The obvious challenge when considering reuse of structure, whether that's in terms of retrofit or renovation of entire buildings, or of reuse of components from one building to another, is the need to know exactly what it is we are working with. Engineering tries to be an exact science, but there will always be some degree of engineering judgement and assumption in action. While our theories strive to be perfect, the world is not, and as safety is critical, we use statistical probability analysis alongside a sprinkle of safety factors to ensure our designs meet remain safe for their lifetime.
In other words, the issue with reuse is quality assurance. For virgin materials, a guarantee is provided by the manufacturer or producer, and while the material itself of, say, a steel beam does not in theory degrade over time, the original guarantee has lapsed, and somehow must be substituted with provisional stamp of quality by a competent person. The material must be determined and its properties understood, using visual and physical grading, checking for defects, and testing of properties where necessary.
At the same time, it can benefit engineers to consider how the work they perform today, and the buildings that are built today, can be adjusted to aid the task of future colleagues who set out to reuse existing structures.
This outlines two related topics reusing existing buildings and preparing current designs and construction for future reuse. While there are a lot of aspects to cover, especially when considering the future, I will here focus on the topic of informational availability as the basis for design.
We'll take them in chronological order:
While the reuse of existing buildings may seem daunting for many structural engineers, the subset of engineers who work with conservation and restoration of heritage buildings will tell us there is no reason to be so hesistant. In fact, in times where global supply chains did not exist and the extraction and processing of raw materials were critically expensive, repair, renovation and reuse were the norm. As described in this study* on reuse, historically materials and elements from one structure were reused in the next. This was the most economic solution at the time. The distribution of cost between labor and material has changed significantly since then, especially post industrialism, meaning the incentive to reuse has all but disappeared, but now that we have become acutely aware of our carbon footprints, common sense will again dictate reuse to be the norm. As we develop towards a zero-carbon world, reuse will in many ways be seen as the starting point, a low-hanging fruit for reducing to zero or near-zero the carbon costs of A1-A3 LCA stages. And as we begin to wider our lens beyond just carbon, and consider the effects on biodiversity, ocean, atmosphere and habitat health, the arguments will only strengthen.
With the perspective of informational availability or non-availability, how does today's practising engineer begin to think about reuse?
First off, whenever there is an existing asset available (where there is no existing asset, engineers must look to (and advocate for) storage banks for materials, as discussed in part 2), archive information should be sought out. Where this doesn't exist, is unavailable or incomplete, information must be gained through site surveys and testing. Surveys are visual inspections with the purpose of understanding, cataloguing and sorting the components in a structure. The type, geometrical and material properties and condition of each are recorded. Testing includes Non-Destructive Testing (NDT), using radar, magnetic, and various other scanning types, and labratory analysis of material samples, but also physical load-testing of whole components.
Load testing, while being the most costly, both in terms of capital cost and the cost to the material (i.e. if you load a beam to failure, you may have reduced the reuse potential entirely), there is also the possibility to use the testing as an argument for using exact material properties. Imagine undertaking a 3-point bending test on a beam, but instead of testing to failure, you test only the necessary amount to develop an understanding of the elastic properties of the exact material in the beam. This may open options for using test-based strength values rather than characteristic strength values, increasing the amount of available use cases, and allowing designers to recoup some of the testing costs. Alternatively, you could test to direct use load, i.e. to a load representing the same effect as the design loads.
Obviously, there is an upper limit to the amount of testing that is economical, and we must be careful to not assume too much based on small sample sizes, but there is certainly room to investigate where the limit of cost-efficiency lies in terms of testing of structural elements.
These actions, to varying degrees incur costs to projects, and so must be weighed up against the possible benefit of being able to forego purchasing new material, and especially the prospect of avoiding the associated carbon emissions.
While there is a lot to talk about considering the technical design and carrying out of construction that can enable or ease future reuse, I'd first like to discuss the informational aspect. Just as current engineers must perform surveys and analyses of existing structure as the starting point for reuse design, so can current engineers aid future colleagues by developing and improving methods of storing construction information for posterity.
The starting point would be construction drawings - these should include all the information necessary to construct the building, and so can also be used to work backwards and develop a theoretical model of the building, which would enable future engineers to compare existing capacities to proposed uses, and understand adequacy and need for strengthening. Many authorities across the world do collect archives of construction drawings as well as other relevant documents, and degree of completeness of this storage has likely increased with the transition to digital formats, but it is far from guaranteed that future engineers will be able to find and access said information. It is vital that robust systems of information transfer through time is secured. Like so many other things, this would be aided by national or international standards for documentation and archival.
In comes the material passport. Like the anthropogenic counterpart, the material passport is intended as a method of identification for individual elements in a building, with a collection of critical information that help describe it. Such information could be material type and subtype, provenance, physical or chemical composition, manufacturer as well as more specific infomation such as LCA data, embodied carbon, structural use and capacity and applicable design code or standard. This information would be relevant for all components in a building meant for disassembly and reuse. For structural elements specifically, salient design information could be included, such as the design intention (i.e. what kind of element is it, how is it working, both locally and as part of a system), design capacities at various limit states, loads applied, reinforcement quantity and layout for concrete, section classification for steel, and all other relevant design assumptions.
However, material passports only get really interesting when we can find way to distinguish them from the other forms of stored information about construction projects. If all the information outlined above is already stored digitally on drawings, in Building Information Models (BIM) and in structural documentation, what use is another digital copy?
The answer here is the layering of information for redundancy and the increase in availability through directly accessible links. I'm talking here about physically accessible links from building component to material passport: QR codes. With a closed server repository of material passports, accessible by scanning the physical code on a given element, we in essence eliminate the barrier to information for future interested parties. This relies, of course, on the reliability of the digital repository and the preservation of the physical codes. Modern cloud servers provide virtually endless redundancy for digital assets, and with the possible addition of blockchain technology the data itself could be secured as immutable (i.e. read only) while remaining transparent and accessible to all. Preservation of the QR codes could with plastic laminated cards, stamps, engravings or laser encoding depending on the material, and could even be provided already during initial fabrication, as a combined effort from designer and manufacturer. If not imprinted, fixation could be done with staples, glue or mechanical fixings. Microchips could provide the same function, and have the advantage of being able to be stored within components, i.e. non-visible, but lack the universality and public adoption that QR codes enjoy.
QR codes also begin to form a story of wider public information spread, where any user of a building is able to investigate and understand where each component came from, its function and its reusability potential. This would again help bring reuse potential to a wider audience, increasing the turnover on materials, and aiding our transition to zero carbon construction. QR codes are also a well established technology, and can be easily created by anyone using one of the numerous free online AR code generators. This means the application to a building project requires little investment and research, limited to the decision of how and where to locate the material passport information - is it a link to a server file, an automatic email reply containing the information or something else?
In summary, wherever there are existing assets, these should be maintained, adapted and reused. Demolition should not be allowed today. It is up to engineers to demonstrate the added-value (or reduced cost) coming from reuse, that can counterbalance concerns regarding the costs of necessary surveys, tests and reprocessing to enable reuse. Creating and affixing QR codes to structural elements should be a workflow engineers and contractors work together to commence now. In the future, also non-structural elements could be included, especially those where the reusability level and/or the input material cost is high. We cannot tell the future, but we should do our very best to enable future engineers to be better.
*Corentin FIVET and Jan BRÜTTING, Nothing is lost, nothing is created, everything is reused: structural design for a circular economy,The Structural Engineer, vol. 98(1), p. 74-81, 2020
