Most concrete crack repairs that fail don’t fail because the product was bad. They fail because the product didn’t suit the crack. A rigid filler goes into a crack that opens and closes with the seasons, or a flexible sealant goes somewhere that needed strength, and a year later the crack is back, usually with the old filler still stuck along one edge.
So before comparing products, it pays to spend two minutes working out what kind of crack you actually have.
First, read the crack
Three questions sort nearly every crack on a driveway, patio, walkway, or garage floor.
How wide is it? A hairline crack you can barely fit a fingernail into is a different job from one you can push a pencil into. Measure at the widest point, not the average.
Is it moving? Cracks in outdoor slabs often open slightly in winter and close in summer, and a crack between two slab sections that rock or shift underfoot is clearly moving. An easy test is to draw a pencil line across the crack at each end, then check it again in a few weeks. If the marks have drifted apart or the crack has grown past them, treat it as a moving crack.
Is one side higher than the other? If you can feel a step as you run your hand across the crack, the slab has moved vertically. That changes the answer completely, and it’s covered in the last section below.
The three filler families, and what each one is actually good at
Cement-based fillers are essentially fine concrete or mortar in a more convenient form. They cure hard, match the look of the surrounding slab well, and suit narrow cracks that aren’t moving. Their weakness is the same as concrete’s: they don’t stretch, so in a crack that keeps moving, the repair simply cracks again along the same line.
Flexible sealants, mostly polyurethane and elastomeric acrylic, stay rubbery after they cure. They won’t add strength, but they keep water out of a crack that opens and closes, which is what actually stops freeze-thaw damage from widening it. The number worth looking for is the movement class under ASTM C920: a Class 25 sealant can handle a joint moving 25% wider or narrower than the width it cured at. We go into how that rating works in our guide to caulk for exterior windows, and the same logic applies to concrete.
Epoxy is the strength option. A two-part epoxy bonds the two sides of a crack back together and is typically much stronger than the concrete around it. It’s also completely rigid, so it’s the right call for a stable crack where strength matters and the wrong call for anything that moves.
Narrow, stable cracks in walkways and patios
For a crack up to about half an inch wide that isn’t moving, a cement-based filler is the simplest fix and the one that blends in best. DAP’s Concrete Crack Filler is a water-activated dry powder: you pack it into the crack and mist it with water, with no mixing bucket involved. DAP rates it for cracks up to 1/2 inch wide and 1/2 inch deep in a single pass, ready for light foot traffic after about 4 hours and for vehicles after about 48 hours.
Check price on Amazon: DAP Concrete Crack Filler. This is an affiliate link (see our disclosure for details).
Cracks that move with the seasons
Driveways and outdoor slabs are where most homeowners meet a moving crack, and this is where a flexible sealant beats anything rigid.
Sashco Slab is an acrylic sealant with a sanded, concrete-like texture, so it looks far less like a caulk line than most flexible options. Sashco rates it for cracks up to 3 inches wide and says it stretches to nearly 300% of the original crack width, which leaves plenty of room for a slab that shifts with soil moisture and temperature. For cracks an inch or wider, Sashco recommends running a bead along each edge and a third down the middle rather than trying to fill it in one pass.
Sikaflex Self-Leveling Sealant is a polyurethane that flows into a horizontal crack and levels itself, which gives a neat flat finish on driveways and garage floors. Sika’s product data sheet lists it as ASTM C920 Class 25, suited to joints from 1/4 inch up to about 1 inch wide, with a width-to-depth ratio of roughly 2 to 1. Because it’s self-leveling, it’s for horizontal surfaces only; on a sloped section it will run.
Check prices on Amazon: Sashco Slab Concrete Crack Filler or Sikaflex Self-Leveling Sealant. These are affiliate links (see our disclosure for details).
Don’t skip the backer rod on deep cracks
A flexible sealant only works if it’s bonded to the two sides of the crack and free at the bottom. If it also sticks to the bottom of a deep crack, it can’t stretch properly and tears the first time the slab moves. Sika’s data sheet calls for a backer rod or bond-breaker tape in the bottom of the joint for exactly this reason. Push a closed-cell foam backer rod slightly wider than the crack into it, leaving about a quarter to half an inch of depth for the sealant on top. It also saves a lot of sealant in a deep crack.
Check price on Amazon: M-D Building Products Foam Backer Rod. This is an affiliate link (see our disclosure for details).
Stable cracks where strength matters
Some cracks aren’t moving but still need more than a cosmetic fill: a cracked step edge, a crack running through a slab where something is bolted down, or a crack on a vertical surface where a pourable filler would simply run out. That’s epoxy territory.
PC-Concrete is a two-part epoxy paste that PC Products says is about three times stronger than concrete once cured. It’s thick enough to stay put on vertical surfaces and overhead, and it can be applied to damp as well as dry concrete, which helps outdoors. The manufacturer lists a working time of about 20 minutes and a cure of about 4 hours at 75°F, so mix only what you can use in that window.
Check price on Amazon: PC-Concrete Two-Part Epoxy. This is an affiliate link (see our disclosure for details).
The one rule with epoxy: only use it on a crack you’re confident has stopped moving. If the slab keeps shifting, a rigid epoxy repair either cracks through or, worse, stays intact while a new crack opens right beside it.
Prep decides whether any of this lasts
Whatever you choose, the repair only holds as well as its bond to the crack walls.
- Scrape out loose concrete, old filler, and weeds, then vacuum or blow out the dust. Filler bonded to dust is bonded to nothing.
- For hairline cracks, widen the top slightly with a chisel or grinder so the filler has something to grip. A crack too narrow to get product into can’t be filled properly.
- Check the label’s temperature range and the weather forecast. Most water-based fillers and sealants need a dry day or more after application, and a rain shower too soon can wash out an uncured repair.
- Keep traffic off for the full cure time listed, especially vehicles. The top can feel firm long before the material underneath is ready to take a tire.
When a crack is no longer a DIY job
A filler seals a crack. It doesn’t fix whatever caused it. InterNACHI, the home inspectors’ association, notes in its guide to inspecting concrete that cracks under about 1/4 inch wide with no significant vertical or horizontal displacement generally aren’t considered a structural threat, and are usually down to ordinary shrinkage. Cracks that go past that point are a different matter, and inspectors recommend having a structural engineer look at them.
In practice, get a professional opinion before filling anything if one side of the crack is higher than the other, the crack keeps getting wider after you’ve sealed it, a slab section has sunk or tilted, or the crack is in a foundation wall with water coming through. Filling those cracks just hides the symptom while the underlying movement carries on.
If you’d rather have a concrete contractor take a look, you can compare local concrete repair pros on Angi. This is an affiliate link (see our disclosure for details).
Quick picks
- Narrow, stable crack, want it to disappear: a cement-based filler such as DAP Concrete Crack Filler.
- Driveway or patio crack that moves with the seasons: Sashco Slab, with backer rod in anything deep.
- Flat driveway or garage-floor crack up to about an inch, neat level finish: Sikaflex Self-Leveling Sealant over backer rod.
- Stable crack that needs strength, or a vertical surface: PC-Concrete epoxy.
- Wider than about 1/4 inch with one side higher than the other: get it looked at before filling anything.
This guide covers wall and slab cracks in general. If the crack is specifically in a garage, driveway, or basement floor, our concrete floor crack filler guide covers the traffic-load and moisture concerns that come with a floor specifically.
Etienne Basson maintains and repairs his own home, and writes practical guides on choosing the right materials and products for repair and finishing jobs: fillers, sealants, adhesives, and similar. He focuses on what actually works for a specific job, not just a spec sheet.