An under-slab void is an empty space beneath a concrete slab where supporting soil has eroded, washed out, shrunk, or settled away, leaving the slab partially unsupported.
In depth
Voids are the hidden problem behind most settled and rocking concrete. The slab may look fine until it cracks, drops, or begins to move, because the empty space beneath it is invisible from the surface. Voids form from washout, soil shrinkage, poor compaction, decaying tree roots, or leaking utilities, and can range from a thin gap to cavities more than six inches deep.
Filling the void restores continuous support. Polyurethane is injected to flow into the space and press against the soil, eliminating the gap and the movement it allowed.
How Acme applies it. During one McHenry project Acme discovered voids over six inches deep beneath a clubhouse floor and injected specialized polyurethane to fill them without lifting, restoring support and avoiding costly slab replacement.
Related terms: Void Filling, Soil Washout, Rocking Slab, Concrete Settlement
Frequently asked questions
How do I know if there is a void under my slab?
Signs include a hollow sound when tapped, a slab that rocks or moves under load, new cracks, or sections dropping. A professional inspection confirms and locates voids.
How are under-slab voids filled?
Polyurethane is injected through small ports and expands to fill the empty space and press against the surrounding soil, restoring uniform support, usually without lifting the slab.