A Modifiable Structural System Made of Precast Concrete Wall Elements: Design and Analysis of a Novel Structural Concept for the Circular Use of Buildings

The use of precast concrete elements for building structures is well established in modern construction practice due to their efficient production and construction methods. Buildings made of precast concrete elements often include concrete walls, which serve multiple functions such as load-bearing, providing structural stability, dividing spaces, and offering sound insulation and fire protection. Concrete has beneficial properties to fulfill these functions. In addition, it is a durable material with the potential for a long lifespan. However, this potential is often not fully realized when a building’s function changes and modifications are required. For example, this may occur when merging apartments, converting residential units into office spaces, or performing other alterations that necessitate openings in existing load-bearing and stabilizing concrete walls. If such openings require costly structural strengthening, they can ultimately lead to the demolition of the entire building.

The built environment is a significant contributor to global greenhouse gas emissions, referred to as carbon or CO₂ emissions. In light of the ongoing climate crisis, there is an urgent need to reduce these emissions. There are many means to this, including promoting the circular use of materials and buildings and minimizing the production of new materials. To support the circular use of buildings and extend their lifespan, more adaptable building designs are needed. Furthermore, new buildings must be optimized to use only the necessary amount of material to ensure resource efficiency.

In this thesis, a novel design concept for precast concrete wall structures is presented and analyzed. The design is based on current efficient precast production methods, yet optimized to enable easier transformations of the buildings, and ensuring a more efficient material usage, compared to current practices. The new concept, modifiable concrete walls,
consists of two distinct zones. A frame zone and a flexible zone in the middle, which can be removed if openings of the walls are needed. The two zones are made of concrete with different strength classes. The strength requirement for the flexible zone is lower than for the frame, and to reduce the CO₂ footprint as much as possible, the ”lowest-feasible” concrete strength and reinforcement quantity in the flexible zone are used and tested.

Throughout the Ph.D. project, the design concept and analyses of the modifiable concrete walls have gone through several phases. Initially, the concept was established and validated through a feasibility study investigating the industry perspective on flexible structures and the modifiable concrete walls. In addition, methods for the production of modifiable wall elements were developed and tested in a concrete element factory to ensure their producibility.

In the next phase, a numerical material optimization framework based on Finite Element Limit Analysis (FELA) was developed to find the combination of concrete strengths and reinforcement quantities, which minimizes the CO2 footprint of the structure. It was used to optimize an example of five-storey wall structure and resulting CO2 reductions of more than 40% were found, compared to conventional concrete walls.

Based on the optimized design, experimental full-scale tests of four individual modifiable concrete walls elements were conducted to gain essential knowledge on the structural behavior of the novel design, loaded with in-plane forces until failure. The experiments provided promising results, with the low-strength flexible zone significantly contributing to the load carrying capacity, and with the first severe crack initiations at load levels well above the capacity of the frame. Furthermore, the results indicated a strong bond connection between the two zones, providing results more similar to a monolithic wall element rather than a frame with disconnected infill.

In the last part of the project, a non-linear finite element model was developed and validated by comparison with the experimental results, with the purpose of having a tool to conduct further investigations of modifiable wall structures. The numerical model was used to analyze a wall structure example with multiple modifiable wall elements and investigate the influence of different modification scenarios. For this example, no premature cracking appeared in the flexible zones from a modification of the adjacent wall element. The research presented in the thesis has resulted in a novel structural concept that enables greater flexibility of concrete structures while significantly reducing the CO₂ footprint. The results of the various analyses demonstrate that the modifiable concrete wall is a viable solution from both the industry, the production and the structural perspective to support the transition towards more adaptable and circular building designs.