As the most commonly used material in construction today, the impact of concrete on the environment cannot be understated.
First used by the ancient Romans, the material – which consists of a coarse granular material mixed with a hard binder such as Portland cement – is a common ingredient in foundations, buildings, pavements, bridges, highways and dams.
The impact of concrete in terms of greenhouse emissions is well understood. According to a study conducted by the World Business Council for Sustainable Development the manufacture of cement – a key ingredient in concrete – serves as one of the chief industrial sources of carbon dioxide, accounting for around 5 per cent of man-made greenhouse gas emissions. The environmental impact of concrete production does not stop there, however.
For one thing, procurement of sand and gravel for use as the granular filler for the material can disrupt the environments from which it is extracted. In-steam mining, for example, has been banned in a number of jurisdictions including Canada and many parts of the United States as it involves dredging of sand and gravel directly from riverbeds and floodplains and can cause erosion, disrupt local ecosystems and affect water quality.
Even the more common practice of open pit mining for aggregates, which avoids threats to aquatic ecosystems, can still lead to habitat destruction, land erosion, noise pollution, and the generation of large volume of silica dust.
The transportation of aggregate also takes its toll, with a 2010 paper by Mohamed Hameed and Abdol R Chini from the University of Florida-Gainesville indicating that trucking of material makes a key contribution to the embodied energy of concrete.
These impacts are set to worsen going forward as concrete production surges, particularly on the back of continued urbanisation in China and India and the associated infrastructure this requires.
The solution may well be the tact commonly employed across a number of other manufacturing sectors – recycling of pre-used materials.
Concrete-based structures are frequently demolished as buildings and facilities are upgraded or replaced, providing the construction industry with an ample source of raw material for use as recycled concrete aggregates (RCA).
During the demolition process, builders can use portable crushing machines to convert up to 600 tons of material per hour into aggregate for reuse, reducing the need for new sand and gravel to be mined and saving on the energy and cost of transporting fresh aggregate to building sites. This also removes the need to send rubble produced by demolition to landfill.
Moreover, acceptance of RCA as being fit for use is growing. A review of recycled concrete aggregates published in International Journal of Concrete Structures and Materials, for example, concluded that the material is a viable option for structural applications, with only modest differences in performance compared to concrete produced using directly mined aggregate.
The review found that the use of RCA produced concrete with comparable splitting tensile strength and only slightly less modulus for rupture, while the performance of RCA concrete beams was found to remain well within standard specifications.
With the viability of RCA now established, all that is needed are measures to foster and abet its usage by industry. One of the most effective tools available for this is the inclusion of a materials assessment benchmark in green building criteria.
In Australia, the Green Building Council of Australia has made a “Concrete” credit an integral part of its set of ratings tools since the introduction of the Green Star system in 2003 and further refined it with the release of a revised version in May this year.
The establishment of the credit serves to provide building owners and members of industry with further motivation to adopt environmental friendly concrete, including those which make use of RCA.