The aim of a circular economy is to directly reuse products and materials where possible and recycle where direct reuse is not possible. This reduces both the consumption of raw materials and the production of unusable waste.
As structural engineers, our design decisions can directly affect how well a building can contribute to a circular economy in many ways.
Firstly, we can reduce the need for replacing buildings in the future by designing:
Secondly, we can improve how many of a structure’s components can be directly reused by:
Low Impact Buildings & Improving Existing Structures
Even if a project brief doesn’t specify a desire for reduced environment impact, we can use our knowledge and experience to produce efficient structures that also benefit the client.
As we are based in London, the majority of our projects are historic renovation works; many on properties over 100 years old. Updating these existing buildings to make them fit for modern life has a substantial inherent environmental advantage over demolishing existing buildings and starting from scratch.
Many of our renovation works lead naturally to improvements in efficiency. For example, a loft conversion will often see the original roof insulation replaced with significantly more effective contemporary materials. Similarly, lots of London houses have uninsulated suspended floors. Adding an insulated basement to such properties isolates the properties from the relatively cold ground below.
Passivhaus is a set of design criteria developed in Europe and becoming ever more popular in the UK. The targets relate to the airtightness and thermal efficiency of buildings with the aim of reducing, or even eliminating, the need for heating and cooling. Building to these standards can produce houses that use up to 75% less energy than a standard UK new-build.
Achieving the Passivhaus standard requires a fully insulated building. While this might sound simple, it prohibits many of the most common details such as a simple column on a concrete footing. In this case, either the entire footing must be within the thermal envelope of the building or the column itself must be isolated from the footing and then insulated up its length until it passes into the thermal envelope of the building. Products such as high-strength cellular glass insulation make these details possible.
At Croft, we have successfully completed several low energy, sustainable homes.
Where other materials are required, there are ways we are able to enhance how they are used; with steel structures, we can optimise for the weight of steel used to reduce both the cost of materials and the embodied carbon of the structure and we can use materials like concrete to create thermal mass which, when used effectively, can capture heat during warm days and retain that warmth to reduce the amount of heating needed during colder nights.
With all these materials, it is possible to reduce the project’s environmental impact by ensuring all structural elements are designed such that their full capacity is used, rather than being overdesigned.
Manufacturing steel from the raw materials is incredibly energy intensive. Fortunately, around 70% of steel is recycled; a fairly impressive record for any major industry. Reforming recycled steel is a very energy-intensive process involving heating to 1200°C. The large nature of the plants required for this process mean that there are relatively few of them necessitating transportation of steel over long distances. All this results in steel having very high values for embodied carbon and energy (see Table 1).
However, steel has many redeeming qualities from an environment point of view. Primarily, steel is an infinitely recyclable material. While it does require large amount of energy, steel can be reformed over and over again without any loss of material nor material quality. This could allow a truly closed loop with no requirement for new raw materials and no waste for a given amount of steel. The manufacturing of steel also produced blast furnace slag as a by-product. This can be ground up and used as a replacement for ordinary Portland cement.
Concrete is a mixture of sand, aggregate, cement and water. The manufacturing process is energy intensive, involving mining and crushing limestone before heating it to 1400°C: the temperature the limestone undergoes chemical reactions to form cement clinker. This takes the form rock-like balls which then need to be ground up into the fine powder we all know as cement.
Concrete is a very popular material due to its low cost and versatility. Concrete is not recycled, but is can be crushed, and repurposed hardcore. Increasingly, replacements for Portland cement like pulverised fly ash and ground granulated blast furnace slag are being used to reduce the environmental impact of concrete. How concrete is used can also be influential on its environmental impact. For example, it can be used to create thermal mass to reduce the heating and cooling needs of a building.
It is also possible to reuse existing concrete foundations to build a new building with the same footprint. If concrete is in place long enough, the cement will actually reabsorb some CO2 from the atmosphere in a process called carbonation.
During their growth, trees absorb CO2, effectively locking it away within the wood. Traditionally in the UK, timber has been used in the construction of floors, roofs and stud partitions. In recent years, it has been gaining popularity; partly due to its environmental credentials but also as a result of its versatility. Modern engineered timber products such as glue laminated timber (glulam), laminated veneer lumber (LVL) and cross-laminated timber (CLT) have paved the way to new ways to build structures using timber. Timber’s many desirable qualities have led to it becoming a favourite for projects like low-energy homes and prefabrication.
The gold standard for sustainable timber is the Forest Stewardship Council (FSC) certification. However, at present, which represents only about 20% of global industrial timber production. Other counterpoints to the sustainability of timber often relate to the preservatives used to protect it and the lack of ability to recycle timber at the end-of-life stage leading to re-release of the sequestered CO2 back into the atmosphere. With regard to the preservatives, using timber only where it is appropriate and correct detailing of timber structures can help prolong the lifetime of the material and mitigate, or even negate, the need for preservatives.
Additionally, improvements in areas such as clean energy will help reduce the environmental impact of the preservatives themselves. In response to the end-of-life criticism, burning wood only releases the CO2 it has already sequestered making it carbon-neutral. This is a claim that very few materials can, or may ever, be able to make. The burning of green waste is also more commonly being used to generate power, only adding to the green credentials of timber.