The Incorporation of Recycled Aggregate Concrete as a Strategy to Enhance the Circular Performance of Residential Building Structures in Spain

1. Introduction

Over the past two decades, rapid urban population growth has led to an intensification of construction and demolition activity [1]. In order to satisfy this increasing demand, the built environment has relied on concrete as its primary manufactured material due to its affordability, versatility, and high strength [2]. Currently, concrete is the most widely used manufactured material, and is extensively employed in structural and non-structural elements [3]. However, the concrete production process has a significant carbon footprint, since it emits approximately 850 kg of CO₂ per ton of clinker produced and requires substantial natural resources [4].

The current practices of the concrete industry are at odds with the objectives set forth by the European Commission (EC), which aim for a reduction of Greenhouse Gas (GHG) emissions of at least 55% by 2030 and a net zero scenario by 2050 [5]. The European Green Deal [5] and the Circular Economy Action Plan [6] establish practices to minimise landfill and enhance material recovery by emphasising the importance of designing for durability, reparability and recyclability [7]. However, fulfilling these initiatives relies on the effective transition to a Circular Economy (CE) model [8]. This regenerative economic system promotes the closed-loop flow of goods through reuse, thereby producing secondary raw materials or serving other purposes. Such practices prevent further resource extraction, optimise material usage, and foster the regeneration of natural systems [9,10,11]. The CE, therefore, encourages sustainable development by acknowledging the interconnectedness of ecological stewardship, social development and economic growth [12,13,14].

The International Energy Agency (IEA) [15] recommends that the concrete industry improve energy efficiency and incorporate alternative fuels and raw materials to align with the net zero scenarios. As a response, several studies have focused on replacing traditional production fuels with biomass or other types of waste [16], as well as reducing resource extraction by including supplementary cementitious materials [17], developing more durable or self-healing concrete [18,19,20], utilising higher-strength concretes [21,22,23], capturing and storing CO₂ emissions [24,25,26], and incorporating Recycled Aggregates (RA) in concrete production [27,28,29,30].

Recycled Aggregate Concrete (RAC) is an innovative material obtained from crushed, screened, and washed RA derived from discarded concrete [31]. It has significant cost-effective potential for the reduction of GHG emissions since the Natural Aggregate Concrete (NAC) mixture usually consists of approximately 70% to 80% aggregate by volume [32]. The EC is interested in using RAC to optimise the 850 million tonnes of Construction and Demolition Waste (CDW) generated in the European Union (EU) annually [33], thereby contributing to the annual demand for 2.55 billion tonnes of aggregate [34]. However, certain studies [27,35] highlight that the mechanical properties of RAC are approximately 10% to 20% lower than those of NAC. This disparity is attributed to the adhered mortar [36] and micro-cracks formed during the recycling or recovery process [37]. Consequently, the water absorption of RAC is approximately ten times greater than that of NAC, while its bulk density is reduced by about 22% [38]. Furthermore, the compressive, splitting, and flexural properties of RAC are diminished by 9.25%, 18.5%, and 17.6%, respectively [39]. Axial compression, shear resistance, and bond strength are also 6% to 24% lower than that of NAC [40].

International standards have considered these findings and have limited the percentage of RA permitted, particularly in structural applications. These restrictions mainly rely on the expected grade of concrete strength and the class of exposure [41]. Furthermore, the RA selected must satisfy requirements on their type, origin, dry density, water absorption, and percentage of contaminants present [42]. As reported by the European Aggregates Association (UEPG) [34], the production of recycled and reused aggregates in European countries is approximately 9.3%. The countries with the highest percentage of RA included in their total annual production are Belgium (29.3%), the Netherlands (24.7%) and the UK (23.9%) (Figure 1). Meanwhile, Germany leads Europe in the total output of RA, by producing 72 million tonnes per year [34].

The integration of secondary raw materials in these countries is supported by established concrete recycling industries and RA standards focused on reducing the percentage of contaminants and on the water absorption capacity [42,43]. For instance, German standards DIN 4226-100 [44] and DIN 12620 [45] set requirements for the use of RA in structural applications. These standards allow the configuration of up to 90% RA sourced from concrete waste (Type 1) and up to 70% RA from demolition waste (Type 2). These percentages are significantly higher than most European country standards, where the permissible RA percentage is approximately 20% [41]. However, there are stricter limitations regarding the acceptable levels of contaminants. Thus, for the RAC Type 1, the maximum allowable rates of mineral contaminants are 2%, for non-mineral contaminants 0.2%, and 1% for asphalt. Otherwise, these percentages for the RAC Type 2 are 3%, 0.5% and 1%, respectively.

In Spain, the design of structures with RAC is included in the Structural Code [46]. This guideline specifies that using RAC is permitted for both mass and reinforced concrete, with a maximum compressive strength of 40 N/mm². Only coarse RA obtained from intact structural or high-strength concrete is deemed suitable for structural applications. The maximum allowable content is capped at 20% of the total coarse aggregate weight, with water absorption not exceeding 7%. In this case, the water absorption of the Natural Aggregates (NA) in the mixture is limited to less than 4.5%. Exceeding the ratio of RA would necessitate additional studies and complementary experimentation. Moreover, the UNE-EN 12620 [47] stipulates that the maximum permissible proportion for lightweight particles is 2%. At the same time, the limits for asphalt and other materials (such as clay, glass, plastics, and metals) are set at 1% and 0.5%, respectively.

Progress in standard requirements aligns with the initiatives of the EC. Indeed, studies examining the transition toward circularity in EU member states [48,49] reveal a slight improvement over the past two decades. This is evidenced by the Resource Productivity Indicator (RPI) (Figure 2), which demonstrates the increasing efficiency of material use over time. This increase is driven by the growth of Gross Domestic Product (GDP) relative to Domestic Material Consumption (DMC) [50]. Resource Productivity Indicator analysis shows that the pace and intensity of this transition vary across different countries, with the Netherlands, Italy, France, Spain, and Germany at the forefront.

However, according to the third and fourth reports of the CE Spanish Strategy (EEEC: acronym in Spanish) [48,49], the reduction in material usage in Spain is still closely tied to economic fluctuations. Thus, the notable rise in the RIP since 2008 results from reduced building sector participation in value generation. Figure 2 illustrates that this trend has moved upward in recent years; however, absolute decoupling has yet to be achieved. The national CE reports [48,49] emphasise that the current focus of the Spanish productive system is on implementing circularity strategies at the End of Life (EOL). While this approach facilitates circularity, it does not effectively promote a transition to a CE model with less resource-intensive development. Consequently, these reports underscore the necessity of integrating circularity strategies from the initial stages of the production process. This shift could enhance the national Circular Material Use (CMU) rate of 8.5% recorded in 2023 (Figure 3) [51], which remains significantly below the EU target of 23.4% by 2030 [49].

Designing structures with RAC presents an opportunity to integrate circularity strategies from the outset. Nonetheless, resistance to change, risk aversion, and limited understanding of CE principles among stakeholders in the Spanish building sector all hinder its broader implementation [48]. Several studies [52,53,54] state that these barriers could be overcome by conducting case study analyses that demonstrate the feasibility and advantages of adopting CE strategies in building projects. This research is particularly crucial for residential projects in Spain, since new multifamily residential buildings account for approximately 70% of annual building construction [55]. The study of these buildings from the CE standpoint is increasingly gaining momentum and is driven by various initiatives. These include the analysis of key circularity indicators in the Spanish building sector [8] and the development of strategies to integrate the framework of Level(s) within the local context [56].

A comprehensive review of the existing literature reveals substantial progress in the evaluation of the carbon footprint of the Spanish residential stock [57]. Furthermore, various case studies promote the adoption of waste reduction principles and off-site approaches at the design stage [58,59], and their implementation in BIM [60,61]. There is also a marked effort to enhance energy efficiency in the envelopes and systems of multi-family residential buildings [62,63,64], alongside ongoing research to achieve nearly zero-energy conditions for buildings in cold rural Mediterranean zones [65]. Despite these advancements, the literature review reveals that the circularity potential of RAC in Spanish residential building structures remains unexplored. However, a notable case study from Sardinia, Italy [66], provides valuable insights for conducting similar assessments. Likewise, the contributions from the Spanish guide on the use of RA derived from CDW [67] and the structural design criteria involving RAC proposed by Tošic et al. [68] are particularly relevant and provide a foundation for future RAC research and application in Spain.

In order to fill this gap in the literature, this work aims to determine the circularity potential of including RAC in the structure of the most commonly constructed type of newly multi-family residential buildings in Spain. For this purpose, the Spanish residential stock is studied to identify the most representative structural system. Then, the circularity assessment is conducted, involving one of the projects carried out by the public housing development enterprise known as EMVISESA [69]. This analysis examined four scenarios, each of which considered a set of distinct assumptions: (i) Scenario 1 (S1), maintaining material linearity throughout its lifespan; (ii) Scenario 2 (S2), implementing circularity strategies at the EOL; (iii) Scenario 3 (S3), considering circularity strategies for all materials, excluding concrete, throughout the entire life cycle; and (iv) Scenario 4 (S4), incorporating circularity strategies for all materials, including concrete, throughout the entire life cycle.

The study thereby examined the entire landscape of the CE transition by evaluating the current state (S1) and potential variations in the process (S2 to S4). Notably, S2 examines the circular performance of ongoing industry initiatives as outlined in the EEEC reports [48,49]. At the same time, S3 reflects the growing interest among the companies involved, especially in the steel industry, in incorporating a higher percentage of secondary raw materials while ensuring compliance with the required physical and mechanical properties [70]. This condition also applies to concrete in S4, since recycled aggregates (RA) must be regarded from the design phase due to stipulations established in the Structural Code [46].

The circular performance is assessed with the CARES Framework (CARES-F) [71], an innovative circularity assessment method that integrates ISO standards, the framework of Level(s), and Life Cycle Assessment (LCA) criteria into the traditional MCI framework to determine the circular performance of residential building structures. This powerful method presents multiple advantages, such as a lifecycle perspective that considers all stages of the building life cycle, as well as all materials involved, including packaging, formwork, and material losses. It also includes transport impacts, biomaterials, and quantitative Key Performance Indicators for Design for Disassembly (DfD) and Design for Adaptability (DfA).

This paper is structured as follows: Section 1 introduces fundamental concepts, the background and the objective of the study. Section 2 outlines the research methodology. Section 3 presents the characterisation of the most commonly constructed type of building structure. Section 4 exhibits the circular performance of the representative models in each scenario. Section 5 discusses the circularity potential of each strategy and future lines of research. Lastly, Section 6 concludes the study by providing the final observations.

2. Materials and Methods

As shown in Figure 4, the research methodology is divided into two phases.

Phase 1, presented in Section 3, studies the Spanish residential stock through the databases from the National Institute of Statistics (INE: acronym in Spanish) [72] and the Ministry of Transport and Sustainable Mobility (MITMA: acronym in Spanish) [55,73]. This analysis identifies key characteristics of the most constructed type of newly multi-family residential building, such as the number of storeys, the materials used, and the representative structural system. In accordance with this information, the structure of a residential building carried out by a public housing development enterprise known as EMVISESA [69] is considered for the analysis in Section 4.

In Phase 2, materials are quantified for each structural element based on the BIM model of the representative structure. This data is subsequently used for the circularity assessment of the structure, which is performed through the CARES-F [71], the circularity assessment framework designed for residential building structures that have already been described in Section 1. The analysis encompasses the aforementioned four scenarios (Figure 4).

The findings of the circularity assessment at the material, element, and system levels are detailed in Section 4, while discussions and conclusions are presented in Section 5 and Section 6, respectively.

3. Characterisation of the Multi-Family Building Model

The third report of the EEEC [48] points out that, in Spain, the behaviour of the construction industry has changed since 2010, particularly in 2015. This shift is evident in the data from the Population and Housing Census 2021 [72], which shows a progressive increase in the construction of residential buildings from 2015 onwards, particularly in multi-family buildings (Figure 5).

The annual reports on building construction developed by the MITMA [55,73] align with this trend, and reveal that 70.71% of construction projects involved new multi-family units (Figure 6).

This article attempts to evaluate the circular performance of the structure of the most commonly constructed type of new-build multi-family residential buildings in the current context of Spain. In order to identify this type, data from the Population and Housing Census 2021 [72] covering the period from 2015 to 2020 was analysed. The findings indicate that a four-storey building constitutes the most predominant newly constructed multi-family building (Figure 7).

The analysis of the MITMA reports [55,73] reveals that approximately 68% of the vertical structural elements in residential buildings are constructed from reinforced concrete (Figure 8a). Unfortunately, there is no specific documentation outlining the typical compressive strength of this concrete. However, it should be borne in mind that the minimum compressive strength stipulated by the Spanish Structural Code for reinforced concrete is 25 MPa. A previous study of Spanish residential construction [74] highlights that this material is also predominant in floor systems. This finding is further supported by reports on building construction [54,68], which show that 77.63% of the floors are unidirectional (Figure 8b).

Considering these most common characteristics, the structural system was considered of a real multi-family residential building located in Seville, Spain. It was developed by EMVISESA [69], the public housing development company in Seville. The building comprises four above-ground storeys and one below-ground level, and accommodates 16 dwellings, parking spaces, and storage rooms. The structure features a reinforced concrete frame with a gross floor area of 2,314 m² (Figure 9).

4. Circularity Assessment of Each Scenario

This section outlines the circularity performance of the building structure discussed in Section 3 across four different scenarios: (i) S1, maintaining material linearity throughout its lifespan; (ii) S2, implementing circularity strategies at the EOL; (iii) S3, considering circularity strategies for all materials, excluding concrete, throughout the entire life cycle; and (iv) S4, incorporating circularity strategies for all materials, including concrete, throughout the entire life cycle.

The evaluation of circular performance was conducted using the CARES-F [71], a circularity assessment tool focused on the circularity of residential building structures. This evaluation consists of three consecutive levels: material, element, and system, each comprising several sub-indices, as illustrated in Figure 10.

A special feature of this framework is that, for the assessment of circularity, in addition to considering the materials that end up as a product, it also includes other materials such as formwork, packaging, and material losses. Another particular aspect at the material level is the transportation impact assessed through the Distance Index (DI). In this study, only distribution centres and waste management facilities within a 20 km radius of the project were considered for all scenarios. This assumption was based on a fuel optimisation criterion, which aimed to reduce GHG emissions and transportation costs. The analysis used logistics software [75] to identify the optimal routes, and focused on roads that permit heavy-good vehicles. It also considered truck capacity based on the type of material and adhered to specifications from the General Vehicle Regulations [76]. In the specific instance of concrete, the capacity of the truck mixer and the discharge rate were determined based on the Spanish Structural Code [46]. Moreover, speed regulations from the General Traffic Regulations [77] were considered in the estimation of the transportation time for concrete, thereby ensuring that the material did not set during transit.

At the element level, the CARES-F [71] normalises the MCI with the mass representativeness of each material and modifies this sub-index with the disassembly potential (DP) factor. The DP considers disassembled connections, the geometry of the edges of elements, prefabricated elements, and materials that do not require periodical secondary finishes to ensure acceptable structural performance over time. This last aspect primarily influences the circularity of the reinforced concrete elements.

Lastly, the system level is analysed by modifying the normalised sub-index derived from the element level with the adaptability potential (AP) factor. The AP considers the geometric characteristics of the structure, as well as the arrangement and load-bearing capacity of the structural elements. In this study, this data is obtained from the building structure presented in Section 3. The building data set is provided in the supplementary data file.

4.1. Scenario 1: Maintaining Material Linearity Throughout Its Lifespan

This scenario assesses the circularity of the building structure under a linear economic model in which resources are extracted, manufactured, and disposed of. As noted in Section 1, this scenario is predominant, and it is therefore assumed to represent the current situation.

Based on this background, it was assumed that all elements in this scenario have been produced through resource extraction and become waste without being maximised in their use through circularity strategies. The only exceptions are biodegradable materials: these undergo a physical, chemical, thermal, or biological decomposition process, which break them down into carbon dioxide, biomass, and water [78].

Following the hierarchy structure of the CARES-F [71], the material level was analysed first. The circularity assessment at this level is defined by the Material Circularity Index (MCI), which considers the material scenario conditions, their technical and functional lifespans, as well as the impact of their transportation on input and output flows. Since the structure is of reinforced concrete, the materials that become structural elements, as well as those involved, including packaging, formwork, and material losses, are taken into account. Table 1 displays the 12 most representative materials based on their total mass, while Figure 11 shows their respective MCI.

The circularity assessment at the element level is represented by the Element Circularity Index (ECI). This index considers the MCI of the materials that constitute each structural element and their individual DP. The outcomes of this analysis for the reinforced concrete elements that comprise the building structure are depicted in Figure 12.

Lastly, the System Circularity Index (SCI) was determined based on the previously defined ECI, as well as on the AP. This evaluation reflects that the SCI for this scenario is 24.1%.

4.2. Scenario 2: Implementing Circularity Strategies at the EOL

This scenario examines the circularity of the building structure outlined in Section 3, whereby a linear input flow of materials and a circular output flow are assumed at the EOL. This scenario was considered based on the third and fourth EEEC reports [48,49], which reveal that current industry efforts focus on implementing circularity strategies at the EOL.

In this scenario, the output flow considers the reusable, recyclable, and manufacturable percentages of materials, as well as the biologically renewable materials present in each structural element. These fractions were obtained from research, reports from material-specific organisations, and government databases that reflect the situation in Spain. Table 2 presents and organises the consulted sources according to the European List of Waste (LoW) codes [79].

In this scenario, the MCI of the most representative materials based on Table 1 are shown in Figure 13.

At the element level, the circularity assessment considers the MCI of the materials constituting each structural element and their corresponding DP. The results of this procedure are reflected in the ECI presented in Figure 14 below.

The SCI is determined based on the ECI previously defined, as well as the AP. This evaluation reflects that the SCI for this scenario is 26.4%.

4.3. Scenario 3: Considering Circularity Strategies for All Materials, Excluding Concrete, Throughout the Entire Life Cycle

In this scenario, the circularity assessment of the reinforced concrete structure is grounded in the criterion that all materials within the input flow, apart from concrete, incorporate a proportion of secondary raw materials, are reused, or consist of biomaterials. Furthermore, circularity strategies are applied to the output flow of all materials.

This scenario reflects the growing interest among companies, particularly in the steel industry, to include a higher percentage of secondary raw materials in their manufacturing processes while ensuring compliance with the required physical and mechanical properties [70]. For reinforced concrete structures, this initiative influences their circular performance due to the integration of rebars. This evaluation of circular performance therefore adopts a perspective that strives, as far as possible, to implement circularity strategies throughout the entire lifecycle. Concrete is excluded from this assessment, since RA must be considered from the design stage because it is dictated by the stipulations established in the Structural Code [46].

For this assessment, the percentages of secondary raw materials specified in the Guideline for the reuse of recycled materials in construction [67] were applied to components made of steel or alloys within the input flow. Regarding plastics, the references provided in Table 2 and the data presented by Döhler et al. [86] were considered. Moreover, information related to packaging in the input flow and the reusable, recyclable, manufacturable, and renewable fractions at the EOL was derived from the references in Table 2.

Based on this information, the MCIs for the representative materials are depicted in Figure 15.

Based on these results and considering the DP of each element, Figure 16 presents the ECI corresponding to each reinforced concrete structural element.

Thus, in this scenario, the SCI based on the ECI and the AP is 26.9%.

4.4. Scenario 4: Incorporating Circularity Strategies for All Materials, Including Concrete, Throughout the Entire Life Cycle

In Scenario 4, RAC is selected as a structural material alongside the considerations outlined in Scenario 3. This approach accounts for the requirements established in the Spanish Structural Code [46] for RAC, which stipulates a maximum allowable percentage of recycled coarse aggregate at 20% by weight of the total coarse aggregate content. It also establishes a maximum compressive strength of 40 MPa for reinforced concrete with RA and a restriction on the origin of RA: it can only originate from intact structural concrete or high-strength concrete. The physical and mechanical properties of the RA and NA that are contained in the mixture are also defined.

As noted in Section 3, the structure of the building considered has been designed with concrete whose compressive strength is 25 MPa. In this analysis, the concrete is considered to have XC0 exposure, namely no risk of corrosion, and hence, according to the Spanish Structural Code [46], its maximum water-cement ratio is 0.65, and the minimum cement content is 250 kg/m³.

The research conducted by Amario et al. [87] explores the feasibility of employing the Compressible Packing Model (CPM) for the proportion of concrete mixtures produced with RAC. To this end, several structural RAC mixtures with varying strength classes and proportions of RA were designed and experimentally validated through mechanical and durability testing. The Recycled Coarse Aggregate (RCA) used was obtained from the debris of reinforced concrete beams produced and tested in laboratory [88]. Given that the properties of the RA and NA examined conform to the specifications of the Structural Code [46], the concrete mixture C25-0120 (Table 3), which corresponds to concrete with a compressive strength of 25 MPa and 20% of RCA, is considered for the circularity assessment of this scenario.

Based on this background, the MCI of the most representative materials of the building structure are presented in Figure 17. Conversely, Figure 18 depicts the ECI associated with the reinforced concrete elements that constitute the structure.

In light of the preceding results, the SCI of this scenario is 34.9%.

5. Discussions and Future Research

The circularity assessment of the four scenarios yields decisive results concerning the transition to a CE model in the Spanish residential sector. The quantitative implementation of circularity strategies throughout the life cycle underscores the critical importance of the implementation of this approach from the design phase. While this assertion strengthens previous studies [14,61,89], the quantitative analysis through scenarios that explore the implementation of strategies at each life cycle phase, and particularly the assessment of the circularity potential of RAC at both the element and system levels from the design phase, provides insights into RAC, structural design standards, Design for Circularity (DfC), CE policy frameworks, and engagement of structural designers and stakeholders in the shift towards a CE model.

In order to analyse these contributions in detail, this section discusses the findings through two different perspectives: (i) the impact of material circularity strategies at all levels in each scenario and (ii) the implications of each scenario at the macro level.

5.1. Impact of Material Circularity Strategies at All Levels in Each Scenario

The circularity assessment of the scenarios studied in Section 4 demonstrates how the adoption of strategies at the material level influences the circular performance of the entire structure and each of its elements.

The evaluation of this measure through scenarios that represent the progressive application of this measure in the lifecycle of the building thereby reveals that the implementation of circularity strategies in concrete at the EOL (S2 and S3) boosts its circular performance by 8.16% compared to the linear flow in S1 (comparative analysis in Figure 19a). Moreover, the incorporation of RCA at 20% by weight of the total coarse aggregate content (S4) results in a 42.82% increase in the circularity of concrete (Figure 19a). A similar trend is observed with steel rebars (Figure 19b), where the introduction of circularity strategies at the EOL (S2) enhances circularity by 31.32% relative to a linear flow (S1). In contrast, the adoption of a circularity perspective from the design phase (S3 and S4) leads to an even more significant increase in circular performance at the material level, with an improvement of 38.27% compared to S1.

However, the analysis of circularity at both the element and system levels reveals that since the circularity assessment scale exceeds the material level, the effectiveness of the RAC is reduced (Figure 20). This decline occurs since the disassembly potential affects the circularity of the elements, whereas adaptability potential is decisive for changes over time at the system level.

The findings demonstrate that replacing traditional materials with RAC enhances the circular performance of the structure by 10.81% (Figure 20b), even in residential buildings that exhibit limited potential for disassembly and adaptability. While this highlights the efficacy of RAC in promoting circularity, the circular performance score of 34.9% in S4 (Section 4.4) emphasises the need to incorporate DfD and DfA criteria to enhance circular performance at both the element and system levels. Notable measures to achieve this include using prefabricated elements with removable connections [90], off-site construction methods [91], and the arrangement of elements to facilitate the adaptability of structure over time [92,93]. Furthermore, circular performance can be further improved at the material level by integrating supplementary cementitious materials, implementing biomineralisation techniques, utilising carbon capture technologies, and employing 3D printing methods [94]. Nevertheless, successful implementation of these initiatives requires supporting standards that build confidence among stakeholders. Several existing standards, such as the Structural Code, only tangentially reference a few of these strategies. Therefore, additional research is essential to broaden their application for structural purposes.

5.2. Implications of Each Scenario at the Macro Level

The circularity assessment through these scenarios provides valuable insights into the macro-level or national scale, particularly since the construction sector accounts for 6% of the Spanish GDP, of which new-build residential construction represents 19% of its activities [95]. Each scenario illustrates a specific stage in the transition to a circular model, founded on the premise of implementing circularity strategies at the material level. In this study, the most circular scenario is that of S4, which incorporates RAC from the design stage.

Thus, the comparative analysis between S1, which represents the current linear model based on extraction, use, and disposal, and S2, which illustrates the approach adopted by certain economic sectors to enhance circularity through strategies implemented at the EOL, reveals a 2.31% increase in the circularity performance of the structure (Figure 20b), with the SCI rising from 24.1% in S1 to 26.4% in S2 (Figure 21).

This modest enhancement in circular performance stems from the limited possibilities for resource reduction of S2, particularly concerning the optimisation of materials, the reuse of elements for similar or different purposes, and the maximisation of their usage as a structural system over time. At the macro scale, this scenario reveals restricted opportunities to achieve the absolute decoupling between GDP and DMC (Figure 2) within the construction sector, since it does not entail the reduction and maximised utilisation of resources at the highest level, but inherently relies on the extraction of raw materials. This partial perspective on circularity also influences the CMU rate (Figure 3), which is diminished due to the adoption of less effective strategies and the loss of perspective in the recovery and integration of secondary raw materials in total consumption. This trend is mirrored in the UEPG report [34], which indicates that the RA production rate in Spain stands at approximately 2.4% (Figure 1).

In order to increase circularity in the manufacturing process and maximise the use of resources, an increasing number of industries that form part of the construction sector are seeking to increase the circularity of materials through the use of a fraction of raw materials. Scenario 3 represents this perspective, leaving aside the cement industry since the inclusion of RA implies its consideration from the design phase. The evaluation of this scenario shows an increase in the circular performance of the structure of 2.81% (Figure 20b) concerning S1 by increasing the SCI from 24.09% to 26.9% (Figure 21). However, its increase concerning S2 is reduced to 0.5% by increasing its SCI from 26.4% in S2 to 26.9 in S3 (Figure 21). Although low, this improvement is not negligible given that the mass participation ratio of materials that include circularity strategies in this scenario represents only 6.67% of the total building mass (Table 1). The evaluation of this scenario on a larger scale demonstrates the very limited possibilities of increasing the circularity of the Spanish construction sector when, with representative reinforced concrete structures (Section 3), the adoption of strategies in concrete is not actively promoted.

The evaluation of the findings in S3 promotes the assessment of S4, in which the use of RA in concrete is considered. Based on its results, the analysis shows an increase in circularity concerning S1 of 10.81% (Figure 20b), 8.0% more than S3 and 8.5% more than S2 (Figure 21). Although this increase is significant, circular performance remains low, with an SCI of 34.9%. These results reflect the need for design standards whose limitations focus mainly on reducing the percentage of contaminants admissible for RA so that the participation of secondary raw materials is greater.

6. Conclusions

The discussion leads to the conclusion that the implementation of strategies at the material level is fundamental in the transition towards a CE model. This is particularly important for concrete, whose production process is resource intensive. While the adoption of RAC as a material-level strategy is effective, it is not enough for structures to meet the EC objectives. Therefore, it is essential to incorporate additional strategies at the material level, such as supplementary cementitious materials, as well as innovative techniques that increase the durability of concrete and reduce GHG emissions during production. Moreover, the integration of DfD and DfA criteria in the design of structures is crucial. Without this integration, efforts made at the material level may be compromised at the element and system levels due to the limited possibilities of reusing elements at the highest possible value and adapting structures to potential future changes in use.

The full effectiveness of these initiatives can only be achieved with the accompaniment of supportive policy frameworks that foster the production of sustainable materials and set clear quantitative targets for each economic sector. Furthermore, it is necessary that design standards promote the adoption of circular design principles at all levels, encouraging designers to integrate these criteria from the early stages of their projects while research delves into the design of structures with innovative materials. The collaborative involvement of these actors would build trust among stakeholders, thereby reducing reliance on raw material extraction and CDW while maximising resource reuse and optimising materials.

In the Spanish construction sector, the effectiveness of these measures would enable, in the long term, the absolute decoupling between GDP and DMC to be achieved, as well as an increase in the rate of CMU.