Concrete driveway crack repair: what holds, what does not

Concrete driveway crack repair starts with sorting the cracks, not buying a tube. A hairline crack under 1/8 inch with both sides level is normal, and it usually needs nothing. A wider crack that hasn’t moved in a year gets routed open with a crack chaser wheel and filled with a sealant that bends. And if one side sits higher than the other, the ground moved. No filler fixes that.

How do you sort the cracks in your own driveway?

You need a tape measure and about ten minutes. Run these four checks standing right in the driveway.

Width. Measure at the widest point, not the average. A crack under 1/8 inch is a whole different case than one a quarter inch wide.

Height. Lay a straightedge across the crack, or run the flat of your hand over it. If one side sits higher than the other, stop measuring width. That step up means the ground under the slab moved.

Does it follow a control joint? Those are the straight cut lines in the slab. A crack that runs along one is the joint doing the job it was cut for. That’s a success, not a failure.

Has it opened since last year? Hardly anybody checks this, and it tells you the most. Pull up a photo from last spring. Or mark the width on the slab with a pencil and check it again next season. A crack that hasn’t moved in a year is dormant, and dormant cracks are easy.

The first pile is usually the biggest. Our concrete repair guide covers what causes each kind of crack. It also covers when a repair stops making sense and a new slab is the better buy.

Why does filler pop out of a crack you already fixed?

A tube of sealant squeezed into a hairline crack is a common driveway fix, and one of the shortest-lived. Sealant works by sticking to two clean side walls and stretching across the gap as the slab moves with the weather. A hairline gives it neither. There’s no width to stretch across, so the first cold night pulls it tight with nowhere to go. And there are no clean walls, because an unopened crack is full of dirt and loose paste. Routing fixes both at once. A crack chaser wheel on an angle grinder cuts the crack into an even groove, back to solid concrete on both sides. That gives the sealant a set width and depth to fill. We price routing and sealing by the linear foot, at $2.55 a foot, because the cutting is most of the work.

Movement is why this matters more on a driveway than in a garage. An outdoor slab in Oklahoma runs through 60 to 70 freeze-thaw cycles in a winter. The surface swings well over a hundred degrees from summer to winter, too. So the crack is a little wider in January than in July. A filler that can’t stretch is a filler that lets go.

How do you route a crack before filling it?

1. Chalk the line. Mark both sides of where you’ll cut, so you follow the crack instead of drifting off it.
2. Set up the grinder. Use a 4-1/2 or 5 inch angle grinder with a crack chaser wheel. That’s a thin diamond blade with a hub that stops it from cutting past a set depth. Use the dust shroud and a vacuum. Cutting dry concrete throws silica dust you can breathe in, and a shop vac without a good filter blows it right back into the air.
3. Cut the groove. Make two passes, one down each side of the crack, to open it to an even width. You want clean straight walls and solid concrete, not the crumbly edges the crack started with.
4. Blow it out and vacuum it. Every bit of dust left in the groove is a spot the sealant can’t stick to.
5. Set backer rod if the groove is wider than about 1/4 inch.
6. Fill it, tool it flush, and leave it alone for the cure time on the label.

Step 4 is the one people skip most. Sealant stuck to dust is stuck to nothing.

Matching the filler to the crack

Most people have never heard of backer rod. It’s a foam rope, a little fatter than the groove. You press it into the bottom before the sealant goes in. Its real job is to keep the sealant from sticking on three sides. Sealant stuck to both walls and the bottom can’t stretch when the crack opens, so it tears. Sealant stuck to just the two walls stretches the way it’s made to.

Should you fill a hairline crack at all?

We’d lose work saying it, but on a hairline crack the after usually looks worse than the before. That’s true on a broom finish most of all. The sealant is a different color and texture than the slab around it. It catches light in its own way, and it won’t take the broom lines. So a thin line you could barely see turns into a dark stripe. And a dormant shrinkage crack under 1/8 inch isn’t letting water into anything that matters. It isn’t growing, and it isn’t making the panel weaker. ACI 224.1R, the industry report on cracking in concrete, treats fine shrinkage cracks as normal, not as a defect. A slab loses water while it cures, so it’s going to crack somewhere, and control joints are there to pick where. If yours cracked at the joint, the joint did its job.

A 1/8 to 1/2 inch crack that hasn’t moved, routed and sealed, is a different story. Before, it’s an open crack that fills with dirt and water. After, it’s a neat line of sealant, flush with the slab.

If an open hairline still bugs you, roll a penetrating sealer over the whole driveway. Skip the bead down one crack. The slab looks even, with no one line called out. And the sealer guards the rest of the slab, too.

Which cracks are telling you something structural?

Any one of these three signs ends the do-it-yourself plan:

• One side has dropped. One panel sits lower than the one next to it. That’s the ground under the slab, not the concrete.
• It opens more each season. A crack that was 1/8 inch last spring and is 3/8 inch this spring is being pulled by something under the slab.
• Water comes up through it after rain. Water is finding a path through the panel from below.

Oklahoma’s clay is behind most of these. It’s expansive soil, which means it swells when wet and shrinks when dry, and it moves whatever sits on it. Our clay soil guide covers how. Filling a crack over moving ground is a repair you pay for twice, because the cause is still there after the sealant cures.

Lift, patch, or replace: what a pro charges

At that point the question isn’t which tub to buy. It’s whether the panel gets lifted, patched, or replaced. A sunk panel that’s still in one piece can be lifted with foam pumped under it, at $9.00 a square foot. Mudjacking, a cement slurry pumped under the slab, runs $4.34 a square foot when the soil can carry the extra weight. A section too broken to lift gets cut out and poured back at $18.06 a square foot. Setup and the drive cost, which is our trip to your place, go on top of each one. Concrete leveling vs an overlay walks through which fix fits a sunken panel. Either way, we make that call standing on your driveway with a straightedge, not from a photo.

Is a cracked driveway a bad driveway?

No. There are two kinds of concrete: concrete that has cracked and concrete that hasn’t cracked yet. That’s just what the material does. The skill is telling the ordinary cracks from the ones with something behind them.

So sort first. Most of what’s in your driveway belongs in the leave-it pile. What’s left splits two ways. One side is a crack chaser wheel and a tube of polyurethane. The other is a straightedge and a talk with someone who can look at the ground under it.