Road building cost 'blow-out' predicted under new NZTA standard for base material

The national quarrying and aggregates organisation fears new New Zealand Transport Agency requirements for roading aggregate will reduce the number of sites making basecourse material and cause a blow-out in the cost of road building.

Aggregate is small chips of stonesourced from rock quarries and river stones, and is used for basecourse - a layer of material under the surface layer of an asphalt road.

Aggregate and Quarry Association (AQA) chief executive Wayne Scott says several recent major roading failures appear to be behind NZTA’s drive to have all M/4 standard basecourse, used on motorways and many other roads, meet additional compliance tests.

However, he says additional shape control requirements from NZTA for M/4 basecourse are seen by construction material technical experts as over the top, and if put in place would blow out the cost of road building.

"We support building durable roads but when roads fail, it’s not necessarily the aggregate that is at fault as the NZTA seems to be implying; there’s a series of decisions around procurement, design and construction processes," Mr Scott said.

He said there are more than 200 quarries across the country making M/4 and AQA members are worried that if the new NZTA requirement continues to creep into project specifications, the volume of compliant M/4 will plummet. 

Early analysis indicates that more than 24 per cent of M/4 will be non-compliant, leading to additional testing costs, associated delays to roading projects and a reduction in sites producing M/4, Mr Scott said.

One major quarry company with strong quality testing procedures in place for its many major roading projects had nearly a third of its M/4 fail to meet the new NZTA requirements, he said. 

Mr Scott says NZTA is being inflexible in its demands.

"NZTA is saying, 'here is the new standard for M/4; make everything to that standard or don’t produce it at all.' That’s not a statistical envelope for us to work within - it’s an unrealistic line drawn in the sand," he said.

"We already have shape control provisions in M/4 - and if NZTA continues to insist on new requirements, it will cause a blow-out in the cost of road building where costs are already under pressure from rising demand for aggregate and increased cartage distances."

A spokesperson for NZTA said they are requiring the new standard to "avoid potential problems with road pavement quality".

"The additional testing requirement is in response to research showing that where base course aggregate doesn’t conform to necessary shape requirements it may cause a failure in the pavement," the spokesperson said.

Mr Scott said he hopes with a new CEO being appointed to NZTA, there will be a fresh opportunity for the AQA to open discussions on the proposed M/4 aggregate specifications.

South Island road.
South Island road (generic). Source: