Most cracks in concrete are normal. Concrete shrinks as it sets and dries, and cracks, ideally at planned joints. Random early cracks rarely affect structure or service life. Worry about cracks where one side sits higher than the other, cracks that keep growing, cracks with flaking or crumbling around them, and cracks over soil that is settling. A mason has to look at those.
Cracks in concrete are normal, and most are harmless. The National Ready Mixed Concrete Association (NRMCA) says no concrete is completely crack free (NRMCA). The real question is which cracks are the slab doing what slabs do, and which are telling you the ground, the water or the mix is wrong. This article sorts them, with what to do about each.
Why does concrete crack in the first place?
Concrete shrinks as it sets and dries. If something holds it back from shrinking, tension builds past what the concrete can take, and it cracks (NRMCA). That is why slabs get joints: control joints are "pre-planned cracks" that tell the slab where to crack. Early cracking is mostly temperature change and the set contraction, and later drying shrinkage can add cracks or widen existing ones (NRMCA).
Most cracking comes from how the slab was designed and built: missing joints, poor subgrade prep, watered down or high slump concrete, poor finishing, fast moisture loss and weak curing (NRMCA). The ACI repair guidance lists typical causes as drying shrinkage, thermal movement, settlement, lack of control joints, overload and restraint (ACI).
Which cracks are usually normal?
| Crack | What it looks like | Usually means |
|---|---|---|
| Plastic shrinkage | Shallow, often 1 to 3 ft apart, appeared within hours of the pour | Surface dried faster than bleed water rose. Mostly cosmetic (NRMCA) |
| Early shrinkage | A crack, often at a spot the joints missed | Set contraction and drying. Rarely affects structure or service life (NRMCA) |
| Crack at a joint | Straight line along a sawed or tooled joint | The joint doing its job (NRMCA) |
NRMCA says most random early cracks rarely affect structural integrity or service life (NRMCA). A hairline crack that has not moved in a year is usually a cosmetic issue.
Plastic shrinkage cracks can still let in chemicals and start later cracking (NRMCA), which is a reason to fill them, not a reason to panic.
Which cracks should you worry about?
NRMCA names two exceptions to "early cracks are harmless": D-cracking, which is freeze-thaw damage in some porous aggregates that starts at joints at the bottom of exterior slabs and shows up later, and alkali-aggregate reaction (NRMCA). Beyond those, these are the signs that move a crack from "watch it" to "call someone":
- One side higher than the other. That is movement. Settlement is a listed cause of cracking (ACI). See why a sidewalk heaves and lift or replace a sinking slab.
- A crack that keeps growing, or widens season to season.
- Flaking or crumbling along the crack, or a rough skin around it. That is surface failure, usually scaling. See why Chicago concrete flakes.
- Cone-shaped holes near cracks where aggregate froze and fractured (NRMCA).
- Rust stains along the crack. If there is steel in the slab, water and salt may be reaching it. NRMCA advises at least 2 inches of cover to keep salt and moisture off steel so rust expansion does not crack the concrete (NRMCA).
- Cracks in a poured basement wall that leak. Some cracking in basement walls is normal, and typical builder warranties only require repair if a crack leaks, shows measurable vertical displacement, or exceeds 1/8 inch (NRMCA). That is a warranty yardstick for basement walls, not a code limit for slabs.
There is no single width in the NRMCA material we can cite that makes a slab crack dangerous. That is why movement, growth and surface condition matter more than a measurement.
On Chicago driveways, garage floors and rear walks, we often find cracks that have run for years without moving, and a few that sit right where a slab has dropped over soil that was never compacted well. The first kind can be sealed and left. The second means the slab needs more than a filler.
How do you check whether a crack is moving?
You do not need tools. Do this once, then again after the next freeze and thaw:
- Photograph the crack with a coin or a ruler beside it, from the same spot, and date the photo.
- Mark each end of the crack with a pencil line. If it runs past the line, it is growing.
- Lay a straight board or a level across the crack. If one side is higher than the other, or the gap under the board has changed, the slab is moving.
- Look at what the crack is near. Downspouts that dump on the slab, a low spot where water sits, a tree root, a gap where fill settled next to a foundation. The cause is often nearby.
- Check after rain and after a hard freeze. A crack that opens and closes with the weather is behaving differently from one that is simply old.
If nothing has changed after a year, you have your answer. If something has, send us the photos and we can tell you whether it needs a visit.
When is a crack really a trip hazard or a safety problem?
A crack by itself is rarely a safety issue. A crack with a raised edge is. A raised edge on a walk, a step or a garage threshold is where people trip, and a slab that has dropped away from a foundation or a stoop can leave a gap. If it is a public sidewalk and you have a notice, see what to do with a sidewalk violation notice. If someone could fall on it today, put a marker on it, and call.
What can be done about a cracked slab?
- Leave it. A tight, stable crack with nothing flaking can wait.
- Fill or seal it. Narrow, stable cracks are a common DIY job. Our driveway crack article covers what is safe to do yourself.
- Inject it. ACI lists epoxy injection under pressure as one potentially effective structural repair. The crack must be clean and free of debris, and the method differs for horizontal, vertical and overhead cracks (ACI).
- Replace the slab. When the slab has moved, or the surface is failing across a wide area, patching the crack does not fix the cause. A subgrade problem, such as soft spots or poor compaction, has to be corrected for the new slab to last (NRMCA).
How are cracks prevented in a new slab?
Joints are the first answer. NRMCA's guidance puts control joint spacing at 24 to 36 times the slab thickness, with a maximum of 15 feet, square panels where possible, and a joint depth of at least a quarter of the thickness (NRMCA). Reinforcement does not prevent cracks, but it reduces crack width and keeps cracks and joints from opening (NRMCA). A vapor retarder placed directly under the slab increases bleeding and greatly raises cracking potential, especially with high slump concrete, so ask how yours is handled (NRMCA).
Cracked concrete on your property? Akats pours and repairs concrete in Chicago, and we will tell you straight when a crack is just a crack. See our concrete service and the Chicago concrete and sidewalk guide.
Common questions
Yes. No concrete is completely crack free. Concrete shrinks as it sets and dries and cracks, ideally at planned joints. Most random early cracks rarely affect structure or service life.
One side higher than the other, a crack that keeps growing, flaking or crumbling along it, or a basement wall crack that leaks. D-cracking, a freeze-thaw damage that starts at joints, is another. A mason should look at any of those.
Often, yes. Narrow stable cracks can be filled or sealed. Structural cracks can be repaired with pressure epoxy injection. If the slab has moved or the surface is failing widely, replacement fixes the cause better than patching.
Sources
- National Ready Mixed Concrete Association: Concrete in Practice 4: Cracking Concrete Surfaces (PDF)
- National Ready Mixed Concrete Association: Concrete in Practice 5: Plastic Shrinkage Cracking (PDF)
- National Ready Mixed Concrete Association: Concrete in Practice 6: Joints in Concrete Slabs on Grade (PDF)
- National Ready Mixed Concrete Association: Concrete in Practice 2: Scaling Concrete Surfaces (PDF)
- National Ready Mixed Concrete Association: Concrete in Practice 40: Aggregate Popouts (PDF)
- American Concrete Institute: RAP Bulletin 1: Structural Crack Repair by Epoxy Injection (PDF)
- National Ready Mixed Concrete Association: Concrete in Practice 7: Cracks in Residential Basement Walls (PDF)
Facts last checked: October 7, 2026.
This article explains general rules and common situations in Chicago. It is not legal advice, and City rules change. For a specific property, check with the Chicago Department of Buildings or a licensed professional.