Voids Under Concrete in Kansas City
The Hidden Cause Behind Most Slab Settlement
Polyurethane foam fills voids beneath concrete slabs, stopping further settlement and restoring proper support. Causes, signs, and how polyjacking fixes them.
Quick Answer
What Voids Are and Why They Matter
Voids form beneath concrete when soil washes out, compresses, or shifts away. Polyurethane foam is injected through small ports to fill the voids and densify surrounding subgrade — stopping further settlement and restoring slab support.
Sunken concrete is the symptom. Voids under the slab are usually the cause. Polyurethane foam is exactly what they were designed to fill.
Expands to Fill
Foam reaches into irregular spaces.
Densifies Soil
Compaction effect as foam cures.
Lightweight
Doesn't add load to already weakened soil.
15–30 Min
Same-hour use after the lift.
1/10th Weight
Far lighter than cement slurry.
5/8" Ports
Small holes, patched flush.
Long-Term
Outlasts cement-based lifting in clay.
How Voids Form
Common Mechanisms in Kansas City
Soil Erosion
Water cutting beneath the slab over time, washing out subgrade.
Clay Shrinkage
Soil pulling back from underside of slab during dry periods.
Poor Compaction
Loose backfill consolidating after construction.
Tree Roots & Trenches
Roots dying off and utility trenches still settling beneath the surface.
Signs You Have Voids
Why Polyurethane Foam Works for Void Filling
You can’t see voids directly, but you can usually see their effects: sunken or sloping slabs that weren’t always that way, cracking patterns following void edges, hollow sound when tapping the slab, water pooling in newly-low areas, and surface that “rocks” slightly under load.
Visible separations at slab joints, settlement that gets worse after heavy rain — if multiple of those apply, voids are probably part of the picture.
Why Polyurethane Is the Right Fix
Purpose-Built for Void Filling
Polyurethane foam expands to fill irregular spaces — even small or oddly-shaped voids. It densifies surrounding soil through compaction effect, adds minimal load, and resists moisture that caused the problem.
Cement-based mudjacking can fill voids too — but slurry’s weight, slow cure, and water sensitivity make it less suited for the conditions that caused the void in the first place.
What Happens If You Ignore Voids
Long-Term Support for Slabs with Washout and Settlement
Untreated voids tend to grow. Water continues cutting through the soil. The slab continues dropping. Cracks expand. What started as a half-inch settlement becomes a structural concern — and the eventual repair gets more expensive.
Catching voids early is one of the most cost-effective concrete maintenance moves a homeowner can make in this region.
Voids Under Concrete FAQs
The most common questions about voids beneath concrete slabs.
- Lightweight material that will not overload weak soils
- Fast cure times, often usable in 15 to 30 minutes
- High lift precision for cleaner leveling results
- Moisture-resistant support for long-term performance
Why Foam Fills Voids Best
The Engineering Behind Stable Lifts
How We Identify Voids
Inspection Checks We Run
Slab Elevation
Sunken slabs lifted back to the original pour grade — measured against fixed reference points so the result is documented, not guessed.
Sound & Feel
Hollow taps, soft spots, or pavement that flexes underfoot — clear signs of void space below. Foam fills it completely.
Surface Cracking
Cracks tied to settlement, not slab failure. Once the support underneath is restored, hairline cracks stop spreading.
Drainage History
Recurring water flow under a slab is the most common void cause. The foam displaces moisture and stabilizes the soil so it stays put.
Free Polyjacking Inspection
If your concrete is sinking, sounds hollow, or has started cracking in places it didn’t before, voids are probably the cause. The fix is straightforward.
Related Concrete Lifting Pages
Related Resources
Hollow sound when tapping, slabs that “rock” under load, water pooling in newly-low areas, and progressive settlement after rain are all signs.
In most cases, yes — polyurethane foam fills the voids and stabilizes the soil. Replacement only needed if the slab itself has failed.
Yes. The foam expands to fill the space available, including small or oddly-shaped voids that slurry would miss.
Polyurethane foam doesn’t shrink, wash out, or degrade. While no soil-related solution is permanent, this method holds up consistently in Kansas City clay.