The Engineers for Social Responsibility invited DR DOUG WILSON, formerly of Auckland University, and professor PHILIPPA BLACK to talk at a presentation on aggregate sustainability. This is a redaction of their presentation with a focus on ‘marginal’ aggregates.
DR DOUG WILSON based his presentation on an MBIE funded, four-year aggregate project that he and his team have been working on for three years now.
About 20 million tonnes of aggregate is consumed (annually) for our public road network and about 10 million tonnes per year in the concrete industry, he says. There has been consistent growth in aggregate use over the past two decades.
“Aggregates are a non-renewable mineral resource and we need to be much better in being able to optimise the usage of that.
“A quarry for example – there is a lot of material within the quarry that doesn’t meet what we would call ‘premium quality material’ and, often, to get that premium quality material we’re wasting a lot of resource just because of ‘over specification’.
“We could be using other materials within the quarry better. And transporting material longer distances, rather than using locally used resources, is a significant problem that we need to be thinking about.”
Generally, high quality aggregate resources are limited and becoming more so in the Auckland region, he adds.
Providing a big picture view of aggregates in this country, Philippa Black says the range of different types of rocks that are quarried and used as aggregates is not evenly distributed.
“Of all the rocks that are actually quarried or extracted and used as aggregates, something like 75 percent of them are produced from greywackes.
“There are a variety of different greywackes. Some produce very high quality material; others don’t have much strength and have undesirable aspects.
“In the North Island 25 percent of aggregates are produced from volcanic rock with the majority in Northland, Coromandel, and the northern part of the Waikato area and Taranaki andesites.
“Andesites from Taranaki are actually gravels and mostly material that has been thrown out of Mount Taranaki.
“Large portions of the North Island actually have no insitu rocks that can be quarried, and gravels are the source of greywacke aggregate.
“In the South Island it is quite different. In Canterbury the material extracted is largely greywackes, extracted from stream gravel.
“But the other thing to note about the South Island is there is much more variable material than the North Island.
“In the South Island most roading aggregates are sourced from alluvial pits or extracted from rivers. Because rivers in the South Island cross a whole lot of geological boundaries, the river gravels tend to be composed of many different rock types. So it’s very difficult to precisely define what the properties are of gravels that are extracted from the South Island because they are basically smorgasbords of gravels.
“Properties of the aggregate derived from gravels are determined from the nature of the ‘mix’ and current specifications don’t recognise the variability of engineering properties in mixtures of rocks.
“This is one of the things that we haven’t quite come to grips with yet. Specifications just simply aren’t adequate to describe the property of material that is a ‘smorgasbord’.
“Many of the greywackes and a lot of the volcanic rocks, because they might have been altered or their sediments have not been very strongly cemented, have fiscal properties that don’t quite satisfy all the specified requirements for roading aggregate material.”
Marginal aggregates are much more abundant than high quality NZTA compliant aggregates, Philippa says.
“And there are large areas of the North Island and southern part of the South Island where greywackes and volcanic rocks don’t reach the specifications required for NZTA.
“So one of the problems that we are trying to deal with is learning how best to use them, and how to treat them so that we can enhance their properties sufficiently for them to be used as premium aggregates.”
Doug Wilson explained that premium quality aggregate, as specified by the NZTA, is determined by a whole series of engineering tests, most often imported from either Europe or Northern America, that were developed from their materials rather than ours.
“Our materials are quite different and geologically quite a lot younger, so they perform quite differently.
“The problem with test methods is that many of them, or almost all of them, are binary pass or fail-type criteria to determine the quality of that material, and there’s a whole series of those tests to determine that.
“And so if you’ve got a very good material that passes all the requirements then it’s determined as a premium quality material. If there is only one aspect of that material that fails it could then be determined a ‘marginal material’. But this doesn’t describe ‘distribution performance’, it’s just a cut line fail or pass situation.
“So one of the things we would like to see in the future is a much better understanding of the distribution of performance of materials rather than just a whole series of binary pass/fail criteria.
“If we understand that much better when we have a marginal material it means we can target that marginality potentially, and improve performance criteria in terms of field performance.
“So what we’re describing there in terms of marginal materials is delving into the geochemistry, looking at basically how materials perform in the environment they’re in, in terms of moisture and pH and other levels in field, and requirements that change over time.
“A lot of Transport Agency state highways in the Auckland region need to have premium quality material. But, given that they’re only about 11 percent of the road network (there’s about 84,000 kilometres of secondary local roads), there’s a much greater opportunity.”
There are a number of consultants involved in this area who find the easiest way to be ‘risk averse’ is to simply cut and paste premium quality specifications into their guidelines, he says.
“There is no incentive for a consultant, or even an agency, to utilise marginal material because they’re taking on potentially a bit more risk.
“So the procurement methods of agencies don’t encourage local usage of marginal materials. If it’s easier to get a premium they’ll use that, and that’s a problem.”
Coping with future growth
Aggregates are a valuable and non-renewable mineral resource, Doug iterates.
“There is an economic imperative to include more ‘marginal aggregates’. Our present testing focuses largely on physical properties and we know very little about the chemical changes that occur when an aggregate is exposed to water or moisture in the wet and dry cycles that pavements go through and so we’re working on understanding that better, and so there’s a need for new tests to be developed.”
Up to 90 percent of our roads and infrastructure are reliant on aggregates, he says. “Local streets range from 4000-10,300 tonnes per kilometre. If you think about increasing the roading network to service new special housing areas, there’s a big demand there.
“Motorways can be anywhere from 20,000-24,000 tonnes per lane kilometre. Asphalt is 90 percent aggregate and concrete is 80 percent aggregate in its mix as well.
“And then we’ve got pipe water services and utilities [power and gas] and if we add these per kilometre there’s a significant development on there.”
Doug says each house will involve about 50 tonnes of aggregate on average.
“That’s taken from Canadian research as we don’t have those statistics – but if you look at a NZIER report on aggregate production in 2007 you can see about 52 percent of the total aggregate is going into roading and then another 21 percent into building.”
In terms of Auckland’s growth and the demand for aggregate if the population is to grow by two million, that will require addition of 400,000 homes, which means about 13,500 homes per year for about 30 years, he says.
“And if we take the average aggregate demand of approximately 250 tonnes per house we can see already around 7.2-8.2 tonnes of aggregate consumed per person, per annum, in the Auckland and the New Zealand metro regions.
“So the Auckland demand is likely to increase from 12 million tonnes annually to approximately 20 million tonnes by 2033.
“Just from that alone we have a huge demand coming our way and yet it’s becoming more and more difficult to actually consent a quarry and actually be able to even change the consent conditions on existing quarries to be able to take more out of them.
“We’ve got some significant problems going forward.”
Philippa Black points out that the remaining quarries producing for the Auckland area are largely on greywacke, with a couple on basalt.
The south Auckland basalts in the Pukekohe area sit below good soils that are prime agricultural areas. Other ”little splodges” of basalt featured in the Auckland city area are built over. Basalts in the Waitakere area are mostly in special ecological areas.
“So there’s actually not many resources available in the Auckland area, and the remaining ones are almost all now on greywackes,” she says.
Doug Wilson adds that a lot of the special housing areas in Auckland have been selected in the north where there are few quarry sites, which means trucking larger distances along transport networks to be able to provide those future housing areas.
In one case study the direct costs of transporting aggregates to the site to build 1000 units and supporting infrastructure was about $13.9 million. And that is not taking into account the environmental costs and wear and tear on the roading network.
Lack of planning
Doug says while access to suitably located aggregates is a vital component of our economy and future growth we, unlike Australia, don’t adequately plan for aggregate supply.
New Zealand is geologically young and very different to most comparable countries, he adds, and yet we import most of our test methods from other countries.
“Which means a lot of our test methods are not necessarily applicable and there’s a lot to do yet to understand the application of those.
“New Zealand planning regimes do not facilitate protection of important mineral resources, there’s no future proofing.
“Transport costs are a significant proportion of cost to site and therefore we need to look at how we minimise haul distances, which leads on to understanding how to better utilise aggregate resources.”
Using treated marginal aggregate will allow us to use resources closer to the project.
“Again, that is an area we want to put more research in. But one of the key areas is that conservatism in the engineering profession means that there is a lack of risk sharing across the agencies to consult to contractors.
“That means that often the selection of aggregates goes automatically to premium quality materials because that is the easiest, less-risk [reason] and we don’t take into account where we can optimise those materials and so alternatives are not often considered because of that reason.”
In the Auckland region these problems have only got worse in terms of planning, rather than better.
“What can we do in terms of local and central government to better value and plan for aggregates? How can aggregate selection be better optimised at both a project and network level?”