The Reason You Should Never Spread Rock Salt on Concrete Driveways

People living in regions with harsh winters are familiar with the hazards of a slick and icy driveway. Under the right conditions, pavement can turn into the equivalent of an ice rink, so that every step you take is bursting with the potential of taking a fall and having to nurse a bruised rib or elbow—or worse.

The solution to this winter calamity is typically sodium chloride, or rock salt, which quickly breaks up ice and creates traction. It’s an easy fix, but unfortunately, spreading salt and other ice melters on concrete driveways can have some unforeseen consequences.

Salt and de-icers work not by melting ice but by lowering the freezing point of water, which is 32°F. Depending on the chemical used, the freezing point could drop to 20°F or even -25°F. (If you’ve used a de-icer to no apparent effect, then it’s because it was simply too cold and remained below the chemical’s freezing point.)

But as temperatures fluctuate, water goes through a freeze/thaw cycle. It might thaw, for example, at 22°F, freeze at 18°F, and thaw again at 22°F. Water seeps into concrete, which is porous. When it freezes (or re-freezes), it expands, causing damage to concrete driveways. (Asphalt driveways are far less susceptible to salt damage, as their aggregate of stones, sand, and petroleum is meant to withstand these freeze/thaw cycles.)

Rock salt may be the worst offender of this type, since it doesn’t lower the freezing point too much—to about 25°F. It’s also hygroscopic, meaning that it attracts about 10 percent more water than would otherwise enter the concrete.

So what should you opt for instead? Other ice melters that drop the freezing point lower mean the water won’t cycle through the freeze/thaw cycle as often. Calcium chloride will drop the freezing point to -25°F, meaning that water entering concrete or asphalt will likely remain liquid instead of freezing and expanding. But don’t use too much, as runoff can seep into lawns and cause damage there instead by killing grass and flowers.

Other melters, like magnesium chloride (0°F) or calcium magnesium acetate (20°F), are more pet- and environmentally-friendly, but have a higher freezing point than calcium chloride. Which one you select should depend on the type of driveway and other conditions. Just remember that rock salt—particularly on newer driveways—can be a concrete killer.