Sunken concrete is rarely the slab’s fault. It’s the ground underneath changing over time. One day, your driveway looks fine. A season later, a corner drops, water starts pooling, and the trip hazard shows up right where people step.
Concrete raising fixes that without tearing everything out. The idea is simple: fill the empty space under the slab, support it again, then lift it back where it belongs using controlled injection.
Why concrete slabs sink in the first place
Most settled slabs are dealing with one (or more) of these problems:
- Soil wasn’t compacted well during the original build.
- Water washed out fine soils and left voids behind.
- Freeze-thaw cycles moved the base around.
- Downspouts or poor drainage kept saturating the soil.
- Tree roots, burrowing, or utility work disturbed the subgrade.
Once there’s a void, the slab starts acting like a bridge. It might hold for a while, then it cracks, tips, or drops.
What “concrete raising” actually means
Concrete raising (also called slabjacking or polyjacking) lifts and stabilizes existing concrete by injecting material beneath it to fill voids and re-support the slab. We focus on polyurethane foam injection, which is lightweight, water-resistant, and cures quickly.
Step-by-step: the polyurethane concrete raising process
1. Inspection and lift plan
A good crew starts by figuring out what moved and why. They’ll check:
- Where the slab is lowest
- Where water is flowing (or pooling)
- Cracks, separation at control joints, and edge conditions
- Nearby drainage issues that might cause repeat settling
This is where the lift plan comes from: where to inject, how many ports are needed, and how much lift is realistic without stressing the concrete.
2. Drilling small injection ports
Small holes are drilled through the slab so the foam can be injected underneath. With polyurethane foam injection, the holes are typically about 5/8 inch (roughly the size of a dime).
These ports are spaced based on the slab size, thickness, and how the voids are distributed.
3. Injecting foam under the slab
Polyurethane foam is created by combining two components at the point of injection. The material starts as a liquid, then expands under the slab, flowing through gaps and into voids.
That expansion does two jobs at once:
- Void filling: it occupies empty space, so the slab is supported again
- Lifting: as the foam expands, it applies upward pressure and raises the slab gradually
4. Controlled lifting and leveling
This part is more “slow and intentional” than people expect.
The crew injects in stages, watching the slab movement, checking the grade, and adjusting as needed so the concrete rises evenly. The goal is usually to remove the trip hazard, improve drainage, and stabilize the slab so it stops moving.
5. Patching the holes
Once the slab is set, the injection ports are patched. One advantage of smaller ports is that the patches are typically far less noticeable than the larger holes used in mudjacking.
6. Cure time and cleanup
Polyurethane foam cures fast. We note that lifted areas are typically usable about 15 minutes after injection.
That short cure time is a big reason homeowners pick raising over replacement when the slab itself is still in decent shape.
Why polyurethane foam is different from mudjacking
Mudjacking can work, but it comes with tradeoffs that matter on real jobs.
- Hole size: polyurethane uses smaller holes (about 5/8 inch) while mudjacking holes are often much larger.
- Material weight: polyurethane is extremely light compared to slurry (about 2 lb/ft³ for polyurethane vs about 100 lb/ft³ for mudjacking slurry). Less added weight means less chance of pushing the soil down again.
- Washout resistance: polyurethane is described as water-resistant and not prone to washing out like some slurries.
- Speed: polyurethane cures quickly, while mudjacking often needs more time to dry before heavy use.
What kinds of concrete can be raised
Foam injection is commonly used for:
- Sidewalks and walkways
- Driveways and garage floors
- Patios and pool decks
- Steps, stoops, and porches
We specifically call out these types of surfaces as typical lift targets.
What to expect on service day
Most projects follow a pretty clean workflow:
- The area is marked and prepped.
- Injection ports are drilled.
- Foam is injected in stages while grade and alignment are monitored.
- Ports are patched.
- The site is cleaned up, and the slab is ready for use shortly after.
If the slab is near a garage door, foundation, or steps, the crew may recommend sealing joints or improving drainage afterward, because water is usually part of the original problem.
When concrete raising might not be the right fix
Raising is a great option when the concrete is basically intact. It’s not the best choice if:
- The slab is shattered into multiple unstable pieces
- There’s major heaving from the roots that will keep moving
- The base is actively washing out, and drainage can’t be corrected
- The surface is so deteriorated that lifting it won’t restore function or appearance
In those cases, replacement or a larger stabilization plan may make more sense.