Tearing out concrete feels decisive. The old slab is ugly, it's been sinking for years, and a fresh pour sounds like the end of the problem.
Sometimes it is. Sometimes it's the start of the same problem on a new slab, because nobody asked why the old one failed.
Here are the questions worth asking: first yourself, then whoever you hire. And yes, we'll tell you when replacement is the better move, even though it's not something we do.
Questions to Ask Yourself First
Why did it sink in the first place?
Concrete rarely sinks on its own. Something under it or around it moved. Figure out which, because the new slab sits on the same ground.
| Likely cause | What it looks like | Will new concrete have the same problem? |
|---|---|---|
| Loose backfill next to the house | Stoop, walk, or patio sinking toward the foundation | Yes, unless the soft soil is dug out or compacted and replaced with a proper base |
| Downspout or sump discharge | Sinking or washout near where water dumps | Yes, unless the water gets rerouted |
| Frost heave | Slab rises in winter, drops back crooked in spring | Yes, unless drainage and the base improve |
| Rotting roots or a ground-out stump | Slow sinking where a tree used to stand | Often, if the organic material isn't removed |
| Thin slab or weak original base | Cracking and settling in the driveway, where you park | Not if the new slab is thicker and built on a better base |
| Surface damage from salt and freeze-thaw | Flaking, pitting, exposed stone on top | Less likely, if the new mix suits freeze-thaw and you skip deicers the first winter |
If the answer is loose soil or water, a new slab only fixes the problem if the replacement includes fixing the ground and the drainage. Ask for that in writing.
Is the old slab actually done?
Walk out and look honestly:
- Broken into several loose pieces? It's done.
- Flaking or crumbling across most of the surface? It's done.
- Pushed up by roots or frost? Probably done, since no lifting method can push concrete down.
- Sunk, but mostly in one or two solid pieces? It might not be done at all.
Could lifting save it?
If the concrete is solid and the problem is height, lifting is worth a look before demolition. Polyurethane foam lifting raises the slab through 5/8" holes, about the size of a dime, and fills the voids underneath as it lifts. You can drive on it the same day, and national cost guides put leveling at less than half the price of replacing a sidewalk, patio, or walkway.
The catch is what lifting can't change. The patches will show up close, existing cracks stay, and you're keeping the concrete you have. If you want a new layout, a new finish, or the surface is shot, lifting isn't the answer.
Replacement timing is typical; a contractor's schedule and the weather can stretch it. New-concrete cure times from Concrete Network.
When Replacement Is the Right Call
Plainly, replacement is the better choice when:
- The slab is broken into many pieces or crumbling at the edges.
- The surface is badly scaled or spalled and you'd hate looking at it even if it were level.
- Roots or frost pushed it up and it has to stay flat, like a patio or garage approach.
- It was built wrong: too thin, no real base, or pitched toward the house.
- You want something different: wider, a new shape, a decorative finish.
If that's you, replace it, and use the questions below to get it done right.
Questions to Ask the Replacement Contractor
Good contractors like these questions. Vague answers tell you something too.
| Ask this | A solid answer sounds like | A red flag |
|---|---|---|
| What will you do about the soil under it? | Digs out soft or organic soil, builds a compacted gravel base, explains how they'll compact it | "We'll just pour on what's there" |
| How thick will it be? | At least 4" for walks and car traffic; 5–6" where trucks or RVs park; thicker edges on driveways | Same thickness everywhere, no reason given |
| What reinforcement, and where in the slab? | Wire mesh or rebar, held up in the slab, not lying in the dirt | "Reinforcement stops cracks" |
| Where will the control joints go? | A layout with roughly square panels, cut deep enough to work | No plan, or very long panels |
| What's the mix for our winters? | Air-entrained concrete rated for freeze-thaw and deicer exposure | Doesn't know what air entrainment is |
| How will it be cured? | A specific method (curing compound, covers, or keeping it moist) and how long | "It cures on its own" |
| Which way will water drain? | Slope away from the house and garage, water directed somewhere sensible | Hasn't thought about it |
| Who handles permits and the utility locate? | Knows your municipality's process and who pulls what | "You don't need anything" without checking |
| What happens if you find soft soil or debris once it's torn out? | Explains in writing how extra digging or base gets handled and priced | No answer until it's mid-job |
A few of those deserve more detail.
Thickness
Common guidance is a minimum of 4" for driveways that see passenger cars and SUVs, and 5–6" where heavier trucks or RVs park. Driveway edges are often thickened, because the edge is where a slab is weakest.
Reinforcement
Wire mesh is common in 4–5" slabs, and rebar in thicker ones. Here's the honest part: reinforcement doesn't prevent cracks. It holds cracks together so the pieces don't spread apart or shift. If someone promises a crack-free slab because of the steel, that's a sales line.
Control joints
Concrete shrinks as it cures, and it will crack somewhere. Control joints give it a planned place to do it. A common rule of thumb: space joints, in feet, no more than 2–3 times the slab thickness in inches. For a 4" slab, that's every 8–12 feet. Joints should be cut about a quarter of the slab's depth, and panels should stay close to square. Ask too about the joint where the new slab meets the house, garage floor, or steps. That gap needs to be there, and it needs sealing once the new concrete cures (joint caulking is something we do).
Permits
Permit rules vary from one city, village, and town to the next, especially for driveway aprons, curb cuts, and public sidewalks in the terrace. Check with your municipality before work starts, and get clear on whether you or the contractor is pulling it.
Pouring in Wisconsin's Shoulder Seasons
A lot of replacement gets pushed into October and November around here. Cold-weather pours can turn out fine, but only with the right precautions:
- Never pour on frozen ground. Snow, ice, and frost have to be gone, and frozen subgrade has to be thawed first.
- Fresh concrete has to be protected from freezing. Industry guidance says concrete should reach about 500 psi before it can handle a single freeze. That's why you'll see insulated blankets on late-season pours.
- Cold slows everything down. Concrete cures best above about 50°F. Below that, strength gain slows a lot, and near freezing it can nearly stop.
- Ask about the plan. Blankets, heated water, accelerators (non-chloride, if there's steel in the slab), and what happens if a hard frost shows up the night after the pour.
If the honest answer is "we'll pour and hope," wait for spring. A sunken slab can sit through one more winter. Concrete that freezes before it gains early strength can be permanently weakened.
After the Pour: Timelines and the First Winter
| Use | Typical wait after the pour |
|---|---|
| Walking | About 24 hours |
| Bikes, scooters, skateboards | About 3 days |
| Parking a car | About 7 days, staying off the edges |
| RVs and large trucks | About 28 days |
Your contractor's guidance wins if it's different, especially in cold weather, when everything takes longer.
The first winter matters most
- Skip deicers the first winter. New concrete is most vulnerable to salt damage early on. Industry guidance says not to use deicers until at least 30 days after proper curing, and fall pours often don't dry out enough before the first snow. Use sand for traction instead.
- Never use ammonium nitrate or ammonium sulfate. These chemically attack concrete. Read the ingredient list before you buy any ice melt.
- Go easy with the shovel and plow. Avoid metal blades that scrape the surface.
- Fix the water you identified earlier. A new slab on the same soggy ground will start the same story over.