Quarry aggregate is arguably one of the most important building materials for the construction industry, as a constituent in many different products across a wide range of applications, from highly specified high-strength concrete to less critical drainage or fill applications and everywhere in between.
Quarry aggregate is an essential and integral part of our existing and future transport networks, including road, rail and airport traffic. It is key to the future of all our existing services we take for granted: water, electricity, waste management etc, none of which could be effectively provided without aggregate materials available for use in concrete – pipes, power poles, support works etc, or many other material applications.
So how healthy is our supply of these important materials into the future? And what price are we paying – locally, nationally and internationally – as part of global greenhouse impact by our choice of smaller aggregate sizes in place of traditional 20mm aggregate?
Historically, quarry operations have targeted their extraction, crushing, screening and stock management to provide a controlled split of aggregate sizes from 20mm, 14mm, 10mm, 7mm down to sand-sized products. In some quarries, other large materials may have been produced such as spalls, gabion, rail ballast, 40mm or 28mm, but the bulk of the aggregate materials were produced in the 20mm minus range to satisfy the demand from the aggregate market.
Because of this demand, the majority of established aggregate quarry plants in Australia were designed to maximise the production of 20mm aggregate, which has been traditionally the largest volume used in concrete, readily used in asphalt and was also used in road sealing. In recent years, there has emerged a growing trend towards using smaller sized aggregates in place of 20mm.
So why is this, such an issue?
In short, existing quarry plants designed to produce 20mm minus aggregate sizes are now expected to make large volumes of 14mm minus which becomes extremely inefficient and energy costly.
To meet the new demand for smaller aggregate sizes, quarry operators are recrushing 20mm, at an additional power and greenhouse emission cost, or simply over-producing excess unwanted 20mm to allow stock on the smaller sizes to build. As a nation focusing very closely on greenhouse gas reduction, we must ask ourselves are we really serious about reducing waste if we allow this to continue?
So how big is the problem?
In Australia, we consume more than 50 million metric tonnes of quarry aggregate per year, with at least one of our major capital cities having one or more major infrastructure projects under way at almost any given time. As such, we need to be responsible about our precious quarry material reserves into the future.
At first glance, the use of recycled aggregates may appear to be an easy solution, however in some cases, the energy required to process, wash etc does not always make them economic or environmentally favourable. In addition, the amount of available recycled aggregate is too small to satisfy even part of our growing needs.
During recent road, tunnel and infrastructure projects in Sydney, there was a real concern that there would not be enough suitable aggregate available in the required sizes to meet the extremely high demand within reduced construction programs, with pressure from government to finish ahead of time for upcoming elections.
Aggregate demand is heavily influenced by construction specifications that outline what materials are required for various applications. While there are well-established reasons for reducing aggregate sizes, in some cases, the trend towards smaller sizes has placed very real pressure on the aggregate supply sector and our precious quarry reserves.
What should we do?
The use of larger 20mm aggregates may need to be revisited for some applications.
? Further use of 20mm no fines concrete for various traditional and possible alternative applications – porous pavements, footpaths, pedestrian trafficked areas etc.
? More widespread use of 20mm aggregate in other concrete applications.
? A different consideration may be to increase the nominal size of our standard aggregates, such that:
? existing 14mm nominal size contains a maximum top size of 16mm
? existing 10mm has a maximum top size of 12mm, and
? existing 7mm has a maximum top size of 9mm.
Whilst this would not completely solve the excess 20mm problem, it would allow greater volumes of more consistent material to be produced, and it would be cheaper and easier to produce larger sizes, in general requiring less input energy and hence reduced greenhouse gas emissions. This approach may not be suitable for all applications, but it may create some much needed cost and environmental benefit.
New quarry plants are now being designed and built with energy efficiency and environmental considerations in mind, with greater flexibility to produce materials to the changing aggregate market. But while the market continues to dictate the need for a 14mm minus range of aggregates over traditional 20mm minus at a higher energy and environmental cost, will the potential energy savings ever be completely realised?
Looking forward into the future, as stockpiles of unwanted 20mm aggregate continue to grow, we need to consider whether we are really making the best use of our precious natural reserves. We cannot afford to waste what little we have, as our reserves will not be around forever.
Michael van Koeverden is a director of a new specialist concrete and quarry technical consultancy company, CQT Services. He is also a current member of Standards Committee CE-012 and a member of the CCAA National and Technical Liaison Committees. He can be contacted on 0431 400 933.