Concrete Spalling Repair: How to Fix Flaking, Scaled and Pitted Concrete

What is the difference between spalling and scaling?

Scaling is the thin top layer flaking off; spalling is deeper pieces breaking away. People use both words for any damaged concrete surface, but they have different causes and different fixes.

The NRMCA defines scaling as local flaking or peeling (opens in a new tab) of the finished surface from freezing and thawing. Light scaling doesn’t expose the coarse aggregate (the stones in the mix), moderate scaling exposes it and can take off 1/8 to 3/8 inch of surface mortar, and severe scaling leaves the stones standing out. InterNACHI describes concrete scaling the same way: the shedding of flakes (opens in a new tab) of hardened concrete at the surface.

Spalling starts below the surface. The Federal Highway Administration lists spalling and cracking over embedded steel (opens in a new tab) as the sign of rusting reinforcement, and thin sheets that pop off a troweled slab are delaminations (opens in a new tab), usually 1/8 to 1/4 inch thick.

Pitted concrete is usually popouts. The NRMCA describes a popout as a small, cone-shaped hole (opens in a new tab) left where a stone near the surface broke, from 1/4 inch to a few inches across, often with part of the broken stone still at the bottom.

NRMCA Concrete in Practice sheets 2, 20, 25 and 40; Federal Highway Administration tech brief on materials-related distress (2015)

What causes concrete scaling?

Water in the surface freezing and thawing, made worse by de-icing salt. Water expands as it freezes, and the NRMCA says de-icers make it worse (opens in a new tab) by keeping the surface wetter and adding more freeze-thaw cycles.

The same NRMCA sheet traces most scaling back to how the slab was made:

• Too little entrained air. Air-entrained concrete holds millions of tiny bubbles that give freezing water room to expand.
• Weak or overly wet concrete that lets water soak in.
• Finishing mistakes. Working the surface while bleed water is on it, or overworking it, weakens the top layer and knocks the air out of it.
• Poor curing, which leaves a weak surface.
• Heavy de-icer use, especially on new concrete.

How much salt damage concrete takes depends a lot on how it was made, because salt damage is freeze-thaw damage with a boost. The Federal Highway Administration explains that the expansive force of the ice is at its maximum when the freezing solution holds about 3% dissolved salt (opens in a new tab), so a weak brine does the most harm.

That’s why InterNACHI finds this damage most often on driveways and garage floors (opens in a new tab). Cars carry frozen slush full of road salt in on their undercarriage, and it melts onto the slab.

What causes concrete spalling?

Usually rusting steel or a weak layer trapped under the surface. Both start below the top, which is why spalls are deeper than scaling.

• Rusting rebar or mesh. Chlorides from de-icing salt break down the layer that protects steel in concrete, and the NRMCA says rust takes up two to four times the volume (opens in a new tab) of the original steel. The swelling pushes off the concrete over it. Rust-colored stains seeping from the concrete are the giveaway, per the Federal Highway Administration (opens in a new tab).
• Delamination. When a slab is troweled while it’s still bleeding, the dense top layer traps air and water underneath and later breaks off. The NRMCA says a hollow sound when tapped with a hammer (opens in a new tab) gives it away.
• Freezing in porous stone. Some stones in the mix soak up water. When it freezes, they crack and push off the surface above them, which is how popouts form.

Is spalling concrete a problem?

Light scaling and a few popouts are cosmetic; spalls over rusting steel are not. The NRMCA calls minor scaling a cosmetic issue (opens in a new tab) that may not need repair, and says most popouts are aesthetic defects (opens in a new tab) that don’t affect the slab’s structure.

The catch is water. The same popout sheet says a large number of popouts make it easier for water and harmful chemicals to get in, which can lead to other damage.

Rusting steel is a bigger deal. The NRMCA says rust reduces the steel’s strength (opens in a new tab) and, left unchecked, can affect the integrity of the structure. On a driveway that means the damage keeps spreading; on a wall, column or beam, have a structural engineer look before anyone patches it.

How do you repair spalled or scaled concrete?

Remove everything loose, then bond a thin resurfacing layer to the sound concrete underneath. The NRMCA says light to moderately scaled surfaces can be repaired (opens in a new tab) this way, and that the repair is only as strong as the surface it’s bonded to.

1. Tap it

   Tap the damaged area and the concrete around it with a hammer. A hollow sound means the top has let go and has to come off, not be covered.

2. Remove the loose concrete

   Chip, scrape or blast away everything that’s flaking or hollow until you reach solid concrete. The NRMCA lists a hammer and chisel, sandblasting, a high-pressure washer or a jackhammer.

3. Stop if you hit rusted steel

   Rusted rebar or mesh needs more than a resurfacer: the NRMCA says the steel needs sandblasting to remove the rust. That’s a contractor job.

4. Clean it

   The surface has to be free of dirt, oil and paint, and left clean and rough so the new layer can grip.

5. Resurface

   Mix and apply a thin bonded resurfacer, following its label for wetting the slab first, thickness and temperature. The NRMCA lists portland cement, latex modified and polymer-modified cement repair mortars for this.

6. Cure, then seal

   Let it cure as long as the label says, then seal the whole slab with a breathable penetrating sealer.

A polymer-modified concrete resurfacer suits a scaled driveway, walk or patio. Plan on covering a whole section between joints: the NRMCA warns that repair material won’t match the color (opens in a new tab) of the original concrete, and an edge at a joint hides the difference.

How do you fix pitted concrete?

Patch each pit, or resurface the slab if there are too many to patch. For pitted concrete repair, the NRMCA says to chip out what’s left of the broken stone (opens in a new tab), clean the hole and fill it with a repair material such as dry pack mortar or epoxy mortar, following the maker’s directions.

A polymer-modified concrete patch fills individual pits. If the patch label calls for a primer, an epoxy bonding agent helps the new material grip the old concrete.

If popouts cover the slab, the same NRMCA sheet suggests a thin bonded overlay instead. Watch for small popouts with a damp-looking or discolored spot: those can come from a chemical reaction in the stone, which the NRMCA says may mean a durability problem later, so have a contractor look first.

When does spalling need a contractor or a new slab?

When steel is showing, when the damage is deep and widespread, or when it keeps spreading. The Federal Highway Administration says materials-related damage in concrete tends to be progressive and difficult to stop once started (opens in a new tab).

• Rusted rebar or mesh. The steel has to be cleaned or replaced and the concrete rebuilt.
• Hollow or flaking over large areas. The NRMCA says extensive delamination may need grinding and a new surface (opens in a new tab), and that repairing excessive, progressing scaling may not be feasible.
• A slab that has also sunk or broken up. A resurfacer won’t level it or hold it together. The repair or replace guide walks through that decision, and the sinking slab guide covers lifting.
• Concrete that holds something up. Spalls on a foundation wall, column or beam go to a structural engineer, along with horizontal basement wall cracks, bowing walls, wall cracks wider than 1/4 inch, offset edges and stair-step cracks in block walls.

Broken step edges and corners are a different repair; the concrete steps guide covers them. For cracks rather than surface damage, start at the concrete cracks guide.

How do you stop concrete from scaling again?

Seal it with a breathable penetrating sealer, go easy on de-icers and keep salty water from sitting on it. The NRMCA recommends a silane or siloxane breathable sealer (opens in a new tab) applied when the concrete is reasonably dry, with late summer after a few dry days as the ideal time.

The Federal Highway Administration adds that properly applied silane and siloxane sealers reduce water getting in (opens in a new tab). Less water in the surface means less to freeze.

PS110’s data sheet reports a 94.0% cut in chloride absorption in a northern exposure test and 98.9% in a southern one, and no scaling in the ASTM C672 freeze-thaw test. For a dense, smooth slab that gets salted every winter, the silane PS109 goes deeper; its data sheet reports an 88% cut in chloride absorption in a northern exposure test.

Seal on a dry day between 40°F and 95°F with no rain for 12 hours, and only after the water drop test shows drops soaking in within 3 to 5 minutes. Skip film-forming topical sealers outside; the penetrating vs topical guide explains why, and the driveway sealer guide matches a sealer to your finish.

Then change the winter routine, per the NRMCA:

• No de-icers on new concrete in its first winter. Use clean sand for traction. The Federal Highway Administration also advises sparing use on concrete a year old or newer (opens in a new tab).
• Calcium chloride or rock salt only, in moderate amounts. Never ammonium sulfate, ammonium nitrate or magnesium-based salts, which the NRMCA says destroy concrete surfaces.
• Hose off road salt that cars drop on driveways and garage floors when the weather allows.
• Fix low spots and drainage so salty water doesn’t pool on the slab.