To align with the objective of the European Green Deal [46] policy of achieving net-zero greenhouse gas emissions by 2050, the construction sector must transition to a circular built environment. Most of the waste in the Netherlands is related to construction and demolition waste, with the industry accounting for approximately 35% of CO₂ emissions [89]. The goal of the circular built environment is to minimise waste by reducing, reusing, recycling, and recovering materials throughout the life cycle of a product [66]. Therefore, making an impact in the construction industry can lead to a significant reduction in CO₂ emissions and contribute to the achievement of the European Green Deal [46].

This research contributes to the "reuse" component of the aforementioned circular built environment goals. Specifically, the focus is on developing a reusable connection between hollow core slabs and steel frames by implementing increased execution tolerances. Increased tolerances are necessary to allow reuse, as alignment-related problems often occur. The research consists of several parts: the design process, structural verification, experimental research on demountability, and environmental impact assessment. The design process comprises a tolerance analysis supported by a Monte Carlo simulation, variant studies, and a comprehensive qualitative trade-off analysis. After the best scoring alternative is determined, the verification part assesses the structural behaviour of the connection in terms of strength and stiffness. This is done using a combination of analytical and numerical calculations. The purpose of the experimental research is to investigate the demountability potential of the reusable connection. The final part of the research investigates the impact of the reusable connection compared to conventional connections for different lifecycle scenarios.

The research demonstrated that incorporating additional tolerances in the connection between the hollow core slab and steel frame is crucial to achieve a reusable construction. Three connection alternatives were generated that can incorporate these tolerances based on a literature review and meetings with experts in the field of building construction. The alternatives were weighted on tolerance inclusion, ease of installation, demountability potential, and costs. The best option was identified as a connection consisting of a square hollow section and a bolted shear stud encased in mortar. This alternative outperformed competitors in terms of tolerances, installation, and costs. However, the demountability potential was identified as a critical part of the connection and, therefore, was further investigated experimentally. The experiments showed an increased demountability potential in situations that include pre-treatment. Vaseline-treated specimens showed no signs of chemical bonding and better lubrication compared to oil-treated specimens, resulting in the lowest resistance and, therefore, the best demountability. The last step of the research investigated the environmental impact of the reusable connection and compared it with the conventional construction technique. Results showed that a marginal addition of 1.3% to the initial environmental impact of the superstructure results in a significant reduction over the full lifespan of the structural elements. This was attributed to the reusability of the connection and the ability to reuse structural elements in another building in a second life cycle.

From the results, it is concluded that a reusable and structurally feasible connection between hollow core slabs and integrated steel beams can be created with a small additional investment upfront, resulting in a substantially reduced environmental impact. The purpose of this study is to provide guidance and persuade decision makers, such as project developers, building owners, and government organisations, to consider implementing reusable construction methods in their real estate projects. By doing so, they can contribute to the objective of the European Green Deal [46] policies and the goal of achieving net-zero greenhouse gas emissions by 2050.

Today, society is becoming more and more aware of the environment and the (negative) influence we have on it. Especially the current workings of our economy have a detrimental effect on the environment. To reduce these effects, a transition is being made towards a circular economy. The construction sector plays a big role in the degradation: in the Netherlands the sector accounts for 50% of the raw material use, 40% of the energy consumption, 40% of the waste production and 35% of the CO2 emission. For the construction sector, one of the changes to establish a more circular economy, is the adoption of a circular design method. This method consists of the construction of buildings, building systems and building elements being designed for disassembly and reuse. This reduces raw material use and the expel of harmful emissions. While we see some developments being made in the construction sector regarding circularity, not many developments can be found in the field of reusable floor systems. For the sizable use of raw materials and expel of harmful emissions during the manufacturing process of floors, a more circular design for floor systems can (eventually) contribute to the sectors transition to circularity. Therefore, this research is focussed on developing demountable connections which can be used in such a circular floor system. The result is a system comprising a prefabricated timber-concrete composite (hereafter: TCC) floor slab connected to steel edge beams with toothed-plate connectors. An important aspect taken into account in the development of this design is the efficiency of erecting, disassembling and reassembling the floor. A multi-criteria analysis is conducted to determine floor slabs viable for use in a reusable floor system. A timber-concrete composite floor slab is chosen and designed. The slab is verified using analytical calculations. Design variants are made for demountable connections at the slab-beam positions at the head end and the side of the slab and for the slab-slab position. A choice between design variants is made by reviewing the fabrication and assembly tolerances and by assuring a non-destructive disassembly procedure. Analytical calculations are performed to verify the chosen connections. A case study building is used for the development of the floor system. The TCC floor slab spans 10.8m, has a total height of 510mm and a width of 1800mm. The used edge beam is an L-section. On the web a toothed-plate connector is adhesively bonded onto which the timber beams are placed to enable shear force transfer between the timber beams and the edge girder. At the top of the web a compression bolt is installed which is fastened after instalment of the floor slabs to ensure force transfer between the bolt and the concrete slab. Two angle sections are screwed to the outer sides of the outer timber beams and bolted to the edge girder to guide the slab to its intended position and to ensure structural soundness by vertically fixing the floor slab to the girder. Lastly the limits of the developed floor system are determined by performing a parameter study of the floor slab and the connection. The floor slab can be designed to span 12.6m. The connection still meets the structural requirements when the system is used in a building of maximally 70m high. This is applicable on many combinations of the building length (10m to 70m) and width (6m to 12.6m), if the right cross-sectional dimensions are used.