Concrete Leveling Guide: Mudjacking vs Foam vs Replace

Last updated: August 20, 2026

A driveway section that dropped 1 to 2 inches. A garage floor sloping a quarter inch per foot toward the door. A sidewalk panel offset past the half inch that ADA guidance treats as a trip hazard. Those three calls come in every week, and in almost every one of them the concrete is fine. The dirt under it moved.

That’s the first thing worth getting straight, because concrete leveling isn’t a repair. It’s an elevation fix. If the surface itself is spalling, cracking, or coming apart, that’s a different conversation and it starts in the concrete repair guide. Leveling raises a sound slab back to grade. It doesn’t make a bad slab good.

Why did the slab drop in the first place?

Settlement means the soil under the slab lost volume or washed out, leaving a void the slab dropped into. Four causes account for most of it, and none of them are the concrete’s fault.

Poor compaction during the original build is the most common. Fill got placed too fast, in lifts that were too thick, and it kept compressing for years under load. Water erosion is next: a downspout dumping at a slab edge, a yard sloped the wrong direction, or a slow plumbing leak that carried the subgrade out a spoonful at a time. Drought does it too, shrinking clay soil until it pulls support out from under the edges. So do tree roots, drawing moisture out of the ground in dry months.

Notice what all four have in common. They’re all still there after the lift unless somebody deals with them.

Sorting out which one you’ve got is usually a walk around the house after a hard rain. A slab that dropped at one corner, right below a downspout, is telling you something different from a slab that dropped evenly across its whole face. Original-compaction settlement tends to show up early in a slab’s life and then slow down. Erosion settlement keeps going, because the water keeps coming. Drought-driven clay movement tracks the season and often partially recovers when the ground gets wet again, which is why the same driveway can look worse in August than it did in April.

Settlement, heave, or a broken slab?

Settlement and heave look similar at a glance but call for opposite fixes, so naming the movement correctly is the first real step. Settlement is the slab dropping into a void where soil lost volume or washed out. Heave is the reverse: the slab pushed up from below, almost always by expansive clay swelling after it absorbs water. A structural crack is neither, and it means the slab itself has broken through, often where it spans a void or carries a load it was never poured for.

Get this wrong and you can make things worse. Injecting material under a heaved slab adds lift to a slab that’s already too high. Lifting a slab that’s broken into pieces just spreads the breaks. The read on a settled slab starts at the low corner, follows the water, and looks at how the cracks move under weight before anybody decides which tool comes off the truck.

How does mudjacking work?

Mudjacking, also called slabjacking, lifts a slab by pumping a cement, sand, and water slurry into the void underneath it. The crew drills holes roughly 1.5 to 2 inches across through the slab, pumps the slurry under pressure until the void fills and the concrete rises to grade, then patches the holes. The slab is ready for traffic in about 24 hours.

It’s the right call for large voids under driveways, patios, and garage floors, for slabs sitting on firm soil that can carry the added weight, and for budget-driven lifts where a patch the size of a soda can is acceptable to look at.

The trade-offs are the same two facts turned around. Cement slurry is heavy, and adding weight on top of soil that already failed to hold the slab up isn’t always the move. And those injection holes stay visible. On a back patio nobody cares. On a stamped front walk, people care.

The hole pattern isn’t arbitrary either. Holes get placed to reach the void and to spread the lift across the panel instead of pushing one spot up and cracking the concrete around it. That’s the difference between a slab that comes up as a unit and a slab that gets a new crack on the way to level.

How does polyurethane foam injection work?

Foam injection, or polyjacking, does the same job with a two-part polyurethane instead of slurry. The crew drills holes about 5/8 inch across, the diameter of a dime, and injects the resin. It reacts and expands within seconds, filling the void and raising the slab, and it cures hard enough for traffic in 15 minutes to an hour.

Two things make foam the better fit on a lot of jobs. Cured polyurethane weighs a fraction of what cement slurry weighs, so it doesn’t add meaningful load to soft soil that’s already struggling. And the reaction is controllable, which means the lift can be brought up precisely and evenly instead of in surges. That control is why foam suits walkways, garage floors, and anything where a quarter inch of overshoot is obvious.

The trade-off is material cost per cubic foot, which runs higher than slurry. On a small precise lift that difference barely registers. On a slab sitting over a large void, it adds up fast, which is exactly the case mudjacking was built for. Matching the method to the slab is the whole job, and it’s what a concrete leveling assessment is for.

When does replacement beat lifting?

Three conditions push a slab out of lift territory and into removal. The slab is structurally broken into two or more sections that move independently. The surface is spalled or delaminated beyond what a cosmetic repair can carry. Or the void and soil problem underneath is too severe to bridge with injection at all.

In those cases the honest answer is tear it out and repour on a base compacted to about 95 percent standard Proctor density. That costs more on day one. Over the 20 to 40 year life of a slab it costs less than lifting a moving target every few years and paying for the same conversation again. The full trade-off, including where resurfacing fits between the two, is in the overlay versus replacement guide.

Mudjacking vs foam vs replacement

Read the bottom row twice. Two of the three columns carry a condition, and it’s the same condition.

What concrete leveling cannot fix

Leveling changes elevation. It does not change soil behavior. If the cause is still active, the slab will sink or heave again, and the injected material has nothing to do with it.

Expansive clay is the clearest example. It can swell with pressures over 3,000 pounds per square foot, which is more than enough to undo a fresh lift in a single wet season. No slurry and no foam is going to argue with that.

So the water gets fixed alongside the lift, not after it. Regrade the soil to fall at least 6 inches over the first 10 feet away from the slab, which is about a 5 percent slope. Redirect the downspouts so they discharge past the edge instead of at it. Where the soil itself is the problem, address the clay. ACI 302.1R covers subgrade support for slabs on grade, ASTM D698 is the standard Proctor compaction reference, and the USDA NRCS soil survey will tell you what’s under your address before anybody guesses.

How do you choose a leveling method?

Choosing a concrete leveling method is a short decision tree, not a guess. The first question is whether the slab still moves as one piece. If it does, it’s liftable. If it’s fractured into sections that move independently, no injection method fixes that, and replacement is the answer no matter how much cheaper the alternative sounds.

Once a slab is confirmed liftable, four site facts decide between slurry and foam: how big the void is, how much weight the soil can bear, what the budget is, and how long the surface can be out of service. Big void, firm soil, patient owner, tight budget points to mudjacking. Soft soil, precise lift, a floor that has to carry traffic this afternoon points to foam.

Downtime is the factor people underweight until it bites. Twenty-four hours off a back patio costs nothing. Twenty-four hours off the only driveway at a working shop, or off a garage floor with a truck that has to be in it tonight, is a real number that just doesn’t appear on the estimate. Foam curing hard in 15 minutes to an hour is worth paying for on those jobs and worth nothing on the others.

Scope and site conditions move the price the same way they move it on any pour. The concrete cost guide walks through what actually drives the number.

One more time, because it’s the sentence people skip: fix the water. A lift on a diagnosed slab holds for years. A lift on an undiagnosed one is a receipt with an expiration date, and figuring out which one you have starts with the repair diagnostics.