Concrete shrinking and pulling away from edges is one of the most common complaints from NZ homeowners after a new slab or pathway is poured. The good news is that shrinkage cracking and edge separation are largely preventable when the right mix, curing, and jointing techniques are used from the start. Here's what you need to know.
Why Does Concrete Shrink?
All concrete shrinks slightly as it dries and cures — this is called drying shrinkage and it's a normal chemical process. As excess water evaporates from the mix, the concrete contracts. If that contraction is restrained by edges, footings, or reinforcing steel, stress builds up and the concrete either cracks internally or pulls away from whatever is holding it.
In New Zealand, our variable climate makes this worse. Hot, dry nor'westerly winds (common in Canterbury and other inland areas) and strong summer sun can dry the surface far faster than the interior, dramatically increasing shrinkage stress. Coastal humidity can slow curing unevenly too.
The main causes of shrinkage and edge separation include:
- Too much water in the mix — a high water-to-cement ratio is the single biggest cause of excessive shrinkage
- No control joints — without planned weak points, concrete cracks randomly
- Poor curing — letting the surface dry out too fast
- Inadequate reinforcing — no mesh or rebar to hold the slab together
- Poorly prepared sub-base — uneven or dry ground that draws moisture out of the fresh concrete
Use the Right Concrete Mix
Start with a low water-to-cement ratio. More water makes concrete easier to work with but dramatically increases shrinkage. As a general rule, don't add extra water to the truck on site just to make placing easier — this is a very common mistake.
For residential slabs, driveways, and paths in NZ, a standard 25 MPa or 30 MPa mix is typically appropriate, but always confirm the specification with your concreter or engineer based on your specific use. Ask your ready-mix supplier about low-shrinkage admixtures — these are chemical additives that reduce drying shrinkage without compromising strength.
If you're mixing your own concrete for small jobs (like a garden path), follow the bag manufacturer's instructions exactly and resist the urge to add extra water.
Install Control Joints
Control joints (also called contraction joints) are the most effective way to manage where cracking occurs. They create a deliberate weak point so the concrete cracks in a straight, planned line rather than randomly across the surface or at the edges.
- For slabs, control joints should generally be spaced at no more than 25–30 times the slab thickness — so a 100 mm thick slab would have joints roughly every 2.5–3 metres
- Joints should be cut to a depth of at least one-quarter of the slab thickness
- Saw-cutting should be done within 6–18 hours of pouring, before random cracking starts (timing depends on conditions)
- Alternatively, tooled joints can be formed while the concrete is still plastic
Where concrete meets an existing structure — a house foundation, a garage wall, a timber edging — always install an isolation joint (a compressible foam strip or similar material). This separates the new slab from the fixed structure so they can move independently, preventing the concrete from pulling away from or cracking against the edge.
Cure the Concrete Properly
Curing means keeping the concrete moist and at a stable temperature for long enough for the cement to fully hydrate. Poor curing is a leading cause of surface shrinkage and edge cracking.
Effective curing methods include:
- Curing compounds — spray-applied membranes that seal in moisture (widely available from concrete suppliers and hardware stores)
- Wet hessian or plastic sheeting — cover the slab immediately after finishing and keep it damp
- Avoid pouring in extreme conditions — very hot, windy, or freezing days dramatically increase shrinkage risk
Concrete should ideally be cured for a minimum of 7 days for standard mixes. BRANZ has good technical guidance on curing practices suited to NZ conditions.
Prepare the Sub-Base Correctly
A dry or uneven sub-base will suck moisture out of fresh concrete from below, accelerating shrinkage. Before pouring:
- Compact the sub-base thoroughly
- Dampen a dry sub-base before pouring (but don't leave standing water)
- Use polythene vapour barrier (typically 0.2 mm) under interior slabs to prevent ground moisture issues and reduce differential drying
Use Reinforcing Steel
While reinforcing mesh or rebar won't prevent shrinkage, it holds the concrete together if cracking does occur, preventing pieces from separating and edges from lifting or pulling apart. For most residential slabs, 665 or 768 mesh (standard NZ designations) is commonly used — your concreter will advise the right specification.
Make sure the mesh is properly supported at the correct depth (typically mid-slab) using bar chairs, not just laid flat on the ground.
When to Call a Professional
For any concrete slab larger than a small garden path, it's strongly worth engaging a qualified concreter. Structural slabs — such as house floors, garage floors, or anything covered by the NZ Building Code — may require a building consent and must meet specific standards. Always check with your local council before starting, as requirements vary. Structural concrete work may also need to be designed or inspected by a licensed building practitioner — you can verify credentials at Licensed Building Practitioners.
If you already have significant cracking or edge separation in existing concrete, a professional can assess whether it's cosmetic or structural and recommend the right repair approach. You can find qualified local concreters on tradedirectory.co.nz.
Sources & Further Reading
- BRANZ – Building Research Association of NZ
- MBIE Building Performance – NZ Building Code guidance
- Licensed Building Practitioners – verify your tradesperson
- Sika NZ – concrete admixtures and repair products
- Ardex NZ – concrete repair and flooring systems
- WorkSafe NZ – safe work on construction sites
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