How to Fill Large Cracks in Concrete, and When a Wide Crack Needs More Than a Filler

How do you fill large cracks in concrete?

Clean and dry the crack, prime it with urethane, pack it with dry sand to 1/4 inch below the surface, soak the sand with urethane, then top it with polyurea and scrape it flush. That’s the outdoor UV kit’s method for wide cracks, which its instructions define as 1/8 to 1/4 inch, and they skip the grinding step. The indoor kit uses no polyurea, as covered below.

The sand is what makes it work in a big crack. It fills the depth so the filler on top doesn’t drain down into the crack.

1. Check the crack

   Lay a straightedge across it and measure the width. If one side is higher, or it’s wider than 1/4 inch, read the sections below on wider cracks and engineers before you fill it.

2. Clean out the crack

   Pull out weeds, dirt and loose concrete, then vacuum it. Wide cracks don’t need grinding.

3. Let it dry

   The kit’s instructions say the crack must be dry before you begin. 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.

What do you need to fill a large crack?

A urethane slab crack kit sized for the job, plenty of dry silica sand, and basic hand tools. The large UV-stabilized kit holds more filler for long outdoor cracks or several of them.

How wide is too wide to fill?

The slab kit’s written method covers cracks up to 1/4 inch. Past that, the instructions don’t give a method, and a crack that wide usually has a cause underneath, like settling soil or a slab with no room to move. Find it before you fill.

The American Concrete Institute’s repair guide says the cause of the damage must be assessed and corrected (opens in a new tab) before concrete is repaired. For a crack wider than 1/4 inch, this guide doesn’t recommend a DIY fill, because no instructions we can point you to cover it. Have a concrete contractor look at the slab and the soil under it.

Check whether it’s still moving. Mark the ends and measure the width at a marked spot every few weeks; the crack monitoring guide shows how. A crack that changes width with the seasons gets treated as a joint, which the next section covers.

Should you use backer rod in a large crack?

Only when the crack is being sealed like a joint. Backer rod is a foam rope pressed into a joint to set how deep the sealant goes and keep it from sticking to the bottom. The slab kit fills cracks with sand instead, so its cracks don’t get backer rod.

A wide crack that keeps opening and closing acts like a joint, and the repair follows. The American Concrete Institute says resin poured into a crack is not effective for repairing moving cracks (opens in a new tab), and that the crack should be routed and sealed to create a joint capable of movement. That joint style seal is where backer rod and a flexible joint sealant come in.

Here’s the joint style seal, start to finish:

1. Open the crack into a clean groove

   Saw or grind along the crack to give it straight, sound side walls.

2. Clean and dry it

   Vacuum out the dust and let the groove dry fully.

3. Set backer rod

   Press in backer rod slightly wider than the gap so it fits snug.

4. Fill with joint sealant

   Fill flush with a flexible sealant made for joints, then keep traffic off it for the cure time on the label.

The backer rod guide covers sizing and depth. If you’re not sure whether you’re looking at a crack or a joint, the crack or joint guide shows how to tell.

How do you repair a large crack in a concrete driveway?

Check whether the slab has moved first. A driveway crack with a raised edge, a sunken section or water running toward the garage is a settlement problem, and filling it won’t stop the movement.

• Flat and stable. Fill it with the UV-stabilized slab kit, using the steps above.
• Moves with the seasons. Seal it like a joint, with backer rod and a flexible joint sealant.
• Sunk, lifted or offset. See the sinking slab guide for lifting options and repair or replace your driveway if there’s a lot of it.

That’s how to repair large cracks in concrete driveway slabs without wasting a kit on one that’s still moving. The driveway crack repair guide covers the rest of the driveway.

For a garage or basement floor, the indoor kit is urethane only, with no polyurea top. Its instructions say to soak the first sand, then add more sand until the crack is full, soak that too and trowel off the excess. Scrape the slight hump flush once it turns light gray.

How do you seal large cracks in concrete so water stays out?

The filled crack is sealed by the repair itself. After it cures, a sealer over the whole slab slows water getting into the concrete around the repair.

That order matters. Concrete Sealers USA’s sealer FAQ says sealers are not fillers or patching materials, and that cracks greater than 1/16 inch should be repaired before sealing. So the answer to how to seal large cracks in concrete is to fill them, then seal the slab.

Water in an open crack does the most damage where it freezes. The NRMCA notes that water in saturated concrete expands as it forms ice (opens in a new tab), which is why a wide crack left open through winter gets wider and its edges crumble.

When does a large crack need an engineer?

When it’s in a foundation and wider than 1/4 inch, or when it shows movement. Those go to a structural engineer, not a kit.

• Foundation wall cracks over 1/4 inch. Start with the foundation cracks guide to see what the crack’s direction means.
• Offset edges. One side higher or pushed out means the concrete moved.
• Horizontal wall cracks, bowing walls and stair-step cracks in block walls. These come from soil pressure or settlement.
• Cracks that keep widening. The NRMCA says to seek professional advice (opens in a new tab) for active cracks that widen with time.

Why do large cracks form in concrete?

Most large cracks start as small ones that grew, or come from the slab being pulled or pushed by something under or around it. The American Concrete Institute lists the typical causes as drying shrinkage, thermal movement, settlement, missing control joints, overload and restraint (opens in a new tab).

Joints are meant to prevent this. The NRMCA describes control joints as pre-planned cracks (opens in a new tab), so a slab with too few joints cracks wherever it needs to. The why concrete cracks guide covers each cause.

How long before you can drive on the repair?

Under an hour with either kit. The outdoor kit’s polyurea top takes light traffic in 30 minutes and full traffic in 60 minutes at 70°F, according to its data sheet, and the concrete must be at least 50°F when you apply it. The indoor kit’s urethane data sheet says the slab is back in service within 20 minutes.

A wide crack takes more sand and urethane, so on the outdoor kit give the first layer its full time before you top it.

What mistakes make large crack repairs fail?

Filling the crack without fixing what made it wide. A slab that’s still settling or heaving opens the crack again, usually along the edge of the repair.

• Pouring filler straight into a deep crack. It drains down and keeps sinking. Pack the depth with sand first.
• Filling a damp crack. Wide outdoor cracks hold water and dirt. Clean them out and let them dry.
• Putting backer rod under the slab kit’s filler. The kit’s instructions use sand. Backer rod belongs under a joint sealant.
• Using mortar in a crack that moves. It cracks loose along both edges within a season or two.
• Filling an offset or sinking slab. Fix the movement first.

The why crack repairs fail guide covers each failure and its fix, and the concrete crack repair hub has the guides for each kind of slab.

Should you seal the slab after filling a large crack?

Yes, outdoors especially. A sealer slows water getting into the concrete around the repair, which matters most where it freezes.

The sealing after crack repair guide covers which sealer to use over a filled crack and how long to wait.