You paid for a new driveway, waited a week to drive on it, and now there's a crack. It feels like you got a bad job.
Before you call anyone, know this: a slab that never cracks anywhere is the exception, not the rule. Good crews expect some cracking and build the slab so it happens in places you'll barely notice.
So the real question isn't whether there's a crack. It's what kind. Below is how to read yours, what can go wrong in a Wisconsin slab's first year, and when it's worth a phone call.
Shrinkage: The Crack Nobody Can Fully Prevent
Concrete is mixed with more water than the cement needs to harden. As that extra water dries out over the following weeks and months, the slab shrinks a little. The ground underneath holds it in place, so something has to give. That something is a crack.
Nobody can stop that shrinkage completely. What a good installer can do is control where it cracks.
Control Joints: Cracks You Planned For
Those lines tooled or saw-cut into your slab every several feet aren't decoration. They're control joints, and their job is to create a weak spot so the slab cracks there, in a straight line, instead of wandering across the middle.
A common rule of thumb is to space joints no more than two to three times the slab thickness in inches, measured in feet. For a typical 4" slab, that works out to roughly every 8 to 12 feet. Joints are usually cut about a quarter of the way through the slab, and panels work best when they're close to square.
So if you look closely and see a hairline crack running down the bottom of a control joint, that's the joint doing its job. A crack that wanders across the middle of a panel is a different story. It usually means the slab started shrinking before the joints could do their job: the panels were too big, the saw cut didn't go deep enough, or the crew didn't get back to cut it in time.
What Else Can Crack a Young Slab
Poor base compaction
The gravel and soil under the slab have to be firm and evenly compacted. If the crew poured over soft spots, loose fill, or topsoil, the slab settles unevenly in the first year or two and cracks over the low spots. This is the big one near the house, where driveways, garage aprons, and stoops often sit on loose foundation backfill.
Early loading
Concrete gains strength over weeks, not hours. A common guideline is to wait about a week before parking a car on a new driveway, and around a month before heavy trucks like a moving van, dumpster delivery, or RV. Edges are the weak point, so a heavy truck turning across the edge of a young driveway is a classic way to crack a corner.
Curing problems
Curing means keeping the fresh slab from drying out too fast so it can gain strength. Industry guidance typically calls for curing over roughly the first 3 to 7 days, with a curing compound, plastic sheeting, or keeping it damp. Skip it, and the concrete ends up weaker at the surface and more prone to cracking and flaking.
A specific version of this happens on pour day: if wind, sun, and low humidity dry the surface before the concrete sets, you can get plastic shrinkage cracks, a set of short, shallow, roughly parallel cracks. They look bad but rarely affect strength.
Cold-weather pours (Wisconsin's fall problem)
Plenty of driveways around here get poured in October and November, when contractors are trying to squeeze in jobs before winter. Fall pours can turn out fine, but they need extra care:
- Fresh concrete has to be kept from freezing until it reaches about 500 psi, which typically takes around two days. Concrete that freezes before that can lose a big share of its potential strength.
- It shouldn't be poured on frozen ground. Frost under the slab thaws later and the base settles.
- Cold slows everything down. Concrete cures best above about 50°F. On cold days, insulated blankets and accelerating admixtures are common tools. If your slab was poured on a cold day and nobody covered it, that's worth asking about.
Deicers in the first winter
New concrete is especially vulnerable to salt and freeze-thaw damage. Industry guidance commonly says to avoid deicers on concrete during its first year. Two products to avoid on any concrete, at any age: ammonium nitrate and ammonium sulfate, which are known to attack concrete. Some others, like magnesium chloride, have also been linked to faster deterioration. Read the ingredients on the bag, not just the "concrete safe" claim on the front.
For the first winter, shovel promptly and use sand for traction. And watch the end of your driveway: the slush the city plow throws there is loaded with road salt.
Hairline vs. Offset vs. Widening: How to Read Your Crack
| Crack | What it looks like | What it usually means | What to do |
|---|---|---|---|
| Hairline in or near a joint | Thin line along the bottom of a control joint | The joint did its job | Nothing. This is normal. |
| Hairline across a panel | Thin crack, both sides level, not getting wider | Shrinkage the joints didn't catch | Monitor it. Consider sealing it before winter. |
| Surface crazing or pour-day cracks | Fine web pattern or short parallel cracks on the surface | Surface dried too fast during finishing or curing | Mostly cosmetic. Mention it to your installer. |
| Offset crack | One side sits higher than the other | Part of the slab has moved, usually from settling | Call your installer. Usually a base issue. |
| Widening crack | Wider this month than last month | Active movement that hasn't stopped | Call your installer. Don't wait for it to finish moving. |
| Corner crack | Crack cutting off a corner, often near the edge | Unsupported edge or early heavy load | Ask about base prep and when it was first loaded. |
| Crack + sinking section | A piece of the slab is lower than it was | Soft or washed-out base underneath | Installer first. If that's not an option, this is a lifting job. |
A simple way to tell if a crack is growing: Make a small pencil mark across the crack and at each end, write the date next to it, and take a photo with a tape measure laid across it. Check again in a month or two. If it's longer, wider, or one side has dropped, it's active.
Questions to Ask Your Installer
A good contractor won't get defensive about these. Their answers tell you a lot:
- How was the base prepared? What went under the slab, how thick, and was it compacted?
- How thick is the slab? And does the joint spacing match that thickness?
- Was the mix air-entrained? Air entrainment is standard practice for outdoor concrete that sees freeze-thaw cycles and deicers.
- How was it cured? Curing compound, plastic, or blankets, and for how long?
- If it was a cold-weather pour, how was it protected?
- What do you consider normal cracking? And what does your warranty cover?
- When can I drive on it, and what should I put on it the first winter?
If you have a warranty, use it before you pay anyone else. That includes us.
When Settling Under New Concrete Is a Lifting Job
Sometimes the slab itself is fine, but the ground under it wasn't. Poorly compacted fill, especially foundation backfill next to the garage or front steps, can keep settling after the new concrete goes in. The result is a garage apron that drops away from the garage floor, a stoop that tips toward the house, or a driveway section that sinks and cracks.
That's not a shrinkage problem. It's a base problem, and it can be fixed without tearing out your new concrete. Foam lifting goes in through a few dime-sized holes, fills the voids under the slab, and raises it back in small, controlled increments. The foam weighs a few pounds per cubic foot, so it doesn't add much weight to soil that's already struggling, and it's waterproof once cured.
Talk to your installer first. If they won't make it right, they're out of business, or the warranty has run out, send us photos.
When lifting isn't the answer: If the cracks are cosmetic, you don't need lifting. If the concrete itself is failing (flaking, crumbling, or scaling across the surface), lifting won't help, because the problem is the concrete, not the ground under it.
Caulk It or Leave It?
For a hairline crack, sealing is optional. For a crack wide enough to hold water, it's worth doing before winter. Every freeze turns that trapped water into a little wedge of ice, and a Green Bay winter has plenty of freezes. We caulk cracks and joints, and it's often done right after a lift. Some homeowners also apply a penetrating concrete sealer once the slab has cured to help it resist deicer damage. Ask your installer about timing.