There is a growing recognition among governments, businesses and the public that we urgently need to transform our take-make-dispose economy into a circular one, in which resources are kept in use for as long as possible while maximum value is extracted.
Designers can adopt strategies to maximise a concrete structure’s potential in a circular economy, and ensure that it is fit for purpose for a world in which resource use is radically lower and no material is wasted. These are outlined below and also, in more depth in this on-demand Circular economy: strategies for concrete buildings webinar and an article on circular economy in Concrete Quarterly magazine, autumn 2021 issue.
Reusing a building structure is the most effective way to keep its materials in use for as long as possible and to extract the maximum value from them and the benefits of retaining and reusing concrete frames are increasingly recognised by designers, developers and planners. The durability of concrete sub- and super-structure means that they can often remain in place while other shorter-lived building elements are replaced, and often well beyond the notional 60-year service life of a building. The methodologies for establishing the suitability of a frame for reuse is well established.
Summary guidance for engineers for assessing the potential for reuse of a concrete structure can be found in the CQ technical article ‘Reusing structures: One step closer to a circular economy’ available here.
More guidance on reuse is here.
Design for future reuse
Where a new structure or building is required, this should be designed to optimise future reuse, so embedding good circular economy practice. Here, the durability of concrete is an advantage.
According to the design standards for a concrete frame located internally – in other words, in an environment classed as “low exposure” – no additional measures are required to achieve a service life of over 100 years compared to 50. (See BS 8500-1, tables A4 and A5, XC1 exposure class.)
The inherent low maintenance requirements of a concrete structure, and its resilience to fire and the impacts of weather, mean that it can remain serviceable over a long period, with the potential for multiple reuses during its lifetime.
Further guidance and exemplars are available in 'Second Life' an article in Concrete Quarterly, winter 2019 issue.
Design for disassembly and reuse
While the majority of examples of reuse of concrete construction elements remain in use on a site. Design of these structures to facilitate the removal and replacement of other shorter lived elements such as windows/ facades, internal walls etc is fairly straightforward and are more reliant on forethought than innovation for implementation.
The concrete structure can also be designed to be disassembled for reuse where required. This is currently mostly confined to temporary structures such as car parks and stadium seating. The upper tiers of the London Olympic Stadium are one such example. The durability and robustness of concrete is an advantage in this regard. Key areas of focus are fixings and standardisation of element size to avoid the need for future adaptation. Examples of other systems developed for disassembly can be found at the Circle House case study and this Fresh Concrete webinar on de- and re-constructable concrete structures.
When concrete does eventually reach the end of its life, it can be recycled. This applies to all concrete, and the process can be repeated again and again in perpetuity to provide a low carbon resource with a range of applications. Feedback from demolition contractors, supported by data from other sources, is that nearly all concrete demolition waste is recycled.
The majority of concrete’s volume/mass is aggregates, and when recycled, it becomes aggregate again. Some of this makes its way back into new concrete, but most is used “unbound” as sub-base materials, fill and hardcore. This plays an important role in the UK’s current circular economy, its use reducing the demand for primary aggregates. The aggregates in concrete may be crushed on site for reuse in a new development on site, or transported to a local recycling centre for processing and distribution.
Crushing concrete for reuse as aggregate substantially increases its surface area, allowing carbon dioxide to be more readily absorbed – a process called carbonation. Exposure to rain has been shown to significantly increase the rate of carbonation during this stage. Carbonation occurs during demolition and while the aggregate is stored on site, but then continues during the concrete’s secondary life if used in groundworks, hardcore and landscaping.
Alternative uses for concrete demolition waste is in development including carbon capture use and storage and the manufacture of recycled concrete paste as a cement replacement.
Use of recycled or secondary material in concrete manufacture
It is currently common practice to include some recycled or secondary material content in the manufacture of concrete, including its cementitious binder, aggregate and reinforcement.
- Recycled content in the manufacture of CEMI, including recycled plasterboard
- GGBS and fly ash (by products of other manufacturing and industrial processes) as supplementary cementitious materials
- Recycled steel reinforcement
- Crushed concrete aggregate (CCA) as coarse aggregate
The extent and type of secondary material used depends on a many factors, but in general are limited by factors other than technical standards for the manufacture of concrete. Limiting factors include availability of appropriate supply; quality control, impact on mix design and ready availability of established supply chains for local, low carbon, natural aggregate.
Recycled aggregates in concrete.
British Standard BS 8500 permits CCA to comprise up to 20% of the coarse aggregate in most designated concrete, without declaration – in other words, unless the concrete is explicitly required to exclude it. According to the standard, it is possible to use higher percentages. In general concrete designated GEN 0 to GEN 3 – that is, non-reinforced concrete used for footings and floor slabs in domestic applications – up to 100% is allowable.
Exposure class may be a limiting factor – in other applications, CCA is permitted provided it can be demonstrated to be suitable for the exposure class conditions. In practical terms, this would mean testing to meet quality protocols.
If concrete is not separated from other recycled construction materials, such as brickwork, the result is a more general recycled aggregate (RA). This has more limited uses within concrete itself, typically limited to mass concrete.
Guidance on the use of recycled aggregate in concrete can be found in the following resources: