Thursday, November 20, 2014

The Science Behind Road Salt

The winter season comes with many things, including snow, ice, cold, and wind.
A byproduct of the season is the salt that is used to keep us safe on the roads and sidewalks.
But why is salt used?

Loading salt into dump trucks to prepare for the winter season
It's simple science. 
Salt lowers the freezing point of water. 

Ice forms when the­ temperature of pure water reaches 32° F. At that temperature, equal melting and freezing are occurring and the water molecules are in a state of dynamic equilibrium. 

When you add salt, the temperature of equal meting and freezing drops. More simply: The freezing point is lowered. This is called 'freezing point depression'.

There are many applications of this process, most notably, the use of salt on roadways to melt layers of ice and snow during the winter months.

Plows, salt trucks, and other traffic traveling over salted surfaces help to break up icy surfaces

On a roadway, if you sprinkle salt on the ice, the salt penetrates the surface layer, turning it to liquid water. With a salt/water brine solution sitting on top of the remaining layer of ice, a continued breakdown of the frozen material ensues. 

Winter Road Treatment & Snow Removal

Eventually, the salt and resulting brine mixture will break down the entire layer of ice or snow and create more traction on road surfaces. The process is further enhanced by traffic traveling over the salted surfaces, breaking up larger chunks of ice.

Check it out!
Take a look at this time lapse video: 
If you ever watch salt melting ice in real-time, you can see the dissolving process happen right before your eyes. With this example (made with the help of the Imagination Station Toledo), we've sped up that transition. The ice immediately around the grain of salt melts, and the melting eventually spreads out from that point.

According to an article by HowStuffWorks, a 10% salt solution freezes water at 20° F, and a 20% solution freezes at 2° F. Because of this, purity and concentration of the mixture matters. 

For example, NaCl, or rock salt, comes in the form of a larger grain, but consists of the same chemical compound as table salt. The chemical name NaCl represents the two ions of a salt grain: sodium and chlorine.

Rock Salt, also known as NaCl has equal parts Sodium and Chlorine
When used to activate a freezing point depression, NaCl breaks down into two components-- Na & Cl. Those ions are responsible for lowering the freezing point of surface ice and water to roughly 15° F, depending on purity and concentration. It is relatively inexpensive and can be dissolved into a solute to make a brine mixture.

Plows and salt trucks often prime the roads with a brine solution
A brine mixture is often used by salt trucks to prime roads before a measurable snowfall or the development of black ice. 

Brine is a solution of salt dissolved in water. 

The brine serves the purpose of preventing a hard freeze at the surface, by lowering the freezing point from 32° to 20° F, depending on purity and concentration. 

Manager of Toledo Streets, Bridges and Harbor, Jeremy Mikolajczyk, says the City of Toledo is planning on trying a method that includes more brining this year with new procedures in place. The lower cost of the brining process is appealing, and once the salt/water solution is mixed, applying it to the streets is easier, too. Because of the moisture in the solution, the brine tends to stick to road surfaces better than dry salt, as well. 

Labor, materials and maintenance cost ODOT $119 million during the 2013-2014 winter.
It was the most expensive winter season ever. 
Unfortunately, brine is less effective on its own, especially if environmental air temperatures are expected to drop below 20° F. For the coldest part of the winter, the method of brining is not enough. 

Some of the coldest 2013-2014 winter season temperatures range between 1° F to -15°. 

2013-2014 Winter Season:
Date: (High/Low)

January 3rd (11°/-12°)
January 7th (1°/-14°)
January 22nd (10°/-10°)
January 28th (3°/-10°)

Temperatures through the 2013-2014 winter season were too cold for salting the roads

With environmental temperatures near 30° F, a surface brine solute will be effective in melting any developing ice.

However, if brine, alone, will only drop the freezing point of water to 20° F, once the air temperature matches or goes below that point, ice will form and adhere to surfaces. The additional use of NaCl (rock salt) will lower the freezing point of water down to approximately 15° F.

Once air temperatures drop to 15° F, however, local municipalities run into that same problem again. At 15° F, the NaCl salt ions become ineffective, and the melting process stops. Below 15°, you can expect fully frozen surfaces, since the salt cannot lower the freezing point beyond that temperature.

But what if we have another year like the winter of 2013-2014?

The Toledo Express Airport recorded 23 days with temperatures below zero, which rendered salt and brine on the roads ineffective.
With air temperatures far below the freezing point, a typical NaCl salt brine or road coat will be ineffective.

So what is the alternative?

Commonly referred to as 'ice melt', an enhanced chemical salt mix can actually be used for a similar price, with better results. The exact chemical compound differs from brand to brand, but ice melt can include a combination of NaCl, MgCl2 (Magnesium Chloride), and CaCl2 (Calcium Chloride).

Cincinnati's salt pile as of Mid-October 2013. 
Officials say it was only 30 percent full. (FOX19)

Due to the variation of the mix, a true freezing depression can range from -2° F to -19° F... a far cry from NaCl alone! While competitively priced, ice melt still has its limitations. 

Over a typical winter season in Ohio, ODOT spreads 630,000 tons of salt-- in 2013-2014, the state used nearly 1 million tons-- over the roadways. At those quantities, price matters. 
A third, more expensive, but even more effective method is pure Calcium Chloride. With Calcium Chloride, or CaCl2, similar to Magnesium Chloride, MgCl2, the compound breaks down into three ions, as opposed to only two ions in NaCl.

Calcium Chloride is less commonly used to melt ice, but it is the most effective. It releases heat as it dissolves, aiding the melting process. This method can be up to 6 times more expensive, but it is also exponentially more effective, melting snow and ice down to -20° F.

Table salt, Rock salt and Ice melt are far inferior to the melting properties of Calcium Chloride.

Regardless of the melting method crews use this winter season, ODOT assures us that they are ready. With a record breaking winter last year, bringing in 23 days with below zero temperatures and over 80" of snow, it would be unlikely that more salt, crews or resources would ever be needed than in the winter of 2013-2014 in Toledo.