Polyurethane foam can lift a concrete slab that weighs thousands of pounds. That sounds simple enough until you stop and think about what is actually happening beneath the slab. The foam is lightweight, the concrete is heavy, and the material is being injected through a relatively small hole. So where does the lifting force actually come from?
The answer starts with understanding what the foam does immediately after it is injected.
Foam Follows the Path of Least Resistance
When polyurethane is injected beneath a slab, it does not automatically push straight upward. As the material reacts and expands, it moves into the areas where it encounters the least resistance. That may be an existing void, a loose area of base material, or another open space near the injection point.
As long as the foam has somewhere easier to go, it will continue occupying those available spaces. This is one reason subsurface conditions can make such a difference from one injection point to another. A tight void with firm material around it may respond very differently than an area with a larger washout, loose base, or an open path that allows the foam to travel farther.
Eventually, though, that available space becomes more restricted.
Confinement Is What Allows Pressure to Build
As the foam occupies more of the open space beneath the slab, it begins encountering greater resistance from the concrete above and from the soil, aggregate, or other materials around it. The polyurethane is still reacting and expanding, but it now has less room to move freely.
That is when confinement becomes important.
Continued expansion in a confined area creates pressure against the surrounding surfaces. Once enough force develops beneath the concrete to overcome the weight of the slab and the other resistance holding it in place, the slab begins to rise.
That is the basic mechanism behind polyurethane lifting.
The foam does not need to weigh anything close to what the concrete weighs. What matters is the pressure that develops as the material expands within a confined space.
Pressure Over an Area Creates Lifting Force
A relatively modest amount of pressure can create a large amount of total force when it is acting across several square feet of concrete.
That is why polyurethane can move such a heavy slab. The lifting force comes from pressure acting against the underside of the concrete over an area, not from the weight of the foam itself.
This also helps separate two things that are sometimes confused: lifting pressure and cured foam strength.
The pressure generated while the polyurethane is reacting is what moves the slab. After the reaction is complete, the cured foam remains beneath the concrete and becomes part of the support system. Its cured physical properties matter at that point, but they are not the same thing as the pressure generated during the lift.
That is why even a lower-density, higher-yield foam can still generate enough pressure to lift concrete in many applications.
Why Every Lift Behaves Differently
The amount of material required before lift begins can vary because confinement is not the same on every job.
A small, well-defined void may become confined quickly. A larger washout or loose base may allow the foam to travel farther before enough resistance develops to move the slab. Cracks, open edges, and other subsurface conditions can also affect where the material goes before pressure starts to build.
For contractors, this is why the slab’s response matters so much during an injection. The material is reacting in conditions that cannot be fully seen from above, and those conditions influence when and where lifting pressure develops.
The Bottom Line
Polyurethane does not lift concrete simply because it expands. It lifts concrete when that expansion becomes confined enough to create pressure beneath the slab.
As available space is occupied, resistance increases. Continued expansion builds pressure, and that pressure acting over an area creates the force needed to move the concrete.
Expansion + confinement creates pressure, and pressure creates lift.
Stay Tuned
Now that we know what creates the lifting force, the next question is where the polyurethane goes before that pressure develops.
In the next Foam Fundamentals article, “How Foam Travels Underground,” we’ll look at how voids, cracks, loose soils, and other subsurface conditions influence the path the material takes after injection.
This is blog #7, check out our other blogs in the foam fundamentals series here:
- Blog #1: What does 2 lb Polyurethane Foam Actually Mean?
- Blog #2: Why are there Different Polyurethane Foam Densities?
- Blog #3: What is Polyurethane Foam Expansion?
- Blog #4: Why More Expansion Isn’t Always Better
- Blog #5: Reaction Speed vs. Spread
- Blog #6: Why Faster isn’t Always better in Polyurethane Concrete Lifting
