By Bennett's Concrete and Excavation, Norwich, NY. Updated October 2026.
The short answer
Concrete shrinks as it dries, so some fine cracking in the first weeks and months is normal. NRMCA's guides describe plastic shrinkage cracks and crazing as mostly cosmetic, and a hairline crack running along or near a control joint is the joint doing its job. Cracks that widen, shift so one side sits higher than the other, or keep growing are the ones worth having looked at.
Four common types
| Type | What it looks like | When it shows | Concern |
|---|---|---|---|
| Plastic shrinkage cracks | Short, parallel cracks about 1 to 3 feet apart, usually not reaching the slab edge | Within hours, while the concrete is still soft | Mostly cosmetic |
| Crazing | A fine web of cracks, cells about 1-1/2 inches across or smaller, rarely over 1/8 inch deep | The next day to the end of the first week | Cosmetic |
| Drying shrinkage cracks | A single fine line, often across a panel or off an inside corner | Weeks to months | Usually cosmetic, watch it |
| Settlement or heave cracks | Wider, with one side higher or lower | Months to years, often after a winter | Points to a base or frost problem |
Sources: NRMCA guides on plastic shrinkage cracking, crazing and joints.
Plastic shrinkage cracks
These form when the surface dries faster than bleed water can rise to replace it, while the concrete is still plastic. NRMCA says they're more likely on windy days, with wind above about 5 mph, low humidity and heat. They're relatively shallow and rarely affect strength. A finisher who spots them early can sometimes close them by refinishing.
Crazing
NRMCA describes crazing as a network of fine random cracks, more noticeable on steel-troweled surfaces and when the surface is drying after rain. It doesn't affect structural integrity and isn't generally a sign of future deterioration. Common causes are extra water in the mix, finishing while bleed water is on the surface, sprinkling water or cement during finishing, and late or poor curing.
Cracks near joints and corners
Control joints are supposed to crack at the bottom of the groove. Sometimes the crack wanders a few inches off the joint, or a crack starts at an inside corner where no joint was cut. NRMCA's joint guide recommends joints every 24 to 36 times the slab thickness, a maximum of 15 feet, and a joint off every inside corner. A crack between joints that are too far apart is a layout issue, not a bad batch. See the control joints guide.
When to call the contractor
- A crack that keeps getting wider over a few months
- One side of the crack sitting higher or lower than the other
- Cracks that show up after the first winter along with a heaved or sunken section
- Rust stains coming out of a crack
- Lots of cracks in a slab that hasn't seen any load yet
Engineer Kim Basham notes that as crack widths approach about 0.035 inch, the stone on each side stops locking together well, and edges can start to chip under traffic. That's why a crack that's opening up deserves attention even if it started as a hairline.
How to keep an eye on it
- Take a dated photo with a coin or tape measure beside the crack.
- Mark each end of the crack with a pencil or marker and the date.
- Check it again after a few months and after the first winter.
- If it has grown longer, wider or offset, call your contractor with the photos.
Should you seal hairline cracks?
Sealing the whole slab with a penetrating sealer helps keep water and salt out of fine cracks, which matters in Central NY. NRMCA's corrosion guide points out that in salt exposure, even narrow cracks can let chlorides reach steel. Wider cracks can be filled with a flexible crack sealant. See our sealing guide, or for cracks with a deeper cause, why concrete cracks. If you have questions about a slab we poured, get in touch.
