Yes, you can absolutely put leftover ready-mix concrete to work in the garden — think stepping stones, garden edging, post footings, or a small pad for a letterbox or garden ornament. The catch is that ready-mix has a limited working window, so you need a clear plan before the truck arrives (or before the drum stops turning). Here's what every NZ homeowner should know.
What Is Ready-Mix Concrete and Why Does Timing Matter?
Ready-mix concrete is batched at a plant and delivered in a rotating drum truck. Once water is added to the mix, a chemical reaction called hydration begins — and it doesn't stop. In typical NZ conditions, you generally have around 60 to 90 minutes from the time of batching before the concrete becomes too stiff to place and work properly. Heat, wind, and low humidity (common in places like Hawke's Bay or Central Otago in summer) can shorten that window significantly.
If you're ordering a full load for a driveway or slab, plan your small garden project as a secondary pour so you're ready to use the leftover concrete immediately after the main job is done — not an hour later.
What Small Garden Projects Work Well?
Leftover ready-mix is well suited to:
- Stepping stones – Pour into timber or polystyrene moulds laid flat on the ground.
- Garden edging – Screed into a shallow trench to create a mowing strip.
- Post footings – Fill holes for fence posts, clothesline poles, or letterbox posts.
- Small pads – A base for a garden tap, ornament, or small shed bearer.
- Retaining wall footings – A small concrete footing for a timber or block retaining wall (check consent requirements — see below).
What You'll Need to Prepare in Advance
Because time is short, have everything ready before the concrete truck arrives:
- Moulds or formwork built and staked in position
- Sub-base compacted — at least 75–100 mm of compacted AP20 (all-passing 20 mm aggregate) or similar well-graded gravel under any slab or pad
- Reinforcing mesh or rebar cut and placed if needed (stepping stones under 75 mm thick generally don't need mesh, but anything structural should)
- Wheelbarrow, shovel, screed board, and float on hand
- A helper — concrete waits for no one
How Much Leftover Concrete Might You Have?
Ready-mix is typically ordered in 0.2 m³ increments (minimum loads vary by supplier — often 0.5 m³ to 1 m³). Overordering by a small margin is common practice to avoid running short on the main job. As a rough guide:
| Project | Approx. volume needed |
|---|---|
| One 600 × 600 × 75 mm stepping stone | ~0.03 m³ |
| 10 m garden edging strip (100 × 100 mm) | ~0.10 m³ |
| 400 mm diameter × 600 mm deep post footing | ~0.08 m³ |
| Small 1 m × 1 m × 100 mm pad | ~0.10 m³ |
Talk to your concrete supplier about how much overage to expect and plan your garden project accordingly.
Curing and Finishing Tips
Once placed, concrete needs to cure properly to reach its design strength:
- Keep it moist for at least three days — cover with wet hessian or plastic sheeting, especially in hot or windy weather.
- Avoid foot traffic for 24–48 hours and heavy loads for at least 7 days.
- In cold conditions (below 10°C, which can occur in southern NZ overnight), protect the pour from frost with insulating blankets.
- A wood float or steel trowel finish works well for stepping stones; a broom finish gives better grip on pads.
Do You Need a Building Consent?
For most small garden projects — stepping stones, edging, or a minor post footing — no building consent is required under Schedule 1 of the Building Act 2004. However, retaining walls over 1.5 m high (or over 1 m if they support a surcharge like a driveway) generally do require consent. Always confirm with your local council before starting, as requirements can vary. The MBIE Building Performance website has useful guidance on consent exemptions.
What to Avoid
- Don't add extra water to loosen stiff leftover concrete — it weakens the mix and reduces durability.
- Don't pour over frozen or waterlogged ground.
- Don't use concrete that has already started to set — if it won't slump off the shovel, it's too late.
- Don't pour down the stormwater drain — hardened concrete can block pipes and disposing of wet concrete incorrectly can breach local council bylaws.
When to Call a Professional
While using leftover concrete for garden projects is well within DIY territory, there are situations where you should bring in a qualified concreter:
- Any structural slab, footing, or driveway that needs to meet NZS 3109 (concrete construction) or the NZ Building Code.
- Projects requiring a building consent — a Licensed Building Practitioner (LBP) may need to be involved. Check LBP for details.
- If you're unsure about sub-base preparation, drainage, or reinforcing requirements.
You can find experienced local concreters on tradedirectory.co.nz who can advise on your specific project or handle the work for you.
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