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
Comments