Joining new concrete to old concrete — sometimes called a construction joint or cold joint — is one of the trickier tasks in concreting. Done properly, with good surface preparation and the right bonding products, you can achieve a strong, durable connection. Done poorly, you'll end up with a cracked, delaminating joint that lets in water and fails early.
Why the Bond Is So Challenging
Fresh concrete doesn't naturally stick well to hardened concrete. The old surface is smooth, dusty, and non-absorbent, which means new concrete poured directly against it will shrink away as it cures, leaving a weak, porous joint. This is why surface preparation and a bonding agent are the two most critical steps in the process.
Step 1 — Prepare the Old Concrete Surface
This is the most important part of the whole job. A clean, rough, sound surface gives the new concrete the best chance of bonding.
- Remove all loose material: Chip away any crumbling, spalling, or contaminated concrete with a cold chisel and hammer, or an angle grinder with a diamond cup wheel.
- Roughen the surface: The old concrete should be mechanically roughened — aim for a surface profile similar to coarse sandpaper. A concrete grinder, scarifier, or even a stiff wire brush can achieve this. Avoid smooth-cut surfaces.
- Clean thoroughly: Remove all dust, oil, grease, curing compounds, and laitance (the weak surface layer). A pressure washer works well. For oil contamination, a degreaser may be needed first.
- Saturate surface dry (SSD): Dampen the old concrete so it won't draw moisture out of the new mix, but make sure there is no standing water when you pour. This is known as the SSD condition and is standard practice.
Step 2 — Use a Concrete Bonding Agent
A bonding agent (also called a bonding primer or adhesive) is applied to the prepared old surface before pouring new concrete. There are two main types used in NZ:
Epoxy Bonding Agents
Epoxy-based bonding agents offer the strongest bond and are ideal for structural repairs, industrial floors, and situations where the joint will be under load or stress. Products from Sika and Ardex are well-regarded in the NZ market. Follow the manufacturer's pot life and open time instructions carefully — epoxy must be applied while still tacky.
Cement-Based Slurry or Acrylic Bonding Agents
For lighter-duty applications such as topping slabs, paths, and driveways, a cement-based slurry (a mix of cement, water, and sometimes an acrylic bonding additive) can be scrubbed into the old surface immediately before pouring. Mapei and Sika both offer acrylic bonding admixtures suitable for this purpose. Never let the slurry dry before placing new concrete — it must be poured while the slurry is still wet.
| Application | Recommended Bonding Method |
|---|---|
| Structural repair or load-bearing slab | Epoxy bonding agent |
| Driveway or path topping | Acrylic bonding slurry |
| Thin overlay (under 40 mm) | Specialist overlay primer |
| New slab abutting existing edge | Expansion joint + dowel bars |
Step 3 — Consider Reinforcement and Dowel Bars
For structural slabs or anywhere the joint will experience movement or load transfer, dowel bars (steel rods drilled and epoxy-anchored into the old concrete) tie the two slabs together and prevent differential settlement. This is standard practice for driveways, garage floors, and footpaths. Your concreter will advise on bar size and spacing based on the slab thickness and load requirements.
Step 4 — Allow for Movement with Expansion Joints
Where two separate slabs meet — for example, a new driveway extension joining an existing one — it is often better to not bond them together at all, but instead install a proper expansion joint (a compressible filler strip) between them. This allows each slab to move independently with temperature changes and ground movement, preventing cracking. In NZ's variable climate, thermal movement is a real consideration, particularly in areas with significant temperature swings.
For guidance on concrete slab construction standards, BRANZ publishes useful technical resources for NZ conditions.
Step 5 — Pour and Cure Properly
Once the bonding agent is applied and the surface is ready:
- Pour and compact the new concrete promptly — don't let the bonding agent skin over.
- Consolidate the concrete thoroughly at the joint using a vibrator or rodding.
- Cure the new concrete for a minimum of 7 days using wet hessian, curing compound, or plastic sheeting. Good curing dramatically reduces shrinkage cracking near the joint.
Building Consent Considerations
Most concrete repair and extension work around the home falls under Schedule 1 exemptions to the NZ Building Act, but this depends on the scope and location of the work. Structural concrete work — such as foundations, retaining walls over a certain height, or anything affecting the building structure — may require a building consent. Always check with your local council before starting. For more information, visit MBIE Building Performance.
When to Call a Professional
While small patch repairs are within reach of a capable DIYer, most concrete joining work benefits from professional expertise. A qualified concreter will assess the structural requirements, select the right bonding system, and ensure the finished result meets NZ standards. Structural repairs, large slabs, and any work near foundations should always be handled by an experienced tradesperson. You can find qualified local concreters on tradedirectory.co.nz.
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