Deicers ... and the Degrees They'll Go To
Understanding the low-temperature effectiveness of deicers can affect your application rate and your bottom line.
Many landscape contractors and maintenance professionals do not use chemical deicers properly. They apply the deicer liberally and wait for it to completely melt snow and ice from sidewalks, driveways and parking lots. This is simply not good practice. It requires too much deicer, is too costly and can lead to other problems.
To use ice melters properly, apply them at rates sufficient to loosen ice and snow from pavement. Once the melting action has loosened the ice, you should remove it mechanically with a plow or a shovel. This method conserves ice melters, holding costs down. It also minimizes potential damage to concrete and vegetation.
Common Deicing Materials and How They Function
There are five basic ice melters. The two most commonly used materials, calcium chloride and sodium chloride (rock salt), occur naturally. Magnesium chloride and potassium chloride also exist in nature, but they are used less frequently. Urea is a manufactured chemical that is also widely used as a fertilizer. The active ingredients in nearly all deicers are made up of one or more of these five materials.
Numerous examples of blended deicers are also commercially available. In their solid form, chemical deicers are incapable of melting ice and snow; they must form a liquid brine. This brine lowers the freezing point of water, thereby melting ice and snow on contact. The melting action continues until the brine becomes so diluted that its freezing point approaches the ground temperature.
Even though the same general behavior is involved for each chemical deicer, there are significant differences in how effectively each works. Table 1 (to the right) shows the volume of liquid generated by identical amounts of chemical deicers over a 20-minute period at two different temperatures.
One reason for these variations is that some chemicals take longer to become a brine that penetrates through ice and snow. Sodium chloride, urea and potassium chloride must come in direct contact with moisture before they can dissolve into a deicing brine. That requires time. Calcium chloride and magnesium chloride, on the other hand, readily attract moisture from the atmosphere to form a brine and begin working more quickly.
The moisture-attracting ability of calcium chloride and magnesium chloride differentiates these two from the other deicing chemicals. But there is another difference that separates them even further. Calcium chloride pellets and magnesium chloride release heat as they dissolve into a brine solution. One pound of calcium chloride releases 290 British Thermal Units (Btu) as it dissolves, raising the temperature of the water. Magnesium chloride has less heat-release capability, about 26 Btu per pound, because it is a hexahydrate salt. In other words, the chemical composition of magnesium chloride is about 50 percent water and 50 percent active ingredient.
The remaining deicers (rock salt, potassium chloride and urea) have the opposite heat effect; they must absorb heat in order to go into solution. A pound of sodium chloride draws 39 Btu as it goes into solution. Urea requires 106 Btu and potassium chloride requires 170 Btu. All of these reactions lower the temperature of the water as the deicers form a brine.
Understanding Low-Temperature Effectiveness
Every deicing chemical has what is called a “eutectic” temperature, the lowest possible temperature at which a deicer brine can dissolve ice. The eutectic temperature only applies to a specific concentration of the deicer in water. No deicer is capable of staying at this concentration for long because the solution is constantly becoming more diluted as the deicer melts more ice and snow. The freezing point gradually increases as the brine becomes more diluted.
While eutectic temperatures have little bearing on actual deicer performance, it is true that the lower the eutectic temperature, the more quickly the deicer can create a brine and the faster it can begin working. And a lower eutectic temperature corresponds to a lower practical melting temperature as well, which is generally accepted as the lowest temperature at which sufficient deicing action takes place within a reasonable length of time. Table 2 (page S6) compares the practical and eutectic temperatures for various deicing chemicals.
Some deicer manufacturers mislead buyers by listing the eutectic temperature in their product’s performance characteristics without indicating what this temperature represents. Check the ingredients on a deicer package and verify performance claims against this table.
Various producers of ice melting chemicals have determined the recommended application rates for their own products.
Although these application rates are good guidelines, most spreaders are not calibrated to accurately deliver a specific volume of material to a specific area. This is because a single setting will not spread each material, from grass seed to fertilizer to ice melter, at the same rate. The spreader’s application rate depends as much on the material you're spreading as it does on the setting of the spreader.
Thus, the best practical recommendation goes back to: Do not overuse any ice melter. Spread just enough to penetrate the snow and ice and loosen its bond with the pavement. If the bond is not broken sufficiently to remove the snow and ice mechanically, try increasing the application rate moderately.
Effects on Concrete
Most common chemical deicers do not chemically attack concrete. Concrete spalling is actually a result of the pressure created by the repeated freezing and expansion of water or brine over a period of time. This damage is much more likely to occur in poor quality concrete that has little or no entrained air, a high water content or poor quality aggregate.
All concrete contains small micropores into which water penetrates. Damage that occurs to concrete is a physical interaction between the concrete and liquid in the micropores, which expands as it freezes. If air voids within concrete are completely saturated with water or brine, hydraulic pressure will result as the liquid freezes. Surface scaling can then occur when this pressure exceeds the tensile strength of the concrete.
Modifying the Freeze-Thaw Cycle
By lowering the freezing point of water, deicers can increase the number of concrete's freeze-thaw cycles. For example, when the temperature fluctuates between 25°F and 35°F, plain water on a concrete surface will freeze each time the temperature drops below 32°F and thaw each time the temperature rises above 32 degrees F. When the temperature fluctuates between 15°F and 25°F, plain water will remain frozen the entire time. But a deicing brine of rock salt and water will freeze each time the temperature drops below 20°F and thaw each time the temperature rises above 20°F. The deicer (i.e., salt) has actually increased the number of freeze-thaw cycles for these conditions.
The best way to minimize the number of freeze-thaw cycles when using a chemical deicer is to select a product with the lowest practical temperature limit. A deicer with a lower practical temperature limit is much more likely to remain in a liquid form once it has formed a brine. The likelihood of the atmospheric temperature fluctuating above and below the freezing point of the brine is reduced as the practical temperature limit of the deicer decreases.
By minimizing the number of freeze-thaw cycles, chemical deicer users can minimize the amount of concrete spalling that occurs because of frequent freezing and thawing of the liquid. To minimize the effects of deicing chemicals on concrete, you should remove snow and ice from pavement once the deicer has broken the bond with the surface.
However, good-quality concrete is your best defense against freeze-thaw cycles. Forty years of testing by the portland Cement Association has validated long-standing recommendations for concrete construction practices that produce durable portland cement concrete when subjected to freeze-thaw exposures. These construction practices include air entrainment, a low water-cement ratio, adequate cement content and an adequate drying period. When properly implemented, these practices produce concrete strong enough to hold up over decades of freeze-thaw cycles associated with winter climates.
Impact on Vegetation
All chemical deicers have the potential to harm plant life if their concentration in the soil rises to abnormally high levels. Chloride ions from sodium chloride, calcium chloride, magnesium chloride and potassium chloride all can be damaging to vegetation. Conversely, all of the chemical deicers listed—with the exception of rock salt—are also commonly used in agricultural applications.
The key to avoiding vegetation damage is controlling the application rate and placement of deicers. Although the ingredients in some deicing chemicals are used as fertilizers, it is important to remember that even fertilizers can cause die back and browning when over applied. The same principle applies for deicing chemicals. The best way to prevent damage to grass, trees and shrubs is to avoid deicer overuse.
You can alleviate the risk of vegetation die back and browning by selecting the most effective deicing product. Using the most effective deicer means that you'll need less of it to clear the same amount of ice and snow, thereby minimizing further exposure of plants to deicing chemicals.
Safe Usage Guidelines
The following guidelines will help to decrease the likelihood of vegetation damage caused by deicing chemicals.
- Any deicing chemical can damage vegetation when over applied. Therefore, follow the manufacturer’s application guidelines closely.
- Whenever possible, select a deicing chemical that melts a greater volume of ice and snow. This reduces the amount of deicer used (lowering the cost) and it reduces the amount of brine entering the soil.
- Plant salt-tolerant trees, shrubs and grass in areas closest to sidewalks, driveways and parking lots. Conversely, strategically place plants with a low tolerance level farther away from these areas.
- Plow or shovel snow and ice away from vegetation. Plow away from grassy areas and pile snow on pavement in parking lots whenever possible. Piling snow atop grassy areas can concentrate deicing chemicals in those areas, leading to vegetation damage.
Application and Storage
Store deicing chemicals in a dry, well-ventilated location. If they are exposed to moisture, they may harden, making them difficult to spread. Keep bags on pallets or otherwise raised above floor level to permit air circulation below the bottom tier.
Unlike other deicers, calcium chloride and magnesium chloride both attract moisture from their surroundings. This makes them more effective as ice melters, but also makes them require extra care once a package of either one has been opened. Keep partial bags of magnesium chloride and calcium chloride in airtight containers to prevent them from drawing moisture from their surroundings and hardening.
Once they have been tracked indoors, sodium chloride, potassium chloride, urea, and mixtures containing any of these chemicals all form a white powdery residue on carpets and flooring. Calcium chloride and magnesium chloride remain in a liquid form, so they do not leave a powdery residue. The following guidelines should prove useful when cleaning up chemical deicers.
- Remove any slush (and residual deicer) from sidewalks and parking lots once the deicer has broken the bond between the snow/ice and the pavement. This reduces the amount of deicer that is tracked indoors by pedestrians, and it also prevents the deicer brine from refreezing.
- Use floor mats inside building entrances to capture moisture, dirt and deicers as they are carried in by foot traffic. This will localize and control tracking, and you can clean the mats as frequently as necessary.
- Vacuuming and steam cleaning will normally remove deicer residues from carpeting; mopping or scrubbing will remove residues from floors.
- Good housekeeping demands that when mud and dirt is tracked indoors, it should be cleaned up promptly. The same guidelines should prevail when deicing chemicals are tracked indoors. This is simply one of the minor inconveniences that nature inflicts on us all.
Standing the Test of Time
Chemical deicers have been in use for more than 75 years, and they will continue to be utilized in the future. While deicers do not pose a health threat, good practices can minimize the effects of normal exposure and control cleanup difficulties.
Greg MacDonnell is marketing manager of calcium chloride products for Dow Chemical Co. (Ludington, Mich.).
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