M
any people, from professional grounds-keepers to home owners, use chemical deicers incorrectly. They apply the deicer heavily, then wait for complete melting to clear paved areas. However, melting every bit of ice and snow on walkways and other pavements simply is not practical. It requires too much deicer, and may lead to other problems, as well.How Chemicals Deicers Work
The two most widely used chemical deicers are naturally
occurring materials, calcium chloride and sodium chloride (rock salt). Sometimes magnesium
chloride, potassium chloride -- two other naturally occurring materials -- and urea, a
manufactured chemical widely used as a fertilizer, are also used. These five chemicals and
combinations of them, account for virtually all deicers available to home owners and
grounds-keepers.
Deicing chemicals are incapable of melting snow and ice in their dry (solid) state. They
first must attract or come into contact with moisture to form a brine (a chemical/water
solution). The brine then penetrates down through the ice and snow until it reaches the
pavement. Once on the pavement surface, it spreads outwards melting and undercutting the
ice and snow for mechanical removal, which means plowing or shoveling.
Even though the same chemical behavior is involved, there are remarkable differences in
how deicers work. When researchers compared the deicing effectiveness of various deicers
at 15oF, calcium chloride pellets melted more ice than any of the other
deicers. At 5oF, the difference was even more dramatic.
One reason for these big differences in deicing is that some chemicals take longer to go
into solution before they can begin penetrating ice and snow. Even though snow and ice are
merely frozen water, moisture is not readily available at freezing temperatures, so
liquefication is slowed. Of all the chemicals, calcium chloride and magnesium chloride
have the greatest ability to attract and retain moisture from its surrounds. Of these two
deicers, calcium chloride is superior.
Exothermic Reactions
There is yet another reason for the deicing action of these
two deicers, heat liberation. As they dissolve, both chemicals liberate heat, which is
known as exothermic reaction. A single pound of calcium chloride releases 290 BTU as it
dissolves. The other deicers have just the opposite effect. Sodium chloride, for an
example, draws 39 BTU from its surroundings as it goes into solution (an endothermic
reaction). Potassium chloride requires even more heat (170 BTU), and urea requires 106
BTU.
For highway deicing, rock salt can sometimes remain marginally effective at lower
temperatures. The friction between vehicle tires and the road surface can generate
sufficient heat to put rock salt into solution. Pedestrian traffic generally would not
create enough heat to make rock salt effective at temperatures much below 15oF
to 20oF.
Breaking The Bond
In order to undercut ice, a deicer must first melt its way
downward, then spread outwards to undercut the bond with the ice and pavement. Pellets are
highly effective at undercutting because they contact only a small area of ice and bore
vertically downward, quickly reaching the ice/pavement interface.
Deicers in chip or flake form are much less effective at penetrating ice. They tend to
work as hard horizontally -- on the ice's surface -- as they do vertically. This
horizontal melting is due it their large surface area, which necessities the chemical to
become so diluted it can become ineffective when it reaches the pavement surface.
Another measure of deicer performance is the ability of the material to provide
long-lasting deicing action. This action directly affects the amount of material required
because the longer the deicing action last the less often reapplication is required.
A factor that affects a deicer's ability to work is the nature state of the material. All
common deicers, with the excepting of calcium chloride pellets and magnesium chloride
flake are solids in their nature state, so they give up moisture freely as they try to
return to their natural solid state from a brine. Calcium chloride and magnesium chloride
are naturally a liquid, so they can maintain an effective melting brine for longer
periods.
Misleading Claims!
Claims of low temperature effectiveness can be misleading.
Manufactures' claims of low temperature effectiveness often are based on the theoretical
lowest temperature to which the deicer can suppress the freezing point of water -- called
the eutectic temperature. However, this temperature can only be reached at the ideal
percent concentration of the deicer in water. All other concentrations will have higher
freezing points.
Because all deicers become increasingly diluted as they melt ice and snow, they remain at
their ideal ice melting concentration only very briefly. Consequently, eutectic
temperature has little bearing on real-life deicing conditions.
A more realistic measure of a deicer's low temperature performance is called the
"practical working temperature." This generally is accepted as the lowest
temperature at which sufficient deicing action is evident within a reasonable time. See
Working Temperatures
Effects on Concrete
The effects deicing chemicals have on concrete is a frequently
misunderstood topic. None of the common deicers chemically attack concrete. But, by
depressing the freezing point of water, all deicers can affect the number of freeze/thaw
cycles concrete goes through. The expansion pressure from water causes concrete to spall.
For example, if the temperature is hovering between 25oF and 35oF,
water on a concrete surface will freeze when the temperature drops below 32oF,
and it will thaw as the temperature rises. This cycle exposes the concrete to one
freeze/thaw cycle.
On the other hand, if the temperature varies between 15oF and 25oF,
water will remain frozen, while a salt brine with a freezing point of 20oF will
go through a freeze/thaw cycle each time the temperature rises and falls. Each deicing
chemical affects the freeze/thaw cycle, and therefore the concrete.
Independent laboratory tests, per ASTM C-672 Standard Test Method for Scaling
Resistance of Concrete Surfaces Exposed to Deicing Chemical, indicate that calcium
chloride brine poses the least threat of damage to concrete of the deicers tested. Deicers
Upon Concrete
In additional, research conducted by the Portland Cement Association indicate that quality
air entrained concrete, properly cured is much more resistant to freeze/thaw scaling than
improperly cured or non-air-entrained concrete. Manufacture's usage recommendations should
always be followed to prevent over applying deicer. Once the bond between ice and pavement
is broken, the slush and residual deicer should be removed to prevent refreezing.
Following this procedure, concrete scaling will be minimized.
Environmental Considerations
All the deicers mentioned hold the potential for damaging grass and
plant biota should their concentration within the soil becomes unusually high. The sodium
in the rock salt can be detrimental to plant life, while calcium, magnesium, and potassium
can be beneficial to soil. The chloride ions are the chief culprit of all the deicers
mentioned except urea, which does not contain chlorides. However, over use of urea will
cause a nitrogen burn to your plant biota. This burn can occur often because the amount of
urea needed to penetrate and disbond ice and snow from the pavement is excessive for plant
life growth when run off occurs.
However, in amounts recommended for sidewalk and driveway deicing, there is minimal chance
of damage to trees, grass, and shrubs. This is especially true if the chemical is used
sparingly -- only to undercut snow and ice -- and the slush is not plowed or shoveled into
grassy or planted areas.
The corrosive effect of deicers on metals has also been the subject of considerable attention and extensive research. Deicers in the salt family -- sodium, calcium, magnesium, and potassium -- can accelerate the corrosion of unprotected (painted) iron and steel by holding moisture against these exposed surfaces. Ordinarily, if the surface's paint, plating, or undercoating is not damaged or broken, corrosion will not occur when deicers are used accordance with manufacturer recommendations.
Another concern of many businesses and homeowners is the visible deicer residue that may be tracked into a building. The deicers that tend to dry quickly leaving an unsightly white residue are rock salt, urea, and potassium chloride and mixtures of these deicers. This residue occurs because these deicers are solids in their natural state. However, since the residue is water soluble, it cleans up readily using plain water or ordinary household cleaner.
Clearly, there are significant differences in how well the various chemical deicers melt ice and snow. When all the performance variables are considered, calcium chloride pellets are most effective. They melt faster, remain effect at lower temperature, penetrate ice better, and less harmful to concrete.
No matter which chemical deicer is selected, it should be used wisely and in according to directions. Do not use a chemical deicer to melt every bit of snow and ice. Use only enough to break the ice/pavement bond, then remove the remaining slush by plowing or shoveling.
More Deicer Information at Anti-Icing
General Deicing Info: Deicing
Chemicals
Deicers Effect on Concrete
Deicer Co-Efficient of Friction
Deicers Effect on Vegetation