Every concrete resurfacing project encounters cracks. The question isn’t whether to deal with them, it’s how. The treatment for a hairline surface crack is entirely different from the treatment for a structural crack with active movement, and applying an overlay without correctly categorising and treating the cracks present is one of the most reliable ways to produce a failed resurfacing job.
Quick answer (BLUF)
Cracks must be assessed and categorised before any overlay is applied. Dormant (stable) cracks get rigid repair; active (moving) cracks get flexible repair; structural cracks with underlying causes get the cause addressed first. Cosmetic crack filling without this differentiation is the shortcut that causes overlays to re-crack and delaminate within months.
Understanding crack types
1. Plastic shrinkage cracks
These form in fresh concrete as it cures, water evaporates from the surface faster than it bleeds from the mix, causing the paste layer to crack. They are typically:
- Fine (under 0.3 mm width)
- Shallow (not through the full slab depth)
- Random or roughly parallel orientation
- Present from the first year of the slab’s life
Assessment: These are surface cracks. They rarely present a structural risk but they do create a pathway for moisture and salt ingress. In Port Stephens coastal conditions, they should be sealed.
Before resurfacing: Fill with a low-viscosity epoxy crack filler or polymer-modified grout, allow to cure, grind flush. The overlay bridges the repair.
2. Thermal and drying shrinkage cracks
As concrete cures over months and years, it shrinks slightly. Cracks form at points of tension, often diagonally from corners, at re-entrant angles (inside corners), and at changes in slab thickness. These are:
- Typically 0.2-1 mm wide
- Usually through or near through the full slab depth
- Develop in the first 2-5 years and then typically stabilise
Assessment: Check whether the crack is active (still widening) or dormant (stable). Measure the width at the widest point, then measure again in 4-8 weeks. If it hasn’t changed, it’s dormant.
Before resurfacing: Dormant cracks, rigid epoxy injection or filler. Active cracks, flexible polyurethane filler, and the overlay must be treated with a crack-bridging membrane or bond breaker strip at the crack location.
3. Settlement/subsidence cracks
Cracking caused by differential movement of the substrate beneath the slab, most relevant in Port Stephens’ sandy soil areas like Anna Bay, Medowie, and Fingal Bay. These are:
- Often diagonal from panel corners
- Sometimes accompanied by a vertical step at the crack (one side higher than the other)
- Can be actively widening if subsidence is ongoing
Assessment: These cracks indicate a substrate problem. Before any resurfacing, the substrate movement must be addressed, otherwise the overlay will simply replicate the same cracking pattern. See our subsidence guide for the appropriate approach.
Before resurfacing: Fix the substrate problem first. Then treat the crack as described for active cracks above.
4. Structural through-cracks (major)
Cracks that have gone through the full slab thickness and show significant vertical displacement (stepped crack) indicate structural failure. These are:
- More than 1-2 mm wide
- With visible differential height between sections
- Often associated with completely broken-out areas
Assessment: Resurfacing over a structurally failed panel is not an appropriate response. The panel may need to be replaced.
Before resurfacing: Engineer assessment to determine whether the panel can be repaired or must be replaced. Resurfacing can then be applied to create a consistent finish over old and repaired/replaced panels.
5. Delamination cracks (map cracking)
Map cracking, a network of fine interconnected cracks forming a polygon pattern across the surface, typically indicates alkali-silica reaction (where reactive aggregate expands over time), carbonation of the concrete surface, or inadequate curing during original placement. These are:
- Confined to the surface paste layer
- Affecting large areas rather than isolated lines
Assessment: These are surface degradation issues. If the underlying concrete below the paste layer is sound (tap test), the affected surface layer can be ground away and replaced with an overlay.
Before resurfacing: Grind back to sound concrete, removing the delaminated paste layer entirely, then apply overlay.
The “active or dormant” test
Before treating any crack wider than 0.5 mm, determine whether it is still moving:
Method 1: Tell-tale crack gauge Attach a simple tell-tale (or just a pencil mark spanning the crack) and photograph it. Return in 4-8 weeks. If the crack has widened, it’s active.
Method 2: Visual check after rain and dry cycle Cracks driven by moisture cycling in clay soils (less common in Port Stephens sandy soils but relevant in some areas) open and close with wet and dry conditions. Checking after rain vs. after an extended dry period shows whether the crack moves.
Method 3: Contractor assessment An experienced contractor can often assess crack activity from the debris pattern in the crack (fresh edges suggest recent movement), associated damage patterns, and site conditions.
Filler products and their applications
| Crack type | Appropriate filler | Properties needed |
|---|---|---|
| Hairline (under 0.3 mm) | Low-viscosity epoxy or sealer | Penetration depth |
| Dormant (0.3-2 mm) | Rigid epoxy | Compressive strength, adhesion |
| Active/moving | Flexible polyurethane | Elongation, movement accommodation |
| Structural void | Rapid-set cementitious mortar + epoxy prime | Volume fill, strength |
For cracks wider than 2 mm, routing (cutting a clean edge) before filling produces a better result than filling a rough, irregular crack.
What happens if cracks aren’t properly treated before resurfacing
Reflective cracking: An overlay applied over an active crack without flexible treatment will re-crack at the same location within months. The crack in the substrate is transmitting movement directly to the overlay above it.
Delamination at crack edges: If crack edges are loose or delaminating, the overlay bonds to loose material that subsequently detaches, taking the overlay with it.
Water ingress through overlay cracks: Once the overlay has cracked, water enters at the crack and migrates under the overlay, accelerating delamination outward from the crack location.
FAQs
My driveway has about 20 small hairline cracks spread across it, do I need to fill every single one?
Not every hairline crack requires individual treatment. Cracks under 0.2 mm that are stable and not associated with delamination can be effectively addressed by the grinding and overlay process, the overlay bridges these fine cracks. Cracks 0.3 mm and above should be individually filled. For a heavily cracked surface, a contractor may apply a general crack-bridging primer coat before the overlay to provide systematic protection against reflective cracking.
Can I fill the cracks in my driveway myself before getting a contractor in?
You can do basic filler work on small, dormant cracks, hardware store polyurethane filler or epoxy filler can be used effectively. However, be honest about your ability to distinguish active from dormant cracks, and about the quality of your preparation (routing, cleaning the crack, applying appropriate primer). Poor DIY crack repair can make the subsequent professional overlay work harder. If you’re unsure, leave the crack assessment and repair to the contractor.
How does the contractor know if a crack will show through the new overlay?
A well-specified overlay system applied over properly repaired cracks will not reflect those cracks, provided the repair was done correctly and appropriate crack-bridging treatment was used for any active cracks. A reputable contractor can give you a realistic expectation based on what they find during preparation. Any contractor who guarantees zero visible movement through the overlay without qualifications, regardless of crack condition, is either overpromising or planning to cut corners on preparation.