A void is empty or poorly supported space beneath concrete. Filling it can restore slab contact, but the durable repair question is why the void developed. If water, leaking utilities, failing joints or unstable fill continue removing support, a freshly filled space can be followed by a new one nearby.
How Voids Develop
- Runoff or irrigation enters a joint/edge and carries fine soil away.
- Backfill placed around a pool, utility trench or addition consolidates after construction.
- Organic debris or unsuitable fill loses volume.
- A broken drain, pipe or underground component changes the local moisture/soil condition.
- Broader subsidence or soil problems reduce support beneath more than one slab.
How Contractors Look for Support Loss
Methods vary by project. Visual elevation changes, joint separation, drainage patterns and crack maps are basic clues. Some contractors use sounding/percussion to identify hollow areas. More consequential projects may involve elevation surveys, probing, geotechnical investigation or other testing. No single “hollow sound” proves the entire cause.
Void Fill Is Not Automatically a Lift
Material can be injected primarily to restore contact without trying to raise the slab substantially. That distinction matters around pool coping, garage thresholds, steps, drains and adjoining fixed surfaces where an aggressive lift could create new stresses or drainage problems.
The Recurrence Checklist
- Where is rainwater going?
- Are downspouts, roof runoff or irrigation discharging toward the slab?
- Are control/expansion joints allowing water into erodible fill?
- Is a pool-deck edge losing material toward the shell or yard?
- Could buried plumbing/drainage be leaking?
- Was the area originally excavated and backfilled?