Arlind Dervishaj, KTH

Concrete is used everywhere—in buildings, cities, and infrastructures. However, due to the large quantities of concrete used worldwide, it contributes to around 8% of global CO2 emissions ​[1]​. While efforts are being made to reduce its carbon footprint, such as by using supplementary cementitious materials, an often overlooked solution is reusing concrete.

The ReCreate project aims to foster a circular economy in the construction industry by reusing precast concrete elements from existing buildings in new construction projects. To support this goal, our study investigated the reuse potential of structural concrete elements, evaluating three key factors: the remaining lifespan of concrete, natural carbonation (ability to reabsorb CO2 over time), and embodied carbon savings achieved by reusing it ​[2]​. Reusing concrete has multiple benefits as it prevents waste, reduces the need for new raw materials, and significantly lowers life cycle CO₂ emissions. However, it is not as straightforward as it looks. The structural integrity of concrete with reinforcing steel can be compromised over the lifetime of buildings, if the right conditions for corrosion emerge, such as from the carbonation of the concrete cover and the presence of moisture at the rebar interface ​[3]​.

Circular Construction concept for concrete

Based on established carbonation models, we proposed a digital approach for estimating the remaining service life of concrete elements. The digital workflow also estimates the CO2 uptake from natural carbonation. We tested the workflow on an apartment building with a precast concrete structure, built in Sweden in 1967 during the Million Program. The building was modelled digitally, and material quantities and exposed surface areas of concrete elements were automatically extracted.

Digital workflow and building model

A key aspect of the study was the comparison of carbonation rates specified in the European standard EN 16757:2022 with rates derived from measurements in the ReCreate project and the literature ​[4,5]​. This comparison revealed that the carbonation rates in EN 16757 may be overly conservative and hinder the reuse of concrete elements. We argue that relying on contextual carbonation rates, such as the ones in our evaluation, from a previous condition assessment, and new on-site measurements, is crucial for making informed decisions about concrete reuse. The study also addresses the recent RILEM recommendation on revising carbonation rates in standards like EN 16757 and CEN/TR 17310:2019 ​[6]​. 

Using carbonation rates from EN 16757:2022, led to the conclusion that most of the precast elements would not be reusable (i.e. carbonated concrete cover and past the initiation phase for service life). The standard assumes a high rate of carbonation for concrete, especially indoors, which reduced the concrete’s remaining service life; concrete cover for indoor elements was expected to carbonate the earliest, 23 years after initial construction. However, when using the contextual carbonation rates derived from the ReCreate project’s investigation and recent literature, all elements were deemed suitable for reuse, with sufficient remaining lifespan. Plaster and other coverings slowed carbonation significantly, extending the service life of concrete. Additionally, carbonated concrete elements can be reused, but further considerations should be made concerning the environment and exposure conditions in the new building. Recommendations from ongoing research in ReCreate are expected for concrete reuse in new buildings.

The study also assessed the CO2 uptake of concrete over its life cycle, including the first service life, a potential storage period prior to reuse, and a second service life when reusing precast elements. The findings indicate that the CO2 uptake estimated using the EN 16757 rates was significantly higher than the estimate based on contextual rates. Additionally, the study demonstrated that the climate benefits of reuse exceeded those of carbonation, which accounted for less than 6% compared to the emissions associated with the production and construction of new precast concrete buildings. This highlights the importance of prioritizing reuse as a key strategy for reducing the climate impact of buildings.

Furthermore, the study investigated the implications of three different allocation methods for assessing the embodied carbon of concrete over two life cycles. The analysis included scenarios with and without carbonation uptake. The results indicated that the Cut-Off method was the most advantageous for reusing the existing building stock, followed by the Distributed approach, while the End-of-Life approach was the least favorable. The study emphasizes that the reuse of existing building stock offers a substantial opportunity for mitigating climate change and fostering a circular built environment.

Comparison of three LCA allocations, over two life cycles

References

​​[1] Monteiro PJM, Miller SA, Horvath A. Towards sustainable concrete. Nat Mater 2017;16:698–9. https://doi.org/10.1038/nmat4930. 

​[2] Dervishaj A, Malmqvist T, Silfwerbrand J, Gudmundsson K. A digital workflow for assessing lifespan, carbonation, and embodied carbon of reusing concrete in buildings. Journal of Building Engineering 2024;96:110536. https://doi.org/10.1016/j.jobe.2024.110536. 

​[3] Angst U, Moro F, Geiker M, Kessler S, Beushausen H, Andrade C, et al. Corrosion of steel in carbonated concrete: mechanisms, practical experience, and research priorities – a critical review by RILEM TC 281-CCC. RILEM Technical Letters 2020;5:85–100. https://doi.org/10.21809/rilemtechlett.2020.127. 

​[4] European Committee for Standardization (CEN). Sustainability of construction works – Environmental product declarations – Product Category Rules for concrete and concrete elements (EN 16757:2022) 2022. https://www.sis.se/en/produkter/construction-materials-and-building/construction-materials/concrete-and-concrete-products/ss-en-167572022/ (accessed November 26, 2023). 

​[5] European Committee for Standardization (CEN). Carbonation and CO2 uptake in concrete (CEN/TR 17310:2019) 2019. https://www.sis.se/en/produkter/construction-materials-and-building/construction-materials/concrete-and-concrete-products/sis-centr-173102019/ (accessed September 26, 2022). 

​[6] Bernal SA, Dhandapani Y, Elakneswaran Y, Gluth GJG, Gruyaert E, Juenger MCG, et al. Report of RILEM TC 281-CCC: A critical review of the standardised testing methods to determine carbonation resistance of concrete. Mater Struct 2024;57:173. https://doi.org/10.1617/s11527-024-02424-9.