How to Repair a Hollow-Sounding Concrete Slab Section

A hollow sound when you tap or walk on a concrete slab is a tell-tale sign that a void has formed underneath — or that the concrete has delaminated from a substrate below. Left untreated, hollow sections can crack, collapse, or become a trip hazard. The good news is that many repairs are well within a capable DIYer's reach, though larger or structurally significant slabs may need a professional concreter.

Why Does Concrete Sound Hollow?

Before you reach for the tools, it pays to understand what's going on beneath your feet. Common causes include:

• Soil settlement or erosion — water washing away fine material under the slab, leaving an air gap
• Tree root movement — roots lifting or displacing fill material
• Poor compaction at the time of pour — fill that was never properly consolidated
• Delamination — a thin surface layer separating from the main slab body, common with older or poorly mixed concrete
• Subsurface drainage issues — blocked or broken stormwater pipes allowing water to scour soil away

In New Zealand's variable climate — from the wet West Coast to the expansive clay soils of Auckland — ground movement and moisture are frequent culprits. If you suspect a drainage issue, fix that first or the void will simply return.

Step 1 — Assess the Extent of the Problem

Use a rubber mallet or the handle of a hammer to tap systematically across the slab. Mark hollow areas with chalk. A dull, drum-like thud indicates a void; a solid slab gives a sharp, dense sound.

Consider the slab's role:

• Is it a structural slab (floor of a house, garage, or load-bearing area)? If so, get a professional assessment before doing anything.
• Is it a non-structural slab (path, driveway, patio)? These are generally safer for DIY repair.

If the hollow area is larger than roughly 300 mm across, or if you can see cracking, significant settlement, or the slab is rocking underfoot, call a qualified concreter. You can find experienced local concreters on tradedirectory.co.nz.

Step 2 — Choose Your Repair Method

There are two main approaches depending on the cause:

A. Slab Injection (Void Filling)

This is the preferred method when the concrete itself is sound but there's a void beneath it. A flowable grout or polyurethane foam is injected through small drilled holes to fill the gap and re-support the slab.

What you'll need:

• Rotary hammer drill with a 12–16 mm masonry bit
• Injection ports (available from concrete repair suppliers)
• Flowable cementitious grout or two-part polyurethane foam
• Safety glasses, gloves, and dust mask (P2 rated)

Basic process:

1. Drill holes at roughly 300–500 mm intervals across the hollow area, angled slightly toward the centre of the void.
2. Insert injection ports and seal around them with rapid-set mortar.
3. Inject grout or foam until you see material appearing at adjacent holes — this tells you the void is filled.
4. Allow to cure fully before loading the slab.

For larger voids, a cementitious grout (such as products from Sika or Ardex) is generally more durable than foam. Always follow the manufacturer's data sheet for mix ratios and cure times.

B. Cut Out and Re-Pour

If the concrete itself is delaminated, crumbling, or badly cracked, the affected section needs to be removed and replaced.

1. Cut around the hollow area with an angle grinder fitted with a diamond blade, or a concrete saw. Wear hearing protection, safety glasses, and a P2 dust mask — concrete dust contains silica and is a serious health hazard. See WorkSafe NZ for guidance on working safely with silica dust.
2. Break out the loose concrete with a cold chisel and club hammer or a demolition hammer.
3. Compact the sub-base — this is critical. If the fill is loose, compact it with a hand tamper or plate compactor before pouring.
4. Prime the edges of the existing slab with a concrete bonding agent.
5. Pour and finish with a suitable repair concrete or rapid-set mortar. For thin sections (under 40 mm), use a polymer-modified repair mortar rather than standard concrete mix.

Cost ranges are a guide only and will vary by region, materials, and scope.

Step 3 — Finishing and Curing

Once repaired, keep the area damp and covered with plastic sheeting for at least three days to allow proper curing — longer in hot or windy conditions. Avoid loading the repaired area until the product's specified cure time has passed.

If the slab is indoors or in a garage, check whether a moisture barrier is required under the NZ Building Code (Clause E3 — Internal Moisture). MBIE Building Performance has useful guidance on moisture management in slabs.

Does This Work Require a Building Consent?

Most like-for-like concrete slab repairs on non-structural elements fall under Schedule 1 exemptions of the Building Act and do not require a building consent. However, if the slab is part of the structural foundation system, or if you're altering drainage, you may need consent. Always confirm with your local council before starting work — rules vary by territorial authority.

When to Call a Professional

Call a licensed concreter or structural engineer if:

• The hollow area is under a house floor slab or load-bearing structure
• The slab is rocking, cracked through, or significantly settled
• The void is large or the cause is unknown
• You suspect a broken underground pipe is causing the erosion (a plumber or drainlayer may also be needed — check PGDB for licensed drainlayers)
• You're not confident in your assessment

A professional concreter can also use specialist equipment such as ground-penetrating radar to map voids accurately before any drilling begins.

Sources & Further Reading

• MBIE Building Performance — NZ Building Code
• WorkSafe NZ — Silica Dust Safety
• Sika NZ — Concrete Repair Products
• Ardex NZ — Flooring and Concrete Repair
• BRANZ — Building Research
• Plumbers, Gasfitters & Drainlayers Board