Road Salt and Your Chicago Building: What It Eats

Road salt rarely "eats" a building the way people picture it. On concrete, deicing salt makes the surface wetter and adds freeze-thaw cycles, and that is what flakes the top layer off. On brick, the damage is mostly about water and salts moving through masonry. The result looks the same to an owner in March: a rough, peeling stoop, a sidewalk that has lost its surface, white crust on a wall. Here is what is actually going on, and what you can change before next winter.

What does road salt actually do to concrete?

The damage has a name: scaling. It is the flaking of the surface of concrete after freeze-thaw exposure, and the National Ready Mixed Concrete Association (NRMCA) says deicers make it worse by raising the saturation of the surface and the number of freeze-thaw cycles it goes through (NRMCA guidance). In plain terms: salty meltwater keeps the top of the slab wet, and a wet surface that freezes again and again loses its skin.

Light scaling leaves the stones in the mix covered. Moderate scaling takes off roughly an eighth to three eighths of an inch of the surface mortar (NRMCA guidance). That is the rough, pebbly look on a stoop or walk that used to be smooth.

Here is the part that matters when you decide who to blame. The same NRMCA source says scaling mostly happens on concrete that was not built to take it: concrete without enough entrained air, weak or permeable concrete, a wrong mix, a surface that was overworked or steel-troweled while bleed water was still on it, or concrete that was not cured properly. Too much deicer, especially on new concrete, is the trigger on top of that (NRMCA guidance). So salt is the accelerant. A well-built slab handles it far better than a marginal one.

Which deicers are safe, and which should you avoid?

NRMCA's guidance is specific, and it is not what most hardware-store labels suggest (NRMCA deicer guidance):

• New concrete, first winter: avoid deicers if you can. Use clean sand for traction instead.
• After that: use moderate amounts. Sodium chloride (common rock salt) and calcium chloride are considered acceptable for concrete.
• Never: ammonium sulfate, ammonium nitrate, or magnesium-based salts. Check the bag. Some "pet safe" or "concrete safe" products are exactly these.
• After the storm: hose off salt that vehicles track in when you can, and fix any drainage that lets salty water pool against the concrete.

That last point is the cheap one. A low spot that holds salt solution next to a step will keep a section of concrete wet all winter. A downspout that dumps onto the walk does the same thing. Sloping water away from the concrete protects it more than any product you buy.

The good-news version of the NRMCA guidance on building new exterior slabs for freezing and deicing: they recommend a higher-strength mix with about 6 percent entrained air, no extra water added at the jobsite, finishing only after bleed water is gone, no steel troweling, proper curing, and a breathable silane or siloxane sealer as an option (NRMCA guidance). If a contractor is quoting a new stoop or walk in Chicago, those are fair things to ask about. Our concrete and sidewalk guide and the post on why Chicago concrete flakes go deeper.

What does salt do to brick?

Here the honest answer is more careful. Brick does not melt either. What hurts masonry is water carrying dissolved salts into the wall and the salts crystallizing inside the brick, where they create pressure that can make the outer face spall or flake off (National Park Service guidance). Freeze-thaw of saturated brick is a second, separate cause of the same damage (BIA and NPS guidance).

The masonry industry's research says the salts in brickwork usually come from the mortar, the backing, the soil or trim, not usually from the brick itself (BIA Technical Note 23A). We are not going to tell you that road salt is the main culprit on a brick wall, because the sources we rely on do not say that. What they do say is that the same ingredients are needed every time: soluble salts, water, and a path to the surface. Take away the water and you take away the damage.

That is why the bottom courses near a salted sidewalk are worth a look every spring, and why the fix is rarely "seal it." The brick industry does not recommend coatings alone to control efflorescence, and says to find and fix the moisture and salt cause first (BIA guidance). Open mortar joints, failed flashing, a leaking downspout or a roofline that sheds water onto the wall are the usual upstream causes. For the white crust itself, see our post on efflorescence, and for the face damage, how freeze-thaw damages Chicago brick.

Does salt reach the steel inside concrete?

It can, and it is slower and more expensive than scaling. Rebar and mesh inside a slab or step rust when salt and moisture reach them, and rust takes up more room than the steel did, which cracks the concrete from inside. NRMCA advises at least 2 inches of cover over reinforcement to keep salt and moisture off the steel, and notes that reinforcement cannot prevent cracking, only limit how wide cracks open (NRMCA guidance). Rust stains bleeding from a crack, or chunks of concrete lifting over a line, are the signs to watch for on older steps and stoops.

How much freezing and thawing is a Chicago building up against?

Enough to take the question seriously. The National Weather Service's 1991 to 2020 normals put Chicago at about 121 days a year with a low at or below 32 degrees Fahrenheit, and about 43 days when even the high stayed at or below 32 (NWS Chicago). Average annual snowfall is 38.4 inches (NWS Chicago). Snow that gets salted and refreezes is the setup every scaling surface faces. The engineering standards even have a name for it: ACI's category F3 is concrete exposed to freezing and thawing, frequent moisture, and deicing chemicals (NRMCA on ACI exposure classes). A Chicago stoop or sidewalk is a textbook F3 surface.

What should you do before and after winter?

Before the first freeze (our fall repair list has the rest):

• Fix the drainage first: downspout extensions, low spots, anything that sends water onto steps or walks.
• On concrete poured within the past year, plan on sand for traction instead of salt.
• Throw out any deicer that lists ammonium or magnesium compounds.
• Seal-coat only if it is the breathable kind, and ask a contractor which one.

After the thaw (the full walk-around is in what to check after winter):

• Look at concrete surfaces in dry light. Rough, pebbly, flaking tops are scaling.
• Look at the bottom of brick walls near walks, stoops and gutters. Sandy mortar, flaking faces and white crust all point to water.
• Mark any crack with rust staining and photograph it.
• Check that water leaves the area. Standing water in April will be standing salt water next January.

Call a pro when steps are crumbling at the edges, a stoop is losing chunks, rust is bleeding from cracks, or brick faces are coming off. Those are repair and rebuild decisions, and the right fix depends on what is underneath. Our masonry and concrete crews can look at the building and tell you which it is. For the whole-year view, see the Chicago building maintenance calendar.

Common questions

This article explains general rules and common situations in Chicago. It is not legal advice, and City rules change. For a specific property, check with the Chicago Department of Buildings or a licensed professional.