North Island minerals draft report

The draft ‘Geologic Inventory of North Island Aggregate Resources’, prepared by PHILIPPA BLACK from the geology school of the environment at the University of Auckland, is now available online.

This comprehensive report is now posted on the technical section of the AQA website and any feedback from members will be tabled and discussed with respect to the next stages of the project.

“The Geological Inventory of Aggregate Resources project is intended to enhance the industry’s awareness of the availability of materials used as aggregates and to provide a matrix of their source and other properties,” writes Black.

The focus is on determining what the differences are between aggregates (mostly greywacke and volcanic) and why there are differences, she says.

The report is written in a two-part format. The first part surveys the relevant physical properties of rocks and minerals to demonstrate how these relate to standard aggregate property tests specified for use in road construction and structural concrete, and discusses what information they provide about their performance.

The second part describes the main types of aggregate currently quarried in the North Island, their general locations as well as their geological (and some engineering) properties. It also includes artificial materials used as aggregate in the roading industry. This approach provides a simple framework that will need little modification when South Island aggregates are incorporated in a few years time, says Black.

“Thanks to the generosity of many quarry operators, a considerable amount of data on aggregate source and production properties has been amassed. However, it is difficult to compare this data because of the variety of ways that the aggregate has been processed and the different testing methods used. In some instances the tests have not been undertaken according to either NZS 4407: 1991 Methods of Sampling and Testing Road Aggregates or NZS 3111: 1986 Methods of Test for Water and Aggregate for Concrete, but to meet a local body or individual contractor’s specification.”

The draft report makes for some very interesting reading in regards to application of aggregate.

“There is a tendency for over specification of aggregate intended for use in secondary and rural roading projects. Processing of material to meet unnecessarily high specifications has the effect of relegating a larger proportion of the aggregate produced by a quarry into the ‘marginal’ category.

“Setting specifications on aggregate to levels appropriate for their intended use would increase the amount of quarried material that can be used and preserve high quality materials for future projects where very high performance is required. Increased use of marginal aggregates produced by quarries close to the point of consumption would also reduce transport costs.”

Interestingly, the report says most of the aggregate property tests used in New Zealand have been developed either in Europe (including the UK), or North America and have been adopted, with minor modifications, to suit New Zealand conditions.

“However, aggregate source rocks of old continental countries are very different to those in New Zealand. In the USA 69 percent of aggregate (crushed stone) produced is carbonate (limestone and dolomite), approximately 16 percent is from intrusive igneous rocks (granites and gabbros) and only about three percent is sandstone, slate and quartzite. In Canada more than 70 percent of all crushed stone production originates from carbonates, the next most common aggregate sources are plutonic rocks (granites, diabases), followed by metamorphic rocks.

The UK production of aggregate is 60 percent carbonate, the remainder derived from plutonic and metamorphic rocks.

Greywackes (and other lithified sandstones) are a very small proportion of the aggregate production in Europe, North America, South Africa and Australia and their sandstones usually contain physically and chemically stable debris that has been through several cycles of sedimentation; many are quartz-rich.

In contrast, more than 70 percent of all aggregates produced in New Zealand are greywacke, the remainder largely derived from young volcanic rocks.”

New Zealand greywackes also have high moisture absorption values and intrinsic shrinkage, it says.

“Many of the volcanic rocks used as aggregate sources in New Zealand are young, erupted in the marine environment, and typically contain zeolite and swelling clays. Thus there are fundamental differences in the nature and properties of rock formed in young active geological environments and the aggregate source rocks of old continental environments. It follows that tests developed for continental source rocks may not be an effective predictive tool for New Zealand aggregates.”

The New Zealand Transport Authority (NZTA) is also questioned over its specifications for roading aggregates.

“This writer concludes that many of the tests included in the suite of aggregate property tests specified by NZTA and other agencies will not adequately inform the engineer about fitness for purpose, or the likelihood of the aggregate remaining sound to the end of the design life, if the aggregate processing methodology is not incorporated in the test report.”