The changing science of salt

De-icing : Every driver's friend in winter is mixed and applied in different ways for maximum benefit.

January 28, 2004|By Dennis O'Brien | Dennis O'Brien,SUN STAFF

It's everywhere these days - spread like chicken feed on steps, showered on parking lots and dumped on roads by the ton.

But the science of salt has changed over the years, with highway crews applying more anti-icing agent - a liquid salt brine - just before each snowfall. Crews also no longer wait for snow to accumulate before spreading salt.

"As soon as there's enough snow on the road to hold down any of that salt, we're putting it down," said Mark Lipnick, a State Highway Administration quality assurance engineer.

He said that state highway crews used to wait until about an inch of snow was on roads before spreading salt.

But research and experience showed that a key to keeping roads clear is preventing a bond from forming between the road and the snow that falls on it. The molecular bond becomes more and more likely to form as temperatures drop.

The salt that crews spread on Maryland highways is made up of two elements, sodium and chloride. The compound is a byproduct of oceans that dried up and formed continents millions of years ago. It melts snow by depressing water's freezing point.

It is one of the most abundant compounds on earth, it has been spread on highways since the 1930s and its use is a result of cost and climate.

Most snow in the United States falls when temperatures are between 25 and 32 degrees Fahrenheit, which makes salt ideal, according to the Salt Institute, an industry trade group based in Washington.

It also is relatively cheap: Maryland - which must clear 16,000 miles of state highways with each storm - pays about $30 a ton. In recent winters, the tab for salt has come to about $30 million a year, state officials say - a figure that does not include the costs for local governments.

Nationally, governments, businesses and homeowners paid $336 million for 13 million tons of salt last year, according to the institute.

But the price may be worth it - particularly if you're a politician.

Michael A. Bilandic was defeated in his bid to be re-elected as mayor of Chicago in 1979 shortly after he failed to sufficiently salt streets during a terrible blizzard. Shortages of salt for Baltimore County's roads in early 1994 forced the county to use slag and contributed to the defeat of County Executive Roger B. Hayden that year.

Lipnick said that salt remains effective until pavement temper- atures - usually a few degrees higher than air temperatures - dip below about 20 degrees Fahrenheit. When temperatures fall below that, SHA crews mix salt with brine, which is basically saltwater, before spreading it. That combination has an effective range down to about 10 degrees, he said

They also apply the brine to several roadways before the snow falls.

In Garrett and Allegany counties, traditionally the chilliest parts of Maryland, the highway administration also sometimes uses a mixture of salt and magnesium chloride, mixing 10 gallons of the liquid compound with every ton of salt.

The state suspended the use of magnesium chloride as a lone de-icing agent last year after officials learned that it created slippery conditions in Montgomery and Prince George's counties. But an SHA spokeswoman said yesterday that the compound has been found safe when used in with highway salt.

Magnesium chloride is expensive, costing about 65 cents a gallon. But it's effective down to zero and sub-zero temperatures, highway officials said.

As state officials juggle different methods for roads, de-icing products on the market also can confuse homeowners looking for something to help clear their sidewalks.

"There are a lot of products out there," said Richard L. Hanneman, president of the Salt Institute, a trade association group in Washington that represents the firms that produce rock salt and other de-icing products.

He said that he recently went to a home-improvement supply outlet to pick up salt for his sidewalks. While more expensive magnesium chloride and calcium chloride were prominently displayed on the shelves, the less expensive rock salt was being kept in the back of the store.

But salt also has drawbacks: it corrodes roads, bridges and sidewalks and seeps into water supplies.

When salt mixes with the water from the melted snow, it seeps into tiny cracks in paved or concrete surfaces and expands when it refreezes, creating cracks as it pushes against the concrete or pavement. Salt corrodes steel by an oxidation process that creates rust, which occupies four times the volume of the steel it replaces. That, in turn, can break up any concrete it is being used to reinforce.

Experts say the key to avoiding cracks and corrosion is to remove the slush formed by the salt and the melting snow as quickly as possible from sidewalks, roadways and metal surfaces.

But road salt also gradually sinks into the earth,, said Kevin Mercer, executive director of the River Side Stewardship Alliance, a Toronto-based environmental group.

Mercer said a five-year study by the Canadian government released in 2000 concluded that salt is damaging waterways and roadside vegetation.

But salt's supporters say there's nothing like it to keep roads clear.

"Rock salt is tried and true," Hanneman said.

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