The incorporation of recycled aggregate concrete as a strategy to enhance the circular performance of residential building structures in Spain

Academic Editor: Paulo Santos Received: 31 January 2025 Revised: 13 March 2025 Accepted: 13 March 2025 Published: 17 March 2025 Citation: Vásquez-Cabrera, A.; Montes, M.V.; Llatas, C. The Incorporation of Recycled Aggregate Concrete as a Strategy to Enhance the Circular Performance of Residential Building Structures in Spain. Appl. Sci. 2025,15, 3265. https://doi.org/ 10.3390/app15063265 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/lice nses/by/4.0/). Article The Incorporation of Recycled Aggregate Concrete as a Strategy to Enhance the Circular Performance of Residential Building Structures in Spain Alicia Vásquez-Cabrera 1, Maria Victoria Montes 2and Carmen Llatas 1,* 1Departamento de Construcciones Arquitectónicas I, Universidad de Sevilla, 41012 Sevilla, Spain; [email protected] 2Departamento de Construcciones Arquitectónicas II, Universidad de Sevilla, 41012 Sevilla, Spain; [email protected] *Correspondence: [email protected] Abstract: The construction industry increasingly relies on concrete to meet growing urban population demands. However, concrete has a high carbon footprint, which contradicts the Sustainable Development Goals and the Circular Economy policies promoted by the European Commission. The use of Recycled Aggregate Concrete (RAC) is a cost-effective circularity strategy to mitigate environmental impacts. Several countries have integrated RAC into their standards and have achieved promising circularity results. Spain is committed to enhancing resource productivity and using circular materials through practices established during the design phase. Although the residential sector plays a significant role within the construction industry, the potential for circularity of RAC in such residential building structures remains unexplored. The present study aimed to fill this gap by assessing the circularity of four scenarios in a multi-family building using a circularity assessment method for residential building structures: the CARES Framework. The results revealed that RAC, following the Structural Code requirements, can enhance the circularity performance at the material level by up to 42.82%, at the element level by 21.68%, and at the system level by 10.81%. These results demonstrated that circularity declines as the assessment levels increase, which underscores the essential integration of circular materials with adaptability and disassembly criteria. Keywords: recycled aggregate concrete; recycled aggregate; circular economy; sustainable buildings; green buildings; green concrete; construction and demolition waste; structural performance; reinforced concrete 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 CO 2 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 Appl. Sci. 2025,15, 3265 https://doi.org/10.3390/app15063265 Appl. Sci. 2025,15, 3265 2 of 27 Deal [ 5 ] and the Circular Economy Action Plan [ 6 ] established 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 – 11 ]. The CE, therefore, encourages sustainable development by acknowledging the interconnectedness of ecological stewardship, social development, and economic growth [12–14]. 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 ]. Additionally, research has focused on enhancing the efficiency of Construction and Demolition Waste (CDW) treatment and reuse, such as through the combined use of dark fermentation in Moving Bed Biofilm Reactors and the bioleaching of concrete debris [ 17 ]. Other strategies aimed to reduce resource extraction by including supplementary cementitious materials [ 18 ], developing more durable or self-healing concrete [ 19 – 21 ], utilising higher-strength concretes [ 22 – 24 ], capturing and storing CO 2 emissions [ 25 – 27 ], and incorporating Recycled Aggregates (RA) in concrete production [28–31]. Recycled Aggregate Concrete (RAC) is an innovative material obtained from crushed, screened, and washed RA derived from discarded concrete [ 32 ]. It has significant costeffective potential for the reduction of GHG emissions since the Natural Aggregate Concrete (NAC) mixture usually consists of approximately 70% to 80% aggregate by volume [ 33 ]. The EC is interested in using RAC to optimise the 850 million tonnes of CDW generated in the European Union (EU) annually [ 34 ], thereby contributing to the annual demand for 2.55 billion tonnes of aggregate [ 35 ]. However, certain studies [ 28 , 36 ] 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 [ 37 ] and micro-cracks formed during the recycling or recovery process [ 38 ]. Consequently, the water absorption of RAC is approximately ten times greater than that of NAC, while its bulk density is reduced by about 22% [ 39 ]. Furthermore, the compressive, splitting, and flexural properties of RAC are diminished by 9.25%, 18.5%, and 17.6%, respectively [ 40 ]. Axial compression, shear resistance, and bond strength are also 6% to 24% lower than that of NAC [41]. International standards have considered these findings and have established requirements for RA, particularly in structural applications [ 42 ]. These criteria include maximum percentages of RA content, as well as stipulations on their type, origin, dry density, water absorption, and acceptable levels of contaminants [ 43 ]. Furthermore, there are limitations concerning the compressive strength of concrete and its class of exposure [44]. In Spain, the Structural Code [ 45 ] permits the use of RAC in both mass and reinforced concrete, provided that the maximum compressive strength does not exceed 40 N/mm 2 . Additionally, it caps the RA content at 20% of the total coarse aggregate weight, highlighting that any surpassing of this threshold necessitates further studies and supplementary experimentation. The Structural Code also states that RA must be sourced solely from sound or high-strength structural concrete, limiting the water absorption of the RA fraction to 7% and the complementary NA fraction to 4.5%. Moreover, the UNE-EN 12620 [ 46 ] stipulates a maximum permissible proportion of 2% for lightweight particles, along with restrictions for asphalt and other materials (such as clay, glass, plastics, and metals) set at 1% and 0.5%, respectively. Appl. Sci. 2025,15, 3265 3 of 27 Other countries have also incorporated the use of RA in structural concrete production [ 47 ]. Each national standard is tailored to the characteristics of its construction sector and aligns with local CE roadmaps. In the EU, member states develop their sustainable policies in accordance with the CE framework established by the EC [ 6 ]. Consequently, several regional standards share similar requirements regarding RA use. Notably, with the exception of Denmark, most EU member states restrict RA utilisation to the coarse fraction [ 44 ]. At a broader European level, the percentage of RA content is a key regulatory criterion. Standards such as LNEC E471 [ 48 ] in Portugal, NEN 5905 [ 49 ] in the Netherlands, the Structural Code [ 45 ] in Spain, and BS 8500-2 [ 50 ] in the United Kingdom limit the RA content to 20%. However, the maximum allowable concrete strength varies across these standards. While LNEC E471, the Structural Code, and BS 8500-2 cap structural concrete production at 40 MPa, NEN 5905 permits a higher threshold of 55 MPa. Conversely, there are standards with specifications that surpass these common criteria [ 44 ]. For instance, the Guideline of the German Committee for Reinforced Concrete (DAfStb) [ 51 ] stipulates that only RA classified as Type 1 (concrete waste) or Type 2 (demolition waste) can be used in the production of structural concrete. According to this guideline, for Type I, the maximum RA content is 35% of the total aggregate volume, and the concrete strength is limited to less than 25 MPa. For Type II, the RA content is capped at 25% (by volume), with permissible concrete strength of up to 35 MPa. Meanwhile, there are stricter limitations regarding the acceptable levels of contaminants. Thus, for RA classified as Type I, the maximum allowable rates of mineral contaminants are set at 2%, 0.2% for non-mineral contaminants, and 1% for asphalt. Otherwise, the percentages permitted for RA classified as Type II are 3% for mineral contamination, 0.5% for non-mineral, and 1% for asphalt. Several studies have examined the impact of RA content on both the structural performance and manufacturing process of concrete [ 52 ]. These studies suggest that RA can be incorporated regardless of its fraction or substitution percentage, provided that the required concrete quality is maintained [ 42 ]. Etxeberría et al. [ 53 ] reported that Recycled Coarse Aggregate (RCA) can be used in concrete mixtures with strengths ranging from 20 to 45 MPa, despite potential strength variations of up to 25%. Limiting concrete strength in this context is crucial, as excessive cement content could undermine the economic and environmental benefits of using RAC [42]. These findings align with the criteria established in European standards. However, several studies [ 47 , 54 ] highlighted the need to supplement these results with specific clauses in structural design codes to enhance the understanding of the structural performance of RAC elements. In this context, McNeil and Kang [ 55 ] argued that, despite variations in behaviour, real structures incorporating RAC comply with the requirements set forth in structural standards. This suggests that recycled aggregates can be used in structural concrete, even if their quality is slightly lower. Nevertheless, integrating these advancements into structural design codes is crucial, as it would mitigate uncertainties associated with their application and promote their adoption in a sector where designers and concrete producers strictly adhere to established guidelines [47]. The impact of standards on the transition towards a CE in the construction sector was highlighted in a report by the European Aggregates Association (UEPG) [ 35 ]. The reported results revealed that the production of recycled and reused aggregates in European countries is approximately 9.3%. Among them, Belgium (29.3%), the Netherlands (24.7%), and the United Kingdom (23.9%) (Figure 1) stand out as having the highest percentages of RA in their total annual production. Meanwhile, Germany leads Europe in the total output of RA, by producing 72 million tonnes per year [ 35 ]. The increased integration of secondary raw materials in these countries coincides with their standards, allowing a Appl. Sci. 2025,15, 3265 4 of 27 higher RA content or the inclusion of RA in concretes with a higher strength than most standards. These developments are supported by established concrete-recycling industries that ensure compliance with the percentage of contaminants and the water absorption capacity indicated in the standards [43,56]. Appl. Sci. 2025, 15, x FOR PEER REVIEW 4 of 28 highest percentages of RA in their total annual production. Meanwhile, Germany leads Europe in the total output of RA, by producing 72 million tonnes per year [35]. The increased integration of secondary raw materials in these countries coincides with their standards, allowing a higher RA content or the inclusion of RA in concretes with a higher strength than most standards. These developments are supported by established concreterecycling industries that ensure compliance with the percentage of contaminants and the water absorption capacity indicated in the standards [43,56]. Figure 1. Percentage of RA production in European countries (Authors’ own based on [35]). In turn, progress in the construction sector’s transition to a CE entails a macro-level impact, since it is classified as a strategic sector in the Circular Economy Action Plan [6]. Indeed, studies examining the transition toward circularity in EU member states [57,58] 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) [59]. The 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. Figure 2. Resource Productivity Indicator (RPI) (2000–2023) (Authors’ own based on [59]). However, according to the third and fourth reports of the CE Spanish Strategy (EEEC: acronym in Spanish) [57,58], the reduction in material usage in Spain is still closely Figure 1. Percentage of RA production in European countries (Authors’ own based on [35]). In turn, progress in the construction sector’s transition to a CE entails a macro-level impact, since it is classified as a strategic sector in the Circular Economy Action Plan [ 6 ]. Indeed, studies examining the transition toward circularity in EU member states [ 57 , 58 ] 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) [ 59 ]. The 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. Appl. Sci. 2025, 15, x FOR PEER REVIEW 4 of 28 highest percentages of RA in their total annual production. Meanwhile, Germany leads Europe in the total output of RA, by producing 72 million tonnes per year [35]. The increased integration of secondary raw materials in these countries coincides with their standards, allowing a higher RA content or the inclusion of RA in concretes with a higher strength than most standards. These developments are supported by established concreterecycling industries that ensure compliance with the percentage of contaminants and the water absorption capacity indicated in the standards [43,56]. Figure 1. Percentage of RA production in European countries (Authors’ own based on [35]). In turn, progress in the construction sector’s transition to a CE entails a macro-level impact, since it is classified as a strategic sector in the Circular Economy Action Plan [6]. Indeed, studies examining the transition toward circularity in EU member states [57,58] 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) [59]. The 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. Figure 2. Resource Productivity Indicator (RPI) (2000–2023) (Authors’ own based on [59]). However, according to the third and fourth reports of the CE Spanish Strategy (EEEC: acronym in Spanish) [57,58], the reduction in material usage in Spain is still closely Figure 2. Resource Productivity Indicator (RPI) (2000–2023) (Authors’ own based on [59]). However, according to the third and fourth reports of the CE Spanish Strategy (EEEC: acronym in Spanish) [ 57 , 58 ], the reduction in material usage in Spain is still closely tied to economic fluctuations. Thus, the notable rise in the RPI since 2008 resulted from reduced building sector participation in value generation. Figure 2illustrates that this trend has moved upward in recent years; however, absolute decoupling has yet to be achieved. The national CE reports [ 57 , 58 ] emphasise that the current focus of the Spanish productive Appl. Sci. 2025,15, 3265 5 of 27 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 Rate (CMR), of 8.5%, as recorded in 2023 (Figure 3) [ 60 ]. This percentage, defined as the portion of materials recovered and reintegrated into the economy relative to overall material use, remains significantly below the EU target of 23.4% by 2030 [58]. Appl. Sci. 2025, 15, x FOR PEER REVIEW 5 of 28 tied to economic fluctuations. Thus, the notable rise in the RPI since 2008 resulted 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 [57,58] 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 Rate (CMR), of 8.5%, as recorded in 2023 (Figure 3) [60]. This percentage, defined as the portion of materials recovered and reintegrated into the economy relative to overall material use, remains significantly below the EU target of 23.4% by 2030 [58]. Figure 3. Circular Material Use Rate (CMR) (2019–2023) (Authors’ own based on [60]). In response to this need, the present study aimed to assess the circularity potential of RAC in the structure of newly built multi-family residential buildings in Spain. For this purpose, the Spanish residential building stock was analysed to identify the most representative structural system. A real, representative structure was then selected from one of the projects carried out by EMVISESA, the municipal housing company in Seville [61]. Subsequently, the circularity assessment was conducted using the CARES Framework (CARES-F) [62], the latest Building Circularity Index (BCI) in a series developed from the Material Circularity Indicator (MCI) proposed by the Ellen MacArthur Foundation [63]. This BCI introduces a novel life cycle perspective, incorporating all stages and materials used throughout the life cycle of a residential building structure. Additionally, it integrates transport impact variables in the material flow, biomaterials, and key performance quantitative indicators for design for disassembly (DfD) and design for adaptability (DfA), achieved through the incorporation of ISO standards, the Level(s) framework, and Life Cycle Assessment (LCA) criteria. The evaluation of the circular performance 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; (iv) Scenario 4 (S4), incorporating circularity strategies for all materials, including concrete, throughout the entire life cycle. Figure 3. Circular Material Use Rate (CMR) (2019–2023) (Authors’ own based on [60]). In response to this need, the present study aimed to assess the circularity potential of RAC in the structure of newly built multi-family residential buildings in Spain. For this purpose, the Spanish residential building stock was analysed to identify the most representative structural system. A real, representative structure was then selected from one of the projects carried out by EMVISESA, the municipal housing company in Seville [ 61 ]. Subsequently, the circularity assessment was conducted using the CARES Framework (CARES-F) [ 62 ], the latest Building Circularity Index (BCI) in a series developed from the Material Circularity Indicator (MCI) proposed by the Ellen MacArthur Foundation [ 63 ]. This BCI introduces a novel life cycle perspective, incorporating all stages and materials used throughout the life cycle of a residential building structure. Additionally, it integrates transport impact variables in the material flow, biomaterials, and key performance quantitative indicators for design for disassembly (DfD) and design for adaptability (DfA), achieved through the incorporation of ISO standards, the Level(s) framework, and Life Cycle Assessment (LCA) criteria. The evaluation of the circular performance 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; (iv) Scenario 4 (S4), incorporating circularity strategies for all materials, including concrete, throughout the entire life cycle. Thus, in addition to identifying the circularity potential of RAC, the entire landscape of the CE transition was examined by evaluating the current state (S1) and potential variations in the process (S2 to S4). Notably, S2 examined the circular performance of ongoing industry initiatives as outlined in the EEEC reports [ 57 , 58 ]. At the same time, S3 reflected the growing interest among the companies involved, especially in the steel industry, in incorporating a higher percentage of secondary raw materials while ensuring compliance Appl. Sci. 2025,15, 3265 6 of 27 with the required physical and mechanical properties [ 64 ]. This condition also applied to concrete in S4, since recycled aggregates (RA) must be regarded from the design phase due to stipulations established in the Structural Code [45]. Novelty and Significance A comprehensive review of the existing literature reveals that recent research on residential buildings in Spain primarily focuses on assessing the carbon footprint of the residential stock [ 65 , 66 ] and enhancing energy efficiency in the envelopes and systems of multi-family residential buildings [ 67 – 69 ]. Furthermore, several case studies advocate for the adoption of waste reduction principles and off-site approaches during the design phase [ 70 , 71 ], as well as their implementation within the BIM environment [ 72 , 73 ]. Regarding circularity, significant efforts have been made to develop strategies that integrate the Level(s) framework into the local context [ 74 ], and to analyse key circularity indicators within the Spanish construction sector [ 8 ]. Likewise, contributions from the Spanish guide on the use of RA derived from CDW [ 75 ] and the structural design criteria for RAC proposed by Tošic et al. [ 76 ] are particularly relevant, providing a solid foundation for future RAC research and application in Spain. However, despite these advancements, the literature review reveals that the circularity potential of RAC in Spanish residential building structures remains unexplored. Therefore, the study presented in this work is inherently innovative. This research is of vital importance to the current residential construction sector, as it addresses social barriers to the transition towards a circular economy stemming from uncertainties regarding the effectiveness of RAC as a circularity strategy at the material level in newly constructed residential structures [ 77 – 79 ]. This contribution is even more significant when considering the current housing shortage, which has made the construction of sustainable residential buildings a primary solution, resulting in this type of building accounting for approximately 70% of all buildings constructed annually [80]. The paper is structured as follows: Section 1introduces fundamental concepts, the background, and the objective of the study. Section 2outlines the research methodology. Section 3presents the characterisation of the most commonly constructed type of building structure. Section 4exhibits the circular performance of the representative models in each scenario. Section 5discusses the circularity potential of each strategy and future lines of research. Lastly, Section 6concludes the study by providing the final observations. 2. Materials and Methods This study aimed to assess the circularity potential of the RAC in the structure of newly constructed multi-family residential buildings in Spain. The research considered four scenarios that encompass the full range of the transition to a CE. The assessment was conducted using the CARES-F [ 62 ], the latest circularity assessment framework in a series of BCIs based on the MCI. This BCI was developed by researchers at the Universidad de Sevilla (Seville, Spain). The analysis followed a comprehensive methodology comprising three phases. The first phase involved identifying the most representative structural system. The second phase focused on collecting empirical data and conducting the circularity assessment for each scenario using the CARES-F [ 62 ]. Finally, the third phase analysed the results obtained. Each phase is detailed in the subsequent sections, with Figure 4illustrating a flowchart of the research methodology. Appl. Sci. 2025,15, 3265 7 of 27 Appl. Sci. 2025, 15, x FOR PEER REVIEW 7 of 28 Each phase is detailed in the subsequent sections, with Figure 4 illustrating a flowchart of the research methodology. Figure 4. Flowchart of the research methodology based on methods from [62,80,81] (Authors’ own). 2.1. Phase 1. Characterisation of the Representative Newly Built Multi-Family Building This study aimed to characterise the circular performance of the structure of representative newly built residential buildings in Spain. To achieve this, the first step involved identifying the characteristics of these structures. The Spanish residential stock was examined through the databases of the National Institute of Statistics (INE: acronym in Spanish) [82] and the Ministry of Transport and Sustainable Mobility (MITMA: acronym in Spanish) [80,83]. This analysis identified key characteristics of the most prevalent type of newly constructed multi-family residential buildings, including the number of storeys, materials used, and the representative structural system. Based on this information, the structure of a residential building developed by EMVISESA, the municipal housing company in Seville, Spain [61], was selected for further analysis in Section 4. 2.2. Phase 2. Empirical Data Collection and Circularity Assessment This phase comprises two sub-phases: scenario formulation and empirical data collection (Section 2.2.1) and circularity assessment of each scenario using the CARES-F framework (Section 2.2.2). These sub-phases are detailed in the following sections. 2.2.1. Scenario Formulation and Empirical Data Collection Four scenarios were analysed to encompass the full spectrum of the CE transition: • Scenario 1 (S1) assessed the construction sector’s current practices by analysing material linearity throughout their entire lifespan. • Scenario 2 (S2) evaluated the circular performance of ongoing industry initiatives, as outlined in the EEEC reports [57,58], by implementing circularity strategies at the EOL stage. • Scenario 3 (S3) examined circularity strategies applied to all materials, excluding concrete, across the complete lifecycle. This scenario highlighted the industry’s growing interest, particularly within the steel sector, in incorporating a higher Figure 4. Flowchart of the research methodology based on methods from [ 62 , 80 , 81 ] (Authors’ own). 2.1. Phase 1. Characterisation of the Representative Newly Built Multi-Family Building This study aimed to characterise the circular performance of the structure of representative newly built residential buildings in Spain. To achieve this, the first step involved identifying the characteristics of these structures. The Spanish residential stock was examined through the databases of the National Institute of Statistics (INE: acronym in Spanish) [ 82 ] and the Ministry of Transport and Sustainable Mobility (MITMA: acronym in Spanish) [80,83]. This analysis identified key characteristics of the most prevalent type of newly constructed multi-family residential buildings, including the number of storeys, materials used, and the representative structural system. Based on this information, the structure of a residential building developed by EMVISESA, the municipal housing company in Seville, Spain [61], was selected for further analysis in Section 4. 2.2. Phase 2. Empirical Data Collection and Circularity Assessment This phase comprises two sub-phases: scenario formulation and empirical data collection (Section 2.2.1) and circularity assessment of each scenario using the CARES-F framework (Section 2.2.2). These sub-phases are detailed in the following sections. 2.2.1. Scenario Formulation and Empirical Data Collection Four scenarios were analysed to encompass the full spectrum of the CE transition: • Scenario 1 (S1) assessed the construction sector’s current practices by analysing material linearity throughout their entire lifespan. • Scenario 2 (S2) evaluated the circular performance of ongoing industry initiatives, as outlined in the EEEC reports [ 57 , 58 ], by implementing circularity strategies at the EOL stage. • Scenario 3 (S3) examined circularity strategies applied to all materials, excluding concrete, across the complete lifecycle. This scenario highlighted the industry’s growing interest, particularly within the steel sector, in incorporating a higher percentage of secondary raw materials while ensuring compliance with required physical and mechanical properties [64]. Appl. Sci. 2025,15, 3265 8 of 27 • Scenario 4 (S4) extended these circularity strategies to encompass concrete by adopting RAC as structural concrete, thereby promoting a more sustainable approach to construction materials. Empirical data were collected from the BIM model of the representative structure established in Phase 1. This virtual 3D building model was developed using Autodesk Revit 2023 (Autodesk Inc., Mill Valley, CA, USA) and integrates a database of its building elements [ 84 ]. The development of this process adhered to the verification flow methodology for BIM models of buildings, as outlined by Andrich et al. [ 81 ]. This methodology comprised three main sequential check phases [ 85 , 86 ]: BIM validation, which evaluated the quality and internal consistency of the model to ensure the extraction of reliable results in subsequent analyses; clash detection, which identified physical conflicts between elements; code checking, which ensured the model’s compliance with relevant standards. 2.2.2. Circularity Assessment of Each Scenario Using the CARES-F Framework The evaluation of circular performance was conducted using the CARES-F [ 62 ], a circularity assessment tool specifically designed for residential building structures. This tool operated on three levels: material, element, and system, each incorporating innovative sub-indices. A distinctive feature of the material level was its comprehensive consideration of all materials used throughout the building’s lifespan, as well as the impact of transportation. At the element level, the CARES-F [ 62 ] normalised the MCI based on the mass representativeness of materials and incorporated a disassembly potential (DP) factor, which accounted for disassembling connections. This factor was crucial in evaluating the circularity of reinforced concrete, as it considered connections and materials that could be disassembled without routine finishes. Finally, at the system level, the normalised sub-index derived from the element level was adjusted with an adaptability potential (AP) factor, which accounted for the geometric characteristics of the structure, as well as the arrangement and load-bearing capacity of its elements. 2.3. Phase 3. Critical Analysis of Results 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 5and Section 6, respectively. 3. Characterisation of the Multi-Family Building Model The third report of the EEEC [ 57 ] pointed out that, in Spain, the behaviour of the construction industry has changed since 2010, particularly in 2015. This shift was evident in the data from the Population and Housing Census 2021 [ 82 ], which showed 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 [ 80 , 83 ] align with this trend, and reveal that 70.71% of construction projects involved new multi-family units (Figure 6). This article attempted 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 [ 82 ] covering the period from 2015 to 2020 were analysed. The findings indicated that a four-storey building constitutes the most predominant newly constructed multi-family building (Figure 7). Appl. Sci. 2025,15, 3265 9 of 27 Appl. Sci. 2025, 15, x FOR PEER REVIEW 9 of 28 Figure 5. Residential buildings constructed in Spain each year (2011–2020) (Authors’ own based on [82]). The annual reports on building construction developed by the MITMA [80,83] align with this trend, and reveal that 70.71% of construction projects involved new multi-family units (Figure 6). Figure 6. Construction projects on buildings in Spain from 2011 to 2020 (Authors’ own based on [80,83]). This article attempted 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 [82] covering the period from 2015 to 2020 were analysed. The findings indicated that a four-storey building constitutes the most predominant newly constructed multi-family building (Figure 7). Figure 5. Residential buildings constructed in Spain each year (2011–2020) (Authors’ own based on [82]). Appl. Sci. 2025, 15, x FOR PEER REVIEW 9 of 28 Figure 5. Residential buildings constructed in Spain each year (2011–2020) (Authors’ own based on [82]). The annual reports on building construction developed by the MITMA [80,83] align with this trend, and reveal that 70.71% of construction projects involved new multi-family units (Figure 6). Figure 6. Construction projects on buildings in Spain from 2011 to 2020 (Authors’ own based on [80,83]). This article attempted 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 [82] covering the period from 2015 to 2020 were analysed. The findings indicated that a four-storey building constitutes the most predominant newly constructed multi-family building (Figure 7). Figure 6. Construction projects on buildings in Spain from 2011 to 2020 (Authors’ own based on [80,83]). Appl. Sci. 2025, 15, x FOR PEER REVIEW 10 of 28 Figure 7. Constructed new-build, multi-family buildings in terms of number of storeys in Spain from 2015 to 2020 (Authors’ own based on [82]). The analysis of the MITMA reports [80,83] 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 [87] highlighted that this material is also predominant in floor systems. This finding is further supported by reports on building construction [54,68], which showed that 77.63% of the floors are unidirectional (Figure 8b). (a) (b) Figure 8. Structural characteristics of multi-family residential buildings in Spain. (a) Materials used to construct vertical structural elements; (b) characteristics of floor systems (Authors’ own). Considering these most common characteristics, the structural system was considered for a real multi-family residential building located in Seville, Spain. It was developed by EMVISESA [61], 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 2314 m 2 (Figure 9). Figure 7. Constructed new-build, multi-family buildings in terms of number of storeys in Spain from 2015 to 2020 (Authors’ own based on [82]). The analysis of the MITMA reports [ 80 , 83 ] 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 Appl. Sci. 2025,15, 3265 16 of 27 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 was selected as a structural material alongside the considerations outlined in Scenario 3. This approach accounted for the requirements established in the Spanish Structural Code [ 45 ] for RAC, which stipulates a maximum allowable percentage of RCA 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 source structural concrete or high-strength concrete. 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 was considered to have XC0 exposure, namely no risk of corrosion, and hence, according to the Spanish Structural Code [ 45 ], its maximum water–cement ratio is 0.65, and the minimum cement content is 250 kg/m3. The research conducted by Amario et al. [ 101 ] explores the feasibility of using the Compressible Packing Model (CPM) to optimise the proportion of RA without compromising structural strength. The analysis highlighted the limitations of the CPM due to the inherent characteristics of RA. Specifically, the CPM assumes that aggregates are used in a dry state, accounting for their water absorption capacity over a 24 h period. However, RA can absorb up to 90% of their capacity within the first five minutes, altering the effective water–cement ratio. As a result, the standard CPM formulation cannot accurately estimate the available water in the concrete mix. Furthermore, the method fails to adequately consider the origin of RCA, despite its significant impact on variations in strength, porosity, and granulometry, all of which affect concrete performance. Finally, the rounded shape and lower density of RCA compared to NA influence concrete compaction and reduce the overall accuracy of the model. To address these challenges, Amario et al. [ 101 ] modified both the mixing procedure and formulation of the CPM. They found that pre-wetting RCA to 50% of their absorption capacity optimally maintains the effective water–cement ratio within desired values without compromising workability or strength. Furthermore, they optimised the mixing process by introducing a new sequence for adding water and admixtures. These adjustments enhance the effectiveness of the CPM in RAC. In the study by Amario et al. [ 101 ], several RAC mixtures with varying strength classes and RA proportions were designed and experimentally validated using the modified CPM procedure. The RCA used were sourced from debris of reinforced concrete beams produced and tested in the laboratory [ 102 ]. Given that the properties of the RA and NA comply with the specifications of the Structural Code [ 45 ], the concrete mixture C25-0120 (Table 3), which corresponds to concrete with a compressive strength of 25 MPa and 20% of RCA, was considered for the circularity assessment of this scenario. The slump of the mix was 45 mm, indicating a plastic and manageable consistency, though with lower fluidity compared to a mix without RCA, due to the greater absorption capacity of the recycled aggregates, their shape, and the reduced presence of fines. Additionally, the durability and reliability of the mixture were verified through absorption, porosity, and permeability tests. These tests showed that water absorption and permeability increase slightly with the addition of RCA but remain within acceptable limits according to structural regulations. Furthermore, the lower density of concrete with RCA does not negatively affect its structural performance under normal conditions. Appl. Sci. 2025,15, 3265 17 of 27 Table 3. Concrete mixture composition and properties developed by [101]. Composition Properties Cement 266.4 kg/m3Compressive strength 25 MPa Free water 170.0 kg/m3% of RCA 20% Natural Fine Aggregate (NFA) 844.4 kg/m3Effective watercement ratio 0.64 Natural Coarse Aggregate (NCA) 803.0 kg/m3 Recycled Coarse Aggregate (RCA) 195.6 kg/m3 Superplasticiser (SP) 2.7 kg/m3 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. Appl. Sci. 2025, 15, x FOR PEER REVIEW 18 of 28 Figure 17. MCI of representative materials in S4 (Authors’ own). Figure 18. ECI of reinforced concrete structural elements in S4 (Authors’ own). In light of the preceding results, the SCI of this scenario is 34.9%. 5. Discussion 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 [14,73,103], 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 Figure 17. MCI of representative materials in S4 (Authors’ own). Appl. Sci. 2025, 15, x FOR PEER REVIEW 18 of 28 Figure 17. MCI of representative materials in S4 (Authors’ own). Figure 18. ECI of reinforced concrete structural elements in S4 (Authors’ own). In light of the preceding results, the SCI of this scenario is 34.9%. 5. Discussion 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 [14,73,103], 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 Figure 18. ECI of reinforced concrete structural elements in S4 (Authors’ own). In light of the preceding results, the SCI of this scenario is 34.9%. 5. Discussion 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 , 73 , 103 ], the quantitative analysis through scenarios that explore the Appl. Sci. 2025,15, 3265 18 of 27 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 4demonstrates 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. Appl. Sci. 2025, 15, x FOR PEER REVIEW 19 of 28 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. (a) (b) Figure 19. Comparative analysis of MCI: (a) improvements in the MCI of concrete of S2 to S4 compared to S1; (b) improvements in the MCI of steel rebars of S2 to S4 compared to S1 (Authors’ own). 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 the adaptability potential is decisive for changes over time at the system level. (a) (b) Figure 19. Comparative analysis of MCI: (a) improvements in the MCI of concrete of S2 to S4 compared to S1; (b) improvements in the MCI of steel rebars of S2 to S4 compared to S1 (Authors’ own) . 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 the adaptability potential is decisive for changes over time at the system level. Appl. Sci. 2025,15, 3265 19 of 27 Appl. Sci. 2025, 15, x FOR PEER REVIEW 19 of 28 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. (a) (b) Figure 19. Comparative analysis of MCI: (a) improvements in the MCI of concrete of S2 to S4 compared to S1; (b) improvements in the MCI of steel rebars of S2 to S4 compared to S1 (Authors’ own). 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 the adaptability potential is decisive for changes over time at the system level. (a) (b) Figure 20. Comparative analysis of ECI and SCI: (a) average increase in ECI of S2, S3, and S4 compared to S1; (b) increase in SCI of S2, S3, and S4 compared to S1 (Authors’ own). 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 [ 104 ], off-site construction methods [ 105 ], and the arrangement of elements to facilitate the adaptability of structure over time [ 106 , 107 ]. 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 [ 108 ]. 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 [ 109 ]. 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). Appl. Sci. 2025,15, 3265 20 of 27 Appl. Sci. 2025, 15, x FOR PEER REVIEW 20 of 28 Figure 20. Comparative analysis of ECI and SCI: (a) average increase in ECI of S2, S3, and S4 compared to S1; (b) increase in SCI of S2, S3, and S4 compared to S1 (Authors’ own). 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 [104], off-site construction methods [105], and the arrangement of elements to facilitate the adaptability of structure over time [106,107]. 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 [108]. 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 [109]. 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). Figure 21. SCI of each scenario (Authors’ own). 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 Figure 21. SCI of each scenario (Authors’ own). 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 CMR 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 [ 35 ], 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 extent to which the restrictive nature of design standards limits the potential for circularity in material strategies within residential building structures. Therefore, it is crucial for design standards to promote the use of secondary raw materials by establishing guidelines that prioritise the optimal quality of concrete [ 47 ]. Such measures would enable more conservative standards to adopt a broader approach to incorporating a higher percentage of RCA [ 43 ] using fine recycled aggregates [ 42 ] and integrating supplementary cementitious materials [ 110 ]. The implementation of these mea- Appl. Sci. 2025,15, 3265 21 of 27 sures in the construction industry would be pivotal, as designers and concrete producers closely follow these codes. Furthermore, it is essential to integrate circularity principles at both the element and system levels, as they enable more efficient resource utilisation in line with the strategies of refuse, reduce, and reuse, ultimately minimising CDW [ 111 ]. Key circular design principles encompass DfD, DfA, Design for Standardisation, Design for Durability, and Longevity and Design for Modularity [112]. 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 [ 108 ]. 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 [103]. 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 [ 113 ]. Furthermore, design standards must promote the adoption of circular design principles at all levels, encouraging designers to integrate these criteria from the early stages of their projects. At the same time, 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 CMR. Supplementary Materials: The following supporting information can be downloaded at: https://ww w.mdpi.com/article/10.3390/app15063265/s1, Table S1: Data Extraction Background; Table S2: Data residential stock; Table S3: Data circularity assessment; Table S4: Summary of the results obtained for all CARES-F variables in the circularity assessment of Scenario 1; Table S5: Summary of the results obtained for all CARES-F variables in the circularity assessment of Scenario 2; Table S6: Summary of the results obtained for all CARES-F variables in the circularity assessment of Scenario 3; Table S7: Summary of the results obtained for all CARES-F variables in the circularity assessment of Scenario 4. Author Contributions: Conceptualisation, A.V.-C. and C.L.; methodology, A.V.-C. and C.L.; software, A.V.-C.; validation, A.V.-C. and C.L.; formal analysis, A.V.-C.; investigation, A.V.-C .; resources, A.V.-C .; data curation, A.V.-C.; writing—original draft preparation, A.V.-C.; writing—review and editing, A.V.-C., C.L. and M.V.M.; visualisation, A.V.-C.; supervision, C.L. and M.V.M.; project administration, C.L.; funding acquisition, C.L. All authors have read and agreed to the published version of the manuscript. Funding: This publication forms part of the following projects: Grant TED2021-129542B-I00, funded by MCIN/AEI/10.13039/501100011033 and by the European Union “NextGenerationEU/PRTR”, and Grant PID2022-137650OB-I00, funded by MCIN/AEI/10.13039/501100011033 and by ERDF, EU. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Appl. Sci. 2025,15, 3265 22 of 27 Data Availability Statement: All data are included within the article and the Supplementary Data File. Acknowledgments: The MCIN funded the FPI scholarship, the subject of the research activities carried out as part of this Architecture PhD study. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations The following abbreviations are used in this manuscript: AP Adaptability Potential CARES-F Circularity Assessment Method for Residential Structures (CARES Framework) CDW Construction and Demolition Waste CE Circular Economy CMR Circular Material Use Rate CPM Compressible Packaging Model DAfStb Guideline of the German Committee for Reinforced Concrete DfA Design for Adaptability DfC Design for Circularity DfD Design for Disassembly DI Distance Index DMC Domestic Material Consumption DP Disassembly Potential EC European Commission ECI Element Circularity Index EEEC CE Spanish Strategy EOL End Of Life EU European Union GDP Gross Domestic Product GHG Greenhouse Gas IEA International Energy Agency INE National Institute of Statistics (acronym in Spanish) MCI Material Circularity Index MITMA Ministry of Transport and Sustainable Mobility (acronym in Spanish) NAC Natural Aggregate Concrete NCA Natural Coarse Aggregate NFA Natural Fine Aggregate RA Recycled Aggregate RAC Recycled Aggregate Concrete RCA Recycled Coarse Aggregate RPI Resource Productivity Indicator S1 Scenario 1 S2 Scenario 2 S3 Scenario 3 S4 Scenario 4 SCI System Circularity Index SDG Sustainable Development Goals SP Superplasticiser References 1. 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