and can help melt snow and ice.
Factors affecting de-icing action
Chemical concentration, time, pavement temperatures, weather conditions, type of road surface, topography, traffic volume, width of application, and, most importantly, time of chemical application all affect the process of melting snow and ice.
Concentration If too much chemical is used, not all of it will dissolve into solution and some will be wasted. Too little chemical may not sufficiently lower the solution's freezing point. The ice will not melt or melted snow may refreeze and waste the chemical. See "Spreading rates" for recommended concentrations.
Temperature The surface temperature of a snow- or ice-covered road determines de- icing chemical amounts and melting rates. As temperatures go down, the amount of de-icer needed to melt a given quantity of ice increases significantly. The graph [below] shows that salt can melt five times as much ice at 30o F as at 20o F. The effectiveness of de-icing is sensitive to small differences in pavement temperatures.
Time The longer a de-icing chemical has to react, the greater the amount of melting (see graph). At temperatures above 20o F both salt and calcium chloride can melt ice in a reasonable time. At lower temperatures salt takes much longer.
|The graph on the left shows that salt melts more ice per pound at higher temperatures. The graph at right shows the comparative time for different compounds to melt 1/8" of glare ice.|
Weather The sun's heat warms the pavement, speeding up melting. Radiant heat may cause the pavement temperature to rise 10o F or more above the air temperature. On clear nights, pavement temperatures will be lower than air temperatures. Use less chemical when temperatures are rising and more when they are falling.
Applying chemicals during blowing snow and cold temperatures will cause drifting snow to stick to the pavement. If chemicals are not used, the dry snow is likely to blow off the cold road surface.
Road surface type Snow and ice melt more rapidly on a concrete surface because it gives up heat more rapidly. Because asphalt absorbs more solar radiation it may have more heat available for melting snow. This is why snow melts rapidly next to bare asphalt pavement areas.
Topography Ice tends to form where topographic conditions, like high banks or vegetation, screen the road surface from the sun. The longer the area is shaded, the more likely that ice will form. Since pavement temperatures are lower in shaded areas, you may need more chemicals there.
Application width Studies show that snow melts faster when salt is applied in narrow strips. The amount of snow melted over a long period of time is the same, however, regardless of application width. If you concentrate spreading (windrowing), you can expose a portion of road surface to the sun quickly. It can then absorb heat and increase the melting rate.
After a road is first plowed, de-icing chemicals are usually applied in a windrow two to four feet wide down the middle of a two-lane road. To remove glare ice or keep snow in a plowable condition, you may want to apply chemicals across a broader portion of the road.
Time of Application Timing is the most important factor in successfully clearing snow by chemical treatment. Early application is critical. Spreading a small amount of de- icer when snow is loose and unpacked melts a little snow and turns the rest to slush. Traffic cannot pack down this slushy snow which is 15% to 30% water. This lets plows remove it easily.
It is better to reapply chemicals as needed than to over-treat initially. Do not plow off the chemical until it has a chance to melt the snow and ice.
A major concern in using chemicals for winter road maintenance is environmental impact. Studies show that soils, vegetation, water, highway facilities, and vehicles are all affected, so it is very important to use chemicals wisely. Most soil and vegetation damage occurs within 60 feet of the road and is greatest close to the pavement.
De-icing chemicals are highly soluble and follow any water flow. Salt concentrations in Wisconsin's surface and ground water have increased since the early 1960's, the [Wisconsin] Department of Natural Resources [WDNR] reports, but aquatic life has not yet been affected that we know of. In drinking water sources, which the WDNR also monitors, salt concentrations are within recognized safe limits. In some reported cases, groundwater carrying de-icing chemicals has contaminated wells, but most of these apparently were caused by seepage from poor storage facilities.
De-icing chemicals can accelerate deterioration in concrete and steel structures. New construction methods are reducing this impact, but highways and bridges do suffer from chemical damage. Vehicle corrosion is also accelerated. Corrosion on vehicles and structures is estimated to be the largest cost impact of chloride based chemicals. Even relatively small amounts of chloride will significantly accelerate existing corrosion.
Localized environmental damage from salt has come largely from stockpile runoff. Since runoff is at maximum concentration, any exposed environmental element receives a very large dose. For that reason, you must prevent stockpile runoff from contaminating ground or surface water by covering the salt and storing it on an asphalt base so rain and melt runoff can't seep in. State regulations require highway agencies to store salt inside a covered, waterproof structure. When this is not possible, stockpiles must be covered with waterproof material and stored on a impervious pad. The facility must be reported to the Wisconsin Department of Transportation.
Spreading rates No two storms are alike, so no single set of standards will give the proper spreading rate for all storm conditions. Generally, however, only apply enough chemical de-icer to permit plows to remove the snow or melt glare ice. Experience shows that it is most effective to spread between 100 and 300 pounds per single lane mile. Do not use any de-icer when temperatures are below its effective range. Normally, 15o to 20o F is considered the lower limit for salt. If de-icing is necessary at lower temperatures, more salt is needed and melting will take much longer. Other chemicals such as calcium chloride and magnesium chloride may be a better choice.
Because melting action spreads across the pavement to lower areas, concentrate de- icers on the center (crown) of two-lane roads and on the high side of curves.
Chute vs. Spreader A spreader with a spinner is the most common way of applying de-icers. A spinning circular plate throws the de-icer out in a semi-circle. Alternatively, a chute can distribute de-icer in a windrow on the road, usually on the centerline.
Spreaders can be equipped with automatic or ground-oriented controls. They automatically regulate application rates as truck speeds fluctuate, so the driver need not adjust the spreader controls. They are proving effective in reducing waste chemicals.
Spreader calibration Calibration is essential for controlling application rates. Different materials will spread at different rates at the same spreader control setting, so you must calibrate spreaders with the material you intend to use. Each spreader must be calibrated separately; even individual spreaders of the same model can vary widely in the amount of material they spread at the same control setting. Furthermore, spreaders operate in a very hostile environment--low temperature, lots of moisture, corrosive chemicals--so, they need to be cleaned and checked every year.
Dry calcium chloride (CaCl) requires special handling and is more costly than salt. However it is effective at temperatures below 0o F and is fast-acting. CaCl actually gives off heat when it dissolves into brine--a very beneficial characteristic. It also draws moisture from the air, providing water for initial brine formation. These unique properties make it valuable in severe conditions.
CaCl is usually stored in moisture proof bags until needed. Otherwise its ability to draw moisture can cause it to cake and form into large chunks.
A mixture of calcium chloride and salt can be very effective. Even a small amount of calcium chloride will start melting at low temperatures. The resulting brine and heat allow the salt to start working. The graph [above] shows how well a mixture (three parts salt to one part calcium chloride) works at lower temperatures.
Pre-wetting salt has become common. Wetting provides moisture to make brine. Faster melting action may be expected. In addition, the wet salt has less tendency to bounce or be blown off the road by traffic. Savings in lost or wasted salt of over 20% to 30% are possible.
While any liquid de-icing chemical can be used to pre-wet, liquid calcium chloride is used widely. Applications of 6-10 gallons per cubic yard of salt are recommended. Calcium chloride has the added advantage of producing extra melting due to its effectiveness.
Using salt brine to pre-wet is becoming more common because of its lower cost. Some agencies are producing their own salt brine solution (23%). Liquid CMA and magnesium chloride are also used.
Some agencies spray the salt as it is loaded into the truck. However, the application is more uniform if truck-mounted equipment is used to spray the salt as it leaves the spreader. This also eliminates the problem of handling pre-wetted salt that is not immediately used.
Savings from losing less salt to bouncing and traffic action can more than pay for pre- wetting. However, these benefits only result with lower application rates.
Anti-icing is a road maintenance strategy that tries to keep the bond between ice and the pavement surface from forming. It involves applying ice control chemicals before or at the very beginning of the storm. Using this strategy often reduces total chemical use and allows a higher level of service to the traveling public.
The strategy most commonly used now is de-icing--breaking the bond between the ice and the pavement. Obviously, this technique is required once the pavement becomes covered with snow or ice. More chemicals are needed to prevent the initial formation of the ice-pavement bond.
Anti-icing is being evaluated for use on high service pavements. To use it, you need accurate pavement condition forecasts to anticipate conditions where anti-icing will be effective. It may also require chemical and equipment types which are different from those used in traditional de-icing.
Various ice control chemicals are being evaluated for anti-icing. Experience shows that liquid chemical applications are more likely to succeed. Liquid salt, magnesium chloride, calcium chloride, CMA, and potassium acetate are being evaluated. Pre- wetted dry chemicals may also prove effective.
Studies during actual storm conditions show that anti-icing produces equal or better road conditions with less chemical use. Liquid chemicals can be applied at fairly low rates (25 to 50 gallons per mile). These liquid chemicals remain on the pavement long enough to work. Several reports note residual effects for several days. The fairly light application rates produce a damp surface rather than flooding it. Of course, the pavement temperatures have to be compatible with the effective operating temperatures for the chemical being applied.
Problems can develop if heavy precipitation continues and the storm gets ahead of the anti-icing efforts. Heavy rain, freezing rain, or intense snowfall rates can cause a problem. Under these conditions you should switch to a normal de-icing approach to accomplish cleanup.
Sand and other abrasives improve vehicle traction on snow- and ice-covered roads. They can be used at all temperatures and are especially valuable when it is too cold for chemical de-icers to work. Sand is the most common abrasive, but slag, cinders, and bottom ash from power plants are also used.
Abrasives used for winter road maintenance have some negative environmental impact. They can clog storm water inlets and sewers.
Cleanup may be necessary in urban areas, on bridge decks, and in ditches. The materials may wash downstream and end up in streams and lakes.
Abrasives must be treated with salt to keep them unfrozen and usable. This salt has the same potential impacts described earlier. In particular, salt-treated abrasives can accelerate vehicle corrosion.
Recent concern has been raised in areas with air pollution. Air pollution from particles less than 10 microns in size (pm 10 ) has been documented from winter abrasive use. As a result, cleaner abrasives and quicker cleanup after the storm are being required in areas with severe air pollution problems.
Some sand and abrasives will be more effective than others. For better traction, use material with crushed or angular particles for better traction. Rounded particles are less effective. Very small particles and dirt are actually harmful to traction. Material larger than the #50 sieve is most effective. To minimize windshield damage, use materials in which all particles are smaller than three-eighths inch.
Combining with chemicals
Treating sand with 50-100 pounds of salt per cubic yard is necessary to keep it from becoming frozen and unworkable. It also helps to anchor the sand into the ice surface, makes the sand easier to load from the stockpile, and makes it spread more evenly from mechanical spreaders.
If slag, cinders, or other abrasives are wet they also need salt to be usable. Add the same amount of salt as for sand. Pre-wetting sand with a liquid de-icing chemical just before spreading has proven effective in embedding the abrasive on icy pavements.
Sometimes de-icing chemicals are mixed more heavily with sand. The sand gives immediate traction and the chemicals may melt the snow later when the temperature rises. To be effective the chemical must remain on the pavement, which is difficult to achieve in most cases. Mixing with sand reduces the salt's melting effectiveness.
Abrasives are usually applied only at hazardous locations such as curves, intersections, railroad crossings, and hills. Rates of 500 pounds to two cubic yards per mile are common. It is important to calibrate spreaders to control application rates.
Since abrasives must stay on the surface to be effective, they should not be used when they will be covered with more snow or when they will be blown off quickly by traffic. Heavy traffic reduces the effectiveness, requiring repeated application.
State regulations also require that abrasives treated with salt meet certain storage requirements. All salt-treated abrasives must be covered from April 1 to October 1 each year. If the abrasive has more than 5% salt by weight (approximately 140 pounds per cubic yard) it is considered the same as salt and must be covered all year and stored on an impervious base.
All salt and abrasive storage facilities must be reported to the Wisconsin Department of Transportation which conducts an annual inspection.