Best Concrete Crack Filler for Each Type of Slab Crack

The best concrete crack filler is the one that matches your crack’s width, location and whether it moves. No single product fills a hairline, a 1/4 inch driveway crack, a control joint and a broken step edge equally well, so the picks below are sorted by crack type.

What is the best concrete crack filler for each type of crack?

Use the table to find your crack, then read the section for it below. Widths are measured at the surface with a ruler or crack width card.

Concrete Sealers USA slab kit, joint kit and sealer data sheets and product pages; NRMCA CIP 3 on crazing

What filler works for hairline cracks?

For one or two hairline lines, a thin squeeze-bottle filler is simplest. Look for a flexible concrete hairline crack filler that says it’s made for cracks too narrow for caulk.

The slab kit also handles hairlines, but with more work. Its data sheet says cracks 1/8 inch and narrower get ground open to 1/4 inch wide by 1/4 inch deep first, so the sand and resin can get in.

A slab covered in fine cracks is a different job. The NRMCA says craze cracks do not affect the structural integrity (opens in a new tab) of concrete, so a penetrating sealer over the whole surface makes more sense than filling each line. The hairline crack guide shows how to tell the patterns apart.

What’s the best filler for 1/8 to 1/4 inch cracks?

A urethane slab kit. You prime the clean, dry crack with urethane, pour in dry silica sand, soak the sand with urethane and, on the UV kit, top it with polyurea that self-levels and gets scraped flush.

The sand is what makes it work in a deep crack. A thin liquid poured straight into a wide crack drains down and keeps sinking; the sand fills the depth, and on the UV kit the polyurea top only covers the last 1/4 inch. The data sheet says cracks 1/8 to 1/4 inch wide are not ground, but they must be clean and dry.

Pick the kit by where the crack is:

• Indoor slabs (basement and garage floors): the urethane slab kit. Its product page says the urethane cures tan and isn’t UV stable, which is fine out of the sun. The small kit covers about 40 to 100 feet of crack, the medium about 80 to 200 feet.
• Outdoor slabs (driveways, patios, walks): the UV slab kit. It adds a polyurea top that the data sheet describes as UV resistant, with 170 percent elongation, curing gray or black.

Neither kit includes the sand, and the small and medium kits need a standard caulk gun. The large crack guide walks through the full method, and the how to repair concrete cracks guide covers each slab.

Is a cement crack filler a good idea?

Not for a crack that still moves. A cement crack filler, like mortar, hydraulic cement or a patching compound, cures hard and rigid. Concrete contracts and expands with changes in moisture and temperature (opens in a new tab), so a rigid fill tends to crack again along its edges.

The American Concrete Institute’s bulletin on gravity-fed resin says that kind of crack filling is not effective for repairing moving cracks (opens in a new tab); a moving crack should be routed and sealed to create a joint capable of movement. That’s why the slab kit and joint kit are flexible.

Cement-based products do have a place: rebuilding concrete that’s broken off. A polymer-modified vertical patch over an epoxy bonding agent rebuilds a chipped step edge, as the concrete steps guide shows. The hydraulic cement guide covers its use on leaking basement cracks.

What filler should go in a control or expansion joint?

A joint filler made to flex, not a crack filler. Joints are cut or formed so the slab can move, and the filler has to move with it.

The polyurea joint kit’s data sheet gives coverage for joints 1/8 to 1/4 inch wide and 3/4 to 2 inches deep, and says the concrete should be at least 50°F. Its product page says the concrete must be 28 days old and puts coverage at about 30 feet of typical joints.

The expansion joint repair guide covers when to use backer rod instead, and the crack or joint guide helps if you’re not sure which you have.

What about crack fillers from the hardware store?

Store crack filler brands, such as Quikrete, DAP, Sika and Flex Seal, come in a few basic types: thin pourable liquids, self-leveling sealants in a caulk tube, cement-based patches and rubberized coatings. Judge any of them by the same questions as the picks above.

• How wide and deep does the label say it fills? Check what it says to put in a deep crack before the filler.
• Is it flexible or rigid? Rigid products suit broken edges, not moving cracks.
• Is it rated for sun and outdoor use? NIST research found ultraviolet light is the primary cause (opens in a new tab) of polyurethane coating breakdown.
• What temperature does it need? Cold concrete slows curing for every product.

The reviews of Quikrete Crack Seal, DAP concrete crack filler and Flex Seal on concrete go through each one’s label against the slab kit. The retail vs pro grade comparison explains the broader difference.

Why do crack fillers fail?

Most failures come from the crack, not the product. A filler goes on a wet or dirty crack, sinks into a deep crack with nothing under it, or goes into a crack that’s still moving.

The U.S. Bureau of Reclamation’s repair guide notes that it’s better to leave most cracking unrepaired (opens in a new tab) than to make an inadequate repair. The why repairs fail guide covers each cause.

When should you not use a crack filler at all?

When the crack is a symptom of something still happening. Skip the filler if:

• The crack is wider than 1/4 inch and you don’t know why.
• One side is higher than the other, or the slab has sunk. The sinking slab guide covers lifting it.
• The crack keeps getting longer or wider.
• It’s in a basement wall. Wall cracks get injected, not filled, and the foundation crack repair kits cover that. Horizontal wall cracks, bowing walls, stair-step cracks in block, cracks with offset edges and wall cracks over 1/4 inch go to an engineer first.

Not sure what you’re looking at? The product chooser asks a few questions and points to the right product, and the epoxy vs polyurethane comparison explains the two main resin types.

Once the filler has cured, the sealing after crack repair guide covers which sealer to use over the repair and how long to wait.

Questions

What is the best filler for cracks in a concrete driveway?

The UV slab kit, for cracks from hairline up to 1/4 inch. Its polyurea top is UV resistant and stays flexible, which matters on a slab that sits in the sun and freezes in winter. The driveway crack guide has the full repair.

Can I use caulk as a concrete crack filler?

A flexible concrete caulk can seal the top of a crack, but it only fills as deep as you push it. On a deep crack it sinks or bridges over an empty gap. The slab kit packs the depth with sand first so the top layer has something to sit on.

Can I fill a crack with cement or mortar?

You can, but a cement crack filler is rigid, so it tends to crack again if the slab still moves. It works best for rebuilding a broken edge or chipped corner, with a bonding agent brushed on first.

Does the filler need to match the color of the concrete?

No filler matches exactly. The UV slab kit's polyurea top cures gray or black, per its data sheet, and the indoor slab kits cure tan. Expect the repair line to show.

How long before I can drive on a filled crack?

For the UV slab kit's polyurea top, the data sheet gives light traffic in 30 minutes and full traffic in 60 minutes at 70°F. Cold concrete slows it down, and the concrete must be at least 50°F to apply it.