Quick Answer
Most hairline and surface cracks in concrete driveways and paths can be patched for $20–$80 in materials. Cracks wider than 10mm, cracks with vertical displacement between edges, or slabs that are actively heaving from tree roots or clay movement need professional assessment — patching won’t fix the underlying cause. In Melbourne’s reactive clay soils (common across Dandenong, Berwick, Cranbourne and Pakenham), concrete cracking is almost inevitable; the question is whether to patch or replace depends on the extent and type of movement.
Types of Concrete Cracks and What They Mean
Not all concrete cracks are equal. Understanding what type of crack you have is the single most important step before deciding to patch or replace.
Hairline Cracks (Under 3mm Wide)
Very common — most concrete develops hairline cracks within 3–5 years as it cures and shrinks. These rarely indicate structural problems and are excellent candidates for DIY patching with a flexible concrete crack filler or polyurethane sealant. Key test: run your fingernail across the crack — if one edge is higher than the other, you have displacement, not just settlement.
Structural Cracks (3–10mm Wide)
Wider cracks suggest more significant movement. These often result from sub-base erosion, clay shrink-swell, or heavy vehicle loads on a slab designed for foot traffic only. Patching is possible but the crack may re-open if the underlying cause is not addressed. Add flexible backer rod before filling cracks in this range.
Wide Cracks or Heaving Slabs (Over 10mm or Displaced)
Cracks wider than 10mm, or any crack where one edge sits higher than the other, indicate active slab movement. In Melbourne, this is almost always caused by reactive clay soils expanding in winter and contracting in summer, or by large tree roots under the slab. Patching these cracks is a temporary cosmetic fix at best — the movement will open them again within one or two seasons.
The Patch vs Replace Decision
| Situation | Recommendation | Reason |
|---|---|---|
| Hairline cracks, no displacement, under 3mm | Patch | Normal concrete shrinkage — structural integrity intact |
| 3–10mm crack, no displacement | Patch with flexible filler + monitor | Movement possible; flexible sealant accommodates minor ongoing movement |
| Any crack with vertical displacement | Professional assessment before patching | Slab has moved; root cause must be identified |
| Multiple cracks forming a network (map cracking) | Replace or resurface | Usually indicates sub-base failure or alkali-silica reaction |
| Slab heaving or dipping visibly | Replace or lift-and-relay | Sub-base failure or tree roots — patching won’t stick |
| Driveway cracks near large trees | Professional assessment | Root intrusion may require root barrier or tree removal before recasting |
How to Patch a Concrete Crack (DIY Step-by-Step)
Step 1: Clean the Crack
Use a cold chisel and hammer to lightly undercut the crack edges — this gives the filler mechanical grip. Remove all loose concrete and dust with a stiff brush, then blast the crack clean with a garden hose or compressed air. The crack must be dry before applying filler.
Step 2: Apply Backer Rod (For Cracks Over 6mm Wide)
Push closed-cell polyethylene backer rod into the crack to a depth of 6–10mm below the surface. Backer rod controls the depth of the sealant fill and allows the sealant to flex properly. Without backer rod, sealant that’s too deep bonds on three sides instead of two — called three-point bonding — and tears apart under movement instead of stretching.
Step 3: Fill with Appropriate Product
For hairline to 6mm cracks: use a polyurethane concrete crack sealant (Sika Concrete Fix, Selleys Liquid Nails or similar). Load into a caulk gun and apply in one continuous bead, tooling smooth with a wet finger. For wider cracks or surface spalling: use a polymer-modified concrete repair mortar (Sika Monotop, Fosroc Renderoc, or premixed concrete patch mixes). Mix to a stiff consistency and press firmly into the void, avoiding air pockets.
Step 4: Cure and Seal
Allow the repair to cure fully (48–72 hours minimum) before vehicle traffic. Once cured, apply a penetrating concrete sealer over the entire driveway or path surface — not just the patch. This protects the repair and evens out the visual difference between old and new concrete.
When to Call a Professional
If the slab is heaving, any crack has visible vertical displacement, or you’re seeing map cracking across a large area, get a quote from a concreter before spending money on patch materials. Many Melbourne concreters offer free assessments, and a new slab poured over a properly prepared sub-base will outlast 20 seasons of patching a failing one.
For driveways near council footpaths or with vehicle crossovers, check with your local council — in some Melbourne municipalities (including Casey, Cardinia and Frankston City) the homeowner is responsible for maintaining the section between property line and the road, including any concrete crossover.
Tips and Gotchas
- Never patch over a wet crack: Sealant and patch mortars don’t bond to damp concrete. Wait for dry weather and at least 48 hours after rain.
- Flexible sealant, not rigid mortar, for moving cracks: If the crack has shown any movement, use a polyurethane sealant not a rigid repair mortar. Rigid repairs in moving cracks always fail within one season.
- Match colour expectations: Fresh concrete patch will never match aged concrete. Manage expectations — the patch will lighten over 3–6 months but won’t be invisible.
- Address drainage first: If water pools on the slab surface and drains toward the crack, redirect it. Persistent water at a crack accelerates erosion of the sub-base and makes any repair short-lived.
- Tree roots near the slab: Patching a crack caused by an active tree root is a treadmill — the root will push the slab again within 2–3 years. Get arborist advice before committing to repairs.
- Control joints should not be filled: The deliberate grooves cut into fresh concrete (control joints) are designed to crack — they control where cracking happens. Don’t fill these with rigid mortar; use flexible sealant only.
- Council footpath responsibility: Call your council before breaking up or replacing any concrete in the nature strip or footpath area — permits may be required, and works on council land need council contractor approval in most Victorian municipalities.
Local Melbourne Resources
- Bunnings — Concrete crack fillers and repair mortars
- Sika Australia — Concrete repair product selector
- Casey Council — Driveway and crossover responsibilities
- Planning Victoria — Permit requirements for works
- Mitre 10 — Concrete repair supplies
FAQ
Why does concrete crack in Melbourne so much?
Melbourne’s reactive clay soils expand significantly when wet and contract when dry — this seasonal movement puts enormous stress on any rigid surface above it. Combined with temperature swings from winter minimums to summer highs of 40°C+, concrete expands and contracts continuously. Cracking is normal; the goal is to manage it, not prevent it entirely.
Can I patch a crack myself if I’ve never done concreting before?
Yes for hairline to 6mm cracks with no displacement — follow the clean-undercut-fill-cure sequence and use a premixed concrete patch product. It’s genuinely a beginner DIY job. Wider cracks with displacement are better left to a concreter.
How long does a concrete patch repair last?
A properly prepared patch in a stable slab should last 5–10 years. Patches in slabs that are still actively moving (clay shrink-swell or tree roots) may re-open within 1–2 seasons regardless of how well the patch was applied.
What is the best concrete crack filler in Australia?
For narrow moving cracks: Sika Concrete Fix or Selleys No More Gaps Concrete & Masonry. For wide static cracks and spalling: Sika Monotop or Fosroc Renderoc Plug. For very wide gaps with active movement: flexible polyurethane backer rod plus sealant.
My entire driveway is cracked — should I resurface or replace?
If the sub-base is stable and the slab is still level, a 40–50mm concrete overlay or spray-on concrete resurfacing can give 10–15 more years of life at lower cost than a full replacement. Get 2–3 quotes from concreters who can assess the sub-base before deciding.