Concrete cracks because it shrinks as it dries, and it's weak when pulled. When the ground, a wall or its own weight holds it in place, it splits to relieve the strain. You can't stop that completely, but control joints, a firm base, a mix without added water and proper curing keep cracks small and put them where you want them.
Why does concrete crack?
Because it shrinks and can’t stretch. Concrete loses water as it sets and dries, so it gets slightly smaller. The NRMCA explains that concrete is weak in tension (opens in a new tab), so when something stops it from shrinking freely, the strain builds until it cracks.
What holds it back is usually the ground under it, a wall or post it was poured against, or just its own length and weight. The longer the slab, the more it has to shrink, and the more likely it is to split somewhere along the way.
Concrete also moves with temperature and moisture for the rest of its life. The NRMCA says it contracts and expands with changes in moisture and temperature (opens in a new tab), and cracks where the design doesn’t make room for that.
What are shrinkage cracks?
Shrinkage cracks are the cracks that come from the concrete drying out, and they’re the most common kind by far. They’re usually thin, with flush edges, and they don’t weaken the slab. The NRMCA says most early random cracks, although unsightly, rarely affect the structural integrity (opens in a new tab) of the concrete.
There are three kinds, each from a different stage of drying:
- Plastic shrinkage cracks form within hours, while the concrete is still soft. The NRMCA says they happen when the rate of evaporation of surface moisture exceeds the rate (opens in a new tab) at which rising water can replace it. Wind, heat and low humidity all speed that up.
- Crazing is a fine web of surface cracks from a weak, water-rich top layer. It’s often caused by finishing while water is still on the surface, or by sprinkling dry cement on to soak it up.
- Drying shrinkage cracks show up over weeks and months as the whole slab dries and shrinks. These are the long cracks that cut across a slab between joints.
The hairline cracks guide shows how to tell them apart and how to treat them.
Why is there a crack in new concrete?
Because new concrete shrinks the most. The NRMCA says that as concrete dries, it will shrink further and either additional cracks may form or preexisting cracks may become wider (opens in a new tab). A crack in new concrete within the first year is usually that.
Timing is a clue to the cause. Cracks that appear the same day or the next morning are plastic shrinkage or crazing, from how the concrete was placed and finished. Cracks that show up weeks or months later are drying shrinkage, often because the joints were too far apart.
A crack in new concrete that’s offset, wider than the rest, or that runs right next to a wall or step is worth pointing out to the contractor. Settlement of the base, or a slab poured tight against something without a gap, shows up early too.
What else makes concrete crack?
After shrinkage, the usual causes are the ground under the slab and things it’s poured against. The NRMCA says the majority of concrete cracks occur due to improper design and construction practices (opens in a new tab), and lists these:
- Missing contraction or isolation joints, or joints in the wrong place.
- A poorly prepared base under the slab.
- Too much water in the mix, or water added on the job.
- Poor finishing.
- Fast drying of new concrete in dry weather.
- Little or no curing.
Outside of those, a few causes come later in the slab’s life:
- Settlement. Soil that wasn’t compacted, or that washes out, leaves the slab without support. It drops and cracks over the soft spot.
- Restraint. A slab poured tight against a wall, footing, post or step can’t move. The NRMCA calls for isolation joints where floors meet walls, columns, or footings (opens in a new tab).
- Freezing and thawing. Water in saturated concrete expands as it forms ice (opens in a new tab), which stresses the surface and causes scaling. Some porous stone in the mix leads to D-cracks that start at joints in outdoor slabs.
- Rusting steel. Rebar or mesh too close to the surface rusts and swells. The NRMCA says at least 2 inches of concrete cover over the steel should prevent cracks from rust.
- Swelling soil. Clay under the slab swells when wet and shrinks when dry, pushing the slab up and letting it down. The heaving cracks guide covers what that looks like.
How do you prevent concrete from cracking?
You can’t prevent it entirely, but you can control where it cracks and keep the cracks thin. The NRMCA is clear that it is not possible to produce completely crack-free concrete (opens in a new tab). These are the practices it recommends to reduce and control cracking.
| Step | What to do | Why it helps |
|---|---|---|
| Base | Remove topsoil and soft spots and compact the soil or gravel under the slab | A firm, even base keeps the slab from dropping in spots |
| Mix | Order a moderate slump, no more than 5 inches, and don't add water on site | Less water means less shrinkage and a stronger surface |
| Finishing | Don't finish while water is on the surface, and don't overwork it | Prevents a weak top layer that crazes and scales |
| Curing | Spray curing compound or keep it covered and damp for at least 3 days | Slows drying while the concrete gains strength |
| Control joints | Cut 1/4 to 1/3 of the slab depth, spaced 24 to 36 times the thickness, 15 feet at most | Gives the slab planned places to crack |
| Isolation joints | Full-depth gaps where the slab meets walls, posts, footings and steps | Lets the slab move without being held |
| Panel shape | Keep panels square, with the long side no more than 1.5 times the short side | Long, narrow panels crack across the middle |
Sources: NRMCA CIP 4 (opens in a new tab), NRMCA CIP 6 (opens in a new tab).
Joint spacing is the one most often gotten wrong on home jobs. The NRMCA’s example: joint spacing for a 4 inch thick slab should be about 10 feet (opens in a new tab). A driveway with joints every 20 feet will usually crack between them.
Weather matters on the day of the pour. The NRMCA lists wind velocity in excess of 5 mph (opens in a new tab), low humidity and high temperatures as the conditions that cause plastic shrinkage cracks. Windbreaks, sunshades or pouring later in the day help.
If you already have cracks, prevention is about the next ones. Sealing outdoor concrete keeps water out of the cracks you have, and the crack or joint guide shows whether your slab has the joints it needs.
Which cracks need more than a filler?
Most don’t. Thin cracks with flush edges are shrinkage, and you can fill and seal them. A crack with one edge higher than the other, one that keeps widening, or one wider than 1/4 inch has a cause underneath that a filler won’t fix.
In a foundation wall, horizontal cracks and stair-step cracks in block are a different matter. Start with the foundation crack guide, and have an engineer look at anything that’s moving. For slab cracks you can fix yourself, the concrete crack repair guide walks through each location.
Questions
Does wire mesh or rebar stop concrete from cracking?
No. The NRMCA says wire mesh and reinforcement in slabs cannot prevent cracking (opens in a new tab). Placed at the right depth, it holds cracks tighter so they stay narrow.
Is it bad to add water to concrete on the job?
Yes. Extra water makes the mix easier to spread but weaker, and it shrinks more as it dries. The NRMCA lists the excessive addition of water on the job among the main causes of cracking.
Is cracking the contractor's fault?
Not always. Even good work cracks, which is why joints exist. Missing joints, a soft base or a slab that wasn't cured are workmanship questions, so check your contract or warranty and your local rules for what's covered.
Does sealing new concrete prevent cracks?
Curing does more. A curing compound sprayed on right after finishing slows drying in the first days, which is when the surface cracks most easily. A penetrating sealer later on keeps water out but doesn't stop shrinkage.