After working with LCA and measuring embodied carbon in structures for even just a short period of time, it starts to become clear that the levers for design typically afforded to structural engineers, such as material selection, construction method and load path development, are very far from being able to approach zero carbon structures. Even when structural engineers are allowed to influence more contentious aspects of the design, such as grid spacings and imposed load values, there is in most cases still quite a large distance to zero. The reason for this is that the extraction, processing and generation of structural elements virgin materials is simply carbon intensive, and even when recycled materials are used (steel), the upfront A1-A3 costs are hard to swallow elsewhere. Its bears stating the obvious, that structures have no way of contributing negatively to their own carbon score. There is no material or building element that is carbon negative. Even when using timber, our most green and sustainable material, we are nowhere near negative emissions when looking at whole-life carbon (which we must, a failure to do so is simply to ignore reality). As discussed in this post, timber is a good material choice, for the carbon storage effect should not be ignored, but it cannot be claimed to remove carbon from the atmosphere in any permanent sense.
When the strategy of offsetting is used to achieve towards-zero carbon scores, it is either totally outside the building system, via forest planting, or it is through energy generation occurring the use phase. Thus we have a dilemma of proportions, in that we must reduce the embodied carbon of our structures to zero by 2050, but we also have no known method of putting negative-carbon-elements in our buildings, to offset the seemingly inevitable (however reduced) positive emissions that will occur. And we know that there emissions will occur, if by nothing else (and that's currently a big if), then through the sheer work-energy required to construct a building.
So while we wait for carbon-negative structures, we must focus on the pathway that is clear and possible right now - reuse. Recycling material is good, but the process still involves a potentially significant energy and carbon cost, and too often value is lost in the process (downcycling vs upcycling). Reuse, and especially direct reuse, where the individual components of a building can participate in a new structure with minimal or no adaptation, is an extremely powerful method for reducing the upfront embodied carbon cost of structures. Despite this, in today's industry, material reuse is very rare. There are many obstacles to reuse currently, both technical (a lack of knowledge on the matter, lack of design for disassembly, irreversible connections) and systemic (lack of teaching on reused materials, lack of an integrated storage and distribution system for reused materials (material banks)).
Recently, I've been very excited by the prospects suggested in studies performed at the Structural Xploration Lab (1) out of EPFL in Switzerland. Coining the term form follows availability, a clever and familiarizing nod to architectural history, the group has developed a methodology for reuse of materials from existing buildings. Through a series of journal articles (3, 4, 5), including case studies, the team at SXL demonstrates a reuse-led design process, where a bank of reclaimed structural components is used in the service of a new structural function. This design approach is, for today's engineers, both unintuitive and unsupported by standards, but also represents a potential for significant reductions in embodied carbon (the studies suggest ranges up to a 60% reduction compared with all new components).
These studies demonstrate the theoretical feasibility and advantages of form follows availability-based design, however a number of practical barriers remain to be addressed.
Stock: currently, there is no integrated and scalable system for the storage, cataloguing and distribution of reclaimed structural components. However, currently in Denmark, a number of small, local retailers sit in this market, and many of the larger demolition companies understand the potential value in resale of deconstructed components, and so either have proprietary webshops/auctions or are creating links to the few online portals that are beginning to emerge. I expect the development here to accelerate. Having a critical mass of catalogued components is critical, if we are to expect wide-spread use of reclaimed building parts, as a certain scale and variety will be necessary.
Design for disassembly: DfD will need to become more standard, to enable the careful deconstruction of obsolete buildings. While the process for timber and steel elements is relatively straightforward (use reversible connections, i.e. bolted, and avoid welds and glues), in Denmark the predominant construction material is concrete; precast and in-situ. Pilot projects have tested replacement of grouted precast joints with mechanical fixtures and chalk grouts, which can be removed using pressure washing, but a lot more research is required here. Even more difficult is the case for insitu concrete where the bespoke and singular nature of the structures makes scalable solutions impossible. The most promising research here also comes from SXL, where concrete walls have been cut into large brick elements that have been used to create a tied arch bridge. It would seem that reusing concrete as a form of masonry, or in compression only structures is a wise pathway.
Material passports: while almost all buildings today are designed and constructed with the help of BIM (Building Information Model), which means all relevant material about the components of a building are stored electronically in a single model, the availability of such documentation at end-of-life is unknown and cannot easily be assumed. One of the main concerns for engineers (and clients) wanting to reuse structural elements is the lack of specific knowledge required for the assessment of structural performance. This may not seem like such a large issue, but it terms of liability and legal responsibility, the risk associated with unknown material properties is significant. Of course it is possible to test materials to ascertain the mechanical properties, but unless extremely efficient and inexpensive test methods are available, or testing only an assumed representative sample of the whole stock is acceptable, testing as a solution at-scale seems unsatisfactory. Another solution is to ensure that the material information can be passed on to the end of life stage, for example via affixed encoder-chips, QR codes or other info-stamps. This would assist designers in transferring elements from one building to a new use in a new structure.
This ties in closely with the idea of Buildings as Material Banks (BAMB) (2), where the urban landscape is viewed as a valuable stockpile of resources for future construction, and where the circulation of materials from one obsolete building to the next new-build form a complete circular loop.
Sources:
(1) Structural Xploration Lab ‐ EPFL
(2) BAMB - Buildings As Material Banks (BAMB2020) - BAMB
(1) Brütting et al. (2019). The reuse of load-bearing components, IOP Conf. Series: Earth and Environmental Science, 225.
(2) Brütting et al. (2019). Form follows availability - designing structures through reuse, Journal of the International Association of Shell and Spatial Structures (IASS), vol. 60, Issue 4, December 2019.
(3) Fivet, C. (2019), The design of load-bearing systems for open-ended downstream use, IOP Conf. Series: Earth and Environmental Science, 225.
