TL;DR — A slab-jacking (concrete-lifting) foam is a two-component (2K) polyurethane: an isocyanate side (polymeric MDI) reacting against a polyol blend. Almost everything a contractor cares about — lift density, set speed, and load-bearing strength — is decided on the polyol side. Adjusting the water level, catalyst package and base-polyol functionality is what turns one chemistry into a 4-lb residential lift, a 6-lb standard slab-jacking system, or an 8-lb commercial-grade foam. This guide breaks down the polyol system from the raw-material formulator's seat.
What slab-jacking foam actually is (the 2K chemistry)
Polyurethane slab-jacking — also called polyjacking, polyurethane concrete leveling or foam jacking — injects an expanding rigid foam under a sunken slab to raise it back to grade. The foam is produced in situ from two liquid components pumped at (typically) a 1:1 volume ratio through an impingement mix gun:
- Part A — Isocyanate: polymeric MDI (pMDI). This side is largely standardized; it supplies the reactive NCO groups.
- Part B — Polyol blend (the resin side): a formulated cocktail of base polyols, catalysts, silicone surfactant, blowing agent (usually water) and optional additives. This is where density, reaction profile and strength are engineered.
The two streams meet, the isocyanate reacts with the polyol hydroxyls to build the polyurethane polymer, and simultaneously reacts with water to release CO2 that blows the foam. Because the formulator controls the Part B recipe, the polyol blend is effectively the "tuning panel" of the entire system.
The polyol system, component by component
A geotechnical / slab-jacking Part B is not a single polyol — it is a balanced blend. Each ingredient does a specific job:
| Component | Role in a slab-jacking foam |
|---|---|
| Base polyols (polyether, high functionality — e.g. sucrose/glycerine-initiated) | Build crosslink density and rigidity. Higher functionality and hydroxyl value = stiffer, higher-strength foam for load-bearing lifts. |
| Catalysts (tertiary amine + metal, e.g. tin or bismuth) | Balance the gel reaction (polymer build) against the blow reaction (CO2). The amine/metal ratio is what sets cream and gel time. |
| Silicone surfactant | Stabilizes the rising foam, controls cell size and keeps cells fine and closed — critical for water resistance and consistent strength. |
| Blowing agent (water; sometimes an HFO co-blowing agent) | Water reacts with isocyanate to generate CO2. Water level is the single biggest lever on density — less water = denser, stronger foam. |
| Optional additives (flame retardant, cell openers, crosslinkers) | Tune fire performance or fine-tune mechanical behaviour for specialized jobs. |
This is precisely why a contractor cannot simply "buy MDI cheaper" and expect the same lift — the performance lives in the engineered polyol blend, not the isocyanate. For how buyers should qualify a Part B supplier, see our 2026 polyol supplier global sourcing guide.
How the formulation sets density (4-lb vs 6-lb vs 8-lb)
Density in this industry is quoted in pounds per cubic foot (lb/ft³) and measured per ASTM D1622 (apparent density of rigid cellular plastics). The formulator dials density mainly through two levers:
- Water content in Part B — more water makes more CO2, so the foam blows lighter and larger. Reducing water raises density and load capacity.
- NCO:OH index — typically held near 1.05–1.15 for these systems; index affects crosslinking and dimensional stability.
It is also essential to distinguish free-rise density (foam allowed to expand unconfined, as on a QC bench) from the in-place / confined density achieved under a slab, which is higher because expansion is restrained. A "6-lb foam" is normally specified on a free-rise basis; the confined density under load can be meaningfully higher. Our PU foam density spec guide covers density measurement and ranges across applications in more depth.
Reaction profile: cream, gel and rise time
For injection under a slab, timing matters as much as density. The catalyst package sets a reaction signature:
- Cream time — when the mixed liquid starts to expand (often a few seconds for fast slab-jacking systems).
- Gel / string time — when the polymer network forms and the foam stops flowing.
- Tack-free and rise time — when the surface cures and full expansion is reached.
A fast, hydrophobic system rises quickly so the slab can be lifted in controlled stages and the foam resists wash-out from groundwater. Too fast and the foam can over-pressure or cure before it travels; too slow and it migrates away from the void. The amine/metal catalyst balance in the polyol blend is the formulator's tool for tuning this curve to the equipment and the job.
Compressive strength by density
Load-bearing capacity is measured per ASTM D1621 (compressive properties of rigid cellular plastics) and scales strongly with density. The figures below are representative industry ranges — always confirm against the supplier's technical data sheet for the specific system:
| Density (lb/ft³) | Typical compressive strength* | Typical use |
|---|---|---|
| ~2.0–2.5 | ~18–25 psi | Void fill / non-structural backfill |
| ~4.0 | ~40–60 psi | Light residential slab and sidewalk lifting |
| ~6.0 | ~60–90 psi | Standard slab-jacking (driveways, floors) |
| ~8.0 | ~90–140 psi | Commercial / heavy-load slabs |
| ~10.0+ | 140+ psi | Structural / industrial floor repair |
*Representative ranges only; verify with the product TDS and ASTM D1621 results. For the heavy end of this range, see our deep dive on high-density (8–10 lb) foam for structural slab lifting.
Equipment, viscosity and safe handling
The polyol blend must match the dispensing equipment. Most slab-jacking is done with high-pressure, heated, two-component proportioners running impingement-mix guns at a 1:1 volume ratio. That means the Part B viscosity has to be pumpable at the rig's process temperature (commonly conditioned to roughly 21–32 °C / 70–90 °F). A blend that is too viscous starves the gun; too thin and the ratio drifts. Formulators tune base-polyol selection to keep viscosity in the equipment's window.
Because Part A is MDI-based, both components require proper PPE and ventilation. Follow recognized guidance on isocyanate handling from OSHA — respiratory protection, skin protection and exposure controls are non-negotiable on a job site.
Sourcing the polyol component
Because the polyol blend is where the engineering lives, the supplier relationship matters more than the headline price per kilo. The questions that actually de-risk a program are: is the Part B formulated specifically for slab-jacking (not a generic rigid-panel polyol re-labelled), does it ship with a real TDS and density/strength data, what is the MOQ and container lead time, and can the supplier tune the reaction profile to your rig? We cover the buyer-side checklist for North America in our slab-jacking foam manufacturer sourcing guide (4/6/8-lb), and the chemistry-vs-mudjacking case in our polyurethane slab-jacking vs mudjacking comparison.
Frequently asked questions
Is the polyol or the isocyanate responsible for foam density?
Primarily the polyol side. Density is controlled chiefly by the water (blowing agent) level in the Part B blend and the NCO:OH index, both set on the resin side. The isocyanate (pMDI) is largely standardized.
Why do two "6-lb" foams perform differently?
Because 6 lb/ft³ describes density, not the full recipe. Base-polyol functionality, catalyst balance, surfactant and closed-cell content all vary, so two foams at the same nominal density can differ in compressive strength, set speed and water resistance.
What density should I use for a residential driveway?
Light residential slab and sidewalk lifting commonly uses ~4-lb foam, while standard driveways and floors typically move to ~6-lb. Heavy commercial slabs step up to 8-lb or higher. Always size density to the load, not the other way around.
Can I run any slab-jacking polyol on my existing rig?
Only if its viscosity and reaction profile fit your proportioner's temperature and ratio window. A reputable formulator will adjust the blend to your equipment rather than expecting your equipment to match a fixed product.
Is the foam stable in wet ground?
Quality slab-jacking systems are formulated to be hydro-insensitive — fast-rising and closed-cell — so they cure and hold even where groundwater is present. This behaviour is engineered through the catalyst and surfactant package in the polyol blend.
About the author — MAE, BlendPolyol technical team. BlendPolyol formulates and supplies polyurethane foam raw materials (polyol systems, catalysts, surfactants and flame retardants) to manufacturers and contractors worldwide.