Hairline cracks are the most common concrete defect that homeowners notice, fine, narrow cracks that can appear in driveways, paths, pool surrounds, slabs, and walls within months of the concrete being poured, or years later as the concrete ages. The question is always the same: do these cracks matter, and what should be done about them?
The answer depends on what type of crack it is, whether it’s active or dormant, and what role the concrete plays in your structure.
What Causes Hairline Cracks
Hairline cracks are defined as cracks with a width of less than 0.3 mm. They are almost universal in concrete, in fact, AS 3600 (the Australian standard for concrete structures) acknowledges that some cracking is normal and expected in concrete elements.
Plastic shrinkage cracks: Occur within the first 24 hours of concrete placement when the surface dries faster than the interior. Very common in Central Coast summer conditions where wind, heat, and low humidity accelerate surface evaporation. These cracks are typically short (100-300 mm), random in direction, and shallow (less than 20 mm deep).
Drying shrinkage cracks: Concrete loses approximately 1 mm per metre of length as it cures over the first 28 days. Without properly spaced control joints, this shrinkage causes cracking in the path of least resistance. These cracks are typically straight, cross the full width of a slab, and extend to significant depth. They should have been control joints but weren’t cut in the right location.
Thermal movement cracks: Repeated cycles of thermal expansion and contraction over years create fine surface cracking, particularly in coastal locations where temperature differences between night and day in summer can be 15-20°C.
Flexural cracks: Where concrete is supported at the edges but not in the middle (a void in the subbase), it flexes under load and develops tension cracks in the middle of the underside of the slab that may extend upward. These are structural cracks, not hairline cracks, even if they start narrow.
Settlement cracks: Where the subbase settles unevenly beneath the slab. These start as hairline cracks but widen over time if settlement continues.
Dormant vs Active Cracks, The Critical Assessment
The most important question about any crack is whether it is dormant (stable, no longer growing) or active (still moving, growing, or subject to seasonal variation).
Dormant cracks, also called dead cracks, have reached their final extent. The concrete has finished shrinking, the subbase has stabilised, and no further movement is occurring. These can be treated relatively simply.
Active cracks are still subject to movement, either from ongoing subbase settlement, thermal expansion/contraction, structural loading, or moisture cycling. Treating active cracks incorrectly (filling them with a rigid material that can’t accommodate movement) leads to the fill material cracking again along the same line.
How to Assess Activity
Mark test: Clean the area around the crack and apply a distinctive mark (a dab of marker or paint) across the crack at several points. Photograph. Re-check at 4 weeks, 3 months, and 6 months. If the mark has shifted or the crack width has changed, the crack is active.
Crack width gauge: Measure the crack width at the same point in summer and winter. A change of more than 0.1 mm indicates active thermal movement.
History: If the crack appeared within the first 12 months, it may still be settling. Cracks that have been present in the same form for 3+ years without visible change are more likely dormant.
When to Seal at the Surface
Surface sealing (applying a flexible crack filler or crack-bridging sealer to the crack) is appropriate when:
- The crack is dormant (confirmed stable)
- The crack is less than 1-2 mm wide at the surface
- The crack is surface-only or shallow (not structural)
- The purpose is to prevent water ingress and further deterioration
- Followed by resurfacing that will provide additional coverage
Products for surface sealing:
Polyurethane crack filler: The most flexible option. Can be applied into the crack, tool-finished flush with the surface, and overlaid. Suitable for cracks that may have minor future movement (small thermal cycling). Available in grey and can be colour-matched to concrete.
Epoxy crack filler (surface application): Used where a harder, more rigid fill is acceptable. Less suitable for cracks subject to any thermal movement, rigid epoxy fills in moving cracks will crack again.
Crack-bridging sealer: Some topical sealers are specifically formulated to bridge hairline cracks, they apply over the surface and create a flexible membrane that spans the crack. These are appropriate for very fine cracks (< 0.3 mm) where direct filling is difficult.
Process:
- Open the crack with a angle grinder (crack chasing tool) to remove dirt and create a slightly wider, V-shaped groove
- Clean with compressed air and/or vacuum
- Apply backer rod (for cracks > 3 mm wide) to control depth of sealant
- Apply polyurethane filler, tool flush
- Allow to cure before applying overlay or sealer
When to Inject with Epoxy or Polyurethane
Crack injection is used when the crack needs to be treated through its full depth, not just at the surface. This is appropriate when:
- The crack extends through the full slab thickness and water is moving through it
- The crack is structural (in a suspended slab or a load-bearing element)
- You need to restore monolithic behaviour of the concrete, bond the two sides together
- The crack is in a wall or elevated slab where a surface treatment alone is insufficient
Epoxy injection bonds the two sides of the crack back together. Once cured, the epoxy is typically stronger in tension than the surrounding concrete. Epoxy injection is the technique for:
- Structural cracks in load-bearing concrete
- Cracks where full-depth water penetration is the issue
- Dormant cracks only, epoxy injection on an active crack will cause the crack to re-open adjacent to the injected zone
Polyurethane injection creates a flexible, expanding foam or gel that seals the crack without rigidly bonding it. It’s the technique for:
- Active cracks where some ongoing movement must be accommodated
- Water-stopping situations (polyurethane foams react with water and expand to fill the crack)
- Cracks in structures subject to ongoing movement
The injection process:
- Clean the crack surface
- Drill holes (typically 45° from the surface into the crack plane) at regular intervals along the crack
- Install injection ports
- Seal the crack surface between ports with epoxy paste
- Inject the chosen product from the lowest port, moving upward when product appears at the next port
- Remove ports and fill holes
- Allow to cure
Injection is typically not cost-effective on residential driveways unless the crack is causing specific problems (water moving through the slab, visible displacement between sides). On a standard residential driveway, surface filling and overlaying is sufficient for most hairline and minor structural cracks.
Crack Assessment and Recommended Treatment
| Crack Type | Width | Movement | Recommended Treatment |
|---|---|---|---|
| Plastic shrinkage (surface) | <0.3 mm | Dormant | Surface seal + reseal or overlay |
| Drying shrinkage (through slab) | 0.3-2 mm | Dormant | Surface fill + overlay |
| Thermal movement | Varies seasonally | Active | Flexible polyurethane surface fill |
| Settlement crack (early) | 1-5 mm | Active | Monitor; address subbase before treating |
| Settlement crack (stable) | 1-5 mm | Dormant | Fill + overlay |
| Structural crack (load bearing) | Any | Any | Engineer assessment + epoxy or PU injection |
| Concrete cancer (rust staining) | Any | , | Structural repair before any surface work |
The Overlay Solution, Covering All Cracks at Once
For driveways and slabs with multiple cracks of varying types, the most practical and economical approach is usually to address the serious structural cracks first (injection or subbase repair), then resurface the entire area with a polymer-modified overlay. The overlay bridges minor hairline cracks and provides a fresh, uniform surface that hides the crack history.
A well-designed resurfacing job places control joints in the overlay over the crack lines in the concrete below, allowing the cracks to continue to exist (which they always will) but directing their movement to controlled locations in the overlay surface where flexible sealant can manage them.
See our guide to resurfacing cracked driveways for more detail on this approach.
Frequently Asked Questions
My driveway has 20+ hairline cracks, do all of them need to be individually filled before resurfacing? Not necessarily. Fine surface cracks less than 0.3 mm wide can be bridged by a quality primer and overlay without individual crack filling. Cracks 0.3-1 mm wide should be filled before overlay. Cracks wider than 1 mm should always be individually treated. A site inspection will determine which approach is appropriate for your specific crack pattern.
Will filled cracks show through the new overlay? Properly filled dormant cracks with flexible polyurethane typically don’t re-open through the overlay. Active cracks, where the two sides are still moving, may eventually reflect through the overlay if the crack keeps moving. This is why assessing activity is so important before treatment.
Is cracking in new concrete from the last 2 years normal or a defect? Some cracking is normal. Hairline shrinkage cracks within the first year are generally normal if properly placed control joints are present. Structural cracking (wide, displaced, or at unexpected locations) within 2 years of construction may indicate a defect and should be assessed for a potential warranty claim.
Does cracking mean my driveway needs to be replaced? Almost never. Unless the slab has cracked into multiple sections that are displacing at different heights, or the subbase has failed and the slab is structurally compromised, cracking is a repairable surface condition. Replacement is a last resort for slabs where repair cost exceeds replacement cost, rare for standard residential driveways.
What’s the cost of crack injection vs resurfacing for my driveway? Crack injection for significant structural cracks: $80 to $200 per crack depending on length and depth. Full driveway resurfacing with surface crack filling included: $55 to $90 per m². For most residential driveways with minor to moderate cracking, resurfacing the whole area is better value than injecting each crack individually.
Worried about cracks in your concrete? Get a free on-site assessment, we identify which cracks matter, which can be bridged, and give you a clear treatment plan and cost.