One of the most important pre-resurfacing checks is identifying whether there’s a void under the slab. Resurfacing over a slab that has subsided, or that has voids forming underneath, is like painting over rotting timber: it conceals the problem temporarily while the underlying issue gets worse. The new surface will crack in exactly the same places as the old one, usually within 12-18 months. Understanding when you need to fix the sub-base before resurfacing saves you from doing the job twice.
Quick Answer (BLUF)
If your concrete slab shows differential settlement (uneven surface with one section lower than another), cracking with displaced edges, or hollow sounds when tapped, there’s likely a sub-base issue that must be rectified before resurfacing. Resurfacing can proceed once the void is filled and the slab is stabilised. The most cost-effective approach for settled slabs is polyurethane foam injection (slab lifting), which doesn’t require breaking up or replacing the concrete.
What Is Slab Subsidence?
Slab subsidence occurs when the material supporting a concrete slab, soil, compacted fill, or gravel base, settles, erodes, or shifts, creating a void between the underside of the slab and the ground beneath it. The slab then “bridges” across the void until it cracks or collapses into the void.
Common causes include:
- Soil erosion from water infiltration (stormwater, burst pipes, poor drainage)
- Organic material decomposition, tree roots that die and decompose, stumps buried under the slab
- Compaction failure in fill material, common in new estates where fill wasn’t adequately compacted
- Clay soil shrinkage, Central Coast clay soils shrink significantly in dry periods, pulling away from the underside of slabs
- Poor sub-base preparation, original slab was poured without adequate compacted gravel base
How to Identify Subsidence
Tap Test
Walk the slab with a solid rod (a length of rebar or even a closed fist) and tap the surface. Solid slab sounds “dead” and flat. A slab over a void sounds noticeably hollow, almost like a drum. This is the fastest field test.
Visual Signs
- One section of slab sitting lower than adjacent sections (differential settlement), a clear sign of sub-base movement
- Cracking with displaced edges: One side of the crack sits higher than the other. This is classic differential settlement cracking and is always a structural issue.
- Cracks that are progressive: Getting wider or more numerous over time indicates an active sub-base issue
Water Test
After rain, look for areas where water pools on the driveway surface. These are typically low points from slab settlement. Puddling is both a safety issue (algae growth risk) and evidence of surface deformation.
The Central Coast’s Subsidence Context
Certain areas of the Central Coast are more prone to slab subsidence:
- Woy Woy and Ettalong Beach: Built on low-lying sandy and silty soils near Brisbane Water. Groundwater fluctuation causes soil movement that affects slab performance.
- Gosford hills and escarpment-adjacent properties: Fill material on steep sites can settle over decades.
- New estates on fill: Woongarrah, Wadalba, and other estates built on previously agricultural land may have fill of variable depth and compaction.
- Properties with large trees: Native trees with aggressive root systems, figs, camphor laurels, can destabilise sub-bases when roots die after tree removal.
Repair Options Before Resurfacing
Polyurethane Foam Injection (Slab Lifting / Mudjacking)
The most cost-effective approach for settled slabs. Small holes (16-25 mm) are drilled through the slab at calculated positions, and expanding polyurethane foam is injected under pressure. The foam expands to fill voids, lifts the slab back to level (or close to it), and cures hard within minutes.
Benefits:
- No demolition of existing slab
- Minimal disruption, driveway is usable within hours
- Can be done on the same day as resurfacing prep
- Cost-effective: typically $800 to $2,500 depending on the area to be treated
Limitations:
- Very large voids may require multiple injection points and significant material
- Cannot fix structural cracking in the slab itself, only the void beneath it
- Slabs that are severely cracked or broken may need partial replacement
Sand and Cement Fill (DIY/Low-Cost)
For small, accessible voids at the edge of a slab, a dry sand-cement mix can be compacted into the void from the sides. This is only practical for edge voids that can be accessed without drilling through the slab. It doesn’t provide the consolidation or expansive fill that foam injection does.
Partial or Full Slab Replacement
When a slab section is too damaged for lift-and-fill, large cracks with displacement, multiple broken sections, or complete structural failure, that section needs to be broken out, the sub-base re-prepared, and new concrete poured. This is more expensive and disruptive but is sometimes unavoidable.
For most residential driveways on the Central Coast with isolated settlement, partial replacement of the affected section (rather than full replacement) is cost-effective, particularly when followed by a full resurfacing overlay that blends the new and existing sections seamlessly.
When Resurfacing Can Proceed Without Sub-Base Repair
Not all cracked or sunken concrete requires sub-base repair. Resurfacing without base repair is appropriate when:
- Cracking is confirmed to be shrinkage/thermal cracking with no differential settlement
- The slab is confirmed to have no voids under it (tap test clean, no water pooling)
- Crack widths are under 3 mm and edges are at the same height
In these cases, crack filling and overlay will produce a durable result. Our driveway resurfacing service includes a slab assessment before quoting to confirm this.
FAQs
My driveway has one section about 10 mm lower than the rest, can it just be overlaid?
A 10 mm height difference is noticeable differential settlement. The overlay will roughly follow the existing profile (though a thick fill overlay can reduce the step). But the underlying cause, a void or settled sub-base, needs to be investigated and addressed before the overlay, or the overlay will crack at the same point.
How much does polyurethane foam injection cost on the Central Coast?
Expect $800 to $2,500 for a residential driveway depending on the number of injection points and volume of foam required. Get an assessment quote first, sometimes the hollow sound is a very localised small void that requires minimal material.
Can I resurface over a slab that has a few hollow spots but hasn’t cracked?
Hollow spots mean voids, the crack hasn’t happened yet, but it will. Filling voids before they cause cracking is much better than waiting. Address hollow spots before resurfacing for the best long-term result.
Will the council or a building inspector care about subsidence?
For residential maintenance (not new construction), there’s no council inspection requirement. But subsidence can be a material defect for property sale disclosure purposes if it’s significant. A building inspector will note it in a pre-purchase inspection report.