A sunken driveway slab that would cost $4,000 to tear out and repour can usually be floated back to grade in a morning with a few pounds of expanding polymer. Concrete lifting foam — the industry calls it polyjacking or polyurethane concrete raising — injects a two-part foam under a settled slab through holes the size of a dime, and the chemical expansion does the lifting. It has largely taken over the slab-raising trade from traditional slurry methods in the last 15 years, and understanding how it actually works tells you when it’s worth the premium and when it isn’t.
The Chemistry Doing the Work
The material is a two-component closed-cell polyurethane: an isocyanate (“A side”) and a polyol resin blend (“B side”) kept separate until they meet at the gun tip. Mixed, they react within seconds, expanding up to 15 to 25 times their liquid volume and generating lift pressures that can exceed several thousand pounds per square foot. Injected as a liquid, the foam flows into voids, follows the path of least resistance, and then expands — first filling empty space, then compacting loose soil, then, with nowhere left to go, pushing the slab upward. Reaction is tunable: fast-rise formulations set for lifting in 15 to 45 seconds; slower deep-injection foams are used to consolidate soil below the slab. Cured density for slab work is typically 3.5 to 6 lbs per cubic foot — strong enough to support traffic loads, light enough not to re-compress the soil that caused the settlement in the first place. That last point is the core engineering argument: cured foam adds roughly 2 to 4 lbs per sq ft under a slab, a rounding error compared to the hundred-plus pounds per cubic foot of traditional fill materials.
The Process, Step by Step
- Assessment: the tech maps the settlement with a level or zip level, locates joints and cracks, and checks for voids by sounding the slab (a hollow drum note under a hammer tap).
- Drilling: 5/8-inch holes — genuinely dime-sized — are drilled through the slab in a grid, typically 3 to 6 feet apart, positioned to lift low corners and support joint edges. Compare mudjacking’s 1-5/8 to 2-inch holes; the small penetrations are a major cosmetic advantage on decorative and stamped work.
- Injection: a heated hose from the rig delivers the two components to a gun fitted with a tapered port in each hole. The tech injects in short bursts, watching a dial indicator or laser level as the slab responds. Lift happens in fractions of an inch per burst, so a good operator can bring a corner up 2 inches and stop within 1/8 inch of plan.
- Sequencing: lifting proceeds low-point first, working joints so adjoining slabs rise together and don’t bind. Trapped edges (against stoops, garage slabs pinned under drywall tracks) get relief cuts if needed.
- Patching: holes are filled with a non-shrink cement grout, leaving pencil-eraser-sized dots as the only evidence.
A typical driveway or sidewalk job runs 1 to 3 hours start to finish.
Cure Time: The Headline Advantage
Foam reaches roughly 90 percent of final strength within 15 to 30 minutes. You can walk the slab immediately and drive on it the same day — most contractors say 30 minutes to a few hours to be conservative. Slurry-lifted slabs, by contrast, want 24 to 72 hours before vehicle loads while the water-heavy mix firms up. For a business sidewalk, a garage you park in daily, or the only driveway you have, same-day return to service is frequently the deciding factor all by itself. The foam is also hydrophobic and closed-cell: it won’t wash out, absorb water, or break down in wet soil, and it’s inert once cured.
Cost per Square Foot and Job Pricing
Polyjacking prices land between mudjacking and replacement:
- Rule-of-thumb rates: $5 to $15 per sq ft of affected slab, driven by lift height and void volume more than area alone.
- Typical jobs: sidewalk sections $300 to $800; a two-car driveway with 1 to 2 inches of settlement $900 to $2,500; garage floors and pool decks $1,000 to $3,000; interior slab floors more, due to access and finish protection.
- Minimums: most rigs charge a $400 to $700 mobilization minimum, so tiny jobs price poorly — batch your settled stoop with the sidewalk.
- Versus replacement: tear-out and repour runs $8 to $15 per sq ft plus demolition and weeks of cure, and new concrete never quite matches the old. Lifting usually saves 50 to 70 percent when the slab itself is sound.
Lifespan and Limits
The cured polymer is permanent for practical purposes — it doesn’t rot, shrink, or wash away, and manufacturers cite service lives beyond 50 years, with many contractors warrantying the lift 5 to 10 years. The honest caveat: the foam is only as permanent as the soil under it. If settlement was caused by an active problem — a leaking downspout washing out fines, a broken irrigation line, deep uncompacted fill still consolidating — the slab can settle again below the foam layer. Good contractors diagnose the water source and fix drainage as part of the job; if yours doesn’t mention drainage, keep interviewing. Foam also can’t fix a slab that’s structurally shattered (many crossing cracks), badly spalled, or curled from finishing defects — lifting recovers grade, not surface condition. And slabs bearing structural loads (footings, stem walls) are a soil-stabilization engineering problem, not a $1,500 polyjack.
Vetting a Polyjacking Contractor
The equipment barrier keeps fly-by-night operators rarer here than in most trades — a proportioner rig, heated hoses, and material inventory represent a six-figure setup — but quality still varies, and a few questions separate technicians from hole-drillers. Ask what foam density they’ll use and why: slab lifting wants 3.5 to 6 lb closed-cell material, and a contractor who can’t answer in numbers is reading a label. Ask how they’ll verify void fill — good outfits sound the slab before and after, and some carry ground-penetrating radar for larger jobs. Ask about environmental credentials if injection is near gardens or wells; cured polyurethane is inert, and several manufacturers (HMI, NCFI, Prime Resins) publish NSF/ANSI 61 certifications for foams safe near potable water, worth requesting in writing near a well head. Ask what happens at the slab’s hidden edges: foam follows least resistance, and an open gap under a sidewalk edge can vent material into the lawn — pros trench-check or dam edges first. Get the lift plan in writing (target elevations at specific points, not “raise it”), the warranty term with re-settlement remedy spelled out, and confirmation that they, not you, own utility locates before drilling; radiant-heat garage slabs and shallow electrical conduits are the classic drill strikes. Expect a professional quote to itemize mobilization, estimated material, and a per-pound or per-job overage policy — foam volume is the honest unknown in every job, and how a contractor handles that uncertainty up front predicts how the whole project goes.
Foam or Mudjacking? The Short Version
Both methods raise slabs; they differ in material and tradeoffs. Foam wins on hole size (5/8 vs ~2 inches), weight on weak soils, cure speed, and washout resistance; slurry mudjacking wins on upfront price — commonly $3 to $8 per sq ft — and uses inert cementitious material some owners prefer. As a quick screen: choose concrete lifting foam for driveways and garages needing same-day use, for slabs over soft or wet soils, and for decorative concrete where big patch holes would show; consider mudjacking when budget leads and the slab is a simple patio or walk on stable soil. The full mudjacking process, mix, and cost breakdown is its own topic — but for most residential slab rescues today, foam is the default the industry has settled on, for the reasons above.
