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Environmental impact of concrete with recycled aggregates an evaluation through 4 case studies
Jeroen Vrijders1 and Lisa Wastiels2 1,2
Belgian Building Research Institute (BBRI), Laboratory for Sustainable Development, Lombardstraat 42, 1000 Brussels, Belgium; +3226557711; E-Mail: jvr@bbri.be Abstract
The use of recycled aggregates in new structural concrete is generally considered as a measure for lowering the environmental impact of concrete. Existing literature considers transport distances and the final performance of the concrete (linked to its composition) as main influencing factors to evaluate the environmental burdens or gains when using recycled aggregates. However, these studies are generally based on generic data and/or on theoretical assumptions (e.g. higher cement content). Between 2013 and 2015, 10 pilot construction
projects using concrete with recycled aggregates were executed in Belgium. On the one hand, these cases provided insights in the equivalency in technical performance of concrete with and without recycled aggregates. On the other hand, the collection of actual data on transportation distances and modes, allowed to refine the LCA model based on specific data. This paper presents the results of 4 case studies, where the environmental impact of using recycled aggregates versus natural aggregates is evaluated by use of LCA calculations according to the European standards EN 15804 and EN 15978. Throughout the cases, different aspects related to the recycling of aggregates are evaluated by consideration of multiple scenarios and comparisons (different replacement ratios, use of concrete aggregates & slags, impact of actual transport distances versus general data). In addition several methodological choices, such as the definition of the end-of-waste-point and the choice of impact indicators and weighting rules, are being discussed.
Keywords: recycled aggregate, concrete, pilot project, LCA, environmental impact. Introduction
The use of recycled aggregates in new structural concrete is generally considered as a measure for lowering the environmental impact of concrete. Existing literature (see references) considers transport distances and the final performance of the concrete, linked to its composition as main influencing factors to evaluate the environmental burdens or gains when using recycled aggregates. However, these studies are generally based on generic data and/or on theoretical assumptions (e.g. higher cement content).
Between 2013 and 2015, 10 pilot projects using concrete with recycled aggregates were executed in Belgium. On the one hand, these cases provided insights in the equivalency in technical performance of concrete with and without recycled aggregates. On the other hand, the collection of actual data on transportation distances and modes, allowed to refine the LCA model based on specific data. 4 case studies were selected to perform the LCA analysis, each with a slightly different approach, in order to study different aspects: different replacement ratios (0-30-50-100%), use of concrete aggregates & metal slags, impact of actual transport distances.
255 Table 1. Overview cases
Case 1 Cycling Path Case 2 Industrial Floor Case 3 Industrial Floor Case 4 Industrial Floor
Methodology & Choices
The main focus of the LCA was to evaluate the environmental impact of using recycled aggregates versus natural aggregates in concrete. The LCA calculations are executed according to the European standards EN 15804 and EN 15978. In order to obtain comparable results, ReCiPe Endpoint (H) v1.12/Europe ReCiPe H/A was used to aggregate and weigh the impacts to a single score (Points). Several methodological choices had to be made.
The first one consisted in the use of specific data versus generic data. From literature, it became clear that 2 aspects can play an important role:
- The actual composition of the concrete, and more specifically the cement content. In order to be able to compare different concrete compositions (with a different level of recycled aggregate) their performance should be equivalent (same strength, same durability). Therefore, the actual compositions of the case studies are used, and their performance measured, in order to compare the results.
- The transport distances of the aggregates. One of the advantages of recycled aggregates is that they are available locally, and thus normally don’t require additional transport. In order to study this these, the actual transport distances and transport modes for the 4 cases were used.
Table 2. Transport (Case 1)
Material Single distance (+ mode)
Concrete (unreinforced) 12 km – concrete mixing truck
Cement (400 kg CEM III/A) 100 km - road truck
Limestone aggregate 100 km – road truck
Sea sand (50%) 150 km - boat
36 km - road truck
River sand (50%) 74 km – road truck
Recycled concrete aggregate 0 km – produced locally - For the other data (impact of cement, of crushing concrete …) existing databases
were used.
A second aspect considers the allocation rules, and more specifically the end-of-waste point. This point determines which impacts should be taken into account in the studied system, and which impacts are to be attributed to other processes and systems. Usually, recycled aggregates for use in concrete are crushed twice, using the fraction 20/40 of the first crushing, in order to obtain high quality aggregate. Several interpretations are possible in order to allocate the environmental impact of the secondary crusher to the recycled aggregate or to the concrete debris, depending on the ‘end-of-waste’ point:
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- Option 1: The 20/40 fraction is already considered as a ‘product’, since it has a market
value. This means the secondary crushing process & sieving is part of the production process of the concrete aggregate. There is a specific environmental impact that is allocated based on economic or weight balance.
- Option 2A: The fraction 20/40 after the first crushing is considered as an ‘intermediate’
and there is a market demand (as resource). However, it needs a second crushing process in order to become a useable ‘product’. The environmental impact is attributed to the recycled concrete aggregate, as is the case in Option 1.
- Option 2B: the fraction 20/40 is considered ‘useless’ as such, and there is no market
demand, so it’s a ‘rest fraction’. The secondary crushing & sieving is considered to be necessary to make a product, and thus, the environmental impact of the process is attributed to the original concrete. The environmental impact of the production stage is zero in this case.
- Option 3: There is only one crushing process to make 0/8 and 8/20. The environmental
impact of the recycled aggregates produced is 0, since the crusher step is attributed to the old concrete.
In general, Option 2B is used for the rest of the results. An evaluation has been done on the sensitivity of this choice.
Result case 1 – Cyclin graph
0%, 50% and 100% of the coarse aggregate were replaced by recycled concrete aggregates in a concrete containing 400 kg CEM III/A. The recycled aggregates are produced directly next to the concrete plant, using a 2-stage crushing process. The transport distances are listed in Table 2. The 3 concrete types obtained a very similar compression strength & met the durability criteria.
For 1 m³ of concrete, the results are shown in the Figure 1 below, using the CEN indicators. The figure clearly shows the beneficial influence of the use of recycled aggregates, although the gain is limited. The most gain (-18%) is visible for “abiotic depletion (ADP)” because less primary aggregates have to be used.
Figure 1. Environmental impact of concrete with 0%, 50% and 100% replacement of coarse aggregate – CEN Indicators
Figure 2. Environmental impact of concrete with 0%, 50% and 100% replacement of coarse aggregate – ReCiPe
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When weighing and aggregating the results using the ReCiPe method, it is also visible that recycled aggregates diminish the environmental impact of the concrete. However, the impact of the cement used stays predominant (dotted and striped areas in Figure 2).
Results other case studies
The results of the 3 other cases were similar: recycled aggregates cause a lower environmental impact, but other factors are dominant in the total environmental impact. Additionally, the following aspects were studied and conclusions drawn.
- The use of reinforcement (2 nets in a 10 000 m² floor) is responsible for 30% of the environmental impact of the concrete floor, thus the concrete accounts for 70%. This effect diminishes the influence of the use of recycled aggregates.
- The allocation of the secondary crusher process (end-of-waste point) to the recycled aggregate or the original concrete debris in the end has very limited impact on the results on concrete level.
- The impact of the use of metal slag aggregates was dependent on the transport distances. It was considered that the production impact of the slags was 0, since they are a rest flux of the metallurgy industry. However, they need to be transported (by road truck), and depending on the distance, the impact is not always positive in 1 m³ of concrete.
Conclusion
LCA is a powerful tool to assess the environmental impact of construction products. Although the method is maturing, still some barriers exist: the existence of accurate data and aspects that are open to interpretation (e.g. allocation of crushing process). LCA has its limits - not all environmental aspects are covered, especially local effects (dust, local depletion of resources …) - but in general some conclusions on the environmental impact of concrete with recycled aggregates can be drawn.
Given the largest impacts are caused by cement (> 1/3) and reinforcement (≈ 1/3), this limits the positive impact recycled aggregates can have: a higher use (50 – 100%) of recycled aggregates causes a lower environmental impact, but the maximum profit is about 10%. Transport of aggregates (primary & secondary) are an important factor.
Acknowledgement
BBRI thanks VLAIO (Vlaams Agentschap voor Innovatie & Ondernemen) who supported the NIB-project Stortklaar Beton voor de Toekomst, the Federal Governmental Service ‘Economie’, who supports the Normen-Antenne Beton-Mortel-Granulaten and the recycling companies who collected the detailed product and transport information (De Brabandere, OBBC, Jacobs Beton, AC Materials).
References
- Braunschweig, A., Kytzia, S., & Bischof, S. (2011). Recycled concrete: Environmentally beneficial over virgin concrete? LCM 2011 - Towards Life Cycle Sustainability Management. Berlin.
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- Knoeri, C., Sanyé-Mengual, E., & Althaus, H.-J. (2013). Comparative LCA of recycled and conventional concrete for structural applications. International Journal of Life cycle
Assessment, 909-918.
