Problems
Void Under a Concrete Slab
A slab that has lost contact with the ground beneath it is carrying load it was not designed to carry alone. The slab may look fine for a long time, and then not.
What is happening beneath the concrete
Slabs on grade are meant to rest continuously on the ground, transferring load straight down. When soil beneath them is carried away by water or compresses further, that contact is lost in patches and the slab spans the gap instead.
At that point the slab is a beam without support in the middle. Whether it cracks, drops, or simply flexes depends on its thickness, its reinforcement, the size of the void and what drives across it.
Signs that support may have been lost
Flex or movement
A gap at the edge
Water disappearing at a joint
Cracking along a line
Soil loss beside the slab
A section that has dropped
Why tapping does not confirm anything
Why voids form
- Water moving beneath or beside the slab, carrying fine material away.
- Concentrated roof discharge or poor grading directing water at slab edges.
- Backfill or trench spoil that continues to consolidate for years after construction.
- Leaking or broken buried lines washing material out.
- Moisture cycling and freeze-thaw working saturated ground beneath the slab.
Local context is on why concrete sinks in Indiana.
What is at risk
- Cracking that turns a repairable slab into a replacement.
- Sudden movement under load, particularly where vehicles cross.
- Progressive erosion — the space grows once water has a path.
- Water being directed toward a structure once the slab drops.
The reason to act on a suspected void before visible settlement is simple: filling the space beneath sound concrete is cheaper than replacing concrete that broke.
How it is addressed
The space is filled by injection so the slab is supported again — and where the slab has also dropped, injection continues until it lifts. Both polyurethane and cement-based slurries are used; which is appropriate depends on the void and the site.
Method detail is on concrete void filling. Filling the space does not stop the water that created it, so drainage usually needs its own attention.
Homeowner questions
Can I see a void from outside the slab?
Sometimes, at an edge or a joint. Often not. What is visible is rarely the full extent of the space.
Is a void an emergency?
Not usually, but it does not improve on its own where water is still moving material. A slab that vehicles cross deserves prompter attention than one nothing loads.
Should I fill it myself with sand or gravel?
Pouring material into an accessible gap does not achieve continuous support beneath the slab, and it can make later injection work harder to do properly.
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