|
 Clearing
winter roads to the bare pavement usually requires deicing chemicals. In
Wisconsin, the most common chemical is salt, which usually comes from mined
rock salt that has been crushed, screened, and treated with an anti-caking
agent. Deicing salt is relatively light — just over one ton per cubic yard —
and comes as a mixture of 0.375-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 deicing chemicals.
Calcium magnesium acetate 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.
Deicing chemicals work by lowering the freezing
point of water. A 23.3% concentration of salt water freezes at -60 degrees F
and a 29.8% solution of calcium chloride freezes at -67 degrees F. These low
freezing points are what makes salt and calcium chloride useful.
Before a dry deicing 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. 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 to consider
 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.
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.
The surface temperature of a snow- or ice-covered
road determines deicing chemical amounts and melting rates. As temperatures
go down, the amount of deicer needed to melt a given quantity of ice
increases significantly. An accompanying graph shows that salt can melt five
times as much ice at 30 degrees F as at 20 degrees F. The effectiveness of
deicing is sensitive to small differences in pavement temperatures.
The longer a deicing chemical has to react, the
greater the amount of melting. At temperatures above 20 degrees F, both salt
and calcium chloride can melt ice in a reasonable time. At lower
temperatures, salt takes much longer.
The sun’s heat warms the pavement, speeding up
melting. Radiant heat may cause the pavement temperature to rise 10 degrees
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.
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.
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.
 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, deicing chemicals are
usually applied in a windrow 2- to 4-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.
Timing is the most important factor in successfully
clearing snow by chemical treatment. Early application is critical.
Spreading a small amount of deicer 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.
Environmental impact
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.
Deicing chemicals are highly soluble and follow any
water flow. Salt concentrations in Wisconsin’s surface and ground water have
increased since the early 1960s, the Wisconsin Department of Natural
Resources 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 deicing chemicals has contaminated wells, but most of
these apparently were caused by seepage from poor storage facilities.
Deicing 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.
Storage requirements
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 an
impervious pad.
Spreading
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 deicer 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 deicer when temperatures are below its effective range. Normally, 15
to 20 degrees F is considered the lower limit for salt. If deicing 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 deicers on the center (crown) of two-lane roads
and on the high side of curves.
A spreader with a spinner is the most common way of
applying deicers. A spinning circular plate throws the deicer out in a
semicircle. Alternatively, a chute can distribute deicer 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.
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.
Calcium chloride
Dry calcium chloride requires special handling and
is more costly than salt. However, it is effective at temperatures below 0
degrees 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.
Pre-wetting
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 deicing chemical can be used to
pre-wet, liquid calcium chloride is used widely. Applications of 6 to 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 handline 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
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 deicing —
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 deicing.
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 deicing approach to accomplish
cleanup.
Abrasives
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 deicers 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.
Concern has been raised in areas with air pollution.
Air pollution from particles less than 10 microns in size 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.
For better traction, use material with crushed or
angular particles. 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
0.375 inch.
Treating sand with 50 to 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 deicing chemical just before spreading has
proven effective in embedding the abrasive on icy pavements.
Sometimes deicing 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 2 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.
This article courtesy of the Wisconsin Department of
Transportation.
Reprinted from Better Roads Magazine
October 2003 |
