Proving aggregate quality properly, before the surfacing goes on, protects the whole pavement and everyone’s margin in it, writes Arne Corrie-Johnston, Civil Contractors New Zealand Technical Strategy and Engagement Manager.
The problem
Here is a question that reaches CCNZ regularly, in one form or another. A contractor supplies basecourse for a council job. Five samples are tested. Four sit comfortably inside the particle size distribution envelope and one nudges outside it on a single sieve. Is the lot compliant?
Under many specifications the honest answer is, it depends on who you ask. That is a problem, because the difference between yes and no can be a stockpile worth tens of thousands of dollars, a programme delay, and a strained relationship between contractor, quarry and client.
The detail
Aggregate is a natural material. No two buckets from the same quarry face are identical, and no sampling and testing regime is free of its own variation. Treating a single test result as a pass or fail gate ignores that reality in both directions. It can condemn material that is consistently well inside specification, and it can wave through material that is drifting toward trouble but happened to test well on the day.
The state highway network has moved on from that thinking. New Zealand Transport Agency’s (NZTA’s) M04 basecourse specification, republished in 2024, introduces aggregate classes and embeds statistical process control for particle size distribution. M03 for sub-base followed in 2025 with a similar architecture. Both ask a better question: not whether one sample passed, but whether the production process is in control and the material as a whole complies.
The gap is everywhere else. Local authority work, the bread and butter for many members, has no consistent statistical acceptance framework. Acceptance rules vary from council to council, and plenty still hinge on the single result.
That variability in the rules, layered on the natural variability of the material, is where the guesswork is. So, what can contractors do to take it out?
What contractors need to know
BPG05 is being rewritten. The CCNZ Best Practice Guideline 05, Quality Assurance of Aggregates for Roads, is under full review by a working group of contractors, quarry operators and the Aggregate and Quarry Association (AQA). The 2019 edition predates the M04 and M03 rewrites.
The focus has shifted. Rather than duplicating the specifications, the new guideline concentrates on what they do not cover: quality systems, lot identification, traceability and consistency over time.
A statistical acceptance tool is in development. CCNZ and the AQA are building a tool that extends statistical acceptance to materials outside M04 and M03, using characteristic values across common basecourse, sub-base and all-passing grades.
The pathway is agreed. A working group review, CCNZ’s Technical Steering Committee – Roading, then NZTA comment, with publication targeted for late this year.
What contractors need to do
Lot and label. Identify lots during production, test before release and put lot numbers on test records. Traceability is the foundation any statistical method stands on.
Get familiar with the class structures in the 2024 M04 and 2025 M03 specifications. They are the direction of travel for acceptance thinking on all networks.
Send us your acceptance disputes. Real examples of good material rejected or arguments over a single result sharpen the case for consistent rules.
The Black Art 2026 – Pavements and Surfacing – Putting the Theory into Practice, was a one-day event on 16 September at Due Drop Events Centre in Auckland. The day was about the black stuff: the art and science of surfacing, low carbon and biogenic binders, chip seal and asphalt compliance.
There was also a strong young presenter line-up and a look beneath the surface at the aggregates every seal and asphalt layer relies on, with a demonstration of the statistical acceptance tool.
Why this approach
Every sealing and asphalt crew knows the practical truth – surfacing is only ever as good as what it sits on. The best binder in the country cannot rescue a basecourse that was never right, and nobody wants to relitigate the base after the black stuff is down. Proving aggregate quality properly, before the surfacing goes on, protects the whole pavement and everyone’s margin in it.
Statistical acceptance is also a shared language. When contractor, quarry and client all read the same characteristic value against the same envelope, disputes shrink and consistency gets rewarded. Producers who invest in process control can prove it and clients get confidence that what is under their pavement matches what was specified, rather than what one sample said on one day.
What is CCNZ doing to take the guesswork out of aggregate acceptance?
CCNZ facilitates the BPG05 working group, is supporting the development of a statistical acceptance tool with the AQA and will take the rewritten guideline through industry and NZTA consultation before publication. If your business has struck acceptance problems on aggregates or you want to test the tool against your own production data, get in touch. The more real examples we can work with, the better the outcomes will be.