Concrete isn't one-size-fits-all — different jobs need different strengths, and using the wrong grade can lead to cracking, structural failure, or a job that won't pass inspection. Understanding the basics of concrete grading will help you have an informed conversation with your concreter and give you confidence that the work is being done properly.
What Does Concrete Grade Mean?
Concrete grade refers to its compressive strength, measured in megapascals (MPa). This is the pressure the hardened concrete can withstand before it fails. The higher the MPa number, the stronger the mix. In New Zealand, ready-mix concrete suppliers typically offer standard grades such as 17.5 MPa, 20 MPa, 25 MPa, 30 MPa, and higher for specialist applications.
The grade is determined by the ratio of cement, aggregate (sand and stone), and water in the mix. A lower water-to-cement ratio generally produces stronger concrete.
Common Concrete Grades and Their Uses in NZ
| Grade (MPa) | Typical Use |
|---|---|
| 17.5 MPa | Non-structural fill, garden edging |
| 20 MPa | Residential paths, driveways (light traffic) |
| 25 MPa | Driveways, shed slabs, footpaths |
| 30 MPa | House floor slabs, structural elements |
| 32–40 MPa | Retaining walls, commercial slabs, high-load areas |
These are general guides — your specific project may require a different grade depending on soil conditions, load requirements, and local council or engineer specifications.
What the NZ Building Code Requires
For any structural concrete work — such as a house foundation or floor slab — the NZ Building Code sets minimum performance standards. Residential timber-framed construction is typically governed by NZS 3604, which specifies minimum concrete strengths for foundations and slabs. For reinforced concrete structures, NZS 3101 applies.
If your project requires a building consent, the approved plans and specifications will state the required concrete grade. Your concreter is legally obliged to follow those specifications. Always confirm consent requirements with your local council — some smaller concrete work may fall under Schedule 1 exemptions, but it's best to check rather than assume.
BRANZ is a useful resource for understanding how building materials perform in NZ conditions, including concrete durability in coastal or high-exposure environments where higher grades or special cement types may be needed.
How to Check Your Concreter Is Using the Right Grade
Ask for the Mix Design or Delivery Docket
When ready-mix concrete is ordered from a batching plant, the truck arrives with a delivery docket (sometimes called a batch ticket). This document lists the mix design, including the specified MPa strength, water-cement ratio, and slump (workability). Ask your concreter to show you this docket — a professional will have no issue doing so.
Check the Quote or Contract
A reputable concreter will specify the concrete grade in their written quote. If your quote just says "concrete driveway" without mentioning MPa, ask them to clarify and put it in writing before work starts.
Look at the Project Plans or Engineer's Spec
If you have consented plans or an engineer's specification, the required concrete grade will be noted on the drawings. Compare this against what your concreter has ordered.
On-Site Slump Tests and Cylinders
For larger or structural pours, slump tests are done on-site to check workability, and concrete cylinders may be taken and sent to a lab for crush testing at 28 days to verify the actual strength achieved. Ask your concreter or project manager whether this is required for your job.
Factors That Affect the Right Grade Choice
- Intended use — foot traffic vs. vehicle traffic vs. structural load
- Reinforcement — steel reinforcing bar (rebar) or mesh changes requirements
- Exposure conditions — coastal areas, freeze-thaw zones (e.g. Central Otago), or chemical exposure may require higher grades or special cement
- Soil conditions — poor or expansive soils may require a stronger slab
- Council or engineer requirements — always the final word on structural work
Red Flags to Watch For
- A concreter who can't tell you the MPa grade they're using
- No written quote specifying the mix
- Concrete that arrives looking very watery (excess water weakens the mix significantly)
- Adding water to the mix on-site without good reason — this reduces strength
- No delivery docket available from the ready-mix truck
When to Call a Professional
For any structural concrete work — foundations, floor slabs, retaining walls (check your local council for the height threshold that triggers consent requirements), or anything covered by a building consent — you should be working with a qualified concreter and, where required, a Licensed Building Practitioner (LBP). Check LBP registration at lbp.govt.nz. For complex or engineered pours, a structural engineer should specify the mix design.
If you're unsure whether your project needs consent or an engineer, contact your local council or visit MBIE Building Performance for guidance.
You can find qualified, experienced concreters in your area right here on tradedirectory.co.nz.
Site Safe
