Recycling and use of recycled aggregates

The issue of recycling concrete - from demolition of buildings or from construction materials surplus - and using recycled aggregates from construction and demolition waste (C&DW) for construction purpose, has been increasingly focused during the last decades. This has been partly from the viewpoint of environmental waste handling, partly as a means of saving natural resources. The resource saving potential is limited, however, as it has been calculated that on a European scale, even a full utilisation of recyclable aggregates will account for maximum 10 % of the annual consumption of aggregates. On a local or National scale - depending on the specific resource availability and waste-handling situation - the impact may be bigger. For this reason a lot of research and practical development in production technology as well as materials utilisation has been undertaken in many European countries, and generally it can be said that this today is more or less a state-of-the-art technology. A main limitation so far has been the lack of standardisation. There is, however, work in hand to have these materials implemented in the European standards for materials and structures, and to make easy-to-use specifications. Several RILEM committees have played a key role in these efforts.

Recycling of demolition waste

It is generally recognised that the technologies for recycling construction and demolition waste (C&DW) back into construction is already widely known and accepted. Thus, it is assumed that BAT on this subject is present and ready to be implemented. However the major hurdle is to establish the economical incentives in order to make recycling an option.

The amounts of C&DW annually generated in Europe are estimated to about 500 kg/capita. This figure is subject to a significant variation but the order of magnitude is correct. The amount of stony/concrete demolition rubble being recycled varies significantly across Europe and the figures are changing rapidly. In Italy and Spain the amount of concrete recycling is about 10 %, in France it amounts to 20 %, in Germany about 80 % and in the Netherlands, Belgium and Denmark 90 % plus.

The principal applications of recycled concrete rubble are as follows:

1. Down-cycling it for road sub-base and back-fill material in connection with construction works. Since the quality and the homogeneity of the recycled concrete material varies significantly due to the presence of bitumen, bricks, gypsum, etc. its performance is mainly suitable for low-tech applications where performance is not top priority. Therefore, sub-base applications are the most interesting when it comes to recycling C&DW.
   
2. Recycling it into concrete as substitute for natural aggregates. This application is possible and the performance of the recycled aggregate concrete can be satisfactory when comparing with conventional concrete. However, due to the mortar and paste adhering to the recycled aggregates the water absorption increases and its mechanical performance diminishes compared with natural aggregates. Therefore, this type of application is more demanding for the quality of the recycled aggregates and it is generally not considered as BAT.

Based on these considerations the application of recycled aggregates into sub-base construction is considered to be best available technology when C&DW is to be recycled into construction. However, this picture may change if local conditions are in favour of using recycled aggregates for concrete production, being the case in parts of the Netherlands and Belgium due to shortage of natural materials.

Another important aspect that may change the picture is transportation. There are no sound environmental reasons for transporting C&DW excessive distances just in order to recycle it. C&DW should be crushed, processed and reused locally in order to make it a sustainable technology.

Another best available technology that should be mentioned is the possibility for the concrete manufacturer to supplement its natural aggregates with recycled aggregates originating from his own production, i.e. reject concrete, surplus production, precast elements, etc. In this way the manufacturer maintains a closed-loop waste stream, which again improves his environmental profile. Furthermore, the risk of pollution from bricks, gypsum, etc. is minimised. In Denmark such a system is currently being implemented, which is mainly on demand from precast producers of structural concrete. Until now recycled aggregates have only been allowed for non-structural concrete in Denmark.

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