How to Repair Cracks in Concrete: The Right Method for Each Crack Width

How do you repair cracks in concrete?

Measure the width, pick the method that fits it, then clean and dry the crack before any filler goes in. Width matters because a filler has to get into the crack to hold, and a wide crack needs something to fill its depth.

Most slab cracks open and close a little through the year. The NRMCA explains that concrete expands and contracts with temperature, and when those volume changes are restrained, cracking results (opens in a new tab). That’s why the methods below use flexible fillers instead of mortar.

How do you measure a concrete crack?

Measure its width at the widest point and check whether the two sides are level. A crack width gauge card gives you a reading in seconds, and a tape measure works for anything wider.

Then lay a straightedge across the crack. If one side sits higher, the slab has moved, and the large cracks guide or the sinking slab guide fits better than this one.

Which crack repair method fits your crack?

The width and the location decide it. Both slab kits fill the crack with sand soaked in urethane, and the outdoor kit adds a UV-resistant polyurea top for concrete in the sun.

What do you need to fix cracks in concrete?

A urethane slab crack kit, dry silica sand and a few hand tools. For driveways, patios, walks and steps, use the UV-stabilized kit; for basement and garage floors, the standard kit.

How do you fill a concrete crack with a slab kit?

Open up thin cracks, clean and dry them, pack them with sand soaked in urethane, then finish the top flush. The outdoor UV kit finishes with a polyurea top; the indoor kit finishes with more sand and urethane. Both methods come from the kits’ instructions and cover cracks up to 1/4 inch wide with flush edges.

Here’s how to fill cracks in concrete with the outdoor UV slab kit:

1. Check the crack

   Lay a straightedge across it. If one side is higher, or it’s wider than 1/4 inch, stop and find the cause first.

2. Open up thin cracks

   For cracks 1/8 inch wide or less, the kit’s instructions say to grind the crack out to 1/4 inch wide and 1/4 inch deep so the filler has room to grip. Wider cracks skip this step.

3. Clean and dry the crack

   Vacuum out the dust and loose concrete. The instructions say it’s critical that the crack be dry, so wait out rain, dew and sprinklers.

4. Prime with urethane

   Wet the inside of the crack with the kit’s urethane.

5. Pack in sand

   Pour dry silica sand into the crack until it’s 1/4 inch below the surface.

6. Soak the sand

   Saturate the sand with more urethane and let it set for about 30 minutes.

7. Top it off

   Fill the last 1/4 inch with the polyurea filler. It’s thin and levels itself.

8. Scrape it flush

   Scrape any overflow flush with a razor scraper.

The indoor kit has no polyurea. Its instructions say to wet the crack with urethane, pour in sand, soak it, then add more sand until the crack is full and soak that too, troweling off the excess. Scrape the slight hump flush once it turns light gray, and the basement floor repair guide has those steps in full.

Prep decides whether it holds. A Kansas State study for the US Department of Transportation found that the durability of partial depth repair is directly related to the bond strength (opens in a new tab), and dust, damp and old sealer are what weaken that bond.

How do you fix hairline cracks in concrete?

Leave them alone or use a thin, flexible filler that wicks into the crack. Hairline cracks are usually shrinkage from curing, and a slab kit’s thicker filler can’t get into them without grinding.

For one or two hairline lines, look for a flexible concrete hairline crack filler made for cracks too narrow for caulk. Clean and dry the crack, apply it as the label says, and wipe the surface clean.

If you’d rather use the slab kit for a uniform look, grind the crack to 1/4 by 1/4 inch first, as in the steps above. The hairline cracks guide covers when they’re normal and when they’re not.

What if the crack is wider than 1/4 inch?

Find out why before you fill it. The slab kit’s written instructions cover cracks up to 1/4 inch, and a crack wider than that usually has a cause underneath, like settling soil or a slab with no room to move.

The American Concrete Institute’s repair guide says the cause of the damage must be assessed and corrected (opens in a new tab) before any concrete is repaired. Fill a crack over a cause that’s still active, and it opens again next to or through the repair.

• Flat, stable and wide. The large cracks guide covers filling it and when a wide crack should be treated as a joint.
• Sunk, tilted or offset slab. See sinking slabs and, for driveways, repair or replace.
• Foundation wall. Wall cracks over 1/4 inch, horizontal cracks, offset edges and stair-step cracks in block walls go to a structural engineer, not a kit.

How do you fix a crack with an old repair in it?

Cut out the old filler, clean back to bare concrete, then fill the crack as if it were new. New filler won’t bond to old filler, paint or sealer.

The urethane’s data sheet says oils and sealers will inhibit proper bonding. The American Concrete Institute’s crack sealing guide likewise says to clean away all dirt, oil, grease, paints, striping, curing compounds, and sealers (opens in a new tab) before filling, and to vacuum so dust isn’t packed into the crack.

Before you refill, figure out why the first repair failed. If it pulled loose because the crack moves, the why repairs fail guide covers what to do differently.

How long before you can use the concrete again?

Under an hour with either kit. The outdoor kit’s polyurea top takes light foot traffic in 30 minutes and full traffic in 60 minutes at 70°F, according to its data sheet. The indoor kit’s urethane data sheet says the concrete is back in service within 20 minutes.

Cold slows every cure, and below 50°F the outdoor kit's polyurea shouldn't go on at all. The cold weather guide covers timing.

The cold weather and timing guide covers repairs in late fall and winter.

What mistakes make concrete crack repairs fail?

Damp cracks, dirty cracks and cracks that are still moving cause most failures. Each one breaks the bond between the filler and the concrete.

• Filling a damp crack. The kit’s instructions call a dry crack critical. Wait a dry day after rain.
• Skipping the grinding on a thin crack. Filler can’t get into a crack 1/8 inch or narrower, so it sits on top and peels. Open it to 1/4 by 1/4 inch.
• Using a rigid patch in a crack that moves. Mortar and concrete patch crack loose along the edges.
• Leaving a ridge. Filler above the surface catches shovels, tires and toes. Scrape it flush before it sets.
• Filling over a cause that’s still active. A sinking or heaving slab reopens the crack.

How do you seal cracks in concrete after filling them?

Fill first, then seal the whole slab once the filler cures. Most guides on how to seal cracks in concrete mean filling them, but a sealer and a crack filler are different products.

Concrete Sealers USA’s own sealer FAQ says sealers are not fillers or patching materials, and that cracks greater than 1/16 inch should be repaired before sealing. A sealer over the whole slab then slows water getting into the concrete and the repair.

The sealing after crack repair guide covers which sealer to use over a filled crack and how long to wait. The concrete crack repair hub has the guides for each kind of slab.