Road Management & Engineering Journal
February 11, 1998
(U.S. and Canada)
Fax: (360) 335-6402
Highway Deicing for Safety and Mobility
Salt was first used in the 1930's in snow and ice control. It wasn't until the sixties that its use became widespread after winter maintenance personnel learned of its effectiveness.
Today, salt is a necessary and generally accepted part of the winter environment. It provides safety and essential mobility for motorists, commercial vehicles and police, fire and other emergency vehicles. Drivers have special responsibilities for careful winter driving. But without salt, there would often be hazardous conditions and even chaos. Yet its use has brought criticism, sometimes justified in the past, when it was used to excess.
Criticism is unjustified when salt is used properly and does not present environmental harm. However, there are those who would like to see it banned and replaced with less effective materials for various reasons, including effects on the environment and salt's contribution to corrosion.
Why Is Salt Used?
Salt is used as the principal deicer because it is the most available and cost-effective safe deicer. It is plentiful in the earth and the sea. The primary type used is rock salt that is mined from the earth. Solar salt, which is evaporated by the sun from seawater is also used. Some 10 million tons of deicing salt is used each year in the U.S. and about 3 million in Canada.
In today's highly mobile society, it is imperative to remove hazardous conditions created by snow and ice as quickly as possible, and to keep roads open to guarantee essential mobility in winter. That makes a deicer necessary. Sometimes salt is used alone with there is ice or too little snow to plow; mostly it is used in conjunction with snowplows.
Salt is used to melt snow and ice, to break bonding to the pavement and to allow snowplows to remove accumulations quickly and efficiently. Professional snowfighters in the U.S. and Canada are learning lessons from European snowfighters in the use of anti-icing techniques and technologies to prevent the ice/pavement bond from forming.
How Does Salt Work?
When salt is applied to ice and snow it creates a brine that has a lower freezing temperature than the surrounding ice or snow. Salt is an ideal deicing material because it is:
Salt is especially effective when it is applied early in a storm event and can form a layer of brine between the pavement and the accumulating ice or snow. The action of vehicle tires enhances its performance.
Why Not Use Alternatives?
Alternatives are just not as practical. Those that are as effective as salt are too expensive and have limited availability. Some have a very detrimental effect on the environment and on pavements. According to a report on the technical, environmental and economic aspects of highway deicing salts by the National Conference of State Legislatures, "Several alternatives to deicing salts have been investigated or tried, but they tend to be too expensive, damaging to highway structures, more toxic than deicing salts or not as effective. The alternatives include other deicing chemicals, pavement heating systems, mobile thermal deicing systems and mechanical equipment."
Abrasives are often cited as practical alternatives, but they have limitations. Disadvantages of abrasives are that they cannot melt snow and ice, offer only temporary traction, are covered up by new snow, large quantities and frequent applications are necessary and they must be cleaned up at great expense. Sand can obstruct and clog drainage ditches and storm drain systems. Also, it is usually necessary to use some salt with abrasives in order to keep the abrasive stockpile from freezing.
Straight salt is more efficient and economical. Used sensibly, it is the best means of providing safe roads in winter by itself or in conjunction with plowing, depending on the situation.
Certain additives are put into salt to keep it from caking. The most frequently used is sodium ferrocyanide, also known as Yellow Prussiate of Soda (YPS). Another is ferric ferrocyanide, also known as Prussian Blue. They are added in amounts of 50 to 100 ppm.
YPS is approved by the Food and Drug Administration as an anti-caking additive in table salt based on exhaustive tests wherein no evidence of toxicity was demonstrated at levels considerably higher than those used in highway deicing salts. Prussian Blue is also used in household bluing, blueprints, blue-black ink and carpenter's chalk. It is also nontoxic to animal and plant life.
One of the major criticisms of salt is that it contributes to corrosion of metal. The basis of much of the environmental concern salt can be traced to salt's acceleration of the corrosion of vehicles.
However, cars will rust even where deicing salt is not used, particularly in warm coastal areas and in wet climates. During the last several years, automobile companies have intensified efforts to protect cars from corrosion by special dipping processes, use of aluminized waxes, zinc-rich primers, galvanized steel and greater use of other noncorrosive metals and plastics. In addition, some give the assembled cars an anticorrosion treatment. Extended warranties by the major auto companies are now offered against rust perforation of automobiles.
Car owners have a responsibility to help protect their cars from corrosion by touching up nicks and scratches, waxing frequently and washing cars regularly, even in winter.
The corrosion of bridge decks and other reinforced concrete structures has been a problem in snow belt areas. Contrary to popular myth, salt does not have a deleterious effect on portland cement concrete (PCC) structures. It is the freeze thaw process and the corrosion of steel reinforcing that can cause problems.
The use of properly cured, air-entrained PCC will prevent damage by the freeze thaw cycle. However, chlorides can migrate through the concrete and can cause corrosion of steel reinforcement with the concrete. The oxidizing metal expands in volume and can cause cracks in the concrete in addition to compromising the design strength of the structure.
Research on the subject is an ongoing process. Various anticorrosion methods have been tried or are now available. The method offering the most promise for old bridge decks is cathodic protection, where a small reverse current halts the rusting process. It is even possible to restore bridges already damaged by corrosion through chloride removal techniques.
Epoxy-coated reinforcing bars and air-entrained concrete and/or high density concrete are used in the construction of new deck surfaces to help limit the corrosion of steel reinforcing. Research into the use of polymer and carbon based reinforcing materials is also underway.
Salt has often been blamed for causing potholes. Potholes are caused by water entering the grade below the surface of the road and then freezing. As the water freezes, it expands in volume. This freezing can cause the road surface to heave. Thawing leaves a cavity or weakened spot beneath the surface. Traffic then causes the surface to collapse into the cavity creating a pothole.
Salt can cause surface spalling of poor concrete or scaling of non-air-entrained concrete. It has not been shown to have any chemical reactions or bad effects on asphalt or the air-entrained concrete that is available today for road building.
Economic Benefits of Salting
A study, Accident Analysis of Ice Control Operations, released in 1992 by Marquette University's Department of Civil and Environmental Engineering concluded that, "As a winter maintenance service, de-icing pays for itself within the first 25 minutes after the first hour that salt is spread on two-lane highways. . . . Then, during the first four hours after the hour of application of salt, the direct road user benefits were $6.50 for every $1.00 spent on direct maintenance costs for the operation." As soon as 71 vehicles drove over the highway, the average direct costs were offset by direct benefits. The study found that costs related to accidents, including medical expenses, emergency services, workplace costs, travel delay, property damage, and administration and legal expenses decrease by 88 percent after application of deicing salt.
Use of salt, in conjunction with a good plowing program, is the fastest and most efficient means of snow and ice removal. The use of abrasives requires at least seven times more materials to treat a given distance of roadway. Studies by the Salt Institute have determined that a loaded salt truck, spreading at the generally accepted rate of 500 pounds per two-lane mile for general storm conditions, can treat a 22.5 mile stretch of roadway, traveling a total of 45 miles. A sand truck requires seven loads, must travel a total distance of 187 miles to treat the same section of road and that truck requires four times more fuel. In more ways than one, salt used in snow and ice control contributes to energy savings.
Using Salt Makes Highway Safe
The same Marquette University [study] cited above concluded that the total number of accidents is 8 times higher before deicing than after on a two-lane roadway (4.5 times higher for multi-lane freeways), the number of accidents involving injuries is 9 times higher before application (7 times higher for multi-lane freeways), and the severity of accidents is reduced by 30 percent after application.
Although no specific statistics are available on the effects of storm-clogged roads on the delivery of emergency service (ambulance, fire, rescue and police), common sense tells us that response times are drastically affected by snow and ice covered streets.
Salt is an essential part of the winter environment and its use is accepted by a great majority of the motoring public. It is one of the major weapons in the battle against snow and ice. It is used to provide safety and essential mobility on roads in winter. Salt is the most plentiful and most inexpensive deicer that is both efficient and safe. It is easy to handle. It is nontoxic to man and animals and will not harm the environment when properly used.
Driving a snowplow is hard work. It requires driving for long hours in the worst conditions. While you, as snowplow operators, are concerned with providing safe travel for motorists, you must not overlook your own safety. Here's a few tips to make your work safer:
Take care of yourself by observing these few tips and keep your shift a safe one.
REMEMBER, SPRING IS JUST AROUND THE CORNER!