Fixing cracks in concrete means cleaning out the damaged area, filling it with the right material for the crack’s size, and sealing the surface so water and salt can’t get back in. Hairline cracks under 1/8 inch are usually cosmetic and safe to patch yourself. Cracks wider than 1/4 inch, or ones that keep growing, usually need a professional assessment before you patch anything.
That’s the short version. The longer version depends on where the crack is, how wide it is, and whether it’s actually moving. Here’s how to work through it.
Concrete cracks because it’s rigid and the ground underneath it isn’t.
Freeze-thaw cycles are the biggest culprit in this part of Illinois. Water gets into tiny pores in the concrete, freezes, expands, and pushes the surface apart. Do that every winter for a few years and hairline cracks show up.
Shrinkage happens as concrete cures. It loses moisture and contracts slightly, which is why you’ll sometimes see thin cracks appear within the first year of a new pour, even on a well-installed slab.
The rest of the usual suspects:
○ Soil movement underneath the slab, especially after a wet spring or a dry summer
○ Heavy loads, like a delivery truck parked on a driveway that wasn’t built for it
○ Tree roots pushing up from below
○ Poor original installation, including a base that wasn’t compacted properly
None of this means the concrete was a bad idea. It means concrete moves, and if you know why it cracked, you’ll know how to fix it and stop it from happening again.
Width and behavior tell you almost everything.
As a rough guide used across the concrete industry, a crack up to about 1/8 inch wide is typically considered a normal shrinkage crack, while anything approaching 1/4 inch or wider is worth a closer look. That threshold shows up repeatedly in trade guidance built on ACI (American Concrete Institute) recommendations for residential flatwork.
Ask yourself:
○ Is it growing? Mark both ends with a pencil and check again in a month.
○ Is water getting through it? Even a hairline crack can let moisture in over time.
○ Is anything else moving? Doors sticking, floors going uneven, or a wall bowing are signs the crack isn’t acting alone.
○ Is it in a foundation wall or a driveway slab? The stakes are different. A hairline crack in a patio is cosmetic. The same width in a basement wall deserves a second look, since even small foundation cracks can point to bigger movement underneath the house.
If the crack is stable, dry, and under that quarter-inch mark, you’re almost certainly in DIY territory. If it’s growing, leaking, or paired with other warning signs, skip ahead to the section on when to call a professional.
For anything under 1/4 inch, this is a weekend job, not a construction project.
Start with prep. Clear out any loose debris, dirt, or old filler with a wire brush, then rinse and let the area dry completely. Most repair materials won’t bond to damp concrete.
Pick your filler based on the crack:
| Crack type | Best material | Why |
| Hairline (under 1/16″) | Concrete crack sealant | Flexes with temperature changes |
| Small (1/16″ to 1/4″) | Polyurethane caulk or epoxy filler | Bonds well, stays flexible through freeze-thaw |
| Slightly wider, uneven surface | Resurfacer | Covers a network of small surface cracks at once |
Apply it. Load the sealant or caulk into a caulking gun and run it along the crack, slightly overfilling so you can tool it flush. For epoxy, mix small batches and press it in with a putty knife, feathering the edges so the patch blends in. Let it cure fully before you drive or walk on it, usually 24 to 48 hours.
One thing worth knowing: skip indoor caulks. They’re not built to expand and contract the way outdoor concrete does, and they’ll pop right back out after the first hard freeze.
Once you’re past 1/4 inch, filler alone often isn’t enough to hold.
For cracks deeper than about half an inch, pack the bottom with sand or a foam backer rod first, leaving room for the patching material on top. This keeps you from wasting filler in a void and gives the patch something to grip.
Concrete patching compound, which has a grout-like texture and sets like new cement, works well here. It’s applied in layers if the repair area is deep, letting each layer set before adding the next.
This is also the point where DIY starts to have real limits. A patch can hold up visually and still fail structurally if the underlying slab has shifted or the base underneath was never compacted correctly. If your driveway has more than one wide crack, or the slab itself is uneven at the crack line, that’s less of a patching job and more of a professional driveway repair and resurfacing conversation.
Foundation cracks get more scrutiny for good reason. A foundation holds up the whole structure above it, so a crack there isn’t just cosmetic risk, it’s a potential entry point for water, and in some cases a sign the ground underneath is shifting.
Watch for these together, not just the crack alone:
○ Uneven floors
○ Doors or windows that stick
○ Visible settling or shifting near the crack
○ Water intrusion after rain
Any one of these on its own might mean nothing. Two or three together, especially alongside a widening crack, is worth a full foundation assessment rather than a tube of caulk from the hardware store. Small and moderate foundation cracks can often be reinforced and repaired without a full rebuild, but that call is easier to make after someone has actually looked at the soil, the slab, and the crack pattern together.
Sealing won’t fix a crack that’s already there, but it does a lot to keep new ones from forming, and it helps a repaired crack stay repaired.
Here’s the logic: most concrete damage in this climate starts with water. Water gets into the pores, freezes, and expands. A quality sealant blocks that moisture from soaking in, which cuts down on freeze-thaw cracking and also protects against de-icing salt, one of the more aggressive things concrete deals with in an Illinois winter.
It’s also worth knowing that professional concrete sealing typically includes a repair step first. Cracks and chips get patched before the sealant goes on, since sealing over a damaged surface just traps the problem underneath a fresh coat. Most sealed surfaces need resealing every two to three years, depending on traffic and exposure.
A good rule: if the fix is cosmetic and the crack is stable, DIY is fine. If the crack is structural, growing, or tied to a bigger project, bring in someone who does this daily.
One example that comes up often: garage floors. A homeowner wants a garage floor coating, sees a few cracks in the slab, and assumes they’ll just coat over them. That’s a mistake. Cracks get filled and the surface gets ground smooth before any coating goes down, because a coating applied over an unrepaired crack will crack again in the same spot within a season. It’s a small step that determines whether the whole project holds up.
The same logic applies anywhere a repair touches something bigger than the crack itself: a driveway that’s cracking in multiple spots, a foundation wall with a crack plus other warning signs, or a slab where you’re not sure if the damage is cosmetic or structural. A quick on-site look settles that question faster than guessing.
See if your crack needs a patch or a professional look.
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○ Under 1/8 inch and stable: Cosmetic. Clean it, fill it with a flexible sealant, done.
○ 1/8 to 1/4 inch: Usually still DIY-friendly with polyurethane caulk or epoxy filler, as long as it’s not growing.
○ Over 1/4 inch, growing, or leaking: Time for a professional look, especially in a foundation wall or a driveway with multiple cracks.
○ Sealing won’t undo existing damage, but it’s the best way to keep new cracks from forming and to protect a repair you’ve already made.
○ Before coating or resurfacing anything, cracks need to be repaired first. Covering damage instead of fixing it just hides the problem for a season.
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How wide does a crack need to be before it’s a problem?
As a general guideline, cracks under 1/8 inch are typically normal shrinkage and not a structural concern. Cracks approaching 1/4 inch, or ones that keep widening over a few months, are worth a professional look, especially in a foundation or load-bearing slab.
Can I use regular caulk instead of concrete filler?
No. Indoor and general-purpose caulks aren’t built to flex with the temperature swings concrete goes through outdoors. Use a polyurethane concrete caulk or an epoxy filler made specifically for exterior concrete, since these stay flexible through freeze-thaw cycles instead of pulling away from the crack.
Will a DIY patch match the rest of my driveway?
Not perfectly. Most fillers and patching compounds dry to a gray tone close to concrete, but small color and texture differences are common, especially on older or stamped concrete. If a seamless finish matters, a professional resurfacing job blends better than a spot patch.
How long does a concrete crack repair last?
A properly prepped and filled crack can last several years, particularly if the surface is sealed afterward. Repairs that skip the cleaning and drying step, or use the wrong filler for the crack width, tend to fail within a season or two.
Does G-Cat Construction repair cracks, or only new installs?
G-Cat Construction handles crack repair as part of driveway resurfacing, foundation work, and garage floor coating prep, in addition to new installations. If you’re not sure which category your project falls into, a free on-site estimate will sort that out.
What’s the difference between a shrinkage crack and a structural crack?
A shrinkage crack forms as concrete cures and loses moisture, is usually thin and stable, and doesn’t affect the slab’s strength. A structural crack is tied to movement, whether from soil shifting, an overloaded slab, or a failing foundation, and typically widens or lengthens over time. Structural cracks need a professional evaluation before repair.
