Concrete Crack Repair: Causes, Fixes & Costs (2026)

Almost every concrete slab in Australia will develop some cracking over its life. That is not a sign of failure. It is the nature of the material. Understanding why a crack formed tells you whether it is safe to patch yourself or whether you need a concreter or structural engineer involved.

Shrinkage and Plastic Cracking

As fresh concrete loses moisture during curing, it shrinks slightly. If the slab cannot contract freely, tension builds and the concrete cracks. This is especially common in hot, dry or windy conditions, which are routine across Queensland, Western Australia, South Australia and inland New South Wales. Plastic shrinkage cracks typically appear within the first 24 hours. They are usually narrow, less than 0.3 mm, and do not indicate structural weakness.

Settlement and Subsidence

When the ground beneath a slab shifts, one section drops relative to another. You get a crack with visible vertical displacement, sometimes called a step crack. Clay soils in Melbourne, Adelaide and parts of Sydney shrink and swell dramatically with moisture change. This movement can open and close cracks seasonally. Settlement cracks often require more than a surface filler.

Tree Roots and Drainage

Roots growing under slabs create uneven upward pressure. Poor drainage concentrates water, which softens subgrade and accelerates settlement. Both produce cracks that return quickly if the underlying cause is not fixed first.

Overloading and Impact

Driveways cracked by heavy vehicles, forklifts or skip bins parked beyond the design load are common. These cracks tend to be wider and may have associated spalling at the edges.

Grab a ruler, a piece of chalk and your phone. Good assessment takes five minutes and stops you wasting money on the wrong product.

Step 1: Measure Width and Depth

Hairline cracks are under 0.3 mm wide. Fine cracks run 0.3 to 1 mm. Anything over 1 mm is considered significant and anything over 3 mm warrants professional advice. Use a crack gauge card if you have one, or hold a credit card edge across the crack. If the card drops in, it is wider than 0.5 mm.

Step 2: Check for Displacement

Run your finger across the crack. If one edge is higher than the other, the slab has moved. Filler alone will not last. The slab may need lifting (mudjacking or polyurethane foam injection) or replacement of the affected panel.

Step 3: Mark and Monitor

Draw short chalk lines across the crack and photograph them. Check back in two to four weeks. A crack that is actively growing, especially one that widens after rain and narrows in dry weather, is likely responding to soil movement. Repair it while it is at its widest, and expect to re-treat it in future unless drainage or root issues are resolved.

Step 4: Identify the Surface Type

Exposed aggregate, polished concrete and stencilled surfaces need colour-matched or specialist products. A standard grey polyurethane filler looks obvious on exposed aggregate. If aesthetics matter, factor in a professional finish.

For hairline to moderate cracks (under 5 mm wide) on driveways, paths and patios, a DIY repair is reasonable. Give yourself two to four hours plus curing time.

Step 1: Clean the Crack Thoroughly

Use a cold chisel and hammer to slightly undercut the edges of the crack (create a V or U shape). This gives the filler something to grip. Blow out dust with compressed air or a brush. Remove any loose concrete, paint or oil contamination. For oil stains, degrease with a diluted TSP solution and rinse well before proceeding.

Step 2: Choose the Right Product

For narrow cracks under 3 mm, a polyurethane concrete crack filler from any hardware chain works well. For wider cracks or those subject to movement, use a flexible polyurethane sealant rather than a rigid epoxy. Rigid epoxies bond strongly but crack again if the concrete continues to move. For structural repairs or where load transfer matters, epoxy injection is the professional standard.

Step 3: Apply the Filler

Follow the product instructions. Most polyurethane fillers are applied with a caulking gun. Fill from the bottom up, slightly overfilling. Smooth with a putty knife wetted with mineral turpentine (for polyurethane) or water (for latex products). Feather the edges so there is no obvious ridge.

Step 4: Cure and Seal

Allow full cure time, typically 24 to 72 hours depending on the product and ambient temperature. Once cured, consider applying a penetrating concrete sealer over the whole surface. This slows moisture ingress that causes future cracking and is particularly worthwhile in coastal areas from the Gold Coast to Fremantle where salt spray accelerates deterioration.

Step 5: Colour Matching

Standard grey filler on a plain slab is usually acceptable. On a coloured or stencilled surface, tinting the filler is difficult to get right. For anything decorative, a concreter who specialises in decorative concrete will achieve a far better result than a DIY patch.

Some cracking genuinely requires a licensed concreter, and occasionally a structural engineer, before any repair work starts.

Cracks That Need a Concreter

• Width over 5 mm or visible vertical displacement between panels
• Cracks in a retaining wall of any height
• Spalling or delamination alongside the crack, suggesting rebar corrosion underneath
• Cracks in a driveway slab that flex when you walk across them
• Any crack in a suspended slab, garage slab over a basement, or commercial floor

Cracks That Need an Engineer First

• Cracks in structural footings or beams
• Cracks that appear suddenly after nearby excavation work or a storm event
• Cracks in a retaining wall showing outward lean
• Multiple cracks forming a pattern across a slab, particularly a map-cracking (crazing) pattern on a structural element

A structural engineer's inspection in a capital city typically costs $300 to $700 for a residential site visit and report. That is money well spent before committing to expensive repairs or, worse, ignoring a problem that worsens.

Costs vary considerably by crack type, access, location and the contractor's rates. The figures below are general market ranges. Always get a written quote specific to your job.

Minimum call-out charges apply across most trades. Expect a minimum job cost of around $300 to $600 for a professional concreter to visit, assess and carry out a small crack repair. If the job is very minor, some contractors will not take it on at all.

Cost drivers include site access (tight suburban blocks in inner Sydney or Melbourne cost more to work in), the number of cracks, the concrete finish type, and whether drainage or subgrade work is needed alongside the repair. Do not accept a phone quote for anything beyond a basic hairline fill.

Fixing the symptom without addressing the cause is a short-term fix. These steps reduce the chance of cracks returning after repair.

Control Joints

If your existing slab lacks saw-cut control joints, a concreter can cut them in retrospect. Joints give the concrete a planned location to crack, typically at 3 to 4 metre spacing for standard residential slabs. In hot climates like Darwin or Townsville, joints are even more important because of extreme thermal cycling.

Drainage Improvements

Water pooling against or under a slab is one of the most common reasons cracks return. Install or reroute stormwater, add channel drains, or regrade garden beds away from the slab edge. This is especially relevant in Adelaide and Melbourne where reactive clay soils swell significantly when wet.

Root Management

If a tree was the culprit, removing the tree alone is not always enough. Roots can persist and decay, leaving voids. A root barrier installed during repair prevents regrowth underneath the slab.

Sealing

A penetrating sealer applied every three to five years slows moisture ingress and reduces the freeze-thaw stress that affects elevated areas like the ACT, alpine Victoria and parts of Tasmania. Coastal areas benefit from sealers that resist chloride penetration.

None of these measures are guarantees. Concrete cracks. Managing the rate and severity of cracking, and repairing early before water gets in and corrodes reinforcement, is the realistic goal.