With the acceleration of global urbanization, most of us will live in cities in the future. As the urban population grows, the shortage of residential buildings has become a more and more severe problem. In response to the rising demand for housing, more and more builders in Europe, East Asia, Australia and other places have begun to adopt a method called modular construction to build houses. By pre-constructing building components off-site, then combining them into a complete modular unit and transporting them to the building site for assembly, this approach can cut construction time in half and significantly reduce on-site labour costs (Hong, 2020). Modular construction is well suited for tall buildings due to its inherent topological modular form and increased number of repeatable modules (Thai, Ngo and Uy, 2020).

However, most builders currently do not consider the concept of circularity when producing modules, let alone how they will be dismantled and reused in the future. It is conceivable that in the end a large number of discarded modular units will be sent to landfills, creating a large amount of solid waste and consuming many resources to dispose of them, while the functionality of the module still can work to fit the demand. Suppose we can consider the concept of circular economy at the beginning of the project and design the modular units to be easily disassembled and reused, in that case, we will benefit not only economically, but also significantly reduce the carbon footprint of the project.

Based on the problem, it can be found that there is currently a research gap between circular economy and modular buildings. And there is still no tool for evaluating the circularity of modular buildings based on their characteristics. Therefore, this study aims to propose a method for circularity measurement based on the characteristics of modular buildings and modular construction.

A review of design strategies for circular construction reveals a number of recognised strategies. Depending on the characteristics of modular construction, design for durability, adaptability, disassembling and reusability can be selected as suitable circular strategies for modular construction. From a structural point of view, an evaluation method called Module Circularity Indicator (MCI) was developed. The method takes into account the structure, the connection method and the loading capacity of modules and assigns a weight to each structural factor based on the results of a questionnaire in order to evaluate the circularity of the modular unit on a numerical basis. Project owners and designers, especially structural engineers, can use this method to evaluate the circularity of modular projects in terms of four dimensions.

A study using student accommodation as a case was carried out to explore the economic and environmental impact of MCI. Through the case study, it was found that a higher MCI score does not lead to better economic returns and that investors need additional incentives to invest in more sustainable modular buildings. Furthermore, a higher MCI can reduce the environmental impact of a project to some extent, but the materials used in the modules are also important. In order to achieve a circular economy in the construction industry, MCI could be used as an indicator to subsidise highly circular modular buildings in order to encourage owners to invest in highly sustainable projects. This will allow the modular building industry to move towards more circular projects and reduce the environmental impact of the construction industry.

The main motivation within this thesis is to explore the possibility of increasing the service life of box-beam bridge girders to aid the ambition of the Dutch government to become a circular economy. The construction industry still follows a more or less linear economy system. Many bridges in the Netherlands are approaching the end of their service lives, and efforts are being made to extend the lives of these bridges. This is but a temporary solution, and these bridges will require replacement within 20-30 years. This provides an opportunity to incorporate circularity within new and upcoming structures from the design phase onward rather than only as an end-of-life practice. Therefore this thesis explores the possibilities of incorporating the strategy of lifespan extension, in the form of a cover increase against chloride-induced corrosion. The exact increase was calculated through the use of a relevant service life design model (DuraCrete) and compared with the Eurocodes. The reliability of the model was checked using a Monte Carlo simulation. Subsequently, the circular aspects of future adaptability were also investigated considering a fictitious scenario of increased traffic load. Hence, it was concluded that a marginal increase in concrete volume and prestressing steel per beam would allow for a more durable and adaptable beam for the future.

The environmental strain and depletion of natural resources caused by the current economic system could be reduced through the transition to a more circular economy, based on regenerative use of resources, where materials are kept in use for as long as possible and their value is retained (Cheshire, 2016; Ellen MacArthur Foundation, 2013; European Commission, 2015; Geldermans & Jacobson, 2015). The building industry has an important part to play in this transition, as it is a heavy user of raw materials and a large producer of construction and demolition waste (Merrild et al., 2016; Rijksoverheid, 2016). The circularity of the building industry can be improved by reducing waste production and preventing the need for additional material input by increasing the service life of existing structures and components (Platform CB’23, 2020; Rijksoverheid, 2018). 
The service life of buildings and building components is usually limited by the frequently changing user demands and their inability to adapt to new requirements (Blok & van Herwijnen, 2005; Durmisevic, 2006; Landman, 2016). In Dutch residential buildings, the building services are often cast into the floors, creating one fixed element (Durmisevic, 2006; Landman, 2016). As a result of this integration, adaptation to changing technical requirements or different spatial topologies results in damage or even demolition of the load-bearing floor, reducing the overall service life of the floor system (Brand, 1994; Durmisevic, 2006). A way to remedy this is by designing the floor systems to be able to adapt to changing circumstances and user requirements through flexible design (Geraedts, 1996; Gijsbers, 2011; Schmidt et al., 2011). This way, flexible design could provide a practical approach to improve the circularity of building components such as floor systems. This leads to the main research question of this thesis: “To what extent can flexibility measures improve the circularity of prefabricated floor systems in residential buildings?”
The main research question was addressed in two parts. First, the theoretical framework for the research was compiled, then the theory was put into a practical context through a case study. As part of the theoretical framework, a literature study was done into the factors that influence the flexibility of design, existing designs for flexible floor systems, and existing assessment methods for the circularity and flexibility of building components. In addition, interviews were held with people involved in different parts of the decision and design process of building projects to identify possible opportunities and challenges for the implementation of flexible design of floor systems. 
As a result of the research into the factors that influence the flexibility of design, seven technical characteristics were identified that should be included in the design process to accomplish flexibility of design. These technical characteristics of flexible design are accessibility, durability, independence, integration, interface simplicity, overcapacity, and standardisation. Accessibility indicates the possibility to access components that need to be adapted without hindrance from other components. Durability regards the capacity of a material or system to continue to be useful after an extended period of time and usage. Independence concerns the technical separation of parts with different service lives and functions, while integration indicates the technical coupling of parts that are unlikely to ever be adapted separately. Interface simplicity requires the use of uncomplicated mechanical connections between elements to allow for easy disconnection. Overcapacity indicates the ability of a design to accommodate higher technical specifications than initially required and allows for flexibility in the application of the design in different functions. Finally, standardisation regards the use of the same components size and construction details throughout the design to ensure compatibility between different parts.
From the interviews, it became apparent that flexible design provides opportunities for both the clients and the building industry itself. Flexible design offers clients the possibility to adapt their buildings to changing circumstances and requirements and plays into the growing interest and willingness of clients to invest in sustainable alternatives. It is expected that the opportunities regarding adaptability to future developments and residual value are especially interesting to professional investors. For the building industry, flexible design can have a positive impact on the efficiency of the collaboration between different disciplines in the design process. Other opportunities for the building industry lie with reduced failure costs and the possibility to increase the standardisation of building components. The main challenges for the adaptation of flexible design in practice, as identified through the interviews, are the conservative nature of the building industry and the possible cost increase of building projects as a result of the use of a novel floor system. 
The literature study into the existing flexible floor designs showed that two types of flexible floor systems can be identified. The first type has the building services incorporated within the structural height of the floor and can be further subdivided into floor systems that are variations of the hollow core slab with grooves through which pipes can be laid and floors systems with a hollow space within the structural height of the floor for the placement of building services. The second type concerns systems where the building services are separated from the structural floor and the building services are either placed on top or below the load-bearing floor.
Through the literature study, a variety of circularity assessment methods was identified. Based on the availability of the full method and the applicability on a building component level, the Building Circularity Indicator method was found to be the most relevant for this thesis. No suitable method was found for the flexibility assessment of building components. Therefore, a new flexibility assessment method was developed based on the seven technical characteristics of flexible design. 
As part of the case study, the building code and stakeholder requirements for the design of a floor system were compiled and the existing flexible floor designs, as well as some traditionally used floor systems, were assessed on their circularity and flexibility to identify what characteristics of the floor designs contribute to their circularity and flexibility and where there was room for improvement. This was used as input for the development of a new flexible floor system. Several design concepts for flexible floor systems were worked out based on the incorporation of the technical characteristics of flexible design and with cost considerations in mind. The design concepts were developed, verified, adapted, and at times discarded, based on the building code and stakeholder requirements. The resulting designs are two versions of a prestressed ribbed concrete floor element. The ribs are placed at the top of the concrete slab. Between the ribs, there is space for the placement of building services and the ribs contain openings for the routing of pipes and cables. The gaps between the ribs are covered with panels, which remain removable for accessibility of the building services, and are filled with a loose granulate material for sound insulation. 
The developed floor systems were assessed on their circularity and flexibility and compared to the existing flexible floor designs and traditional floor systems. This comparison showed that the developed floor systems have the highest flexibility score of all floor systems included in the comparison and also have circularity scores that are among the highest. The high circularity score as a result of a design based on flexibility and cost considerations indicates that the circularity of a floor system is improved by the inclusion of flexibility measures in the design. When looking at all the floor systems included in the comparison, a significant positive correlation between the circularity and flexibility scores of the floor systems was found. In conclusion, the implementation of flexibility measures can improve the circularity of prefabricated floor systems in residential buildings.