One of the most tedious winter chores I can think of is removing s
now from my sidewalks and driveway around my house. Here in the midwest U.S., annual snowfall amounts vary greatly from year-to-year. In the last five years, we’ve seen some record-breaking amounts (in the area of 70 or 80 inches).
Some would think that using salt to melt snow and ice is a pretty straightforward procedure - and it is. But it can be a bit of a two-edged sword if you don’t know how to do it properly. As
a professional whose job it is to train snow plow operators and oversee highway snow removal, I have seen firsthand how using salt properly can make for a really safe road, and how using it improperly can literally turn an interstate into a skating rink.
How does salt work?
There's a lot of chemistry that goes into how salt works. Some organizations have spent a lot of money trying to determine the best ways to spread it or mix it with other chemicals to make it work better, with varying degrees of success. But really, salt’s effectiveness depends primarily on three things:
1. Whether or not it was laid before the storm
2. The temperature of the pavement
3. Whether or not precipitation (snow, sleet, ice rain) is falling
The first point really gets at how salt works. Salt acts as a bond-breaker between ice / snow and the pavement. Spread before the storm, salt prevents snow and ice from bonding to the pavement, making removal much easier. If it’s applied after snow and ice have bonded on the pavement, then it must melt through these layers to the pavement surface before it can begin to break the bond. Obviously, this is a much less effective way to use salt, but it’s also the most common practice used to fight snow and ice.
That leaves the other two points. The effectiveness of salt’s ability to melt snow is highly dependent upon the snow’s thickness and the pavement’s temperature. The bar chart above gives a good visual idea of how much more salt is required as the thickness of snow increases as the pavement temperature drops.
For example, to gauge how the snow’s thickness affects things, compare the 1” bar at the left to the ¼” bar. To be accurate, snow that’s 4 times thicker requires 5 -8 times as much salt to melt.
Now compare the 1” bar at 32 degrees Fahrenheit to the one at 20 degrees. A similar relationship emerges. In this case, a ten-degree drop in temperature requires five times as much salt to melt the same thickness of snow.
That helps us understand how important it is to use salt only on thin layers of snow and ice, and to apply it at warmer temperatures - around 15-20 degrees Fahrenheit (-10 to -7 degrees C).
You may think that the temperature limitation seems a bit much. If you live in a cold, wintery climate as I do, you know how cold things get. But in reality, most snow storms (at least in more temperate regions) tend to occur at 15 degrees Fahrenheit and above.
A small increase in thickness or a small drop in temperature results in a large increase in the amount of salt required to completely melt the snow.
What about other kinds of salt?
Not all salts are the same.
Most people probably don’t know that a “salt” is a generic term for a chemical compound with a crystalline structure that results when an acid and a base react. They can have a variety of rich colors like blue, red, and purple), and serve a wide variety of other purposes.
As for melting snow, most other salt types are chloride-based like regular table salt (sodium chloride, or NaCl). Potassium chloride, magnesium chloride, and calcium chloride are three examples. Some agencies even mix beet juice with salt to melt snow!
Calcium chloride is one of the most effective snow-fighting salts and it shows up in a lot of commercial products. The next time you go to the store, check the contents on a bag of the various salts they have available. Chances are, you’ll see calcium chloride as the primary ingredient.
So why is calcium chloride it better?
Regular salt (sodium chloride) cannot melt snow if it gets too cold because the reaction that occurs when it melts is endothermic - it must absorb heat from the environment in order to melt snow. By contrast, calcium chloride’s reaction is exothermic - it gives off heat, as it reacts with snow.
I like to think of it this way: Sodium chloride may melt snow, but calcium chloride burns it.
The big problem with these more powerful salts is that they damage pavement (actually, all salts will do this) and yes, I have the pock marks in my driveway and sidewalk to prove it. That’s because salt eats minerals. Concrete is made up of lots of minerals (like lime). Therefore, salt eats concrete - it’s a corrosive chemical. So when using salt, especially the more aggressive kinds, they should be applied sparingly. It’s also important to remove the salt after its done it’s job. Shovel or clear the pavement as well as you can after the storm.
Don’t let salt-melted snow refreeze where it is.
What about liquid salt?
Salt (no matter what kind) always works better as a brine - salt dissolved in water.
Brine acts much more quickly to melt through snow. For that reason, most highway maintenance agencies across the world use liquid salt along with dry salt when fighting snow and ice.
One of my staff told me that they guys that use liquid salt on their routes are typically done two hours sooner after the snow stops falling than the guys who use only dry (rock) salt.
There is one caveat with brine, however. Brine functions best at a particular concentration (around 28%). But, as brine melts more snow and ice, it becomes diluted by the snowmelt - the water that was once snow. The more dilute the brine becomes, the less effective it is. Worse yet, as temperatures drop, a diluted brine will cease to melt ice and start to become ice itself - and if conditions are right, it can happen very quickly. In my work, on rare occasion, we’d see what we call a “flash freeze”. Not fun - especially when it happens on an interstate.
Experience dictates that brine should be used down to about 15 degrees Fahrenheit (-10 degrees C). It can continue to work in temperatures 5 to 10 degrees colder, but the length of time that it will be effective is greatly reduced. If rain or snow is falling at the same time, this will also reduce its effectiveness by further diluting the brine.
Five Quick Tips on Using Salt to Melt Snow and Ice
If you keep these principles of how salt works to fight snow and ice in mind, you can greatly improve the performance of the salt you put down and likely save a lot of salt that you might have otherwise wasted (buying it by the bag at the store gets expensive during the winter around here!).
Just keep the following five tips in mind when you're out spreading salt:
- The more powerful salts (like calcium chloride) may work better, but they also do more damage to your pavements.
- Salt is a bond breaker. It’s best applied before the snow and ice hit the ground.
- If you apply salt to snow and ice on the ground, it’s best to use it around 20 degrees Fahrenheit (-9 degrees Celsius) and on about ¼ inch (½ - 1 cm) of snow.
- Don’t leave salt you’ve applied in place. Make sure to clear it off with a shovel, or it’ll eventually do damage to your pavement.
- Don’t apply salt while snow is falling. It’s a lost cause, because the precipitation will quickly dilute the resulting brine and render it useless.