The environmental assessment of recycled aggregate concrete (RAC) for structural use and its comparison with natural aggregate concrete (NAC) has been studied in previous research [4
]. These studies evaluated the environmental impact of production of 1 m3
of concrete with NA or RA, respectively. In contrast to these studies, in this study two use cycles of NAC or RAC were evaluated. The main reasons for this consideration were that ordinary concrete is necessary for every RA, and although the recycling process is similar to crushed aggregate production, in addition to recycling, inventory data in the previous studies also included mobile plant transportation to the demolition site and landfilling of recycling waste [4
]. This study responds to the philosophical question of whether the demolition and recycling process is the end of the ordinary concrete life cycle or if it is the beginning of the recycled aggregate life cycle. The results of this consideration are discussed and compared in this chapter.
and Figure 7
show the impact category indicator increase or decrease (in percentages) of Scenarios 2, 3, and 4 over the impact category indicators of Scenario 1. In these figures all category indicators for Scenario 1 are presented as a 100% value, while category indicators of other concrete types were calculated as percentages of increase or decrease compared to Scenario 1. The percentage of recycled materials in concretes is also mentioned in Figure 6
shows that Scenario 1, which is two use cycles of a concrete element with only natural aggregate (NAC + NAC), had the largest indicators for all impact categories, which is shown as 50% for each use cycle. Scenarios 2 and 3, where the natural aggregate is partially replaced by recycled aggregate in the second use cycle from the first use cycle of a concrete element with natural aggregate, had lower indicators for both use cycles. Scenario 4, which is two use cycles of concrete with full replacement of natural aggregate, partial replacement of natural sand, and partial replacement of cement (NAC + RAC C100.F30.P5), had the lowest indicators for all impact categories because it used the lowest amount of cement, which has mostly the highest environmental impact.
The increase in all category indicators of Scenario 1 compared to Scenarios 2, 3, and 4 ranged from 20% to 40%. There are a few reasons for this result: higher natural aggregate consumption, higher landfilling, and for Scenario 4, lower cement content. In Scenario 1, the landfilling was assumed for 2 m3 of concrete and the natural aggregate consumption was a much more polluting process than the recycling of concrete, which was assumed in Scenarios 2, 3, and 4. However, this should be considered as realistic for the Czech Republic, since in this study the real data for the recycling process were collected from recycling centers in the Czech Republic to reproduce the real recycling process in Czechia.
The indicators for all impact categories of Scenario 2 (NAC + RAC 100), Scenario 3 (NAC + RAC C100.F30, and Scenario 4 (NAC + RAC C100.F30.P5) were similar—the difference was below 10%. The indicators for most impact categories (except POCP) of RAC mixtures in the second life cycle were similar compared to indicators of NAC (i.e., a decrease below 2%, which can be considered as negligible). The category indicator POCP showed the higher impact of the second use cycle for Scenarios 2, 3, and 4 in comparison with Scenario 1. Nevertheless, the summary of both use cycles showed lower total impacts in this category indicator. However, using part of the NAC from the first use cycle as a partial replacement of natural resources in the second use cycle could reduce the indicators for all impact categories in a total of two use cycles by nearly 40% in all impact categories. In the case of RAC concretes, it should be noted a clear benefit in terms of waste reduction and minimization of natural mineral resources depletion.
shows the contribution of different phases in the concrete production process to the total impacts, for all concrete types and calculated category indicators. The contribution of the cement production was by far the largest contributor to all of the category indicators and for all concrete types, and varied from 25% to 80%, depending on the category indicator and the concrete type. The largest contribution of cement production was for Scenario 1 (GWP), while the lowest was for Scenario 4 (EP). The main reason for this is the well-known fact that large CO2
emissions are produced during the calcination process, in clinker production, and in the use of fossil fuels [19
The contribution of the coarse natural aggregate (gravel) production phase was smaller for all category indicators and concrete types in comparison with the cement production phase. It varied from 0.6% to 6.1% depending on the category indicator and scenario. The largest contribution of natural aggregate production was for Scenario 1, where only natural aggregate was used for manufacturing of concrete mixtures. In other scenarios, the natural aggregate was partially replaced by the recycled concrete aggregate in the second use cycle. Moreover, more energy was consumed for the natural aggregate production than for the production of recycled concrete aggregates and, hence, category indicators based on emissions were higher in this case. Although the recycling process is similar to the crushed aggregate production process, LCI data for RCA include crushing, separating, and transport machines powered by diesel fuel. The impact of the recycling process was evaluated as the production and combustion of diesel fuel; the impact of the recycled aggregate production was lower in all impact categories in comparison with natural aggregate production (see Table 6
The contribution of the natural sand production phase was also small and varied from 0.1% to 1.7%, depending mostly on the category indicator. It was largest for Scenarios 1 and 2 because natural sand was used in both use cycles.
Landfilling was the second greatest source of impacts. The contribution of the landfill phase ranged from 7% to 30%. It was largest for Scenario 1 because the NAC was fully landfilled after both of the use cycles of the concrete structural element. In Scenarios 2, 3, and 4, part of the NAC from the first life cycle was not landfilled but used as a partial replacement of primary sources in the second life cycle. Due to this fact, fewer waste materials were disposed to landfill. The highest impact (between 20% and 30% for all scenarios) of landfilling was for category indicators ADP fossil and AP compared to the lowest impact, which was for GWP and ranged between 7% and 10%.
The diesel consumption phase was one of the greatest sources of impacts for the ADP fossil category indicator, and it varied between 19% and 23% depending on the scenario. On the other category indicators, the diesel consumption ranged from 1% to 5%. The diesel consumption phase included diesel used for trucks and for the recycling process.
Transport was the third-largest source of impact for the AP, EP, and GWP indicators, ranging from 5% to 23%. The largest transport contribution was for Scenario 1 for all impact categories, as the NAC was assumed to be transported to landfill twice. In Scenarios 2, 3, and 4, on the other hand, only part of the first use cycle was landfilled, reducing the amount of transported materials.
These results show that the cement production phase was by far the largest contributor to the of all assessed category indicators. The contribution of gravel production was below 5%, and the contribution of natural sand production was below 2%. Even if there are some uncertainties regarding the quality of data, it would not affect the results significantly. The recycling process was included in the diesel consumption phase, which was crucial only in the ADP fossil category indicator. In the other category indicators, the diesel consumption was below 5%.
The LCA results for 1 m3
of NAC or RAC showed that the impacts of aggregate production phases were slightly larger for RAC than for NAC in previous case studies [4
]. On the contrary, some studies have presented a lower embodied energy and lower greenhouse emissions of RA compared with natural aggregate. In comparison with NA, the embodied carbon emissions are reduced by around 25% [18
]. The reduction of emissions is also connected with the transportation of aggregate, especially where recycled aggregate is used near the demolition site [14
The present study found that the impacts of aggregate production phases were lower for RAC in comparison with NAC. There are two reasons leading to this fact; the first is the consideration of recycling and transportation phases, and the second is the saving of primary material in the second use cycle. The highest impact in all studies was determined from cement production (which confirmed the well-known fact that cement production is the largest contributor to all category indicators for concrete production, due to its large energy consumption), the use of fossil fuels, and high CO2
emissions during the calcination process in clinker production [19
]. In comparison with the cement production phase, it can be said that the impact of the aggregate production phase is negligible for all impact categories, with no significant differences between all types of aggregate. In contrast to previous studies where two scenarios of aggregate transportation were compared and it was found that transportation had the second-highest impact and could influence the whole LCA of concrete [4
], in this study transportation was considered to be the same for all aggregate types. This consideration is supported by the availability of both aggregate types in the Czech Republic.
In this case study, the higher beam height was designed to compensate for the lower strength class of concretes with RCA, which had no significant effect on the compared impact categories. Another way to compensate for the decline of RAC properties is to add cement to RAC mixtures. There are some studies [8
] in which this solution is used. In the compared impact categories (e.g., energy use, climate change (global warming), acidification, respiratory effects, land use, and gravel use) no significant differences were found. For instance, the comparison showed differences in energy consumption and global warming potential between NAC and RAC of up to 3% [10
The results indicate the positive impact of cement replacement by RCP in all impact categories. The RCP was separated during the second phase of the recycling process, where only pure concrete fragments were crushed and sieved to limit the soil content in RCP. In contrast, some studies have been published on different ways to obtain RCP. These procedures mostly separate attached cement mortar from the aggregate surface from waste concrete. Nevertheless, these procedures are not efficient for the reduction of CO2
emissions. One such procedure involves heat treatment and abrasion [65
]. In another method, the cement paste and aggregate can be completely separated by heat treatment between 300 and 500 °C, where the attached cement mortar is separated from aggregate during this process and then separated cement mortar is milled to cement powder [26
]. Finally, another way of separating attached cement mortar from the aggregate surface is by microwave heating [25