Concrete
As one of the most durable and robust building materials with high fire resistance and a relatively simple and low-cost production process (i.e., requiring few raw materials), concrete is commonly used in construction worldwide. It can also be moulded into various shapes and dimensions of different sizes, so it can be used to build virtually any type of structure, from small residential buildings to large industrial facilities.
Environmental impact
The toll concrete takes on the environment is mainly related to cement production, an energy-intensive process resulting in considerable greenhouse gas emissions. According to the Global Cement and Concrete Association, cement production accounts for about 5-8% of global greenhouse gas emissions. To reduce the energy intensity of the cement production process, in addition to process automation, proper maintenance, and operation of machinery and equipment, several technologies can be applied (e.g., more efficient grinding technologies or the use of waste heat to generate energy). Additionally, clinker can be replaced with other materials (e.g., calcinated clay), or cement can be partially replaced, for example, with waste from combustion processes (i.e., slag or fly ash).
The University of Cambridge researchers found a strikingly innovative and feasible method of recycling cement while recycling steel, which does not add any significant costs to concrete and steel production. It occurs that the used cement (from crushed concrete waste) is an effective substitute for lime flux used in steel recycling to remove inpurities. Normally, this lime flux ends up as a slag – a waste product from the process, but when replaced with cement, the slag becomes recycled cement that can be used again. More information about the process development and a link to the research paper can be found here.
How to reuse and recycle
Direct reuse of concrete elements retaining their original function: elements that are free of design, technological and functional defects can be reused as part of their original function, as exemplified by the use of existing walls, basement ceiling, elevator shaft, and entrance walkway in the Brunnerstasse 9 project by Brandlhuber & Emde & Burlon in Berlin, or the use of a large slab in a bungalow project in Berlin by Carsten Wiewiorra.
In ENTRA's KA13 project, a Norwegian complete full-scale circular project, reusing approx. 160 m2 of hollow core slabs resulted in an 89% reduction of GHG emissions compared to using new slabs. However, due to the project's pioneering character (i.e., the need to establish procedures for selective demolition, testing, and correctly documenting the properties of the concrete elements, etc.), the costs of this procedure were 5-6 times higher than for using new slabs. However, if their reuse becomes more common and better known by stakeholders, the costs will significantly go down, according to the project team.
Pre-cast concrete elements were standardised in the 60s; therefore, only the elements manufactured after 1969 should be considered for reuse.
Direct reuse of concrete elements in other than its original function: concrete elements can also serve a new function, such as using concrete blocks as garden and sidewalk slabs, as was the case with the Urban Outfitters HQ project by D.I.R.T. Studio in Pennsylvania, USA.
Norwegian standard NS 3682:2022 covers the reuse of hollow core slabs.
In Iceland, direct reuse of concrete is possible, but changing the concrete mix throughout the years in the past has to be considered.
Recycling of crushed concrete: crushed concrete can be used as a source of unhydrated cement, waste aggregate (which can be used for both structural and non-structural elements, such as drainage), and, ultimately, as a base for roads, parking lots or filling pits on a construction site.
In the case of recycled crushed concrete, one of the biggest challenges today is maintaining the required parameters of the final product. Therefore, recycled concrete is often used in road construction or for non-load-bearing elements.
In Vasakronan's KAJ16 project, a Swedish complete full-scale circular project, over 10,000 t of crushed concrete from demolition was reused as an aggregate in the building of a new building.
In Iceland, in the Háteigsvegur 59 project, a concrete mix with a lowered carbon footprint was used, and part of the aggregates used in its production came from secondary sources (crushed concrete slabs). You can read more about experiences with such reuse and lessons learnt here.
Reuse classification of concrete
Glass
Glass has become popular in modern construction due to its aesthetic aspects and transparency, which allows large amounts of natural light to enter a building, improving the comfort of tenants and users.
Environmental impact
The environmental impact of glass production is associated predominantly with raw materials consumption (i.e., quartz sand, sodium carbonate, and calcium oxide) and high-intensity production processes resulting in pollutants emission (e.g., greenhouse gases, nitrogen oxides, sulfur oxides). Additionally, due to their fragility, the transportation and assembly of glass components require more vehicles and special equipment and, thus, more fuel-consuming. Lastly, glass affects energy consumption during the building operation as its relatively low thermal insulation can amplify heat loss and increase energy demand for heating. Glass is, however, a material that can be easily recycled without losing its original properties. Recycling glass can require much less energy than manufacturing it, so the greater the amount of glass recycled, the less energy is used to produce new glass, minimising its environmental impact.
How to reuse and recycle
Steel
Steel is one of the primary materials in the construction industry because of its high strength, durability, relatively low weight, fireproofness, and weather resistance. Its remarkable versatility (strength and ductility) enables its use in various structures and components, from frames and columns to roof trusses and beams.
Environmental impact
The steel production process generates air pollutants, such as, for instance, sulfur dioxide, nitrogen oxides, and particulate matter, waste (e.g., steel slag), and is associated with significant water consumption. Moreover, the process is energy-intensive, and its general energy consumption is estimated at 10 GJ/t of steel produced (exact values depend on the type of steel, its finish, application, etc.).
How to reuse and recycle
Wood
Wood as a building material has many advantages, as it is renewable, durable, resistant to corrosion, has natural insulating properties, and its production and processing take a lower toll on the environment than other prevalent building materials (i.e., concrete or steel). Nowadays, wood is used in the construction of houses (both as a load-bearing and decorative material), as well as in roof structures, window frames, doors, floors, railings, or stairs.
More about the future of Icelandic timber and the possibilities of its reuse and recycling can be found in the recordings from the "Íslenskar timburvörur fyrir byggingar" event in 2023.
Environmental impact
Wood material production usually has a lower environmental impact than conventional materials (i.e., concrete or steel) as its processing is less energy-intensive. Similarly, its use in construction is associated with a lower toll on the environment as it can reduce the building's energy consumption due to its excellent insulating properties preventing heat losses. Nevertheless, emphasis should be placed on sustainable forest management, which minimises the negative impact of the timber industry on biodiversity and forest resources.
How to reuse and recycle
Sources and further reading
1) International Energy Agency, Driving Energy Efficiency in Heavy Industries - Global energy efficiency benchmarking in cement, iron & steel, 2021
2) Salgado F. and Silva F., Recycled aggregates from construction and demolition waste towards an application on structural concrete: A review, Journal of Building Engineering, 52, 2022
3) Chen H.M. et al., Reclaiming structural steels from the end of service life composite structures for reuse - An assessment of the viability of different methods, Developments in the Built Environment, 10, 2022
4) The CIRCON's project website and compendium
5) Yeung J. et al., Understanding the total life cycle cost implications of reusing structural steel, Environment Systems and Decisions 37, 2016
6) Risse M. et al., Eco-efficiency analysis of recycling recovered solid wood from construction into laminated timber products, Science of The Total Environment 661, 2019
7) Kromoser B. et al., Circular economy in wood construction - Additive manufacturing of fully recyclable walls made from renewables: Proof of concept and preliminary data, Construction and Building Materials, 344, 2022
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