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Most concrete cracks are shrinkage cracks you can fix yourself. Clean the crack, let it dry and fill it with a flexible urethane filler, using a UV-stable one outdoors, or a thin filler for hairlines. Cracks wider than 1/4 inch, cracks with one edge higher than the other and cracks that keep growing have a cause underneath that a filler won't fix.
How do you repair cracks in concrete?
Clean the crack, let it dry, and fill it with a flexible filler suited to its width and location. Indoors, a standard urethane filler works. Outdoors, use one with UV stabilization, because sun breaks down fillers that don’t have it.
Here’s the basic method to fix concrete cracks in any slab:
Check the crack first
Lay a straightedge across it and measure the width. If one side is higher, or it’s wider than 1/4 inch, read the section on cracks that aren’t a DIY repair before you buy anything.Clean it out
Scrape out loose concrete, dirt, weeds and old filler, then vacuum or blow out the dust.Let it dry
Most fillers need dry concrete. Wait a day or two after rain, unless the product says it bonds to damp concrete.Fill the depth
Deep cracks get filled below the surface first so the top filler isn’t wasted. Slab kits use dry silica sand soaked in urethane; some joint sealants use foam backer rod instead.Top it off
Fill the last 1/4 inch level with the surface and scrape off any overflow.Let it cure
Keep traffic off it for the cure time on the label.
The how to repair cracks in concrete guide goes through each step in detail, including how to deal with old failed repairs.
Which repair method fits your crack?
Width and edges decide it. A hairline crack needs a thin filler that can get in, a wider crack needs a thicker filler and something to fill its depth, and a crack that has moved needs its cause found before anything else.
| Crack | What to use | Guide |
|---|---|---|
| Hairline, flush edges | Thin flexible filler, or a penetrating sealer over the whole slab | Hairline cracks |
| Up to 1/4 inch, flush edges | Flexible urethane crack filler | How to repair cracks |
| Deep crack up to 1/4 inch wide | Sand soaked in urethane to fill the depth, with a polyurea top outdoors | Large cracks guide |
| Wider than 1/4 inch | Find out why first, then a large-crack repair | Large cracks |
| One edge higher, or growing | Not a filler job until the cause is fixed | Sinking slabs |
| Straight line in a groove | Probably a control joint; joint filler if you seal it | Crack or joint |
Most cracks you’ll see are the first two kinds. The NRMCA says most early random cracks, although unsightly, rarely affect the structural integrity (opens in a new tab) of the concrete. They’re still worth filling outdoors, because water that gets in freezes, and the NRMCA notes that water expands as it forms ice (opens in a new tab).
Where is the crack?
Each location has its own problems: sun, salt, car weight, water or trip hazards. Find yours in the picture, then go to that guide.
- Driveway. Sun, car weight, salt and weeds. Use a UV-stable filler and deal with any gap where the driveway meets the garage or house.
- Garage floor. Fill before you coat or seal, since cracks show through coatings.
- Patio. Sun and water, plus joints that need resealing.
- Sidewalk. Raised edges are a trip hazard that filling doesn’t fix.
- Steps. Cracks, broken edges and steps pulling away each need a different fix.
- Pool deck. Most cracks at the coping are the joint, which needs a flexible joint filler.
- Basement floor. Indoor cracks, plus water and radon questions.
Cracks in a basement or foundation wall are a separate topic, with their own rules. Start at the foundation cracks guide.
What’s the best way to fill a concrete crack?
With a flexible filler that bonds to concrete. Concrete moves a little all year: the NRMCA says it contracts and expands with changes in moisture and temperature (opens in a new tab). A filler that stretches stays in the crack; one that can’t, pops loose.
Urethane slab kits are made for this. Pick by where the crack is and how much of it there is:
| Where | Kit | Notes |
|---|---|---|
| Basement floor, small job | Urethane slab kit, small | Indoor only, no UV protection |
| Garage or larger indoor floor | Urethane slab kit, medium | More filler for longer or more cracks |
| Large indoor job | Urethane slab kit, large | Many cracks or a big floor |
| Driveway, patio, steps, pool deck | UV slab kit, small | UV-stable for sun exposure |
| Large outdoor job | UV slab kit, large | Long driveway cracks or several slabs |
For outdoor concrete, start with the small UV kit and size up if you have a lot of crack length.
Indoors, the small urethane slab kit, medium kit or large kit covers it, and the large UV kit handles big outdoor jobs. The product chooser narrows it down by crack, width and location if you’re not sure.
Is cement crack repair different from concrete repair?
No, it’s the same job. Cement is the powder in concrete that binds the sand and stone together, so a “cement” driveway or floor is concrete, and cement crack repair means the same thing as concrete crack repair.
Where it matters is the product. Cement-based patches are rigid: they suit rebuilding a broken step edge or filling a hole, but they don’t flex in a crack. A Kansas State University study for the U.S. Department of Transportation found that patch durability is directly related to the bond strength (opens in a new tab) to the old concrete, which is why cement patches need a bonding agent and careful prep.
Concrete repair also covers more than cracks. A flaking or pitted surface is spalling or scaling, and a slab that has dropped is a sinking slab. Neither is fixed by filling cracks.
When is a concrete crack not a DIY repair?
When the slab has moved. A crack with one edge higher than the other, a crack wider than 1/4 inch, or one that keeps growing has a cause underneath, usually settling soil or water. Filling hides it until the crack opens again.
Lay a straightedge across the crack and mark the ends with a pencil and the date. If it’s level and the marks haven’t moved after a few months, it’s safe to fill. The why concrete cracks guide explains the causes.
Slab cracks near the house deserve a closer look if they line up with cracks in the foundation wall, or if doors and windows have started to stick. Those are signs of movement in the house itself, and a structural engineer is the right person to judge them.
Why do concrete crack repairs fail?
Usually because the crack wasn’t clean and dry, the filler was rigid, or the slab is still moving. A filler can only hold as well as its bond to the sides of the crack.
The fix is in the prep: clean out every bit of dust and old filler, wait for dry concrete, and use the right filler for the width. The why repairs fail guide goes through each cause, and the cold weather guide covers timing.
Once the repair cures, sealing the slab keeps water out of new cracks. The sealing after crack repair guide covers which sealer to use and how long to wait.
Every guide in this section
Is it a problem?
How to fix it
- How to repair cracks in concrete
- Large cracks in concrete
- Driveway crack repair
- Garage floor crack repair
- Patio crack repair
- Sidewalk crack repair
- Pool deck crack repair
- Concrete step repair
- Why your crack repair keeps failing
- Cold weather and timing
Cost, insurance and who to call
Related problems
Questions
Can I fill concrete cracks with mortar or concrete mix?
You can, but a rigid patch in a thin crack usually breaks loose. The slab keeps moving a little with temperature and moisture, and a cement patch can't stretch with it. Flexible fillers last longer in cracks; cement patches suit broken edges and holes.
Does caulk work on concrete cracks?
Only a sealant made for concrete. General-purpose caulk isn't made to bond to concrete outdoors, flex with a slab or stand up to sun and foot traffic.
What temperature do I need to fix concrete cracks?
Follow the range on the product label. The polyurea top in the outdoor UV slab kits calls for concrete at 50°F or warmer, and every filler needs dry concrete, so spring and fall are usually easiest.
Will the repair be invisible?
No. A filled crack shows as a thin gray line. Scraping the filler flush with the surface keeps that line as thin as possible.