A concrete slab can look fine from the surface and still have a problem underneath it. One of the clearest examples is a highway joint. The pavement may still be at the correct elevation, but when a heavy truck crosses the joint, the slab edge can deflect downward and then rebound after the load passes. That movement can be a sign that support has been lost beneath the slab. This is where undersealing comes into play.
What Is Undersealing?
Undersealing is the process of introducing material beneath a slab to restore support where contact has been lost. Unlike a typical lifting project, the objective is not necessarily to change the elevation of the concrete. In many cases, the slab is already where it needs to be. The issue is that the material beneath it is no longer providing the support it should.
Highway pavements are a good example. Over time, water intrusion, erosion, pumping of fines, or other base problems can create unsupported areas near joints and slab edges. From the surface, the pavement may still appear level, but repeated heavy wheel loads can cause the unsupported area to move. As trucks pass over the joint, the slab edge may deflect and rebound, placing additional stress on the pavement and potentially contributing to continued deterioration.
In that situation, the goal of polyurethane injection is not to noticeably raise the slab. The goal is to fill the unsupported area and help reestablish contact beneath the concrete so loads can be transferred more effectively into the supporting material below.
Why Support Matters
Concrete is designed to work with the material beneath it. When support is lost under part of a slab, the concrete may begin bridging across that unsupported area instead of transferring the load into the base. The slab itself then has to carry more of that load.
On a highway, that can mean repeated movement every time a heavy vehicle crosses the affected area. Over time, that repeated deflection can contribute to cracking, faulting, additional loss of support, or other pavement problems. The same basic issue can occur beneath industrial floors, loading areas, warehouse slabs, or other heavily loaded concrete where support has been lost even though the slab has not visibly settled.
This is why elevation alone does not always tell you whether a slab is performing properly. A slab can be level and still need support underneath it.
How Undersealing Differs From Lifting
During a typical lifting project, movement is usually the result the contractor is looking for. Polyurethane is injected, the slab begins to rise, and the contractor controls the lift as the concrete approaches the desired elevation.
With undersealing, significant movement may be exactly what the contractor is trying to avoid. The objective is to place material into the unsupported area while keeping the slab close to its existing elevation. That means the contractor has to think differently about injection location, material placement, and how the slab responds during the work.
Some projects may involve both lifting and undersealing. A settled slab may first need to be brought back toward the correct elevation, and then additional unsupported areas may need to be addressed. In other situations, the concrete may already be at the correct elevation and only require support beneath certain areas.
The distinction comes down to the objective of the repair. If the problem is elevation, the focus is lifting. If the problem is lost support, the focus is undersealing.
The Bottom Line
Undersealing is about restoring support beneath concrete, whether or not the slab has visibly settled. Highway joint deflection under heavy truck traffic is a good example because the pavement may look acceptable from above while movement under load reveals a support problem underneath.
Concrete lifting and undersealing are closely related, but they solve different problems. One primarily corrects elevation. The other primarily restores support.
Stay Tuned
In the next Foam Fundamentals article, “Choosing the Right Foam for the Job,” we’ll bring together the concepts we’ve covered so far and look at how the application helps determine which polyurethane formulation makes the most sense.
This is blog #9, 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
- Blog #7: What Actually Lifts Concrete
- Blog #8: How Polyurethane Foam Travels Underground
