How do you fix cracks in concrete?

Control joint depth

⅛–⅓ of slabNRMCA CIP 4

Joint spacing

24–36× thickness15 ft max, NRMCA CIP 4

Crack width limit, deicers

0.007 indurability, NRMCA CIP 25

Crack width limit, marine

0.006 indurability, NRMCA CIP 25

Rust volume

2–4× the steelNRMCA CIP 25

Short version

• Identify the crack first — the same gray line can mean a finishing problem or a foundation problem.
• Fine surface networks (crazing) and narrow shrinkage cracks are cosmetic and stay that way.
• Mark the ends of a crack, date the mark, and measure it monthly; a crack that moves has an active cause.
• Vertical offset, steady widening, water pushing through, or rust staining move the job out of DIY range.
• Most slab cracks trace back to jointing, subgrade, added water, finishing, or curing — the five items NRMCA CIP 4 lists.

What kind of crack are you looking at?

Cracks fall into a handful of recognizable patterns, and the pattern is the diagnosis. NRMCA CIP 4 illustrates seven common forms: plastic shrinkage cracks, cracks caused by improper jointing, cracks from continuous external restraint, cracks where an isolation joint was left out, D-cracking from freezing and thawing, craze cracks, and settlement cracks.

Plastic shrinkage cracks are short, roughly parallel, and appear while the concrete is still soft. Jointing cracks show up mid-panel where joints were cut too shallow, too late, or too far apart — CIP 4 recommends joints cut ⅛ to ⅓ of the slab thickness, spaced 24 to 36 times the thickness and no more than 15 feet apart. Restraint cracks run where the slab is tied to something that will not move with it, like a wall or a column with no isolation joint. Craze cracks form a fine surface network, and CIP 3 treats them as a surface condition, not a strength problem.

When did the crack appear?

Timing narrows the cause faster than width does. A crack that opened within hours of placement, before the concrete hardened, is plastic shrinkage: CIP 5 attributes it to surface water evaporating faster than bleed water can rise, which is why hot, dry, windy days produce it and why it clusters in the middle of open slabs.

Cracks that appear over the first weeks are usually drying shrinkage finding a path — the slab is getting shorter, and if the joints are not doing their job the concrete makes its own. Cracks that appear months or years later, in concrete that was fine, mean something outside the concrete changed: the subgrade washed out, a tree root lifted a panel, frost heaved a footing, or embedded steel started corroding. CIP 25 describes that last case — rust occupies two to four times the volume of the steel it came from, so it cracks and spalls the cover from the inside.

Is the crack active or dormant?

An active crack is still moving, and no repair holds until the movement stops or the repair can move with it. Test it rather than guess: draw a line across the crack at each end, write the date next to it, and measure the width with a crack comparator card at the same two spots every month. Include a summer and a winter reading, because a crack that opens in cold weather and closes in warm weather is thermal movement, not failure.

A dormant crack that has not changed across a full temperature cycle can take a rigid repair such as an epoxy or a cementitious filler. An active crack needs a flexible sealant in a properly cut reservoir, or it will simply reopen through whatever you put in it. Filling an active crack with a rigid material also transfers the movement somewhere else, which usually means a new crack an inch away.

How do you tell a cosmetic crack from a structural one?

Displacement is the clearest dividing line. Run a hand across the crack: if one side sits higher than the other, or the two faces have separated so the crack passes daylight, the slab is moving in more than one plane and something under it or around it has failed. Width alone is a weaker signal — ACI 224R deals with crack width tolerances by exposure condition, and CIP 25 gives durability-based limits of 0.007 inch where deicing salts are used and 0.006 inch in marine exposure, but those are corrosion thresholds, not structural verdicts.

Treat these as stop signs: diagonal cracks running from the corners of a door or window opening, horizontal cracks in a basement or retaining wall, cracks accompanied by bowing, cracks that leak under pressure, and rust stains bleeding out of the crack. Cracks in beams, columns, suspended slabs, and anything holding back soil belong in the same category regardless of width.

General information, not engineering advice. Structural work should be designed by a licensed engineer.

What each crack type usually calls for

When should you call an engineer instead of a store?

Call an engineer when the crack is telling you a load path or a soil condition changed. That includes vertical offset across the crack, a crack that has widened measurably in a single season, horizontal cracking in a foundation wall, cracks in structural members, cracks in a slab supporting a wall or column, and any crack that appeared after nearby excavation, a plumbing leak, or a heavy vehicle got on the slab.

Repair guidance from ICRI starts from the same principle: identify the cause, then select the method. A repair applied without knowing the cause fails on the same schedule the crack formed on, and a filled crack that hides ongoing settlement is worse than an open one you can watch.

Which repair method matches your crack?

Once the diagnosis is done, the method follows the crack type and location. For narrow, dormant cracks in flatwork, see how to fill cracks in concrete for the sealant and filler options. For a driveway, where slab movement, freeze-thaw, and vehicle loads all apply at once, see how to repair cracks in a concrete driveway.

If the panels on either side of the crack have sunk rather than split, the crack is a symptom of a void and lifting comes before filling — see do-it-yourself concrete lifting foam for what that involves and where it goes wrong.

Two rules cut across every method. Match the material’s flexibility to the crack’s movement — rigid where nothing moves, elastomeric where something does — and prepare the crack before filling it, because a sealant bonded to loose edges and dust fails at the bond line rather than in the material.

Timing matters too. Fill cracks when the slab is at a middle temperature rather than at a seasonal extreme, since a crack sealed wide open in January closes on the sealant in July, and one sealed at its tightest in August tears itself open the following winter.

Common mistakes

• Filling a crack before finding the cause — the repair fails on the same timeline the crack formed on, and now the movement is hidden.
• Using a rigid filler on a moving crack — epoxy in an active crack either debonds or forces a fresh crack alongside it.
• Reading width instead of displacement — a wide dormant crack can be harmless while a narrow crack with vertical offset is not.
• Treating rust stains as cosmetic staining — CIP 25 describes rust expanding to two to four times its original volume, so the crack is being driven from inside.
• Skipping the seasonal check — one measurement proves nothing; a crack has to be watched across a hot and a cold month before you can call it dormant.