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  • Friday 22 Sep 2017
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Road Salt, Ice Salt, Snow Salt

USING SALT and SAND for WINTER ROAD MAINTENANCE:

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. 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.

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.

SPREADING:

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.

 

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