Concrete Recycling

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Concrete is the most important construction material in terms of volume and turnover. Its success comes, among other reasons, from its amazing versatility and its availability in most parts of this planet, being essentially a local material. However, this 21st century sees humanity facing its most difficult challenge since its appearance on this planet: how to keep it livable for the next generations, with a population approaching 10 billions of inhabitants, and a general shortage of nonrenewable resources? Most informed people agree nowadays with the necessity of turning from linear to circular economy. Concrete cannot miss this major paradigm change. Although the mineral stock which can be used to produce concrete is almost unlimited— limestone, clay, and hard rocks—opening new quarries at accessible distances from cities becomes difficult in many countries. As a matter of fact, in the usage competition of land (residential, agricultural, industrial), the third category often loses the battle.ii. Landfilling construction and demolition materials (C&DM) is more and more banned by public policies, as in Europe since the beginning of the millennium (1999/31/CE and 2003/33/CE European directives). iii. There is a social pressure to shorten material transportation distances from production to use sites, in order to limit CO2 emissions and impact of truck traffic on the populations. Hence, demolition materials are mostly generated in cities, where new construction sites need to be fed. Using C&DM in new construction is a double win, tackling transport generated by both material export and import at the urban zone scale. iv. Finally, concrete must catch up with regard to others materials (steel, asphalt, etc.
where recycling processes and streams have been established for years. It is not the place to perform a comprehensive life cycle analysis of concrete as a construction material compared to other solutions. Let us just note that, although concrete solutions have many merits, their ability to incorporate their own waste is an important aspect according to most environmental assessment methods. What are the most significant data to assess the potential of concrete recycling? At the European scale, 196 Mt of recycled concrete aggregates (RAs) were produced in 20151 (mostly from C&DM). About 40% of the total aggregate productions were devoted to either ready-mix or precast concrete, which constituted a market of nearly 1 Bt. Therefore, according to this simple calculations, there would be a potential to substitute 20% of the aggregate phase by RA in the whole concrete production. We will see later that this figure
is an underestimation of the potential, if we account for all inert materials coming from demolition (including natural rocks or granular materials from road subbase courses). In addition, to reach 20% as a mean percentage of recycling rate in concrete means to be able to reach locally much higher values, depending on the resource geography. Although general statistics for Europe were not found regarding the proportion of recycled aggregate in the aggregate consumption of concrete industry, it was found that in UK, this amount was about 5% in 2008. This country being the most advanced in this respect (followed by the Netherlands and Belgium),1 the figure for the whole Europe is probably not higher that some 1%–2%, to be compared with the 20% potential. So it can be concluded that the European concrete industry could recycle substantially, but is not doing so significantly at the moment. A large proportion of current RA streams is consumed in road subbases and embankments. Is this situation satisfactory? Should it be considered as fully sustainable in the present and near future? We don’t think so for the following reasons: • The RA stream will probably increase in the next years, because a large stock of residential (especially
in Europe). The need to cut the global energy expense in buildings leads not only to vast retrofitting plans but also to demolition/reconstruction given the difficulty to provide thermal insulation to old buildings; • In the same countries, there are less and less new roads to be built, but mainly maintenance
works to perform. As a matter of fact, these works deal with the road top layers and only require a low amount of good-quality natural aggregates. Therefore,
the stream is growing, and the well is shrinking; • Even if RA are excellent materials to build subbase layers, less noble materials (mixed C&DM, low-grade natural aggregates, municipal solid waste incineration bottom ash, nontoxic industrial wastes, etc.) can be used for the same purpose. Therefore, reserving
RA for new concrete is a way to make a better use of the whole waste materials resource by the civil and construction sectors.

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