Advancing Circular Buildings: A Review of Building Strategies for AEC Stakeholders

Abstract

The uptake of a circular economy (CE) in the building sector is challenging, primarily due to the complexity associated with the design process and the dynamic interaction among architects, engineers, and construction (AEC) stakeholders. The standard and typical design process and construction methods raise concerns about building life cycles. Buildings should not only fulfill current needs, but one also needs to consider how they will function in the future and throughout their lifetime. To address these complexities, early planning is required to guide designers in holistically applying systems thinking to deliver CE outcomes. This paper outlines a critical review of CE implementation in buildings, with a proposed trifecta of approaches that significantly contribute to the development of circular buildings (CBs). The findings outline a proposed visualized framework with a conceptual formula that integrates CE design strategies to simplify and enhance AEC stakeholders’ perception of the circularity sequence in buildings. By strategically integrating loop-based strategies with the value retention process (VRP) and design for X (DFX) strategies, along with efficient assessment tools and technologies, it becomes feasible to embrace a CE during the design phase. The outcome of this review informs AEC stakeholders to systematically and strategically integrate the critical dimensions of a CE throughout the building life cycle, striking a balance between environmental concern, economic value, and future needs.

1. Introduction

The building sector is resource-intensive, responsible for substantial material consumption and global waste production [1]. Annually, around one-third of the waste generated from building construction, renovation, and demolition ends up in landfills or is incinerated without any further use [2,3]. While many countries place more emphasis on reducing the operational energy consumption of buildings, the embodied environmental impacts resulting from the extraction of materials, manufacturing, construction, maintenance, and the disposal of building materials have been largely neglected, imposing an enormous burden on the environment [4,5].

The 2022 global status report by UNEP highlights the urgency for all new buildings to achieve net-zero status for both operational and embodied emissions by 2050. However, this sector is currently falling short of expected targets and alarmingly, building energy use and carbon intensity have increased by four and five percent, respectively, since pre-pandemic levels in 2020 [3]. In response to this concerning trend, scholars and AEC stakeholders advocate for the adoption of CE principles as a comprehensive approach to efficiently manage resources while minimizing the harm to natural environments. Specifically, since 2015, the European Union Action Plan (EUAP) has aligned EU policies with a CE, supporting the EU’s commitment to achieving the United Nations (UN) Sustainable Development Goals (SDGs) [6].

It is evident that AEC stakeholders collectively have a significant role in reclaiming emissions produced in all phases of buildings, potentially up to 36% [7]. However, CE development in the building sector faces many challenges due to the conservative and risk-averse nature of AEC stakeholders, predominantly driven by client considerations of time, cost, and quality [8]. This challenge is further compounded by barriers such as a lack of codes and regulations, fragmented supply chains, and split and insufficient incentives, which are frequently highlighted in several studies [9,10,11,12,13]. Moreover, incorporating CE strategies in buildings is complex, requiring the simultaneous consideration of various factors during the design phase, including the dynamics of process, materials, and stakeholder involvement [14]. Given the longevity of buildings, integrating key CE design strategies from the outset is critical to prevent long-term environmental and societal burdens resulting from many initial misguided decisions. To counter the prevailing and entrenched linear decision-making trend, a shift toward a comprehensive CE approach is necessary to effectively implement circular strategies throughout the life cycle of buildings. However, the literature presents a wide range of CE guidelines and approaches, leading to confusion in identifying the main factors essential for CB development and structured design strategies from the perspective of AEC stakeholders. The simplified approach to addressing the CB complexity among these guidelines is notably lacking in the existing literature. Given the existing overlaps in CE design strategies in practical scenarios, this paper aims to:

• identify key factors essential for the development of circular buildings;
• propose a simplified CB design-thinking approach, to inform AEC stakeholders to effectively embrace circularity.

Additionally, visual presentations and the conceptual framework of the CB structure will facilitate an understanding of stakeholders’ roles in the supply chain, thereby promoting the adoption of CE strategies in the building sector.

1.1. Theoretical Background and the Importance of CE in the Building Sector

The concept of a CE emerged in the early 1990s, drawing inspiration from a variety of nature-based approaches and theories, such as biomimicry, blue economy, green growth, natural capitalism, eco-efficiency, cradle to cradle, performance economy, sustainability, and industrial ecology [15,16,17,18,19,20]. These diverse concepts, evolving over the last half-century, share both similarities and differences at various levels [21]. Among all these concepts, sustainability and the CE have received extensive attention over the last couple of decades; however, this also causes confusion among AEC experts due to a lack of clarity regarding how these concepts relate to each other. Scholars emphasize that these theories are linked through the shared recognition of the environmental consequences of human actions [22]. The circular economy serves as a pathway towards achieving sustainability, particularly within the building sector, by revamping resource usage and reducing environmental impact [22,23].

The CE, as a holistic system, emphasizes the importance of achieving harmony between the built and natural environments, providing an effective alternative to the linear “take-make-use-dispose” economic model [24]. It supports industrial practices by minimizing material extraction, reducing degradation, conserving resources and energy, and encouraging the regeneration of input sources [25]. Numerous definitions of the CE are found in the literature, indicating its growing significance [26,27]. A commonly cited definition emphasizes an economic system that replaces the “end-of-life” concept with reducing, reusing, recycling, and recovering materials at different levels, which aims to achieve sustainable development, creating environmental quality, economic prosperity, and social equity while benefiting current and future generations [26]. Ultimately, with the CE in place, economic growth will decouple from material consumption and the closed cycle of materials will ultimately lead to a low-carbon economy [22]. Acknowledging the significance of the building industry, the European Commission (EC) in 2018 prioritized the transition to a circular economy within this sector, considering its substantial economic contribution of about 9 percent due to significant waste generation each year [28,29,30]. Additionally, the EU has outlined several milestones to achieve a fully circular built environment, with a primary goal of achieving 100 percent circularity in the tendering process to reach a resource-efficient building stock [31]. While there has been some attempt to realize circular buildings, the EU has not achieved fully circular buildings in real practice.

Implementing a CE not only offers significant environmental benefits but also generates new jobs, strengthens supply chains, and contributes to economic growth [32,33]. According to the International Labor Organization [34], transitioning towards a CE could create up to 6 million jobs globally. Embracing a CE will improve material efficiency, potentially reducing up to 80% of the total embodied greenhouse gas (GHG) emissions from buildings [4]. In the absence of action towards circularity, this sector will face resource shortages in the near future. For instance, projections in Europe indicate that materials such as zinc and chromium, which are mostly used in facades, roofing, and technical building services, will encounter supply shortages within 20 years [35]. Additionally, the rising cost of construction materials, such as cement and metals, illustrates the challenges facing the industry in many countries [35]. Considering the expected increase in the global population by 41% [36] and the projected doubling of the global building stock by 2050, a radical shift in decoupling from ongoing environmental concerns and worldwide economic uncertainties is necessary [37]. Consequently, the need to adopt CE practices in the building sector is becoming even more pressing.

From Linear to Circular Buildings

The first prototype of a circular building emerged during the London Design Festival in 2016, introduced by Arup Associates [38]. While the CE is a broad concept applied across various sectors, a CB specifically pertains to the building sector. CBs gained significant momentum beginning in 2021, when the Dutch government formally endorsed CBs with the intention to integrate circular materials and components into buildings and structures by 2030 [31]. However, unifying the pathways towards CBs is challenging due to the variety of interpretations of the CE, leading to diverse directions. Scholars such as Pomponi and Moncaster [39] and Leising, Quist and Bocken [40] provide insights into CB definitions, as well as efforts undertaken by non-governmental organizations such as the Ellen MacArthur Foundation (EMF) [41], and recently the Circulair Bouwen 2023 [31]. These contributions mainly encompass key principles of resource conservation, environmental protection, economic and ecological responsibility, and well-being contribution.

The transition to CBs is gradually progressing and is considered inevitable. However, to design, construct, operate, and deconstruct buildings aligned with CE principles requires the careful consideration of several factors. These factors include, but are not limited to, the nature of buildings (new, existing, heritage, etc.), building typology (residential, commercial, industrial, institutional, mixed-use, etc.), materials (biological, technological), building layers (site, structure, skin, services, space plan, stuff furnishing), location, phases, and various stakeholders [42,43,44]. Extensive support throughout the implementation of circularity is necessary, starting from the initial stages of the design and pre-design phases [45,46,47]. Planning for CE strategies from the very beginning ensures that manufacturing and construction processes adhere to the circular construction approach [48]. Further, research indicates that 80% of material environmental impacts across the life cycle are determined during the design stage [33]. Although the development of CBs covers comprehensive factors and goals, its current focus in many building sectors primarily revolves around resource use and waste management [20]. The literature mainly emphasizes life cycle analysis and building materials, while the importance of the design process is largely overlooked [38].

2. Methods

A systematic and critical review was conducted to identify the relevant literature and key factors for CB development, and the main design strategies adoption by AEC stakeholders. This approach ensures a thorough and unbiased assessment of existing research, highlighting gaps and trends essential for advancing the field.

The primary source of information for this review was the Scopus database as shown Figure 1, recognized for its comprehensive coverage of the scholarly literature and high-quality, peer-reviewed data [46]. The search strategy was designed using a combination of selected phrases and keywords, categorized into three main areas: “Circular building/s”, “Circular design”, and “Building End of Life”. The keyword “Circular design” was expanded to include “Circular design” “AND “building” OR “building sector” OR “building sector” OR “AEC”. The timeframe of 2004 to 2024 was selected to conduct an all-encompassing search of the literature relevant to CBs as well as the CE in the building and construction sector. This included all peer-reviewed journal articles, conference papers, and review papers, all published exclusively in English.

The selection criteria for the literature review followed the PRISMA guidelines [49]. The primary focus was to map the existing literature within the selected themes. The subject areas were narrowed to the following fields: engineering, environmental science, social science, energy, business, management and accounting, materials science, and economics, econometrics and finance. A total of 359 articles were identified as a result of this targeted approach.

To maintain the quality and relevance of the review, a multi-stage screening process was employed. The first stage involved screening the titles and keywords of the selected articles to ensure alignment with the research objectives and to eliminate duplicates and unrelated topics. In the initial screening, a total of 155 articles were eliminated. The second stage involved a further detailed review of the full abstracts of the remaining 204 articles.

The following exclusion criteria were established for data extraction during the analysis of abstracts and full article screening:

• Papers focusing solely on construction and demolition waste (CDW) management were excluded;
• Papers analyzing buildings purely from a green or eco-energy perspective, without a consideration of CE principles, were excluded;
• Papers focused exclusively on urban and infrastructure were selected as being out of the scope;
• Papers focused exclusively on materials science and engineering without viewing materials through the perspective of the CE in buildings were excluded.

Only documents that focused mainly on the CE in buildings or the building and construction sectors were included. After applying these exclusion criteria in the third stage of reading through the full articles, 101 articles remained. Additionally, all selected articles were thoroughly reviewed using Google Scholar. During this cross-checking process, five additional articles were added due to their close alignment with the subject of circular buildings.

A total of 106 papers were selected for final review and analysis. The analysis was summarized into the following categories:

• Overview and descriptive analysis of CB development through the trend of years of publication, countries, themes, and types of documents analyzed;
• Critical analysis of key factors of CB development and the main design strategy factors for AEC adoption.

The gap this review paper addresses is advancing the field of circular building development by simplifying the complexity of CE practices through the building life cycle. This simplified approach presents a tangible empirical application for AEC stakeholders to embrace circularity from the design phase through to the entire building life cycle. Ultimately, through the review process and analysis, the key factors of CB development and design strategies adoption were visually interpreted and conceptually integrated for transition to AEC stakeholders.

3. Results and Discussion

The results are outlined in two main parts: a descriptive analysis of the selected articles, representing trends in CE research within the building and construction sector, and a critical analysis identifying key factors for CB development, including a detailed integration of design strategy factors for AEC stakeholders.

3.1. Descriptive Analysis and Trends

From the selected articles, the progress and increased focus on the CE in buildings are evident in the trends over the years (Figure 2), as well as scholarly attempts and publications worldwide (Figure 3). Additionally, the co-occurrence diagram (Figure 4) highlights the most frequently associated keywords extracted through VOSviewer, representing the evolving trends and keywords used by authors over time.

Rapid developments in CBs have occurred in the last five years. Although the initial attempt to review was limited to the literature from the last two decades, the earliest articles identified based on the selection criteria date back to 2010. The main trends in the relevant articles are presented from 2016 onwards. Additionally, during the screening process, only three articles published before 2016 met the inclusion criteria, corresponding to the years 2010, 2013, and 2014. This resulted in gaps in the intervening years (Figure 2).

The findings highlight that while the concept of the CE began to evolve in the late 90s [17], initial scholarly exploration and adoption within the building and construction sector gained momentum around 2010. Notably, the number of publications had a significant increase after 2018, primarily attributed to research related to BAMB (building as a material banks), especially conference papers focused on building material passports and reversible building design [50]. Moreover, the heightened attention from 2019 onwards can be largely attributed to various rules and regulations worldwide, particularly within the European context. For instance, the “sustainable use of resources” mandated in European building products since 2013 [51], the Paris Agreement in 2015 aimed at reducing global GHG emissions and curbing temperature rise to below 1.5 degrees Celsius, and international commitments to support the SDGs [52]. In addition, a series of industry reports by influential non-governmental organizations such as the Ellen MacArthur Foundation (since 2012), which has focused on the building and built environment sectors since 2016 [41], have further contributed to this increase.

EU countries such as the Netherlands lead the way by contributing the most to both CB knowledge and real-life case studies. This is followed by Belgium, the United Kingdom, Italy, Denmark, and Portugal. These significant contributions are primarily attributed to several EU policies and initiatives, including the European Green Deal [53] and the Circular Economy Action Plan [6]. Some notable real-life case studies that contribute to circularity initiatives in buildings include the ABN AMRO Circl building and the temporary courthouse in Amsterdam, as well as the Circular Pavilion at the Paris Expo in France [54,55,56]. In particular, these countries have progressed beyond only waste management strategies, with each having a specific roadmap and policy program dedicated to the CE in the building and construction sector. This progression is also evident in the scholarly contributions of the research from European countries (Figure 3). However, the EU has yet to implement fully circular buildings in real practice.

Additionally, the most associated keywords in this field have been identified using a keyword co-occurrence network created by VOSviewer. Figure 4 highlights the frequent pairing of terms such as “circular economy”, “buildings”, “life cycle”, “circular buildings”, and “circular designs”. The network analysis of co-occurring terms for 111 selected keywords, organized into 7 clusters, illustrates the development of the circular building (CB) field over time. The size of each node represents the frequency of each keyword, while nodes with lighter colors indicate the latest and emerging keywords that highlight trends requiring further study, such as “design process”, “circular building design”, “artificial intelligence”, and “digital technologies”. It is evident that the evolving trends, particularly in the “design process”, underscore the growing need for more design-thinking practices and related research.

Furthermore, the distribution of document types among the selected articles reveals that 56 percent are journal articles, 34 percent are conference papers, and less than 10 percent are review papers. This underscores the need for more review papers in this field to provide a comprehensive overview of existing research, which is essential for informing progress and trends in CBs, particularly at the design stage of buildings. It is also noteworthy that existing review papers generally focus on a selection of journal articles. However, several conference papers in this review provide original content that explores the practical applications of CBs.

3.2. Critical Analysis and CB Design Thinking

Implementing robust design-thinking processes is essential for the development of CB practices. The fundamental step occurs during the design phase [57]. The subsequent life cycle of buildings and their components relies heavily on the initial design choices, which must be guided by non-linear systems thinking [38]. This focus on design aligns with the core principle of the CE, which defines it as a system driven by design [58]. Through the implementation of technical design solutions and CE strategies, the efficiency of buildings can be significantly enhanced [38]. However, the ambiguity and challenges in achieving CBs during the initial phase are primarily due to uncertainty among AEC stakeholders about where to begin and what is required to construct buildings in a circular manner. Addressing this requires incorporating key aspects of CE strategies during the design phase while considering the intrinsic characteristics of buildings. A simplified design-thinking approach is necessary for the transition towards more CB practices, as shown in Figure 5. To understand CB design thinking, this section will first conceptually formulate the key approaches to CB development, and second, formulate a simplified design-thinking approach to assist in design strategies adoption and the development process of AEC projects.

3.3. Key Approaches to CB Development

To comply with CE principles, the management of various factors throughout the life cycle of buildings is essential. This approach highlights the main design factors, including circularity indicators, that need to be considered during the design phase [59,60]. The following management classification is visually presented in a trifecta configuration (Figure 6) and described in detail throughout this section. The term “management” is used because it encompasses the holistic oversight and coordination of activities and factors necessary to ensure that circularity principles are integrated at every stage of the building life cycle, from design to end of life. By adopting a management perspective, we can systematically address and integrate the multiple dimensions and stakeholders involved in implementing CE principles effectively.

• Resource management: This encompasses indicators related to building materials, components, products, energy, and water factors and their associated values.
• Design management: This includes a combination of factors related to CE design strategies for constructing buildings, and the necessary tools for evaluation process, supported by advanced technologies for broad digitalization.
• Collaboration management: This covers factors related to stakeholder and supply chain involvement, including AEC interactions, and introduces new collaboration methods and business models in the building and construction industry [61].

The effective management of these key factors is essential to consider throughout the building life cycle, and a lack of attention to any one of these directions can leave the CB development incomplete, hindering overall progress. By following this structured approach, AEC stakeholders can navigate the complexities of CB development, and make informed decisions from the project’s inception.

To facilitate an understanding of the process and the navigation of the key factors involved in CB development, each management approach is conceptually formulated in the format of Equation (1) throughout the paper. This method allows technical experts and AEC designers to better digest the complexities of true CB development.

$$\mathrm{C}\mathrm{B}=\mathrm{D}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{g}\mathrm{n} \mathrm{M}\mathrm{a}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}+\mathrm{R}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{u}\mathrm{r}\mathrm{c}\mathrm{e} \mathrm{M}\mathrm{a}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}+\mathrm{C}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{a}\mathrm{b}\mathrm{o}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{M}\mathrm{a}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{m}\mathrm{n}\mathrm{t}$$

(1)

Table 1. Classification of key factors of CB development.

Design Management	Description	Sources	No. of Papers
CE Loop *	Factors of 10Rs and design strategies.	[1,2,4,5,13,14,20,29,31,33,35,38,42,57,58,60,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110]	65
Tools	Assessment factors of LCA, BIM, LCC, MP, MFA, Circ-Flex, etc.	[1,2,5,35,37,57,58,67,83,88,100,103,111,112,113,114,115,116,117,118,119,120,121,122,123], [22,50,52,81,86,92,95,96,105,122,124,125]	37
Digitalization	Factors of AI, digital twins, Block chain, IOT, etc.	[81,93,126,127,128]	5
Resource Management
Materials	Indicators of bio-based, renewable, innovative, secondary, and low impact materials, components product, etc.	[2,5,20,29,35,37,46,50,52,58,63,67,81,83,86,88,89,90,93,94,97,99,101,104,114,115,116,118,119,120,122,124,127,128,129,130,131,132,133]	39
Energy	Indicators of carbon (CO2), GHG, etc.	[1,67,88,91,93,96,100,103,112,117,119,121,122,130,133,134,135]	17
Water	Indicators of recycled and grey water.	[46,80,136]	3
Collaboration Management
Value	Factors of take back scheme, business model, tax incentives, reverse supply chain model.	[4,29,40,75,76,89,90,93,96,130,137,138,139]	13
Share	Factors of sharing system, innovative ownership model, etc.	[33,40,60]	3
Education	Factors of knowledge exchange, actor learning, traceability, etc.	[29,33,40,60,68,93,108,112,135,137,139,140]	12

* Detailed CE loop-based strategy including the relevant Rs, and design strategies are expanded further in Section 3.4.

presents the general focus of each of the selected sources on these main factors. It outlines the three management approaches, including the associated factors and the related key indicators extracted from the selected sources during the review process, providing a comprehensive overview of the essential elements for CB development.

While several indicators and factors have been proposed by scholars, the variety can add to the complexity of adopting CBs. Therefore, this research will focus on the main key factors, which will be expanded upon in the following sections. It should be noted that the partial application of the trifecta of approaches and their associated factors has been implemented in real case studies within the building sector; however, there are no case studies or examples that fully cover all aspects of the trifecta of approaches across the value chain in a circular manner.

3.3.1. Approaches to CB Resource Management

Managing resources within the context of CBs involves overseeing the three key indicators of materials, water, and energy. All resources, both solid and non-solid, such as material, components, products, water usage, carbon emissions, and energy consumption, must undergo CE regenerative and technical cycles. The consumption and flow of materials (used and discarded), water (used and recycled), and energy (consumed and emitted) need to be positive throughout the life cycle of buildings [refer to conceptual Equation (2) presented below]. This applies to all levels and layers of building applicability, from individual materials and components/products to the entire building function, optimizing and retaining resource value.

$$\mathrm{R}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{u}\mathrm{r}\mathrm{c}\mathrm{e} \mathrm{M}\mathrm{a}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}=\left(\mathrm{M}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{I}\mathrm{n}/\mathrm{O}\mathrm{u}\mathrm{t}\right)+\left(\mathrm{E}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y} \mathrm{I}\mathrm{n}/\mathrm{O}\mathrm{u}\mathrm{t}\right)+\mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r} \mathrm{F}\mathrm{l}\mathrm{o}\mathrm{w}$$

(2)

In resource management, the application of nature-based resources such as bio-based, eco-efficient, low-impact, and renewable materials, aligns with the regenerative principles of a CE [31,141]. However, implementing only bio-based materials and components does not necessarily result in low environmental impacts throughout the building life cycle [115]. Purposeful hybrid methods that combine both biological and material cycles lead to better CB outcomes [58,67,114]. Additionally, controlling the upfront emissions of resources can be achieved by using locally sourced bio and technical materials [32].

Furthermore, the embodied energy and carbon inherent in buildings will result in less material used (including reused and recycled materials) and less water consumption [130]. Effective mechanisms are required for recycling water consumed in buildings, allowing for a second life use or its reuse (as greywater) and providing methods for efficient water flow and use within the building’s structure [46]. Resource management in circularity concepts requires focusing on existing building stocks and the effective combination of building components to minimize the quantity while providing future use opportunities for the next life or more lives [22].

An effective CB outcome depends on the combination of resources that are coming in, being used, and being exhausted across the entire building process. Each of these factors influences the others, enabling the whole building system to minimize environmental harm by addressing both embodied and operational emissions throughout the building life cycle. It is noteworthy that, in contrast to the existing literature, an indicator of waste is not specified in Table 1, since in accordance with CE principles, no material is regarded as waste; instead, all resources are viewed as having an endless cascading cycle, which can continue to be used over and over again in their second and subsequent lives.

3.3.2. Approaches to CB Collaboration Management

To achieve an innovative system change toward CB, a new form of network collaboration among the main actors is necessary [142]. This can be harnessed by a combination of descriptive factors, as shown in conceptual Equation (3), namely education, share, and value.

$$\mathrm{C}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{a}\mathrm{b}\mathrm{o}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{M}\mathrm{a}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}=\mathrm{E}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}+\mathrm{S}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{e}+\mathrm{V}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}$$

(3)

Education and enhancing knowledge exchange towards CB development is a critical pillar in establishing collaboration among main actors. This includes but is not limited to informing AEC stakeholders about CE principles and enhancing communication and traceability through advisory teams or project managers involved in the design and construction process [29,31]. This facilitates information exchange and directs interactions among stakeholders under one umbrella, rather than sequential interaction between each discipline [125]. The entire project team should be educated and engaged in circularity discussions and goals. While general training programs and initiatives like EURECA-PRO, promoted by the European Commission (EC), aim to create a global educational hub for promoting the CE, there are currently no targeted and unified training programs specifically developed for AEC stakeholders [143], including undergraduate programs for the disciplines underpinning AEC.

In addition, sharing systems in both products and services, such as specifying the multi-actor ownership of buildings and spaces, emerge as key elements of CBs [136]. Facilitating shared and long-term ownerships for contractors to extend responsibilities along the entire building supply chain will increase resource efficiency in CBs [40].

On top of that, key factors of value management in CB development are becoming increasingly critical, particularly in terms of value creation and value propositions [144]. The adoption of new business models fosters value creation, trust among stakeholders, and alignments of similar goals across the value chain [44,66]. Nußholz, Rasmussen and Milios [130] promote business model innovations for CBs to enable secondary material use. Further, stakeholder networking facilitates value creation through innovative collaboration, involving cooperation from clients in all disciplines throughout the building’s life cycle [66]. Effective collaboration amongst stakeholders can be fostered by establishing a shared vision and managing data transparency, material flows, and responsibilities throughout the building’s life cycle [40,41]. While no universally accepted frameworks exist for collaboration, initiatives like the “Better Building Initiative” and national programs such as Belgium’s Green Deal Circular Building and Denmark’s VCØB offer promising solutions by unifying materials, design, and processes and promoting stakeholder engagement [29,145].

It is important to identify quantifiable indicators within collaboration management, recognizing that merely implementing circular materials and components does not automatically lead to CB outcomes. The current efforts underscore the need for the further development of collaborative tools and methodologies to enhance multi-stakeholder collaboration in the building sector. Achieving a true CB necessitates a paradigm shift, active stakeholder involvement informed by a CE, and the establishment of shared and innovative business models within the building and construction industry [136].

3.3.3. Approaches to CB Design Management

In the critical review of selected articles, three key CE design approaches were identified to formulate design management in CB. These factors, forming the conceptual Equation (4), include the CB loop-based process, assessment tools, and the digitalization approach. Systems thinking in the design process and the efficient integration of these three factors can lead to CB development and assist in mindful decision-making actions of AEC stakeholders.

$$\mathrm{D}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{g}\mathrm{n} \mathrm{M}\mathrm{a}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}=\mathrm{C}\mathrm{B} \mathrm{L}\mathrm{o}\mathrm{o}\mathrm{p}+\mathrm{T}\mathrm{o}\mathrm{o}\mathrm{l}\mathrm{s}+\mathrm{D}\mathrm{i}\mathrm{g}\mathrm{i}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}$$

(4)

3.3.4. CB Loop-Based Process

The process directions required for CB development under the CE principle, referring to designing and implementing methods and strategies, lead to “narrow”, “slow”, and “closed” process loops of the building cycle. Buildings should be designed to incorporate these loop approaches to effectively manage resources and reduce the environmental impact of buildings [136]. Each of these processes can be identified as a direct goal of CBs.

The “CB slow loop” aims to extend the use of the building and its components by providing methods for adaptability and flexibility. This approach expects the design components to retain value for as long as possible through reuse, repair, and remanufacturing [35,52]. On the other hand, the “CB closed loop”, focuses on strategies for closing the cycle of resources so that materials can be reused or recycled after reaching the end of their lifespan. This strategy emphasizes the importance of technical and biological inputs and outputs in achieving the closed loop strategy by applying recyclable or biodegradable materials [35,136]. In contrast to the other two strategies, the “CB narrow loop” approach does not affect the speed of the flow of materials or components and does not involve any service loops (e.g., repair) [108]. This approach aims to optimize the consumption of resources such as raw materials, energy, and water. By using fewer resources per component, this strategy adopts drastic resource efficiency measures [108,146]. Furthermore, narrowing loops encourages rethinking space use and the sharing of buildings [52,62].

The consideration of multiple loops will generate a cascading system where materials and components are transferred through a series of different forms one after the other [35].

Each loop encompasses specific sub-elements such as design for X (DFX) strategies and value retention process (VRP) methods, which need to be integrated as shown in the conceptual Equation (5). Detailed subcategories of each loop and the effective combination of each strategy will be elaborated in the Section 3.4.

$$\mathrm{C}\mathrm{B} \mathrm{L}\mathrm{o}\mathrm{o}\mathrm{p}=\mathrm{V}\mathrm{R}\mathrm{P}+\mathrm{D}\mathrm{F}\mathrm{X}$$

(5)

3.3.5. Assessment Tools

Alongside the CB loop-based process, which provides designers with specific pathways for design actions, certain assessment tools are essential to make these designs measurable. These tools represent a critical step in supporting the transition to a CB [37]. The selected CB loop-based design strategies need to be tested using several existing assessment tools. Some of the main tools are LCA (life cycle assessment), BMP (building material passport), MFA (material flow analysis), and BIM (building information modeling) [52,146,147,148,149]. These tools evaluate design outcomes in terms of targeted impacts, requiring control throughout the whole cycle of buildings. Depending on the priority impact factors derived from environmental, economic, and social concerns, the impacts of GHG emissions, carbon footprint, material weight and quantity, water usage, cost, and social marketing effects will be measured using these tools to foster CB development [139]. The initial analysis of the literature review revealed several tools currently utilized or under development to assess the contribution of the CE to the building and construction industry [2,86,96,119]. However, many countries, including those in the EU, are still working on a CE evaluation system to measure circularity in buildings [150].

The application of these tools needs to be evaluated in the early design stages of buildings to ensure the optimal choices for environmentally, economically, and socially led solutions [52]. There is some concern that these tools may not adequately assess CE strategies, specifically when considering the cradle-to-cradle impact of selected resources [115]. For instance, BIM has limitations when high-quality data are required from manufacturers for the deconstruction phase [151,152]. LCA needs further development to predict multi-cycle scenarios for the long-term environmental impact of components [37,153]. Additionally, there is uncertainty regarding material flow in the demolition phase, which adds to MFA limitations in assessing the CE [154]. In general, the lack of transparent information regarding the end-of-life condition of materials can make choosing and comparing design variants difficult [72]. However, ongoing research of scholars working in this area aims to further develop and improve these tools [148,155].

3.3.6. Digitalization

Digitalization assists in the design process through the application of various digital technologies such as artificial intelligence (AI), the Internet of Things (IoT), and digital platforms (DPs) [81,92,126]. The integration of these technologies across the entire supply chain enhances connectivity and information exchange. This process is crucial for accelerating the transition to CBs, as it allows for the real-time tracking and management of resources throughout the life cycle of a building. AI technology assists in optimizing design processes by using data-driven techniques, such as neural networks, to generate and evaluate multiple design choices [126]. Several scholars have highlighted the application of machine learning models in forecasting the carbon footprint of buildings [156], predicting the quantity of recyclable materials during the deconstruction phase [157], and estimating future scenarios of resources through AI algorithms [81]. By using the existing data, these models address several uncertainties of AEC regarding the end-of-life phase of buildings.

The IoT further enhances digitalization by connecting physical objects through embedded electronics, sensors, and network connectivity, enabling real-time data exchange and monitoring [128]. However, it has limitations, particularly in the lack of transparency and structured data management processes required for high-quality data analysis [158,159]. The EU Action Plan (2020) emphasizes the role of innovation and digitalization in tracking, tracing, and mapping resources, ultimately dematerializing the economy and reducing dependency on natural resources [6]. One of the recognized DPs in the development of CBs is the platform of Madastar, which records material details and documentation of use in real time, using web-based technologies [59]. By integrating advanced digital technologies, the industry can achieve significant reductions in costs, increased production efficiency, improved quality, faster project completion times, and substantially better circular design outcomes.

This fast-growing movement towards CBs needs to be fully supported by these technologies. While AI, IoT, and DPs are essential, there are plenty of other digital technologies available, such as robotics, additive manufacturing, digital twins, and blockchain technology [81,93,126,127]. These technologies collectively contribute to the overall enhancement of design management and circular building practices [128].

3.4. Simplified Design Strategies Approach towards CB Developmnet

In each of the three CB loop-based approaches, the combination of the certain VRP method and the DFX strategy can complete the approaches. The review revealed some overlaps and similar concepts in practice between the VRP method and the DFX strategy, which needs to be simplified from an AEC perspective for adoption.

Value retention process (VRP): Through the VRP method, a hierarchy of CE strategies is defined for retaining resource value [118]. Conceptually, these strategies have been characterized by the “R” strategies [33]. Initially, the “R” list prioritized three main actions: “reduce, reuse, and recycle”, which led to the reconsideration of resource flow in buildings [10]. Over time, the list expanded to include additional actions and levels of strategies, starting with “refuse”, followed by “rethink, reduce, reuse, repair, refurbish, remanufacture, repurpose, recycle”, and ending with “recover”, which is essentially waste to energy conversion [46]. Several scholars have proposed structuring the VRP method by developing more “R” strategies [65,68,72]. Recently, Çimen [65] introduced three more Rs, “renew, refill, and replant”, for CB development, which refer to the use of renewable energy and green materials, consumable components, and standardized products, respectively.

Implementing VRP methods in the design process can reduce the building industry’s reliance on new resources to bring economic benefits [160]. The “reuse” strategy involves applying the direct use of secondary materials and components, without major process rectification, promoting the extension of resource use and harnessing the CB slowing loop. Specifically, the direct “reuse” of materials and components has more economic and environmental benefits than recycling strategies as it requires minimal energy consumption to convert the material and components into new products [45]. This has been tested with reused concrete blocks, and cement compared to recycled concrete [121]. On a larger scale, reuse represents adaptive reuse, involving the direct reuse of entire buildings and structures [57]. In this research, we included the regenerative strategy under the VRP method. This strategy, with its nature-based approach and minimal environmental impact, ensures that resources are returned to the system, thereby harnessing the closed cycle of CBs [77,80].

Design for X (DFX) strategies: Following value-retention processes in design management, the development of CBs can be greatly enhanced by applying several individual design strategies, which are referred to as design for X (DFX) strategies in this paper. X is identified as there are many strategies recognized in the literature, such as design for disassembly (DFD), design for adaptability (DFA)/flexibility, design for modularity, design for durability, design for longevity, design for ease of maintenance, design for optimization, etc. [73,161,162,163]. These strategies play a crucial role in enhancing the development of CBs by extending the life of buildings and supporting low-carbon practices in construction activities [33,35,108]. For instance, the case study of the fully detachable building at the Courthouse in Amsterdam estimated a reduction of 2000 tons of CO₂ compared to new construction, as the building was relocated and repurposed for office use after five years. Additionally, the case study of building at Loughborough University demonstrates a high level of adaptability and modularity, allowing for the easy reconfiguration of spaces to meet various functions and future needs, even after several decades [164]. As a part of the objective of this paper and to avoid confusion for AEC stakeholders through the process of design, it is imperative to distinguish the most effective CE design strategies from the variety of options presented in the literature [95]. Through a critical review of the articles, the most relevant and essential design strategies with associated value for the development of CBs were identified, as shown in

Table 2. Detailed CB loop and subsequent design strategy factors.

Loops	Strategies		Sources	No. of Papers
CB Narrow Loop	VRP	Refuse	[65,68]	2
		Reduce	[65,68]	2
		Rethink	[65,68]	2
	DFX	Design for optimization	[57]	1
		Dematerialization	[57]	1
		Design for share spaces	[52]	1
CB Slow Loop	VRP	Reuse	[2,5,64,65,68,72,78,81,82,83,88,94,97,99,103,104,109,112,116,118,133]	21
		Repair	[65,68]	2
		Refurbishment/Renovation/Retrofit	[4,60,65,68,101,116,133]	8
		Re-manufacture	[65,68]	2
		Repurpose/Adaptive reuse	[57,65,66,68,74,78,111]	7
	DFX	Design for adaptability	[14,35,42,66,70,73,83,107,133]	9
		Design for flexibility	[66,88,102]	3
		Modular & Prefab design	[64,70,89,97,98,118,133,165]	8
		Design for low maintenance	[99]	1
		Design for durability	[35,38]	2
CB close Loop	VRP	Regenerative	[77,80,87,92,100]	8
		Recycle	[2,5,65,67,78,81,103,106,109,112,116]	11
		Recover	[65,68,81,88,97]	5
	DFX	Design for disassembly	[4,13,14,42,57,70,72,82,88,94,99,103,105,106,109,134,165]	17
		Design for deconstruction	[69]	1
		Demountable design	[94,111,165]	3
		Reversible design	[29,38,42,98,99]	5
		Standardization	[70]	1

.

All the extracted design strategies can be classified under the following main categories: design for disassembly (DFD), design for adaptability (DFA), and design for optimization (DFO), as shown in Equation (6). These classifications are based on the substantial impact on resource management, and carbon emissions to some extent. Additionally, all these strategies contribute to the three CB loop-based process in design management.

$$\mathrm{D}\mathrm{F}\mathrm{X}=\mathrm{D}\mathrm{F}\mathrm{D}+\mathrm{D}\mathrm{F}\mathrm{A}+\mathrm{D}\mathrm{F}\mathrm{O}$$

(6)

Design for disassembly (DFD): DFD stands out as a pivotal strategy central to CBs and construction practices, focusing on making buildings demountable, allowing for the ease of reuse, repair, remanufacturing, or recycling after the initial design life [22,147]. Reversibility and design for deconstruction, both related concepts, emphasize the property of returning a process, system, or device to its original state [8,166]. Moreover, this strategy has been tested in comparison with conventional building types for its potential material savings and greenhouse gas emissions reduction [30,153,167]. For example, studies on life cycle costing (LCC) indicate that circular design alternatives, such as demountable and reusable wall assemblies, have life cycle costs that are 10% to 17% lower than conventional alternatives [122]. DFD has been argued to be more environmentally beneficial when lighter materials such as timber and steel are used, as opposed to concrete [103]. However, despite the potential of DFD, less than 1% of buildings are currently demountable, primarily due to technical barriers and building complexity [22]. The majority of organizational and technical challenges related to end-of-life (EOL) stages reflect conventional buildings’ design linearity. Addressing these challenges requires action at the design stage, such as utilizing existing components, extending building use stages, and focusing on EOL considerations from the outset [70]. Developing CBs will require DFD, as landfilling should not be an option for the construction and demolition process. Additionally, landfilling construction and demolition (C&D) materials does not make logical sense, as construction materials are mostly inert, and can be used repeatedly as long as their integrity is maintained.

Design for adaptability (DFA): This key design strategy will provide flexibility by retaining the ability to relocate components or a building layer to another place if space expansion is required. This strategy harnesses the slowing loop process that results in a longer lifespan for buildings [52]. Several scholars evaluated the impacts of DFA on building life cycle, especially in terms of reductions in cost and time, both in the initial construction phase [73] and also during the operation stage through renovation [168]. The strategy contributes to the transition to CBs by improving long-term usability, enabling future renovations and replacements, and supporting future changes and transformations of building functionality if required [38,52]. Also, adaptability and flexibility should be the highest for components with the shortest life span of buildings [73]. The development of CBs relies heavily on this strategy to be implemented and considered at the earliest stage of the design process to maximize physical adjustability and ensure buildings’ usefulness or value is preserved. In Table 2, durability and modularity design has been allocated to the CB slow loop strategy and under the category of DFA. These methods focus on creating long-lasting, resilient, and regular dimensions in components and products, which reduce the need for frequent repairs and replacements and enhance the building and components’ lifespan [35]. Additionally, modularity, with its potential for flexibility and relocation, supports the adaptability aspect of design for AEC stakeholders [165].

Design for optimization (DFO): The primary focus of this strategy lies in the rethinking and redesigning of buildings to enhance the efficiency of resource flow, floor space utilization, shared spaces, and energy performance [52]. DFO approaches minimize the use of virgin materials [38] and reduce the overall quantity of materials in buildings, including measures to decrease transportation needs [118]. Encompassing efficient design, while highlighting less material consumption and the efficient management of energy [20], DFO represents an essential design strategy for circularity in building design. At the planning stage, DFO emerges as the critical strategy that designers should prioritize, offering design alternatives aimed at optimizing the utility of buildings over time [22]. The implementation of this strategy contributes significantly to maximizing the life cycle cost by reducing both material usage and energy consumption in building projects [95]. In the context of this paper, optimization is defined as a set of strategies that not only reject excessive material use but also ensure effective utilization, all while adopting a circular approach to rethinking the entire building life cycle.

Integrating CB systems thinking necessitates considering key design factors throughout the entire life cycle of a building. This approach is crucial for optimizing design strategies towards CB development Figure 7.

AEC stakeholders have the highest capacity to optimize design and construction processes effectively. Their influence is crucial in guiding project teams to compare design alternatives and select the most circular outcomes. Ultimately, the following Formula (7) is designed to encompass all the discussed design factors, aiming to provide a simplified and tangible understanding of AEC towards CB adoption.

$$\mathrm{C}\mathrm{B}=\left[\begin{array}{c}\underset{\mathrm{D}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{g}\mathrm{n}-\mathrm{M}\mathrm{a}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}{\underbrace{\left[\underset{\mathrm{L}\mathrm{o}\mathrm{o}\mathrm{p}}{\underbrace{\left(\underset{\mathrm{D}\mathrm{F}\mathrm{X}}{\underbrace{\left(\mathrm{D}\mathrm{F}\mathrm{O}+\mathrm{D}\mathrm{F}\mathrm{A}+\mathrm{D}\mathrm{F}\mathrm{D}\right)}}+\mathrm{V}\mathrm{R}\mathrm{P}\right)}}+\mathrm{T}\mathrm{o}\mathrm{o}\mathrm{l}\mathrm{s}+\mathrm{D}\mathrm{i}\mathrm{g}\mathrm{i}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\right]}+}\\ \underset{\mathrm{R}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{u}\mathrm{r}\mathrm{c}\mathrm{e}-\mathrm{M}\mathrm{a}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}{\underbrace{\left(\mathrm{M}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{I}\mathrm{n}/\mathrm{O}\mathrm{u}\mathrm{t}\right)+\left(\mathrm{E}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y} \mathrm{I}\mathrm{n}/\mathrm{O}\mathrm{u}\mathrm{t}\right)+\mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r} \mathrm{F}\mathrm{l}\mathrm{o}\mathrm{w}}}+\\ \underset{\mathrm{C}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{a}\mathrm{b}\mathrm{o}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}-\mathrm{M}\mathrm{a}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}{\underbrace{\mathrm{E}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}+\mathrm{S}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{e}+\mathrm{V}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}}}\end{array}\right]$$

(7)

4. Conclusions

The imperative to shift towards a CE in the building sector and the subsequent development of CBs is underscored by growing environmental and economic concerns. However, AEC stakeholders face significant challenges necessary for this transition. Notably, the scarcity of qualified individuals with a comprehensive understanding of logical combinations of design strategies and the effective utilization of tools, often compounded by a lack of digital technology support, poses hurdles. One prevailing misconception hindering progress is the tendency to associate a CE solely with short-term financial gains limited to the recycling process. This undervalues the comprehensive impact across the entire value chain and at various levels, from global to local and individual, encompassing buildings and materials. Moreover, theoretical explorations suggest that the lack of trust between stakeholders, which leads to social issues, should be included in and beyond the construction process as part of the circular design.

Through a comprehensive literature review, this paper identified critical factors essential for CB development. It presents a holistic theoretical background to CE in buildings, describing WHY CB is important, identifying the critical factors through the trifecta approaches of WHAT is required, and proposes a simplified framework along with a conceptual formula to inform AEC stakeholders on HOW CB strategies can be integrated and applied through the life cycle of buildings. By simplifying and optimizing the integration of design strategies, a visual guideline is proposed for AEC experts, which serves as a practical pathway to CBs at the early stages of building design. The framework facilitates the implementation of CB design thinking for AEC stakeholders, aiming to utilize design strategies to keep the building as viable as possible, both within existing time and across various life cycle scenarios, including up-cycling or down-cycling. Emphasizing multi-level design applications throughout the life cycle of buildings, along with the incorporation of advanced tools and digital technologies, ensures that it adds substantial value to buildings and potentially other built environment assets, facilitating a more sustainable and circular approach to the building industry.

The recommendations in this review emphasize the importance of simplifying circular building (CB) adoption when considering the dynamic nature of building and construction industry. Given the inherent complexity of the sector, AEC stakeholders require practical and straightforward guidelines. Future research should focus on the development of a CB certificate or similar type of transparent approach that integrates the trifecta approaches outlined in this review. A transparent system for achieving circular outcomes should be established, guiding AEC experts in every aspect of their projects through the process of design and construction of buildings to operations/maintenance to the deconstruction of buildings to advance the development of more circular buildings for the future.