What Is Concrete Lifting Foam?

What it is made of

Structural lifting polyurethane is supplied as two liquids, usually called the A side and the B side. One is an isocyanate; the other is a resin blend containing polyols, catalysts, and a blowing agent. Kept apart, both are stable. Combined, they react immediately.

The reaction does two things at once: it forms a rigid polymer and it generates gas that inflates that polymer into millions of tiny sealed cells. The result expands many times its liquid volume within seconds, which is where the lifting force comes from.

Density, strength, and why both matter

Density is the number that separates products. Insulation foam runs about half a pound per cubic foot and has no business under a slab. Structural lifting foam typically runs between two and four pounds per cubic foot, and higher-density formulations exist for heavy commercial loads.

That density buys compressive strength well beyond what residential slabs and vehicles impose — typically several thousand pounds per square foot, far more than the soil it is replacing could carry. At the same time, at roughly 2–4 lbs/ft³ the material adds almost no load to ground that already proved it could not hold weight. High strength with negligible weight is the entire engineering argument for foam.

Closed-cell means waterproof

Each cell in cured lifting foam is sealed from its neighbors, so water cannot travel through the mass or soak into it. Groundwater flows around the foam instead of eroding it, which is a meaningful difference under Middle Tennessee slabs where the water table and heavy rain events do the damage.

Mudjacking slurry is porous by comparison. Water works through it, carries fines away, and slowly recreates the void that was filled. Closed-cell polymer does not give water that path.

Is it safe?

During injection the components are handled with trained technicians and appropriate protective equipment, because uncured isocyanate is an irritant. That is a job-site concern, not a homeowner one — you are not near the gun.

Once cured, which takes about fifteen minutes, the polymer is inert. It does not off-gas in service, it is not a food source for mold or insects, and it does not leach. The same chemistry family is widely used in potable water infrastructure and in building insulation. It is also a stable, non-decomposing material, which is why it does not need replacing later.

Foam vs. mudjacking slurry, side by side

• •Weight: about 2–4 lbs/ft³ for foam versus roughly 100 lbs/ft³ for slurry.
• •Hole size: 5/8 inch versus about 1 5/8 inch.
• •Cure time: roughly 15 minutes versus 24–72 hours before full use.
• •Water resistance: closed-cell and waterproof versus porous and erodible.
• •Void filling: expands into small cavities and compacts loose soil versus flowing where the pressure takes it.
• •Cost: usually somewhat higher up front, with a lower likelihood of a repeat visit.

How much foam a job takes

Material volume is set by the void, not by the slab area you can see. A driveway with a shallow half-inch dish may take a fraction of what an identical driveway takes when a washout has hollowed the base.

This is also why probing matters during the estimate, and why flat-rate quoting protects the homeowner. If pricing is per pound, the customer absorbs the uncertainty of an unknown void; if it is flat-rate, the contractor does.