How Do You Fill a Void Under a Concrete Slab? Pumping Options Explained
By PumpX Team ·
A slab that sounds hollow when tapped, has settled, or has a visible gap beneath has a void. Whether the cause is erosion, poor compaction, a leaking pipe or a shrinking subgrade, the common repair is to pump material into the void through small holes. Here are the typical options and what to think about first.
Find out why the void exists
Filling a void without fixing the cause just delays the problem. Ask:
- Is water getting under the slab (broken pipe, bad drainage, downpipes)?
- Is the soil washing out or settling?
- Is the slab structural, such as a garage floor, a suspended section or a foundation?
- Does the void keep growing or the slab keep cracking?
If any answer points to a structural issue, involve a structural or geotechnical engineer before pumping.
Common fill options
| Option | How it works | Typical use |
|---|---|---|
| Cement grout or mortar-based slurry | Pumped through drilled holes to fill voids and support the slab | Larger or load-bearing voids |
| Flowable fill (low-strength, high-flow mix) | Very fluid mix that fills voids without vibration | Bigger voids and trenches |
| Slabjacking (mudjacking) | Slurry injected under pressure to fill voids and lift the slab | Settled paths, patios, driveways |
| Polyurethane foam | Injected expanding foam that fills and can lift | Light-duty lifting; manufacturer-dependent |
Which suits you depends on the void size, slab loading and the contractor's experience. We cannot recommend a specific product for your case, and you should follow the supplier's data and any engineer's specification.
How the pumping job works
- Survey. Tap or sound the slab and mark the hollow areas. Some contractors use scanning methods to map voids.
- Drill injection holes. Small holes, spaced to suit the void and material.
- Inject in stages. The material is pumped in slowly while monitoring slab movement. Too much pressure can crack or heave the slab.
- Monitor. Operators watch for the slab lifting, material coming out of cracks or other holes, and flow slowing.
- Patch the holes once the fill has set.
Why a pump matters here
Void filling needs controlled, low-volume delivery, not a rush of concrete. Small grout pumps or small line pumps suit the work because the operator can meter flow and stop at once. Gravity pouring rarely fills a void completely.
Mix considerations
- The mix must be fluid enough to run into the void but with enough body to harden into a support.
- Ordinary concrete with large aggregate will not pass through small injection holes or fine hoses.
- Test for water leakage or a high groundwater table first, since that affects what material survives.
Safety and practical notes
- Pressure. Injecting under a slab can lift it. Go slowly and watch.
- Services. Check for pipes, conduits and underfloor heating before drilling.
- Dust. Concrete drilling dust is a respiratory hazard; control dust and wear protection.
- Hose pressure. Treat pressurised hoses with care and relieve pressure before opening any coupling.
When to hire out
Small cosmetic voids at a patio might be a hands-on job for an experienced DIYer using a suitable grout. Anything under a house, garage, shed with machinery or driveway taking vehicles is better handled by a contractor with the right pump, with an engineer's input if structural.
Practical tips
Start pumping at the hole furthest from the lowest point, or as the contractor advises, so material pushes air and water toward other holes rather than trapping it. Keep a record of how much material each hole takes; a hole that swallows far more than expected suggests a larger cavity than you thought. Let the fill cure before putting heavy loads back on the slab. If water is the root cause, fix drainage first, or the void will return.
Key takeaways
- Work out and fix the cause of the void first.
- Grout, flowable fill, slabjacking and foam are the common solutions.
- Controlled, staged pumping with monitoring prevents damage.
- Use a mix suited to the injection hole size and the load.
- Get an engineer involved for structural slabs or recurring voids.
You can find contractors in the concrete pumping directory and check small pump options in the machine specs library. Find pumping operators and equipment on PumpX at pumpxchange.com.
Original question on Reddit: r/Concrete: Help filling under slab