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Use a flexible urethane filler for slab cracks up to 1/4 inch, with a UV-stable kit outdoors, and a thin filler that wicks in for hairlines. Joints get a flexible joint filler, and cracks in poured basement walls get polyurethane foam injection. Cracks wider than 1/4 inch, cracks with one side higher and sideways cracks in a basement wall need their cause found before any product goes in.
Answer three questions about your crack and the chooser shows what to use, or tells you when no product is the right answer.
Product chooser
1. Where is the crack?
2. How wide is it?
Match it to the line that looks closest, or measure the widest point with a ruler.
Use a UV-stable slab kit
Outdoor cracks need a filler that flexes through the seasons and holds up in the sun. Clean and dry the crack first; that is where most repairs fail.
What should you use to fill cracks in concrete?
The crack’s location, width and movement decide it. Slabs, joints and basement walls each take a different material, and a crack that’s still moving needs a flexible one.
| Your crack | What to use | Why |
|---|---|---|
| Hairline crack in a slab | Thin filler that wicks in, or a penetrating sealer | Thick fillers can't get into a crack this narrow |
| Slab crack up to 1/4 inch, indoors | Urethane slab kit | Flexes with the slab and fills the full depth |
| Slab crack up to 1/4 inch, outdoors | UV-stable urethane slab kit | Sun breaks down fillers without UV protection |
| Control or expansion joint | Flexible joint filler | Joints are meant to move, so the filler has to stretch |
| Crack in a poured basement wall, dry or leaking | Polyurethane foam injection | Fills the crack through the wall and works when wet |
| Dry crack that needs a structural bond | Epoxy injection | Bonds the two sides back together, but it's rigid |
| Broken edge, hole or chipped corner | Polymer-modified patch over a bonding agent | Rebuilds missing concrete; fillers only fill gaps |
| Wider than 1/4 inch, offset, or sideways in a wall | No product yet | The cause has to be found first |
Our picks for each crack:
- Indoor slab cracks: the urethane slab crack repair kit, small ($84.95), or the medium kit ($149.95) for a garage floor.
- Outdoor slab cracks: the UV-stabilized slab kit ($109.95), made for driveways, patios, steps and pool decks.
- Joints: the polyurea joint filler kit ($159.95), which covers about 30 feet.
- Poured basement walls: the 10 ft polyurethane foam injection kit ($204.95).
- Hairlines: a thin, flexible hairline crack filler.
The Concrete Sealers USA kits are sold online, not in stores. If you’re weighing them against store brands, the retail vs pro-grade guide compares concrete repair products by what’s in the tube.
What’s the difference between concrete crack filler and sealant?
A filler fills a crack’s depth, and a sealant stretches. Rigid fillers and resins suit cracks that have stopped moving, while a flexible sealant bonds to both sides and moves with a crack that opens and closes.
The American Concrete Institute draws the line by movement. Its repair bulletin describes thin resin as a way to seal cracks that are not moving (opens in a new tab), like shrinkage cracks and settlement cracks that have stabilized. For moving cracks, it says it’s most often necessary to rout and seal the crack to create a joint capable of movement (opens in a new tab).
How do you know if your crack is moving? ACI defines an active crack as one whose width changes with time (opens in a new tab). Outdoor slab cracks usually are, because the NRMCA notes that concrete contracts and expands with changes in moisture and temperature (opens in a new tab).
That’s why most crack kits for slabs are flexible urethane or polyurea, not epoxy or cement. The words get mixed up on labels, so it helps to know the five kinds of product:
| Product | What it does | Use it for |
|---|---|---|
| Crack filler | Fills the depth of the crack, often rigid | Cracks that have stopped moving |
| Crack sealant | Bonds to both sides and stretches | Cracks and joints that open and close |
| Penetrating sealer | Soaks into the surface to repel water | Whole slabs and very fine cracks, after repairs |
| Patch or resurfacer | Cement-based, rebuilds missing concrete | Broken edges, holes and scaled surfaces |
| Injection (foam or epoxy) | Pumped in to fill a wall crack through its thickness | Basement wall cracks |
A sealer is the one people most often buy by mistake. It protects the surface and keeps water out of the slab, but it doesn’t fill an open crack. Fill first, then seal.
Should you use epoxy or polyurethane on a concrete crack?
Polyurethane for most basement wall cracks, especially leaking ones, and epoxy only for dry cracks that need a structural bond. Polyurethane stays flexible and works in wet cracks; epoxy is rigid and needs a clean, dry crack to bond.
ACI material on leaking wall cracks notes that flexible hydrophobic polyurethane foam grout typically retains volume after curing (opens in a new tab) and when the surrounding concrete dries. It lists epoxy injection for dormant cracks, as a structural repair.
A wall crack that needs a structural bond is one an engineer should look at first. The epoxy vs polyurethane guide compares them in detail.
Can you use mortar, caulk or hydraulic cement on concrete cracks?
Sometimes, and it helps to know when. Cheap materials work for the right job, and fail fast in the wrong one.
- Mortar or concrete mix suits broken edges and holes when used with a bonding agent. In a thin crack it can’t flex, so it usually breaks loose. The concrete step repair guide shows the edge patch method.
- Caulk works only if it’s a sealant made for concrete. General-purpose caulk isn’t made to bond to concrete outdoors or stand up to sun and foot traffic.
- Hydraulic cement sets fast, which makes it a useful temporary plug for an active leak. It’s rigid, so it isn’t a long-term crack fix. The hydraulic cement guide covers when it works.
- Self-leveling sealant from a store can be fine on a small indoor crack. Outdoors, check that the label says it’s UV-resistant.
Which concrete cracks shouldn’t you fill?
The ones that are still moving or that point to a bigger problem. A product hides these without fixing them, and can hide the evidence an inspector needs to see.
- Horizontal cracks in a basement wall, or a wall that bows inward.
- Stair-step cracks in a block wall.
- Any crack wider than 1/4 inch.
- Cracks with one side higher than the other, in a wall or a slab.
- Cracks that keep getting longer or wider.
For a foundation wall, have a structural engineer look first. For a slab with one side lower, start with the sinking slab guide. The when to worry about foundation cracks guide shows how to read width and direction.
Questions
Can I fill concrete cracks with silica sand and epoxy?
Sand packed into a crack and soaked with resin is how the urethane slab kits handle depth, so the idea works. The difference is the top layer: a flexible top moves with the slab, while a rigid epoxy top can crack loose if the crack is still moving.
Does Flex Seal work on concrete cracks?
It can cover a crack for a while, but it's a coating, not a filler, and it doesn't fill the crack's depth. The Flex Seal on concrete guide goes through where it holds up and where it doesn't.
Can I seal the concrete after filling the cracks?
Yes, and outdoors it's worth doing. Let the filler cure first, then use a penetrating sealer. The sealing after crack repair guide covers timing and which sealer to use.
What's the best concrete crack filler?
The one made for your crack's width and location. A filler that's great on a garage floor can fail outdoors in the sun, and nothing fills a crack that's still moving. The best concrete crack filler roundup compares them by crack type.