Designing Temporary Use: Prototyping a Framework towards Material-Wise Projects

Abstract

The growing awareness of the value of temporary use projects in reactivating unused buildings has led to their increased implementation and acceptance by local authorities and building owners. Encouraging such reactivation can help to address the large number of unused spaces in cities. Additionally, the construction sector’s substantial contribution to environmental damage amplifies the imperative for material-wise approaches to building reactivation, particularly when undertaken on a temporary basis. This study therefore delves into the overlooked dimensions of materialization and design within the realm of temporary use, shifting the discourse from the conventional emphasis on awareness raising to a more holistic approach encompassing sustainability and circularity. Through an in-depth case study analysis of nine pioneering temporary projects in and around Brussels, supported by semi-structured interviews and site visits, this paper first delves into the ‘material approach’ and the ‘design’ of temporary use projects. Second, through a comparative analysis of the cases, a framework of guidelines is prototyped, structured around three perspectives—materialization, design, and stakeholders—and their actionable steps, guidelines, and attention points. The proposed framework, presented and evaluated for the first time in academic research, serves as a proof of concept for the analysis and guidance of temporary use through three integrated perspectives towards resource efficiency. This innovative approach offers a holistic perspective that goes beyond mere standardization and awareness raising, acknowledging the complex dynamics inherent in temporary use projects and providing a comprehensive framework for analysis and action. Ultimately, translating this prototype into a user-friendly pamphlet holds potential to advance the knowledge in this field and facilitate the more effective and sustainable utilization of resources in temporary contexts.

1. Introduction

Cities play an immense role in the transition towards a circular economy. They are a vast collection of resources, both material and immaterial. Take, for instance, the urban environment. It is composed of material infrastructure, such as buildings, and immaterial resources, such as social dynamics. The combination of such elements makes the city an urban place and central to the envisioned change.

Demographic growth, social segregation, urban sprawl, spatial inefficiency, and economic disparities, however, threaten sustainable urban development [1]. Together with climate change, they have raised awareness of the impact of the extensive and exhausting systems and industries at play, among which is construction. This sector contributes significantly to the emissions of global greenhouse gasses. Moreover, it depletes virgin resources and has the highest level of waste production, accounting for one third of all produced waste [2]. Encouraging this sector to transition towards sustainable and circular urban development is therefore key, as confirmed by the European Commission in its Green Deal and Circular Action Plan [3,4]. Fortunately, within the current built environment, many opportunities are present that could support the transition towards circular cities. Reimagining vacant and underused buildings through temporary use projects is one of them.

1.1. Emergence and Recognition of Temporary Use Projects

Urban environments are densely populated areas, where little space seems to be free and available for new activities. However, this could not be further from the truth. Unused spaces can be found throughout cities, in open (e.g., brownfields) as well as densely built areas [5,6,7,8,9]. Within the latter, transformation processes are taking place to reimagine and reactivate unused and neglected spaces [10]. However, due to the time needed for the design and planning processes of such transformation projects [11,12], these unused spaces remain unoccupied for even longer periods.

Instead of remaining unused, these spaces can be activated through temporary use projects. Temporary use projects, also referred to in the literature as ‘temporary urbanism’ by Bishop and Williams [13], are short-term interventions that can have a direct impact on the surroundings and on future planning. Here, in the phase between vacancy and transformation, they temporarily organize and house various innovative activities [14,15,16,17,18].

Temporary use is a growing phenomenon in cities and has been accelerated by several European research projects, among which are ‘Urbact’ (2016), ‘Urban Catalysts’ (2013), and the ongoing ‘T-Factor’ [17,18,19]. These research projects focus on the potential of temporary urbanism, where temporary use is implemented as a means of placemaking, promoting transformation projects [20,21,22,23]. Such low-cost, temporary interventions can inspire a wider discussion about the replanning process [21]. In this regard, temporary activities can be analyzed on the ground and adapted to fit within the future project.

Temporary use provides the freedom to experiment with certain methods before extensively applying them, offering local collaborators the opportunity to achieve fast results. This experimentation can give designers and planners more certainty in making deliberate decisions when the conditions are unpredictable or traditional planning strategies prove to be inadequate [20]. However, the challenges lie in translating the positive experiences and tested strategies into the actual implementation of the final project.

In Europe, the organization and implementation of temporary projects in unused buildings is starting to be put forward by urban planners and policymakers [19], as they are starting to recognize the benefits of activating unused buildings through temporary projects in terms of social, cultural, environmental, and economic aspects.

Table 1. Benefits and challenges of temporary use (TU), gathered through an in-depth literature review of various publications; data from [9,19,20,24,25,26,27].

Benefits	Challenges
Social
An unused space can become a local meeting and gathering place for the neighborhood.	Homogenization: Allowing only successful enterprises and one demographic group can lead to the exclusion of underprivileged and marginalized communities, which are already disadvantaged by normal planning.
Underprivileged and marginalized communities have a chance to participate.	Gentrification: Using TU as a way to attract a new population group and highlight the future transformation project and thereby excluding and driving away the original community living there.
New and missing activities can be presented in the neighborhood.
Real societal needs and wants are revealed.
TU boosts social and cultural inclusion by bringing a vast group of society together.
Decrease in vandalism through social control by activating unused spaces.
Cultural
The history of a building is remembered by its continuous usage, and the development of cultural and creative activities is linked to the perception of this location.	The historical value is not always evident and can be contradictory. The history of a building is composed of layers and is open to interpretation.
Innovation is stimulated through experimentation.	Administrative challenge: long administrative process, which includes obtaining the necessary building permits, to organize TU.
Embracing uncertainties in urban planning.	Slow collaboration between various stakeholders.
Citizen participation in urban planning.
TU as an instrument to create future-fit cities and respond to actual societal needs.
Environmental
TU promotes local vegetation, natural habitats, and microclimates, while improving public space with greenery.	Limited information exists addressing the utilization of materials in the context of the temporary use of vacant buildings. This raises the question of how the temporary nature of such projects impacts the handling and utilization of materials.
Testing ground for innovative and sustainable practices, including preservation, reuse, and recycling.
The reuse of buildings minimizes construction waste and uses already occupied spaces in an efficient way.
Economic
Maintaining and using vacant buildings leads to avoiding high reparation costs due to decay and degradation, which would be present if the building remained unused.	The owner avoids vacancy taxes.
Small investments can transform temporarily unoccupied buildings into attractive spaces, where young entrepreneurs and start-ups can perform their economic activities.	Low investment security.
A vacant building’s revitalization can increase the value of the neighborhood and in turn increase the value of the real estate market.
TU can boost the local economy by introducing new activities.
Initiatives and organizations that are not financially successful can affordably rent a space to test and start their activities.

gathers the benefits and challenges of temporary use (TU), which are subsequently grouped according to four categories on which temporary use can have an impact, i.e., social, cultural, environmental, and economic aspects.

Moreover, as governmental entities are recognizing that the benefits of temporary use projects outweigh the challenges [28], as also noted in Table 1, they are taking steps to stimulate it. As a result, tools are being developed that aim to simplify the process of temporary use and make it more accessible. As such, one of the deliverables of the European research project ‘Urbact’ was the roadmap ‘The Temporary Use Value Creation Plan’ [29]. This roadmap, developed in 2018, is a tool that helps to envisage a range of future possible developments in a temporary use project. It sets a framework in which an agreement between a candidate for temporary use and a vacant space owner can be made and allows both parties to explore a temporary use project together. Thus, it addresses several questions that need to be considered at the start of such a project [29,30]. The aim is to devise a common project where both parties benefit from the temporary use project.

Similar endeavors have been noted in Belgium. In Ghent, for instance, the city promotes temporary use actively among its residents through a six-step guide that covers various aspects of temporary use. This overview helps initiators and organizers to gain insights into all phases of temporary use and guides them in their decision-making [31].

Recently, various public stakeholders in Brussels collaborated to create a new online tool for the organization of temporary use projects, named ‘temporary.brussels’. This online tool, active since December 2022, was developed by perspective.brussels (the local planning department of Brussels), citydev.brussels (the regional development institution), and multiple partners, with the main aim of supporting the initiators of a temporary project in finding a suitable space and the owners of unused spaces in finding a suitable coordinator [32].

Such a matchmaking tool could be helpful if used by the various Brussels actors, such as the large building owners and the various organizations activating unused spaces and managing and coordinating temporary activities. However, its effectiveness is still uncertain as the tool is in its early stages of existence.

Apart from the matchmaking tool, since 2018, Brussels has published a report that gathers all of the legal and administrative information needed when seeking to develop a temporary project [33]. However, as this guide was initially not user-friendly, it has since been adapted and included on the new website, ‘temporary.brussels’.

Finally, actors in Brussels also co-developed a charter with the aim of aligning temporary use practices, smoothing the stages of project development, and facilitating access to temporary uses. By signing the charter, the Brussels actors commit to collectively following certain guidelines.

Two guidelines that are put forward in the charter are as follows: ‘Ensure sufficient space for projects with a social purpose, including cultural or emergency housing projects, and for projects that contribute to the switch to a circular economy and the climate transition’ and ‘Respect the environment and the architectural and heritage qualities of the property’ [34]. According to these guidelines, temporary use projects should thus integrate circular design and be material-efficient.

1.2. Material Usage in Temporary Use

Currently, most research focuses on how temporary use projects can become tools for urban regeneration (e.g., the European research projects Urban Catalysts, T-Factor, and REFILL), by raising awareness and standardizing and legalizing temporary use. However, the opportunities that temporary use projects offer for circular construction are not fully grasped. Some noteworthy publications on temporary use, however, have briefly addressed the aspect of material usage in temporary projects.

The research of Oswalt et al. (2013), presented in their book ‘Urban Catalysts: Strategies for temporary use’, highlights that temporary initiatives often operate with a restricted financial capacity, resulting in the reuse and recycling of resources with minimal investment [18]. Thus, in addition to reusing existing buildings, they also make use of existing materials with minimal intervention for their infill.

Furthermore, the book ‘Leegstond’ (2018), by the Brussels non-profit organization Toestand, which focuses on activating vacant buildings for social and cultural purposes, briefly considers materials and reuse in the context of temporary use, followed by some crucial guidelines and permits for connections, facilities, and safety [24]. In fact, temporary activities should adapt to the previously built space and, therefore, in their essence, need to be very adaptable and flexible to allow the coexistence of the fixed structure and the temporary infill.

Joja emphasized, in their article titled ‘Temporary use of abandoned buildings’ (2021), that temporary infills are modular, use locally available materials, and favor easily assembled and disassembled constructions, allowing single parts to be reused and recycled at other places [35]. As such, variable designs allow simple modifications of temporary structures to be adapted to their sites.

1.3. Towards a Circular Construction Economy

The strategies described in the limited literature addressing material usage in temporary use align, in fact, with the principles put forward in a circular economy. The concept of a circular economy (CE) has been of interest since the 1970s, stemming from various streams of thought, including ‘regenerative design’ and ‘cradle to cradle’ [36]. However, recently, the transition towards a CE has been gaining momentum as policymakers and local authorities are recognizing that our current linear model of ‘take–make–use–waste’ is not sustainable. Therefore, the transition towards a CE is supported and accelerated among various sectors by the European Commission in its Green Deal and Circular Action Plan [3,4].

An important sector in which to transition towards and implement CE principles is the construction sector, as emphasized by the European Commission and Ellen MacArthur Foundation. This is because the construction sector holds significant responsibility as both a major consumer of resources and a contributor to waste generation [37]. Thus, to overcome the high material consumption associated with this linear economy, a transition is crucial.

In a circular construction economy, materials are reintroduced as valuable resources for construction purposes [6]. As such, the continuous flow of materials should be ensured through practices such as sharing, maintenance, reuse, manufacturing, and recycling to keep products and materials in circulation, thereby closing material loops.

Circular strategies supporting the closure of material loops have been further developed and adapted to the context of the design practice. As such, circular design qualities, approaches, and concepts have been gathered and presented by the Architectural Engineering (ARCH) research group of the Vrije Universiteit Brussel.

Moreover, since specific design choices, such as extending the service lives of buildings and closing material loops, are considered key strategies in the transition towards a circular construction sector, the research by ARCH presents complementary design approaches, qualities, and concepts. In their publication ‘Building a Circular Economy’, a circular design tool is presented in which approaches, sixteen design qualities, and concepts are introduced to offer guidance to professionals when developing a suitable building concept and aid them in making well-considered and informed decisions [38]. Figure 1 illustrates the toolkit.

This toolkit, publicly available on the website of ARCH, aims to support the shift towards a circular practice. As such, the approaches (in blue) of Design for Longevity, Design for Disassembly, and Design for Reuse are put forward, in addition to the most typical concepts (in green). Moreover, by considering and discussing circular design qualities (in orange) at the beginning of a project and setting common ambitions, the design can enable the more effective reuse, recycling, or renewal of buildings and building components [38].

1.4. Reseach Objectives

Several tools and guidelines have emerged to support practitioners in the transition towards circular construction and the implementation of temporary use initiatives. However, research has yet to explore these two aspects together. Existing studies on temporary use predominantly focus on its implementation, awareness raising, and the standardization of the practice, neglecting the thorough examination of materialization and design.

To effectively advance towards circular construction with closed material loops, the temporary activation of unused buildings must adopt a material-wise approach where existing buildings and materials are efficiently utilized and reused to limit the environmental impact of the construction sector. This necessitates the development of a framework to guide practitioners and researchers in decision-making processes towards material-wise temporary projects.

This study, centered on the Brussels Capital Region, seeks to prototype such a framework for material-wise temporary use. Accordingly, it aims to explore the materialization and design of temporary use projects in vacant properties to be transformed, while shedding light on the pivotal roles of various actors in this endeavor.

2. Methods and Materials

To move towards material-wise temporary projects, and to comprehensively investigate the materialization and design of temporary use projects, qualitative research via case study analyses was undertaken. The methodological approach encompassed four sequential phases: (1) case study selection and data collection through semi-structured interviews; (2) an in-depth case study analysis from two perspectives—the materialization and the design of temporary use projects; (3) a case study comparison and the proposal of an assessment framework based on three perspectives; and (4) the evaluation of the proposed framework of guidelines.

First, the case studies’ selection involved a selection process to capture the diversity and intricacies inherent in temporary use projects. Nine temporary projects across Belgium were purposefully chosen, with eight situated in the Brussels Capital Region and one in Leuven. This specific geographical area was selected for its goals towards circularity and the activation of vacant buildings. To support these ambitions, local policymakers are taking action and are facilitating this transition. In addition to their geographical locations, the case studies were selected based on their national recognition and their positions in the activation period, i.e., situated in the middle or nearing the end of their activation periods. Notably, the chosen case studies were well-documented projects in Belgium, facilitating access to relevant articles and publications for supplemental information. In addition, this selection was supported by the feasibility of conducting in-depth investigations to procure detailed insights, collected between September 2022 and July 2023. Visits to the temporary projects were conducted, accompanied by ten interviews with key stakeholders, including seven temporary project managers, two designers, and one client. An overview of the selected temporary projects is presented in Section 2.1 and a brief introduced of each case is included in the Supplementary Materials (S1: Project Fiches).

Second, semi-structured interviews were conducted to elucidate the actual practices and perspectives. Section 3 presents the findings of these interviews, for each case, around two overarching topics: (1) ‘the material approach’ and (2) ‘the design of temporary use projects’. These two topics were the focus of the interviews since they are under-researched in the current practice of temporary use. The interviews provided valuable data regarding the conceptualization, execution, and materialization of temporary use projects, facilitating a nuanced understanding of the matter. The questions of the semi-structured interviews were structured as follows: a general exploration of material approaches, the detailed gathering of the used materials in spatial interventions, and a conclusion through a set of final questions related to the challenges, opportunities, and possible improvements of material usage in temporary use.

The third phase involved the iteratively structured analysis and comparison of the selected cases of temporary use projects, presented in Section 4. By identifying commonalities and disparities in the material and design approaches and the involved stakeholders across various spaces, recurring patterns and variations were discerned. This iterative and comparative analysis, resulting in a table underscoring the discussed and observed aspects, served as the basis for the prototyping of a framework of guidelines aimed at facilitating the design and evaluation of temporary use projects. This prototype, structured around three key perspectives—materialization, design, and stakeholders—delineates essential elements, best practices, and critical considerations.

The fourth and final phase involved a workshop with stakeholders to evaluate and validate the prototype. This led to valuable insights and outcomes regarding the proposed framework of guidelines for material-wise temporary use projects, presented in Section 5.

2.1. Materials

This section briefly situates the nine temporary projects selected as case studies: Allée du Kaai, WTC 1, Molenwest Square, Circularium, Le Tripostal, Studio Citygate, Circle Park, FLOW, and Maakleerplek. Their contexts, ambitions, activities, and key stakeholders are presented in the Supplementary Materials (S1: Project Fiches).

Figure 2 illustrates the eight temporary projects located in the Brussels Capital Region. The ninth temporary project, Maakleerplek, is located in Leuven, a city in the east of Brussels; as such, it is not situated on the map.

3. Findings

This section presents the detailed data gathered through the semi-structured interviews and the literature findings. Subsequently, it describes the nine selected temporary projects, one by one, from two perspectives: ‘the material approach’ and ‘the design of the temporary project’.

3.1. The Material Approach

By analyzing and comparing the material approaches of the nine temporary projects, several commonalities and differences were observed.

Table 2. Overview highlighting the presence of recurring material approaches, represented by ‘x’, across the analyzed cases.

	Materialization
	Material Reuse	Tendering	Reuse Handbook	Onsite Reuse	Reuse Network	External Reuse	New Materials	Support Residents
Allée du Kaai	x			x	x		x	x
WTC 1	x			x	x	x	x	x
Molenwest Square	x	x				x	x
Circularium	x			x	x	x	x	x
Le Tripostal	x				x		x	x
Studio Citygate	x			x	x	x	x	x
Circle Park	x					x
FLOW	x					x	x
Maakleerplek	x	x	x			x	x

presents an overview, underscoring the presence of key approaches and aspects of the materialization of the temporary use projects.

This overview underscores the presence of material reuse across all examined cases, manifested through a dynamic interplay among onsite, network-based, and external material reuse strategies. A notable observation is that of the contrasting material approaches between projects initiated via project calls and those through design competitions. The former exhibit heightened collaboration in material reuse within their network, exemplified by initiatives such as material banks or resident engagements. In contrast, projects stemming from design competitions often navigate through a more rigid tendering process, resulting in reduced flexibility, limited spatial experimentation, and diminished resident participation in material integration throughout the temporary project’s lifecycle.

To ensure comprehension and preserve the subtleties inherent in the material approaches, detailed descriptions of the varied approaches are provided on a case-by-case basis, recognizing the uniqueness and contextual dependencies of each project.

3.1.1. Allée du Kaai

To materialize and define the design of the project, the non-profit organization Toestand relied on various actors. For the necessary basic facilities, such as lighting and sanitary facilities, Toestand could rely on the client. For the design and organization of the project, they could rely on their network, which they have built over the years by providing space for people. Members of their network, including various sociocultural organizations and other partners, spontaneously offered surplus or unused materials. Moreover, the choice to work with reclaimed materials was driven by their limited budget, as emphasized by a representative of Toestand. This budget was acquired through subsidies but preferably allocated to social work. However, if necessary, new materials and production tools could be acquired.

Since 2018, Toestand has included a physical material depot, ‘In Limbo’, onsite. In Limbo is a collective material depot that facilitates the reuse and donation of existing materials, building products, furniture, etc., within the sociocultural sector in Brussels. Toestand refers to their design and material approach as an ‘amateur style’ since they build all components themselves. By organizing building activities that are open to everyone, the users themselves become involved and responsible for their space [39] (pp. 131–132).

Due to the temporary setting, Toestand’s emphasis is on minimal functionality, rather than perfection. Their aim is to build quickly, often resulting in easy assembly and thereby removal. According to them, temporality also means manufacturability, as they seize the freedom to experiment with less caution, since there is no risk of failure in the long term. As such, the site becomes a laboratory where failure is allowed and where the space can constantly evolve to satisfy newly emerging needs [39] (pp. 42, 142).

3.1.2. WTC 1

The materialization of the temporary project in the WTC towers started from components that were already present. The temporary use project focused on small interventions showcasing the building’s qualities, often involving more removals, like partitioning walls, instead of additions. Further interior design involved users bringing their own materials and furniture to the site.

The project focused on the environmental impact by utilizing the available materials, but the architectural office did not actively pursue material reuse initially. Experience from the WTC towers fostered an increased awareness of material reuse in subsequent projects and inspired considerations for the future of the materials in the redevelopment of the WTC towards the future transformation project, ZIN.

The temporality of the project had an influence in terms of investments. Additions or removals needed to be cost-effective and commensurate with their project’s duration. Thus, the temporariness fostered modesty compared to permanent setups. Some floors remained as found, devoid of interior arrangement, offering temporary users a sense of freedom.

3.1.3. Molenwest Square

For the architectural office that won the design competition, the temporality of Molenwest Square meant that it had to be constructed quickly and easily dismantled and relocated again at the end. Therefore, the ability to move the entire project to another location was an important boundary condition in the design. The challenge was to find a way to move the project without complete disassembly, while maintaining its shape. In addition, the costs needed to remain low for both construction and dismantling. The site itself was affected as little as possible and was deployed as a play area. Moreover, nature was taken as an advantage.

Since Molenwest Square was a public contract, all components had to be designed in advance, and the materiality had to be defined in the tendering process. Several factors made it difficult to work with salvaged materials, such as tight deadlines and the need for neutral specification texts, common in public contracts. To ensure the correct expertise of the contractors, the architects divided the tender into three parts. The contractors were carefully selected to align with the architects’ vision, particularly regarding the use of recycled containers.

3.1.4. Circularium

The temporary project manager Makettt strived to utilize the existing infrastructure in the best way possible by matching residents to the correct spaces, and thereby reducing the need for interior adjustments and additional facilities. This approach kept the costs low and emphasized material efficiency. Additionally, the residents could then design their allocated spaces in the large warehouse to suit their needs. Since the reusable materials, such as fences and storage racks, remained onsite, they could be collected and distributed among the residents.

Moreover, by selecting residents that valued reuse and a circular economy, Makettt indirectly ensured the environmental impact of the materials in the temporary project. For example, BatiTerre, a resident, operates on the site, selling and reclaiming used building materials. This enables the residents to utilize BatiTerre’s reclaimed materials to design their personal workspaces or implement them in their activities. As such, Regglo, another resident of Circularium, crafts furniture using the materials reclaimed by BatiTerre. Additionally, The Free Shop offers various used materials for free. This thoughtful mix of residents creates a dynamic that promotes a local circular material flow.

3.1.5. Le Tripostal

Over the years, the non-profit organization Communa has built up a large network with other sociocultural organizations and various partners, on whom they can rely in terms of providing and exchanging materials and products. Moreover, the residents of the temporary project were free to design their own spaces. As such, they brought their own materials to the site. However, Communa supported them with their expertise, workforce, and network to provide the needed materials and knowledge. As such, they were responsible for ensuring the necessary adjustments and reparations of the vacant building (e.g., heating, water, electricity, etc.) to make the building functionable.

Reuse is not the main goal of Communa. Their primary mission is to occupy the building and offer the space to the people who need it. Usually, they are required to act in a very short and limited period. In this context, Communa found that it is often simpler to employ new materials rather than to reuse damaged materials that require adjustments and reparations. Furthermore, the time and effort involved in designing a component that is reuseable for future projects can be high.

However, due to budgetary constraints, there is partial reliance on the use of second-hand materials in their projects. Moreover, as Communa gains experience in creative solutions, they strive to incorporate more circular practices for their material usage. As such, they try to reuse as much material as possible from one project to another, and, if this is not feasible, they store standardized building projects and furniture that could easily be reused in the future.

3.1.6. Studio Citygate

Entrakt, a for-profit company, was responsible for defining the main material approach in the temporary project as the building owner, the regional development institute citydev.brussels, was not involved in the functioning of the project. Thus, at the start of the project, Entrakt decided to reclaim some materials, like false ceilings, while construction waste was discarded or downcycled. ‘New’ materials entered the site for the construction of workspaces and offices. For this, Entrakt proactively looked for parties offering materials for free or at a lower price, and they recovered materials from their other projects. In addition, a collaboration with Retrieval, a reclamation dealer, facilitated material recovery from a dismantled building in Louvain-La-Neuve.

However, a large amount of new materials was still purchased for the functioning of the project. As the building has to be left empty at the end of the temporary project, it must be remembered that all components that are brought in must be removed later.

Residents also take responsibility for the materials brought into the project, aiding in space arrangement or activities. Regular deliveries, like pallets, contribute to the materialization of the project. Unused items are stored for the residents, while the rest is collected for periodic disposal. Entrakt and the residents avoid large investments for short-lived outcomes. Thus, they prioritize functionality over aesthetics, often using recovered materials due to economic reasons. Additionally, Entrakt avoids constructing fixed structures to facilitate dismantling at the project’s end.

3.1.7. Circle Park

Entrakt manages both the Circle Park and Studio Citygate projects simultaneously on adjacent sites. Circle Park differs in its location, being built on vacant land, and thus requires various materials to be brought to the site for its functioning and activities.

Entrakt chooses to avoid building fixed structures as they would have to be demolished after the project. They opt to work with second-hand containers due to the ease of transport, the low cost, and the ability to use the containers for multiple purposes. The short project duration is another reason for which they do not wish to make large investments and try to keep the costs low with reclaimed materials.

However, a concrete plan for the future of the used materials, including the containers, a DJ booth, and a grandstand, does not yet exist. It relies on finding a new location for the containers. In Circle Park, more so than in Studio Citygate, the design of the temporary project is a dynamic process that evolves as they respond to the users’ needs and learn from the project’s successes and failures. They are progressively shaping the final project.

3.1.8. FLOW

While this project did not prioritize the use of recycled materials, the aim was to prevent wastage. FLOW was designed with the concept of ‘design for disassembly,’ allowing for its potential relocation in the future, as requested by Pool Is Cool due to the uncertainty of its duration on the site. With a limited budget, reclaimed materials were mainly used from a financial point of view. These materials were recycled from Decoratelier’s old projects or second-hand initiatives. However, many new materials were also integrated into the project.

The design was constructed from a modular grandstand system composed of pallet racks and a timber frame. This structure, known as Scenographies, was developed by Decoratelier. The modularity of the system allows the structures to easily respond to different conditions and be reused at other locations [40]. Pool Is Cool also chose to collaborate with Decoratelier due to their extensive network in Brussels, including young builders, facilitating participatory work. Decoratelier’s approach involves preparing identical elements in their atelier before assembling them onsite. They invite local youth to participate in crafting wooden components, designed for easy reproduction by inexperienced individuals. This inclusive approach fosters community engagement, making FLOW a neighborhood project.

The modularity of Decoratelier’s Scenographies provides flexibility for the project’s future, yet some materials will inevitably become waste. Specific components were tailored to FLOW’s location, making it impractical to reuse them elsewhere. Budget constraints prioritized cost over durability, meaning that some materials may require early replacement.

3.1.9. Maakleerplek

To realize this project, the City of Leuven initiated a design competition. A team, composed of an architectural office, a contractor, and a technical advisor, was selected to create and design the temporary project, which was not only financed by the City of Leuven but also subsidized by Flanders and Europe.

However, with a limited construction budget and project duration, the designers of POLO decided to opt for targeted building strategies to make the site ready for use. In addition, a number of essential interventions and repairs were ensured, such as operational and technical installations and fire safety adaptations.

The designers decided that all adaptations to the site needed to be clearly emphasized. Moreover, together with the City of Leuven, they searched for partnerships to ensure responsible material use and reuse. As such, there was a desire to source materials from other construction sites within the city and reuse as much as possible from the current site. Additionally, since working with leasing contracts for materials is challenging in the context of a public project, the designers decided to work with a buyback guarantee for some materials, such as the scaffolding.

The designers also developed a circular handbook that included an inventory of the components’ dimensions and quantities, a detailed construction methodology, and instructions on how to assemble and disassemble the material components for reuse. For POLO, the temporality of the project did not only mean ‘design for disassembly’ but also robustness in the sense that the materials of the structure had to be strong enough to be reused. With this mindset, the majority of the interventions were designed to be reversible and were guided by the general principle of ‘keeping it simple’.

3.2. Designing the Temporary Project

By analyzing and comparing the material and design approaches, other recurring and differing considerations, strategies, and spatial interventions were observed.

Table 3. Overview underscoring the presence of key observations, marked by ‘x’, in relation to designing temporary use projects across the nine case studies.

	Design
	Temporality and Prolongation	Functionality and Aesthetics	Experimentation	Fixed Design	Low Budget and Investments	Design for Disassembly	Modularity	Reuse	Shared Spaces	Distributed Spaces: Box-in-Box	Distributed Spaces: Partition Walls	Distributed Spaces: Visual Separation
Allée du Kaai	x	x	x		x			x	x	x	x	x
WTC 1	x	x	x		x			x	x	x	x	x
Molenwest Square	x	x		x	x	x		x	x	x
Circularium	x	x	x	x	x	x	x	x	x	x	x
Le Tripostal	x	x	x	x	x			x	x		x
Studio Citygate	x	x	x	x	x			x	x	x	x
Circle Park	x	x	x	x	x		x	x	x	x
FLOW	x	x		x	x	x		x	x	x		x
Maakleerplek	x	x	x	x	x	x	x	x	x	x	x

presents an overview, underscoring the presence of the key observations across the cases.

Throughout the nine case studies, the design observations could be broadly categorized into three main aspects. Firstly, considerations and challenges inherent in the development of the temporary use projects were thoroughly examined and discussed with the interviewees. Secondly, alongside reuse, two additional strategies emerged within the temporary use context, offering potential for future reuse but not consistently implemented. Lastly, a focus was placed on the types of spaces generated by these projects. During the semi-structured interviews, these design observations were extensively discussed, recognizing their significant impacts on project materialization.

To ensure comprehension and preserve the subtleties inherent in the design variations in temporary use, detailed descriptions of the varied approaches are provided on a case-by-case basis.

3.2.1. Allée du Kaai

The layout of the space was constantly changing. If a component was needed, Toestand and the residents briefly discussed it and built it. If it was unsuccessful, they changed it again. Some aspects were kept, and others were added or completely rebuilt. There was no concrete plan at the start of the project regarding how the space should be constructed.

However, there was a general idea to share the space. Rather than dividing it with walls, they shared it among the users, fostering collaboration and allowing for more partnerships as not all users had their own defined rooms. According to Toestand, residents and users themselves are best suited to fill empty spaces and are therefore free to shape the space. The final space layout is illustrated in Figure 3.

The Yellow Atelier (Figure 3, nr 1), an enclosed meeting space built from reclaimed materials, hosted various creative activities in one corner of the open space in one of the hangars. At one point during the project, this space also housed the bar. However, due to the lack of permanence, the bar (Figure 3, nr 3) shifted to several locations before finally settling next to the communal kitchen. The bar was constructed of repurposed furniture, such as kitchen cabinets and cupboards.

The communal kitchen (Figure 3, nr 2) in turn was also enlarged and constructed in a way that divided the large open space of the hangar into smaller compartments. As such, steel frames, wooden boards, and polycarbonate sheets were added to construct the dividing walls [39] (pp. 43–45). Initially, the agreement was that the kitchen’s structure would be demountable and removable, but, eventually, due to the impending demolition of the hangar, some elements were allowed to remain—for example, the steel bars of the kitchen walls.

For the spatial layout, a range of loose and fixed elements were introduced in the hangars of Allée du Kaai. In the central hangar, volunteers built concrete skate ramps (Figure 3, nr 4) using surplus materials donated from a nearby concrete plant. However, in the foundations of these skate ramps, various types of waste, such as old glass, a fallen tree, and a mound of earth, were reused. The loose elements, such as the yellow atelier, the bar, and the kitchen wall, could be recovered at the end of the temporary project; however, the fixed structures, such as the skate ramps, needed to be demolished. Figure 3 visualizes the fixed and loose spatial interventions introduced in Allée du Kaai in red, and it briefly presents the utilized materials for each design intervention.

3.2.2. WTC 1

At the beginning of the temporary use project, some floors were divided into small office spaces, while others were left vacant and stripped to the shell. Major players like the architectural and design offices 51N4E, Architecture Workroom Brussels (AWB), and Vraiment Vraiment reinterpreted the space with minimal intervention and created shared areas. They removed walls and added elements like curtains (Figure 4, nr 1), plants, tables (Figure 4, nr 2), and new walls, transforming the space into a hybrid space where the residents organized their areas. Moreover, 51N4E provided support by creating a model of the floors, acting as a toolbox to test different layouts. The primary aim was to establish a sense of hybridity by positioning shared facilities and considering private and individual workspaces.

The 16th floor, housing 51N4E and AWB, featured architectural considerations like meeting spaces, a shared garden, and a library. Additionally, 51N4E created an experimental greenhouse (Figure 4, nr 3) by replacing the infills in old partition walls with plastic films, allowing light entry. However, the floors where smaller actors were located rather exhibited the functional use of the space.

For the ‘You Are Here’ exhibition, minimal interior decoration was added, which could easily be removed later. In addition to the strategic placement of carpets and cabinets, large hanging curtains contributed to the perception of different spaces across the large open floors.

Additionally, there was an exhibition box (Figure 4, nr 4), The Replica, an installation created by Recyclart FABRIK, replicating their former café storefront. This box-in-box structure hosted art exhibitions on the first floor of WTC 1’s former bank space. Constructed mainly from wooden and metal materials, likely new, they were reclaimed by Recyclart afterward [40]. Figure 4 visualizes the described spatial interventions in WTC 1 in red, and it briefly presents the introduced materials for each design intervention.

3.2.3. Molenwest Square

The designers started from vacant terrain that could take many different shapes. To create interior spaces, they opted to quickly convert containers (Figure 5, nr 1) due to the requirement for easy mobility. Two containers were joined side by side to create a wide space. In addition to the necessary facilities, these classrooms contained a workspace for craft workshops or a space to assist children with homework. Between these containers, they placed an arched roof (Figure 5, nr 2) to create a covered outdoor play area for the neighborhood.

They used reclaimed containers because these were available in large stock, and it was appealing that they already had a history. They needed containers of two meters and ninety centimeters high, instead of the classical container of two meters and forty-five centimeters, to maintain the proper height after a finish layer was added. The containers were then joined two by two. This was achieved by cutting out a side wall and then anchoring both containers with bolts. A small column was placed to ensure the strength of the structure. Thus, when the project must be dismantled, it will simply require the loosening of the bolts and the removal of the rubber seal between the two containers. Consequently, the containers, including their finishes, can be moved again one by one on a truck.

The finishing of the containers was provided by sandwich panels. These were all new materials, but they were attached in such a way that they could be disassembled. This was achieved by connecting the sandwich panels to a rail below and above. The panels consisted of a metal finish and rock wool inside.

The structure of the arches that provide shelter in the open space was constructed from a prefabricated Meccano system. This is a standard system that is often used for temporary storage in outdoor spaces. This arch structure can be completely disassembled and rebuilt. Figure 5 visualizes the described spatial interventions in Molenwest Square in red, and it briefly presents the introduced materials for each design intervention.

3.2.4. Circularium

To design this temporary project, Makettt assigned residents a surface area in the existing building and provided guidance on space design and technical aspects. The planning of the layout involved mainly logistics, such as coordinating deliveries to prevent resident interference and ensure easy access to supplies.

Recovered fences (Figure 6, nr 1), initially situated on the roofs and around the plot’s perimeter, serve as materials to divide the warehouse. As such, these fences partition the large open area into smaller sections. Additionally, Makettt seeks residents who can use the space as it is. For example, the Microfactory, a multidisciplinary production place, occupies various sections of a former car dealership, which used to include a spray booth, metal workshop, and garage joinery. The newly designed space is structured into a small makers’ village, offering co-working spaces for craftspeople around organized storage racks. Here, individual makers can rent small workspaces positioned on, under, and around these racks. Larger ateliers, like textiles and ceramics studios, are defined by reused partition walls (Figure 6, nr 2), salvaged from an old school, creating smaller offices within spacious areas.

To create comfortable office spaces within large workshops, smaller enclosed areas were built using the box-in-box principle (Figure 6, nr 3). These offices were constructed atop or beneath the existing steel structures of the car storage racks. The start-up Konligo, developing innovating temporary scissor structures for events, sustainably built their office focusing on future adaptability and material reusability from the design stage onwards. It features reversible mechanical connections for easy modification and includes reclaimed materials salvaged by RotorDC and In Limbo, which are reclamation dealers in Brussels [41].

The common kitchen area (Figure 6, nr 4) is enclosed by a wall produced with repurposed high-speed cross-laminated timber (CLT). The integrated CLT was repurposed from a modular pavilion for the Habitat Festival in Leuven by the collective bøøt, which sewed the CLT beams into building blocks. This approach of reusing CLT beams as building blocks facilitates simple assembly and disassembly.

In the outdoor area, scaffolding (Figure 6, nr 5) was reused to create an interesting structure that acts as meeting place for the residents. Figure 6 visualizes the described spatial interventions in Circularium in red, and it briefly presents the introduced materials for each design intervention.

3.2.5. Le Tripostal

At the beginning of the temporary project, there was little planned for the layout of the large open space of Le Tripostal. Therefore, at the front of the building, eight boxes (Figure 7, nr 1) were built to divide the open space. These boxes were allocated to various associations, where they could decide and build themselves the interior arrangements according to their needs. However, Communa learned that allowing the various organizations to conduct the building was not the most efficient means of working. Firstly, time is required for different organizations to start to work together. Moreover, all individuals must follow the rules regarding legislation and fire safety. Thus, to inform and support the various organizations in the project, and to work more efficiently in the limited period, a technical team oversaw the building and unbuilding of components (e.g., the addition of interior walls to create the boxes).

In addition to the boxes, a bar (Figure 7, nr 2) was constructed in the polyvalent space, where events were organized. This bar was produced from recuperated materials only. As such, 200 wooden slats, recovered from old furniture, became the cladding of the new bar. In addition, a cloud-like form was created from 500 jerry cans that hung above the bar and acted as a chandelier.

In their hallway, which sometimes was transformed into an exhibition space, Communa decided to create some wooden constructions from reclaimed materials (Figure 7, nr 3), which acted both as benches and space dividers. In addition to these temporary lightweight structures, fluid elements such as curtains were also introduced as space dividers.

To make the building accessible to everyone, a ramp was constructed in concrete at the entrance. Moreover, a lift with three steps was bought to ensure accessibility.

Figure 7 visualizes the described spatial interventions in Le Tripostal in red, and it briefly presents the introduced materials for each design intervention.

3.2.6. Studio Citygate

The layout of the spaces in this large building was not straightforward and consequently involved some planning. The intention was mainly to create a logical division, aligning the advantages of each space to the appropriate user. For example, the basement was divided into music studios to avoid noise pollution.

Upon entering the building, Entrakt encountered vast open floors that were later divided into smaller atelier spaces using metal-framed partition walls with OSB boards (Figure 8, nr 1). The open space on the third floor was more difficult to divide into an office floor. Moreover, office spaces demand higher comfort than craft workshops. A solution was provided by the Be-Modules (Figure 8, nr 2), designed by Dzerostudio Architectes, as modular office boxes of 20 m² each. They are dismountable, constructed without glue or nails, using mostly salvaged materials from a building in Louvain-La-Neuve. Retrieval collaborated in material recovery, including white melamine panels, windows, glass doors, etc. The former false ceiling of the space was dismantled, and the panels were used as insulation for the Be-Modules. Entrakt wishes to disassemble the modules and transfer them to another project. However, this building was yet to be found at the time of the interview.

The residents built their own communal kitchen with materials provided by Entrakt. The openings were constructed with windows taken from the building in Louvain-La-Neuve. Furthermore, they built a concrete bar in the event hall. This will have to be demolished at the end of the project.

Studio Citygate has an indoor and outdoor skate park. The creator of the skate parks used palettes as the base structure and repurposed leftover granite pieces from a tombstone maker for skateboarding surfaces. While parts of the indoor wooden park may find a new use, the outdoor concrete park will be demolished. The concrete contains various debris, like bricks, cellular concrete blocks, and porcelain toilet bowls from the building’s basements [42,43,44]. Figure 8 visualizes the described spatial interventions in Studio Citygate in red, and it briefly presents the introduced materials for each design intervention.

3.2.7. Circle Park

Initiating a temporary project on vacant terrain differed from doing so within a vacant building. It required more creativity to visualize how they wished to design the place. They decided to divide the site into several zones: a container village, a sociocultural event zone, and a sports area.

Their initial need was shelter for visitors and temporary users from the rain. Opting for second-hand containers (Figure 9, nr 1), they found an easily removable solution for the short-term project. However, in winter, these containers remain empty due to a lack of insulation. Positioned at both entrances, three containers stacked atop each other serve as eye-catching gateways. The container village provides offices and workshops for makers. Alongside the concert area, a line of open-sided containers offers space to relax and enjoy drinks, creating cozy corners. Some containers were also converted into a bar or food stand. The relocation of the containers after Circle Park has not yet been planned. These shipping containers are not demountable but can easily be moved as one box on a truck. The advantage of the containers is that they can be used for various purposes. Whether they will be sold or moved to another project remains to be determined. Of course, transportation carries financial costs, so it would be preferable to move them immediately for a period of more than one year.

Concerts and parties frequently take place at Circle Park. Consequently, a covered DJ booth (Figure 9, nr 2) proved to be a valuable addition. This canopy is composed of a metal frame supporting wooden beams and corrugated polycarbonate sheets, likely newly purchased.

The beach volleyball courts (Figure 9, nr 3) are encircled by salvaged train ties. The purchased sand will remain there and can be reused for the future construction site. Figure 9 visualizes the described spatial interventions in Circle Park in red and briefly presents the introduced materials for each design intervention.

3.2.8. FLOW

The outdoor swimming pool of FLOW (Figure 10) was constructed with the modular system Scenographies designed by Decoratelier [45]. The interplay between seeing and being seen is typical of Decoratelier. This concept is also conveyed in the design of FLOW. Inspired by old swimming pools, such as that of Sint-Joost in Brussels, FLOW has a passerelle. From the passerelle, one can see from the upper to the lower level and vice versa. This relationship is very important also because the viewer is not seen equally well everywhere. Therefore, different locations in the project offer a different experience. There is a stage where one can be seen from the center of the space, but there are also more secure places where one can hide from the crowds.

Most of the wood used for the floors, podium, stairs, and walls is new material. The intention is to reuse this wood in the future when the project relocates. However, challenges arise due to the quality of the inexpensive pine, which is mostly untreated. The planks aligned with the pallet rack rhythm are more likely to be reused, while wood of specific sizes may be lost during relocation. The pine multiplex panels in the changing rooms are easily removable and placed between the rack structures.

The stairs were one of the main elements that were predominantly prepared in the atelier. For the atelier, there was a list detailing the specific elements required. Decoratelier then created diverse templates indicating precise locations for screwing or drilling. These templates enabled the production of many identical elements that could be assembled later. As a result, all of the stairs were crafted using approximately three distinct shapes. This approach facilitated the involvement of inexperienced young individuals from the neighborhood in the construction of the swimming pool. The same method was also applied to the balustrades. A specific template was produced in which the bars were placed to create the desired shape. These bars were sourced from the port of Antwerp and originally intended for transportation.

FLOW features a small kiosk doubling as a bar, designed by Italian artists using arches sourced from a nearby construction site, reinforced with new screed reinforcement nets from Pool Is Cool’s stock. The roof is a patchwork of surplus liner off-cuts. Containers were initially used for material storage during construction and later repurposed for valuable items and additional storage needs. These containers were either bought second-hand or rented. Figure 10 visualizes the spatial intervention in FLOW in red and briefly presents the introduced materials in Scenographies, the modular tribune system.

3.2.9. Maakleerplek

Since Maakleerplek was created in the context of a design competition, its spatial organization and material implementation choices were made from a designer’s perspective. As such, four main strategies were put forward by POLO: the scaffolding, the circular façade, the containers, and the wall. Moreover, these strategies followed circular principles since this was one of the objectives of the City of Leuven.

The four building strategies restructure the space and provide a distinctive identity to the project. The ‘scaffolding’ (Figure 11, nr 1), for instance, primarily serves as an improvement to the circulation. Here, an external tailor-fit elevator was introduced to make the project accessible for all. In addition to the circulation, the scaffolding acts as a visual landmark since it stands outside, is covered by printed banners, and is illuminated at night. In fact, the addition of scaffolding, a standardized and modular structure, is a well-known and normal practice in construction.

However, the designers also included other creative solutions to improve and design the interior spaces of the former mill. As such, the ‘circular façade’ (Figure 11, nr 2) building strategy, employed by the designers, enclosed the large space beneath the silos. This demountable system consisted of translucent polycarbonate panels and steel profiles. Moreover, the reuse and modularity of the components was ensured since there was a desire to repurpose the components in the later phase of the temporary project, when the transformation of the silos would begin, to construct a greenhouse-like roof in a courtyard on the site.

Underneath the silos, there was a cathedral-like space that was transformed into a large workshop. The large space was divided into several zones, including a low-tech lab with a workshop for makers and repairers. This zone was demarcated by ‘the wall’ (Figure 11, nr 3), a structure composed of vegetable crates filled with rock wool to dampen the noise. The crates used were acquired with a buyback guarantee, ensuring that they could be repurchased at 60% of their original value at the end of the temporary project. Moreover, a part of the crate wall was rebuilt at another location that needed the temporary compartmentalization of a large space.

In addition to the crate wall, new demountable and adjustable partition walls, designed by JUUNOO, a Belgian start-up, were installed around the sanitary area, kitchen, and cafeteria.

Another method employed to partition the large space was the implementation of a box-in-box system, which they called ‘the containers’ (Figure 11, nr 4). For this, former shipping containers were repurposed and served as workshops, offering improved interior comfort. After being crafted by the Belgian company IdFabric in Tanzania, they were delivered as fully assembled units, complete with the necessary facilities and the ability to connect to water and electricity suppliers. In addition to the containers, second-hand greenhouses were introduced in the space to create smaller enclosed working areas. Figure 11 visualizes the described spatial interventions in Maakleerplek in red and briefly presents the introduced materials for each design intervention.

4. Discussion: Prototyping a Framework of Guidelines

The analysis of the case studies reveals that each temporary project is unique and depends on its context and actors. However, multiple recurring elements and processes have been observed in relation to crucial aspects. The materialization and the design perspectives were discussed in depth in the previous section. However, a third perspective emerged, encompassing the stakeholders. Partnerships and collaborations with various entities are essential in ensuring responsible material use and reuse. As such, the proposed framework of guidelines is structured around three crucial perspectives: (1) materialization, (2) design, and (3) stakeholders.

Together, these three elements constitute a comprehensive framework enabling practitioners to co-create a common vision for the materialization and design of material-wise temporary projects.

Table 4. Summary of guidelines, attention points, and actionable steps to assess and develop temporary use projects, presented according to three perspectives: materialization, design, and stakeholders.

Materialization	Design	Stakeholders
Case: Project call Step 1. Asses onsite reuse potential. Step 2. Reuse within network. Step 3. Search for external reuse opportunities. Step 4. Use new materials responsibly. Step 5. Encourage and support material reuse of residents. Case: Design competition Step 1. Design with reuse. Step 2. Detailed reuse description in tendering. Step 3. Develop reuse handbook.	Considerations 1. Temporality and prolongation 2. Functionality and aesthetics 3. Experimentation and fixed design 4. Low budget and investments Circular design strategies - Design for disassembly - Modularity Spatial interventions - Shared space - Distributed space (box-in-box, partition walls, visual separation)	Client Temporary project manager Resident/User Designer …

presents a summary of the guidelines across the three perspectives and their actionable steps, considerations, and attention points.

Practically, an overview of these guidelines and their critical elements that can be considered are presented and discussed below. Following the cases studied above, these critical elements point towards the best practices and points of attention. For this reason, they are formulated here as possible recommendations.

4.1. Materialization of Temporary Use

Several steps have been observed for the materialization of temporary use projects. This results in the proposal of a responsible material workflow when developing temporary use projects and integrating materials for spatial arrangement. We focus first on projects initiated through project calls and look afterwards at the process in design competitions.

4.1.1. Workflow for Temporary Use Projects through Project Calls

STEP 1. Assess the Potential of the Existing Space and the Onsite Materials

First, to organize and create a temporary project, various building materials and products need to be acquired and brought to the existing site, regardless of whether it is an unused building or vacant plot of land. However, in the case of organizing a temporary infill in an existing building, lower quantities of materials might be needed. Nonetheless, in both cases, the temporary project managers or designers start by assessing the opportunities of the existing space and the reuse potential of the existing materials onsite and within the existing building.

As such, in Circularium, the original fences were reused to create partition walls and to divide the large open space of the hangar into smaller sections. In Studio Citygate, the false ceiling panels could be deconstructed and reused as insulation in the BE-Modules, which were developed according to the ‘box-in-box’ principle and acted briefly as office boxes.

In Studio Citygate and Allée du Kaai, few or no large quantities of reusable building materials remained onsite. However, several materials perceived as construction waste that remained onsite could be reused by integrating them into the foundations of the concrete skateparks. As these permanent structures were built, it was noticed that, with the prospect of demolition, temporary use would be more invasive, and the existing buildings were handled with less caution compared to when a renovation was planned.

STEP 2. Look for Potential Collaborations for Material Exchange within One’s Network

After assessing the onsite materials’ reusability, the interviewed actors—mostly the temporary project managers—mentioned that they relied on their vast networks of partners and past collaborations for the donation of second-hand materials and products. This is because they often operated with limited financial capacity. As such, their own previous temporary projects, former residents, sociocultural initiatives, and other organizations and companies are resources that can be called upon to determine the availability of a suitable material or product that could possibly be reused.

In this manner, the furniture and exhibition walls constructed of reused wooden boards in Le Tripostal were collected, transported, and reused from their current project, Maxima. In Circle Park, the grandstand was a donation from a theater initiative. Moreover, in Allée du Kaai, the initiative In Limbo, a material bank, was housed, which supplied various organizations with reclaimed materials and products for their temporary project.

STEP 3. Look for External Deconstruction Projects

In addition to the network of temporary project managers, external projects can be a source of reclaimed building materials and products. In addition to interior elements, such as doors, radiators, sanitary facilities, and furniture, larger building products and materials, such as partition walls, glazing, wooden cladding, etc., are also increasingly salvaged in larger transformation projects and can thus be found on the reclamation market.

This resulted in the reuse of reclaimed partition walls in Circularium from a former school for the construction of the communal kitchen. In Studio Citygate, the BE-Modules integrated reused finishing and glass panels, reclaimed from a building owned by the Wallonia-Brussels Federation. In addition, former sea containers were reused in the temporary use projects Circle Park, Molenwest Square, and FLOW.

STEP 4. Look for New Materials and Use Them Responsibly

Not all necessary materials are found on the reclamation market. As such, new materials and building products need to be acquired for the construction of some interior arrangements and technical installations in temporary projects. For instance, new OSB boards were bought to create partitions and construct a wall between the existing columns in Studio Citygate.

In some cases, the temporary managers decided to acquire specific new materials due to the time constraints regarding the start of the temporary project. For the non-profit organization Communa, for instance, adopting a reversible approach and reusing materials is considered a bonus rather a primary concern, since their main ambition is to have a positive impact on the neighborhood and focus on community engagement.

STEP 5. Support Residents in Making Responsible Material Choices

A temporary project’s residents are responsible for the design of their allocated spaces.

In large hangars and warehouses, interior arrangements and office boxes are necessary to divide and design an open space. Through the well-defined and common ambition of reuse, temporary projects can promote the local flow of materials among residents. Moreover, temporary project managers and designers can support and guide residents in making well-informed design decisions.

In the case of Circularium, the residents were responsible for the design and construction of their enclosed workspaces. As such, the residents Konligo, Regglo, and MicroFactory designed their own workspaces. Moreover, as the reclamation dealer BatiTerre was one of the other residents, material reuse was facilitated.

In Studio Citygate and Le Tripostal, it was the temporary project managers that divided the open floorplan and created individual ateliers. To support the residents, Communa has a technical team that is responsible for ensuring that construction takes place according to the regulations and norms.

In Allée du Kaai, a collaboration was observed between Toestand and the residents. This collaborative experimentation resulted in the continuous transformation of the space and collective building and teaching moments.

4.1.2. Workflow for Temporary Use Projects through Design Competition

A noteworthy aspect to highlight in the process of acquiring and (re)using building materials is that the workflow differs when the temporary project is a public contract in collaboration with the government and when designers are involved in the temporary project’s creation.

In two cases, a competition was launched for the design of the temporary project and its construction. In particular, for Maakleerplek Leuven and Molenwest Square, the ambition was to design a sustainable temporary project that adopted a circular approach and prioritized reversibility. Due to these objectives set by the public authorities, the design and architectural offices decided to design as much as possible with second-hand materials and demountable or reusable structures. However, due to the context of designing within a competition framework, the design and the necessary building materials and products had to be partially determined before starting the development of the temporary project.

In Maakleerplek, the designers even developed a circular handbook that included an inventory of the components’ dimensions and quantities, a detailed construction methodology, and instructions on how to assemble and disassemble the material components for reuse.

4.2. Design in Temporary Use

In this section, three observed critical elements in the design that could or should be considered when designing temporary use projects are presented: design-influencing aspects, circular design strategies, and spatial interventions. These critical elements point towards best practices or points of attention.

4.2.1. Aspects to Consider during Design

Consideration 1. Temporality and Prolongation

The phenomenon of temporarily reactivating unused buildings and sites is becoming common practice. The actual duration of such temporary use projects is often uncertain, resulting, in many cases, in the prolongation of the initial activation period. For example, Allée du Kaai was initially planned for a period of four years but was ultimately able to host its activities for eight years (2014–2022). Similar prolongations can be seen in the projects Circularium, Studio Citygate, Circle Park, FLOW, and Molenwest Square.

The uncertainty about the duration of the actual activation period, coupled with the awareness that the project is a temporary infill in anticipation of a building or site’s future transformation, influences the ways in which materials are applied in various aspects of the design of a temporary project. In many cases, the temporary project needs to be set up within a short notice period, where limited time is available for design. This usually results in fast decisions for spatial interventions and structures that are not constructed to last.

By designing for temporality at the beginning of a temporary use project, the necessary structures could be built, adapted, and improved when the temporary project is prolonged, or they could be disassembled and removed at the end of the project’s activation period.

Consideration 2. Functionality and Aesthetics

The functionality of the space should take priority over its aesthetics. It is not necessary for all components to be perfectly finished; they only need to function properly. This was clearly emphasized by the various temporary projects’ managers, such as Toestand, Communa, and Entrakt, when discussing the temporary projects that they coordinated.

However, the stakeholders also emphasized that the spaces needed to be clean and that a certain aesthetic is important to ensure visitors and users of the temporary project.

Consideration 3. Experimentation and Fixed Design

Temporary use offers the freedom for the spatial arrangement to evolve according to the changing program and to be easily adapted according to the residents’ needs. Without the constraints of long-term planning and the pressure of perfection, the existing space becomes a platform for experimentation and learning, allowing for a better understanding of what is successful and what is not.

Consideration 4. Low Budget and Investments

Temporary projects’ managers and residents see little value in making large investments for short-term projects. Therefore, a low-budget approach seems to be taken. This includes the reuse of common reclaimed building materials and products. It must be mentioned that this type of reuse falls into the low-budget category as it is implemented in the context of do-it-yourself activities and bricoleurs, which involve individuals reusing small quantities of reclaimed building materials and not large-scale construction projects.

A prime example of this experimentation and low-budget approach is the project Allée du Kaai. The freedom to experiment was secured by the lack of a predetermined design for the large warehouses. The openness of the design program and the physical space allowed constant changes in the layout of the site and the division and arrangements of the space over the years.

Moreover, initially higher investments in the design of circular and reusable structures can lead to a cost reduction in the long term. Examples include the tribune system in FLOW; the walls, such as the crate wall in Maakleerplek and the CLT block wall in Circularium; and the box-in-box offices (e.g., Konligo’s office in Circularium, the yellow atelier in Allée du Kaai, the use of containers, etc.).

4.2.2. Circular Design Strategies to Consider

Design for Disassembly

Experimentation and constant adaptation could result in a large amount of waste production if the materials’ and products’ integration hinder the adaptation and adjustment of the already constructed structures. Fortunately, permanent structures, such as concrete or stone walls, are avoided. This decision, arising due to the temporary nature of the project, ensures the low-budget approach as well as the objective that all components that are implemented in a temporary project can be extracted at the end of it.

Moreover, the latter is the main reason that constructions that are light and easy to dismantle and deconstruct are preferred. As such, design for disassembly or deconstruction (DfD) strategies are key, implemented in a certain way in all of the nine temporary projects studied.

Notable examples are the office boxes of Konligo, a resident in Circularium, and the BE-Modules in Studio Citygate. These offices were built with reclaimed building materials, such as wood, aluminum, and even steel, and incorporated reversible connections that allow easy reconfiguration and dismantling when they need to be moved.

Modularity

Additionally, modularity proves to be an asset in temporary projects. This strategy allows, in a similar manner, easy disassembly and assembly or downscaling and upscaling. Examples of this strategy are the repurposed cross-laminated timber (CLT) beams used to create a CLT building block wall in Circularium for a communal kitchen and the division walls in Maakleerplek, which were built by reusing and stacking standardized vegetable crates. Furthermore, the case of the modular tribune system, Scenographies, in FLOW highlights the important aspect of upscaleability through modularity.

Moreover, in some cases, especially those where a temporary use project is organized on vacant land, containers seem to be the main choice to quickly construct indoor spaces, sheltered from outdoor conditions. In fact, the reuse of containers aligns with the principle of modularity and easy assembly and disassembly, as these containers can be stacked on top of each other and can be easily reused elsewhere.

In conclusion, a design for temporality is approached in two manners. The intention is to design and construct either a system that can be dismantled into separate components for diverse reuse possibilities or a system that can be dismantled and reconstructed elsewhere with limited modifications.

4.2.3. Spatial Interventions to Consider

The Creation of Shared Spaces

The notion of shared space emerges as a crucial aspect in temporary use projects. These shared spaces differ according to the temporary use project and its residents. As such, hybrid and evolving common spaces emerge and are facilitated in projects where collaboration and exchange between the residents is encouraged (e.g., Maakleerplek, Circularium’s Microfactory, Allée du Kaai, etc.). These spaces are open workspaces and allow the residents to freely utilize the existing space, according to their needs.

In other cases (e.g., Studio Citygate, Circularium, and Le Tripostal), the residents are allocated to specific zones by the temporary projects’ managers, taking into consideration logistical movement and the existing infrastructure. However, a common trend of disconnecting heavy and loud workspaces from quieter and more tranquil offices and ateliers is identified.

The Creation of Distributed Spaces

In addition to common spaces, the recurring phenomenon of compartmentalization was identified in all analyzed cases. Since the existing spaces are large open areas, a need arises to divide the existing space into smaller, more human-scale areas. Three different methods were distinguished to shape the spatial layout during temporary use.

First, the space is compartmentalized through the installation of walls or physical barriers. Within this approach, a further distinction can be made between partition walls and walls constructed using building blocks. In the former scenario, the partition walls are typically constructed using lightweight structures, such as the OSB panels used to separate the workshop spaces in Studio Citygate, the wooden walls in Le Tripostal, the reused partition walls in Circularium’s Microfactory, or the innovative circular JUUNOO walls in Maakleerplek. In the latter scenario, the walls consist of modular building elements creatively crafted from reclaimed materials. Examples include CLT block walls and walls produced from jerry cans, crate walls, and containers.

Second, the ‘box-in-box’-principle is employed to enclose and insulate spaces, such as offices, in large open areas. This approach is often necessary due to the poor conditions of the buildings (e.g., insufficient insulation and openings with drafts). By creating such smaller enclosed spaces, a more comfortable interior climate can be achieved, thereby eliminating the need to heat the entire space. Examples include the BE-Modules in Studio Citygate, Konligo’s office in Circularium, and the reconverted containers in Maakleerplek.

Third, the more subtle division of the space can also be observed. This entails strategically placing certain objects in the space to create a visual and tangible sense of separation. These minimal interventions, such as shelving units, planters, curtains, carpets, etc., help to organize the space. WTC 1 provides a good example of how small and limited interventions can still be very effective in transforming floors into dynamic and versatile spaces. Similarly, in Le Tripostal, the exhibition area takes shape through the placement of benches and curtains.

4.3. Stakeholders in Temporary Use

The responsibility for the design of the temporary use project, the definition of the spatial layout, and the compartmentalization vary from project to project. From the analyzed temporary use projects, several actors were identified that play a role in the design of a temporary use project. Accordingly, the identified actors and their responsibilities are elaborated.

4.3.1. The Client

The first observation is that the clients (e.g., local policymakers, municipalities, private building owners) do not take responsibility for designing the temporary use project themselves. However, they do have the responsibility to put forward sustainability goals for the project and to ensure that the temporary project managers implement them. As such, in some cases, the clients financially support the temporary project managers. These partnerships can be identified as ‘controlled’ collaborations. This was the case for Allée du Kaai, WTC 1, and Circularium. However, the main objective of introducing a circular approach in the project was only present in Circularium. In the other temporary projects, the objectives of the temporary projects remained limited within the predefined themes for the temporary activities.

4.3.2. The Temporary Project Manager

It is the temporary project managers that usually create and define the layout of the spaces. As such, they are not only tasked with ensuring the coordination and the daily functioning of the temporary use project, where they manage the existing building and facilitate the activities of the various residents, but they also make design and materiality decisions.

In the case of WTC 1, little space designing was needed since the existing buildings were already well equipped for various activities. Small interventions and redecorations were introduced by the coalition of organizations managing the project.

For the other temporary projects, however, the spaces needed to be designed and spatial interventions were implemented. In Studio Citygate and Circle Park, the temporary project’s manager, Entrakt, was responsible for this. In Studio Citygate, they decided to divide the existing open space into smaller workspaces and ateliers using OSB panels to create walls between existing concrete columns, whereafter they could initiate an open project call to find suitable residents. For Circle Park, as it was a vacant plot of land, they looked for structures that could be easily transported onsite, were flexible in use, and could be easily removed and reused afterwards. With these objectives, they decided to acquire and reuse containers. The main objective of Entrakt was to start the temporary project as soon as possible. The circular principles that they introduced were primarily due to budget and time constraints. However, as both projects evolved, the importance of materiality and circularity increased, and more awareness was given to these aspects.

Makettt, the manager of Circularium, on the other hand, first further developed the client’s ambitions regarding circularity, favoring local resource loops, and productivity. They searched for residents whose activities could be organized in the existing industrial setting and aligned with the former productive activities. As such, residents were favored that needed large open workspaces. Therefore, few modifications were needed to the existing building. Moreover, by setting certain objectives related to material usage and sustainability, Makettt selected innovative start-ups that shared the same mindset and ambition to shift towards a circular economy. This resulted in selecting residents that had material knowledge and who were eager to experiment with circular construction.

4.3.3. The Residents and Users

The residents play an important role in designing the temporary use project and their allocated spaces. However, their approach towards circularity, reuse, and design for deconstruction depends on their interests, ambitions, and knowledge. In Circularium, the various start-ups were eager to experiment with circular construction. As such, innovative structures were constructed onsite following the box-in-box principle and both modular and DfD strategies.

Meanwhile, in Allée du Kaai, collective building events were introduced with the residents as well as the local community to provide training about various building techniques. During these events, the residents collaborated in designing and building the temporary use project.

In other cases, such as Studio Citygate, Le Tripostal, and WTC 1, the residents did not have a notable influence on the spatial design of the project. They were only responsible for furnishing their allocated spaces.

Moreover, in the cases of Maakleerplek, Circle Park, FLOW, and Molenwest Square, the temporary project designs were established prior to the selection of residents. As such, the residents were not included in the design process.

4.3.4. The Designer

Lastly, with the increased recognition and implementation of temporary use, the emergence of design competitions for temporary use projects has been noticed in Belgium. As such, designers and architects are increasingly becoming involved in designing short-term projects that reactivate vacant areas. Temporary use projects were previously a bottom-up activist statement in response to wasted infrastructure. Nowadays, they are increasingly introduced by building owners and public authorities. These stakeholders have the power to raise the ambitions and demand sustainable and efficient material use in temporary projects. Through design competitions, designers and architectural offices propose their interpretations of the temporary use project; they develop an approach to the spatial layout of the existing space from an architectural and material perspective. As such, they can include circular design strategies, as observed in Maakleerplek and Molenwest Square.

Moreover, in FLOW and WTC 1, the designers were responsible for coordinating the temporary use projects. For WTC 1’s spatial design, little was needed. The designers aimed to use the space as it was and only included minimal interventions (e.g., curtains, plants, and bookcases). For FLOW, a more detailed design was needed. The designers developed a modular system around the swimming pool that could be disassembled if needed.

5. Evaluation and Validation

The current guidelines and roadmaps for temporary use primarily focus on standardizing practices and raising awareness [30,31,32,33,34], but they lack integration across three crucial dimensions, namely materialization, design, and actors, along with their corresponding steps and critical elements. The framework proposed in this paper fills this gap by providing a proof of concept for the analysis and fostering of temporary use projects through these interconnected lenses. The resulting compendium of guidelines, attention points, and actionable steps aims to support researchers and stakeholders involved in temporary use projects, including clients, project managers, residents, and designers, in their endeavors towards material-wise temporary use projects.

A prototype of this framework underwent evaluation during a workshop involving four stakeholders, including two temporary project managers, a developer, and a planner from the Brussels planning department. During this workshop, the stakeholders assessed the feasibility and utility of the proposed framework.

Initially, the stakeholders expressed appreciation of the research efforts and the proposed framework’s focus on the design aspect of temporary use projects. They emphasized the significance of tools and guidelines offering insights into safety and legislative considerations in the field of the temporary use of vacant buildings.

Furthermore, the stakeholders highlighted the value of a framework that emphasizes the material perspective in the design of temporary use projects. They suggested that such a framework should not be rigidly structured as a checklist, given the uniqueness and contextuality of each temporary use project. However, the stakeholders endorsed the idea of a pamphlet showcasing pioneering projects and innovative approaches integrating circular design and reused materials in temporary use initiatives. Additionally, they proposed that this pamphlet should offer practical solutions for safety, heating, and other relevant aspects.

In conclusion, translating the prototyped framework into user-friendly guidelines could provide a valuable tool for the design of material-wise temporary use projects. By showcasing inspiring practices of material reuse and offering practical solutions, such guidelines have the potential to elevate temporary projects into catalysts for circular cities, where resources are valorized and reused in closed cycles.

6. Conclusions

In this comprehensive study, we moved beyond the prevalent emphasis on awareness raising and the standardization of temporary use to investigate the critical yet understudied domains of materialization and design within this context. Drawing upon an in-depth case study of pioneering practices in and around Brussels, complemented by insightful interviews, discussions, and site visits, a framework of guidelines articulated around three pivotal areas—materialization, design, and stakeholders—was prototyped.

Firstly, our examination of materialization revealed two distinct practices for the initiation of temporary use projects: project calls and design competitions. We identified recurring steps within these practices that contribute to actions for the materialization of projects, with a focus on reuse and the reduction of material impacts. Additionally, we highlighted the importance of setting high standards for circularity at the outset of the design competition and the development of a reuse handbook to facilitate deconstruction and material reuse.

Secondly, our study uncovered several design elements relevant to material-wise projects, which were translated into guidelines and subdivided into concrete attention points. These included considerations such as temporality, circular design strategies, and spatial design to create shared and distributed spaces.

Lastly, we identified key stakeholders—clients, temporary project managers, and designers—and their responsibilities in driving sustainability and circularity objectives within temporary use projects. Establishing common ambitions for circularity and material usage emerged as a crucial step towards realizing material-wise temporary projects.

Our prototype framework, introduced for the first time in academic research, serves as a proof of concept for the analysis and fostering of temporary use projects through three interconnected lenses. The resulting guidelines, attention points, and actionable steps aim to support researchers and practitioners—including clients, project managers, users, and designers—in designing material-wise temporary use projects. Translating this framework of guidelines into a user-friendly pamphlet could further support practitioners in innovatively addressing design and material reuse challenges in temporary projects.

While rooted in the analysis of nine projects in and around Brussels, we recognize the potential for broader application and adaptability across diverse cultural and geographical contexts. Even if limited by the number of case studies, the study revealed recurring aspects that likely exist in other temporary projects. Therefore, we advocate for the continued refinement and application of such guidelines in various settings and sectors, expanding the scope to encompass a wider range of temporary projects. We suggest the further investigation of this framework in two ways: firstly, by evaluating its usefulness for new initiatives; secondly, by employing a retrospective approach, utilizing the framework of guidelines to analyze and assess past temporary projects.