Spalling, pitting and freeze-thaw damage: causes and fixes

Four kinds of surface damage

People use "spalling," "pitting" and "flaking" loosely. The names matter, because each has a different cause and fix.

What you see	Name	Depth	Usual cause
Thin flakes peeling off in patches, sometimes exposing stone	Scaling	Light: no stone showing. Moderate: up to 1/8 to 3/8 in[1]	Freeze-thaw, deicers, weak surface[1]
Small cone-shaped holes, broken stone at the bottom	Popouts	1/4 in to a few inches across[2]	Porous stones swelling as they freeze[2]
Thin sheets that sound hollow, then break off	Delamination	About 1/8 to 1/4 in layers[4]	Finishing that sealed the surface while it was still bleeding[4]
Larger, deeper chunks breaking off	Spalling	Deeper than scaling	Rusting steel[3], curled edges hit by traffic[5]

"Pitting" usually means scaling or popouts, depending on whether the pits are shallow patches or cone-shaped holes.

Scaling: the classic freeze-thaw problem

NRMCA defines scaling as local flaking or peeling of a finished surface from cycles of freezing and thawing. It starts as small patches and can merge into large areas. Light scaling does not expose the coarse aggregate. Moderate scaling exposes it and can remove 1/8 to 3/8 inch of surface mortar. In severe scaling the aggregate clearly stands out^[1].

Why it happens. When saturated concrete freezes, the water in it expands and creates stress. Each cycle adds damage. Deicing salts make it worse by keeping the surface wetter and adding freeze-thaw cycles. Concrete made with entrained air has millions of tiny bubbles that give the freezing water room to expand^[1].

NRMCA lists the main causes of scaling^[1]:

• Concrete with no entrained air or too little, especially at the surface.
• Low-strength concrete that lets water in.
• The wrong mix for the job.
• Heavy deicer use, especially on new concrete.
• Poor finishing, such as working the surface with bleed water on it.
• Poor curing that leaves a weak surface.

What good exterior concrete looks like. For slabs that stay moist, freeze and get deicers, NRMCA recommends ordering 4,000 psi concrete consistent with ACI 332, about 6% total air for typical 3/4 or 1 inch stone, a slump of 3 to 5 inches with no extra water added on site, a broom finish rather than hard troweling, and proper curing^[1]. If you are replacing a scaled driveway in Ohio or Illinois, ask the contractor to put those items in writing.

Popouts: cone-shaped pits

A popout is a small cone-shaped hole left when a stone near the surface breaks from internal pressure. Part of the broken stone often sits at the bottom of the hole. They range from about 1/4 inch to a few inches across^[2]. Most come from porous, lightweight stones (often low-density chert) that soak up water and crack when it freezes. They usually show up in the first year^[2].

NRMCA says most popouts are cosmetic and do not affect structural performance or long-term service life, but a large number of them let water and chemicals into the slab^[2]. A second type, from a chemical reaction in the stone (alkali-silica reaction), tends to be small with a damp or discolored spot and may point to a later durability problem^[2].

Fix: chip out the remaining stone, clean the hole and fill it with dry-pack mortar, epoxy mortar or a similar repair product. Color matching is hard. If there are too many to patch one by one, a thin bonded overlay can restore an even surface^[2].

Delamination: hollow-sounding sheets

Delaminations are thin layers, typically 1/8 to 1/4 inch, that separate from the concrete below^[4]. They form when the surface is troweled tight while the concrete underneath is still bleeding, trapping water and air. You can find them by tapping with a hammer and listening for a hollow sound^[4]. They are most common on hard-troweled floor slabs^[4].

Fix: remove the loose layer and patch over clean concrete; large areas may need grinding and an overlay^[4].

Spalling: the deeper kind

Spalls are larger, deeper breaks. Two causes matter most on home concrete:

• Rusting steel. Steel in concrete is protected by the concrete's high alkalinity. Chlorides, often from deicing salt, break that protection and rust starts. Rust takes up two to four times the volume of the original steel, which cracks, delaminates and spalls the concrete over it^[3]. NRMCA recommends at least 2 inches of cover over steel in slabs to keep salt and water away^[7].
• Curled edges. Slab edges and corners that curl up at joints are unsupported. NRMCA notes they can chip or spall when traffic hits them^[5].

Fix: spalls over rusting steel need the rust dealt with, not just a patch. That is work for an experienced repair contractor, and for anything structural (a porch slab over a space, a stair landing that spans), a structural engineer.

The deicer question

NRMCA's advice on deicers is specific^[1]:

• Avoid deicers on new concrete in its first winter if you can. Use clean sand for traction.
• Use moderate amounts. Calcium chloride and sodium chloride (rock salt) are considered acceptable.
• Never use ammonium sulfate, ammonium nitrate or magnesium-based salts. They chemically attack concrete. Magnesium-based salts are used on roads and can be tracked onto your driveway by cars.
• Hose off salt that drips from cars onto driveways and garage floors when weather allows.
• Fix drainage so salty water does not pool on the slab.

That makes the common claim "rock salt ruins concrete" half true. The damage comes from heavy use, new concrete, poor-quality concrete and the wrong chemicals.

Sealers help, within limits

NRMCA recommends a breathable silane or siloxane-based sealer or water repellent made for concrete slabs, applied to reasonably dry concrete per the maker's directions. It names late summer, after a few dry days, as an ideal time^[1]. A sealer reduces how much water and salt get in. It does not fix poor air content, weak surfaces or cracking.

Cold-weather pours

Damage often starts with when the slab was poured. NRMCA says fresh concrete freezes at about 25°F, frozen concrete can lose more than 50% of its potential strength, and concrete should be protected from freezing until it reaches about 500 psi, roughly two days. Saturated new concrete should be protected from freeze-thaw cycles until it reaches about 3,500 psi^[6]. Be wary of a late-fall pour with no plan for blankets or heat.

Repair or replace?

Condition	Usual answer
Light scaling, small area	Leave it, seal it, cut back on deicers
Light to moderate scaling over a sound slab	Remove loose material, then a thin bonded resurfacing[1]
Scattered popouts	Patch[2]
Many popouts	Patch or overlay[2]
Severe, spreading scaling	Often replacement; NRMCA says repair may not be feasible[1]
Spalls over rusting steel	Qualified repair; engineer if structural[3]

NRMCA warns that a resurfaced layer is only as strong as the concrete it is bonded to, and the repair material will not match the original color^[1]. HomeGuide puts resurfacing at $3 to $7 per square foot and says it lasts 8 to 15 years on a sound base^[8].

When to call a pro

• Scaling or popouts are spreading every winter across a driveway or walk.
• Rust stains, exposed steel or spalls on steps, landings or porch slabs.
• A hollow sound over a large area of a garage or patio slab.
• You are replacing exterior concrete in a freezing climate and want the mix and curing specified in writing.