Concrete Spalling: Scaling vs. Spalling, Salt Damage and How to Repair It

Concrete spalling is surface damage where pieces of a slab flake, pop or break away, and in freeze-thaw country it shows up most on driveways, walks and steps after a salty winter. Much of what homeowners call spalling is really scaling, a shallow loss of the top mortar that can be resurfaced, while deeper breaks over rusting steel are a structural problem that a topping won’t fix.

Scaling vs. spalling: name the damage first

People use “spalling” for any concrete that’s coming apart, but the fix depends on which problem you have. NRMCA’s CIP 2 on scaling defines scaling as flaking or peeling of the finished surface from freezing and thawing. Light scaling doesn’t expose the coarse stone. Moderate scaling exposes it and can take 1/8 to 3/8 inch of surface mortar. In severe scaling the stone clearly stands out.

Damage	What you see	Depth or size	Usual cause	How serious
Scaling	Flaked patches, stone showing	1/8 to 3/8 in when moderate	Freeze-thaw and salt on a weak top	Cosmetic when light
Popout	Cone-shaped pits, broken stone at the bottom	1/4 in to a few inches across	A porous stone near the surface freezes	Usually cosmetic
Delamination	Thin sheet lifts off; sounds hollow	1/8 to 1/4 in	Early or heavy finishing traps bleed water	Cut out and patch
Corrosion spall	Rust stains and cracks, then chunks break off	Down to the steel	Chlorides from salt rust the rebar or mesh	Structural

True spalling is the deep kind. NRMCA’s CIP 25 on corrosion explains that rust takes up two to four times the volume of the steel, so rusting bars push the concrete cover off from inside. CIP 20 on delamination notes that corrosion makes thicker separations than finishing problems do.

Why concrete flakes after winter

Soaked concrete that freezes is under stress, because water expands as it turns to ice, and every cycle adds damage. CIP 2 says deicers make it worse by keeping the surface wetter and adding cycles. Air-entrained concrete has millions of tiny bubbles that give the ice room.

NRMCA’s usual causes: too little entrained air at the surface, low-strength concrete, the wrong mix, too much deicer (especially on new concrete), poor finishing and too little curing.

The Pennsylvania Aggregates and Concrete Association’s scaling sheet says never to finish while bleed water is visible or add water to the surface, and NRMCA adds that overworking the surface reduces the entrained air in the top layer. Our guide to how long concrete takes to cure covers curing.

Poured a new slab this fall? Our free concrete cure time calculator turns your pour date and average temperatures into dates to walk, drive and seal, and shows how long to keep deicers off it.

Climate changes the risk. PACA calls scaling common in the colder Northeast, where freeze-thaw and deicers drive it. In warm climates scaling is rarer, though NRMCA notes surfaces can still peel from early steel troweling. Near the coast, sea water supplies chlorides that rust steel. To see how many freeze-thaw cycles your area gets in a year, check freeze-thaw cycles by state.

Salt damage: which deicers hurt concrete

NRMCA and PACA agree that rock salt (sodium chloride) and calcium chloride are generally acceptable on good-quality, dense concrete. PACA’s deicer guidance says magnesium chloride, potassium chloride, calcium magnesium acetate, urea, ammonium sulfate and nitrogen salts cause much higher rates of deterioration.

Never use ammonium sulfate or ammonium nitrate, the fertilizer-type melts; both groups call them too aggressive, and NRMCA adds magnesium-based salts to its never-use list. PACA also warns that “safe” on the front of a bag often comes with conditions in the fine print.

Some of the worst salt arrives on tires. NRMCA notes that magnesium-based salts pre-treat roads and cars carry them onto driveways and garage floors, so hose off that slush and fix spots where salty water ponds. Our guide to salt on pavers covers how much ice melt to use.

Our free ice melt calculator works out how many pounds and bags of ice melt to spread per application from the rate printed on the bag label.

Salt on new concrete

Keep all deicers off new concrete for its first year, as PACA advises. PACA also says never to use them before the concrete reaches at least 4,000 psi, and NRMCA recommends clean sand for traction instead. A late-fall pour is the most exposed; see our guide to how cold is too cold to pour concrete.

Related: when you can drive on new concrete without damaging it

How to tell if it’s cosmetic or structural

Walk the slab with a hammer before you buy anything.

1. Check the depth. Flaked mortar with stone showing is scaling. Pieces broken out down to a bar or mesh is spalling.
2. Tap it. CIP 20 says a delamination sounds hollow when tapped with a hammer or dragged with a heavy chain.
3. Look for rust. Brown stains, cracks in a straight line or exposed steel point to corrosion. Steel too close to the surface rusts sooner; our article on rebar vs. wire mesh shows where it belongs.
4. Count the popouts. NRMCA’s CIP 40 says most are cosmetic and appear in the first year, but a large number lets water in. Small, damp-looking discolored spots can be alkali-silica popouts, a possible sign of later trouble.
5. Compare it to last spring. Damage that grows every winter needs action sooner.

How to repair spalled concrete

Scaled surfaces: a bonded resurfacer

NRMCA says light to moderate scaling can be repaired, while severe, progressing scaling may not be feasible to repair. A repair is only as strong as the surface it bonds to, so remove every loose bit first with a hammer and chisel, sandblaster, pressure washer or jackhammer. The color won’t match the old concrete.

Quikrete’s Re-Cap resurfacer data sheet shows what the work involves:

• Pressure wash at 2,500 psi, then dampen the slab with no standing water
• Apply from a feather edge of 1/16 inch up to 1/2 inch, in layers no thicker than 1/4 inch
• Work in sections of about 144 square feet and keep control and expansion joints open
• Apply between 50 and 90°F; walk on it after 8 hours and drive on it after 24

A 40-pound bag covers about 16 square feet at 1/4 inch. A 12 by 20 foot slab is 240 square feet (two sections of 120 square feet), so it needs about 15 bags at that thickness (240 ÷ 16 = 15).

Re-Cap goes on bare concrete only, not painted or sealed surfaces, and Quikrete warns that old cracks can show back through if the slab moves.

Popouts, delaminations and corrosion spalls

For popouts, CIP 40 says to chip out what’s left of the stone, clean the hole and fill it with dry-pack or epoxy mortar; too many to patch calls for a thin bonded overlay. For delaminations, cut out the loose layer, clean underneath and patch, or grind and overlay large areas. Where steel has rusted, CIP 20 says to blast the rust off the steel before patching. On steps, a porch or anything carrying load, have an experienced concrete contractor or an engineer look first.

When to replace the slab instead

Replacement usually beats repair when scaling is severe and spreading, steel is rusting in many places, or the slab has also cracked and settled. If you replace it, order the concrete for a wet, frozen, salted slab:

Spec	Target
Strength	4,000 psi
Entrained air	6% with 3/4 or 1 in. stone (PACA: 5 to 7%)
Water-cementitious ratio	0.45 maximum
Slump	3 to 5 in., no excess water added at the site
Finish	After bleed water is gone; broom finish

Thickness and base are their own decisions; see our guide to how thick a concrete driveway should be. If you’re replacing a patio or walk anyway, our online patio design plans set its layout, size and materials; the Full Yard plan ($895) adds a grading and drainage concept so salty water runs off instead of sitting on the surface.

Protecting a slab that’s still sound

PACA says penetrating silane or siloxane sealers give the highest level of resistance to deicer damage. NRMCA says to apply a breathable concrete sealer when the slab is reasonably dry, ideally in late summer after a few dry days. If the slab needs resurfacing, do that first, because Quikrete says its resurfacer goes only on bare, unsealed concrete. Then shovel before you salt, salt in moderation and rinse off what the car brings in.

DIY or hire a pro

Light scaling, a handful of popouts and a resurfacer on a small slab are fair DIY jobs. Quikrete gives about 20 minutes of working time at 73°F, so have enough help to finish each section in one go. Hire a pro when large areas sound hollow, you see rust or exposed steel, the damage is on steps or a porch, the slab has cracked and settled, or you’re resurfacing a whole driveway.

Related: All our concrete guides in one place

Frequently Asked Questions

What is the difference between scaling and spalling?

Scaling is shallow flaking of the top mortar from freezing and thawing, often worsened by salt. Spalling is deeper: pieces break off, usually because steel inside is rusting and swelling.

Can you put salt on new concrete?

No. PACA says to keep deicers off concrete for its first year, and NRMCA recommends clean sand for traction instead.

Can spalled concrete be repaired?

Light to moderate scaling can be resurfaced with a bonded cement-based topping once the loose concrete is removed. Corrosion spalls need the rust cleaned off the steel before patching, and severe, spreading damage is usually better replaced.

Does sealing concrete prevent spalling?

A penetrating silane or siloxane sealer helps; PACA says it gives the highest level of resistance to deicer damage. It won’t save concrete that was badly mixed, finished or cured.

Why is my concrete driveway flaking?

Usually it’s scaling: freeze-thaw and deicing salt on a surface that had too little air, extra water or overworking during finishing. Magnesium salts tracked in from treated roads make it worse.

Sources: NRMCA, CIP 2 Scaling Concrete Surfaces; NRMCA, CIP 20 Delamination of Troweled Concrete Surfaces; NRMCA, CIP 25 Corrosion of Steel in Concrete; NRMCA, CIP 40 Aggregate Popouts; Pennsylvania Aggregates and Concrete Association, Protecting Concrete From Deicer Damage; Pennsylvania Aggregates and Concrete Association, Scaling; Quikrete, Re-Cap Concrete Resurfacer data sheet (No. 1131-47, Jan. 2023)

Written and reviewed by Thayne Muthler, owner of Muthler Landscaping & Construction (PA HIC #PA212584). How we check facts: editorial policy.