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

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.

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] 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 [9,10,11]. 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]. 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 [19,20,21], utilising higher-strength concretes [22,23,24], capturing and storing CO₂ emissions [25,26,27], and incorporating Recycled Aggregates (RA) in concrete production [28,29,30,31].

Recycled Aggregate Concrete (RAC) is an innovative material obtained from crushed, screened, and washed RA derived from discarded concrete [32]. 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 [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/mm². 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.

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 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].

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.

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 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].

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 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 [67,68,69]. 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 [77,78,79]. 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 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

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 4 illustrating a flowchart of the research methodology.

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].
• 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 5 and 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).

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).

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 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; (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 [62], 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 the logistics software CargosApps 2.0 [88] (IMPARGO, Berlin, Germany) 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 [89]. In the specific instance of concrete, the capacity of the truck mixer and the discharge rate were determined based on the Spanish Structural Code [45]. Moreover, speed regulations from the General Traffic Regulations [90] 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 [62] 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 was 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, these data were 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 assessed 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 was predominant, and it was 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 breaks them down into carbon dioxide, biomass, and water [91].

Following the hierarchical structure of the CARES-F [62], 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 made of reinforced concrete, the materials that become structural elements, as well as those involved, including packaging, formwork, and material losses, were taken into account.

Table 1. Representative materials of the building structure (Authors’ own).

LoW 1 Code and Description		Representative Material	Participation Mass Ratio
170101	Concrete	Concrete (H-25)	93.33%
170405	Iron and steel	Steel rebar (B 500 S)	2.82%
170405	Iron and steel	Small steel material	1.08%
150111 *	Metallic packaging containing a hazardous solid porous matrix	Metallic packaging	0.55%
170405	Iron and steel	Supplementary steel material or special parts	0.54%
170201	Wood	Pinewood planks (shuttering)	0.45%
170201	Wood	Pinewood boards (shuttering)	0.43%
170405	Iron and steel	Welded wire mesh (ME B 500 T)	0.13%
170604	Insulation materials	EPS waffle pod for slabs (0.6 × 0.6 m)	0.11%
150103	Wooden packaging	Wooden packaging	0.10%
170405	Iron and steel	Steel panel formwork (50 × 50 cm)	0.10%
150101	Paper and cardboard packaging	Cardboard box	0.10%

¹ European List of Waste (LoW) [92]. * Potentially hazardous waste.

presents 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 and 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 explored the circularity of the building structure outlined in Section 3, examining a linear input flow of materials during the initial phases and a circular output flow of materials at the EOL. The analysis was based on the findings from the third and fourth EEEC reports [57,58], which indicated that the construction sector currently prioritises the implementation of circularity strategies primarily at the EOL stage. This approach aligns with Directive 2008/98/EC [92], which established a target for the recovery of 70% of CDW by 2020.

In this scenario, the output flow considered the reusable, recyclable, and biologically renewable materials in each structural element. These fractions were derived from research, reports, and databases provided by governmental and non-governmental organisations (Table 1). These sources were selected for their accuracy and up-to-date relevance to the construction sector at the national level.

Table 2. Sources consulted for the analysis of S2 (Authors’ own).

LoW Code and Description		Title	Author	Year
170101	Concrete	Spanish Guide to Recycled Aggregates from RCD	GEAR [75]	2012
170405	Iron and steel	Guideline for recycled materials reused in construction Trade in Recyclable Raw Materials (database) Galvanised steel and sustainable construction (report)	Ihobe [64] Eurostat [94] EGGA [95]	2016 2022 2021
170201	Wood	Contribution of Recycled Materials to Raw Materials Demand (database)	Eurostat [96]	2020
170203	Plastic	The CE of plastics (report) Packaging and environmental sustainability	Plastics Europe [97] Emblem et al. [91]	2022 2012
150111 *	Metallic packaging	Metal Recycling Factsheet	EuRIC AISBL [98]	2022
150103	Wooden packaging	Packaging Waste by Waste Management Operations (database) Trade in Recyclable Raw Materials (database)	Eurostat [99]	2023
170604	Insulation materials		Eurostat [94]
150101	Paper and cardboard packaging			2022

* Potentially hazardous waste.

organises the consulted sources according to the European List of Waste (LoW) codes [93].

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 considered 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 was 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 was 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 [64]. For reinforced concrete structures, this initiative influences their circular performance due to the integration of rebars. Therefore, this evaluation of circular performance adopts a perspective that strives, as far as possible, to implement circularity strategies throughout the entire lifecycle. Concrete was 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 [45].

For this assessment, the percentages of secondary raw materials specified in the Guideline for the reuse of recycled materials in construction [75] 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. [100] 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 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. Concrete mixture composition and properties developed by [101].

Composition		Properties
Cement	266.4 kg/m3	Compressive strength	25 MPa
Free water	170.0 kg/m3	% of RCA	20%
Natural Fine Aggregate (NFA)	844.4 kg/m3	Effective water-cement 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

), 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.

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. 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 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 the 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 [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).

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 measures 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.