Load-Bearing Reinforced 3D-Printed Concrete

3D-printed concrete has primarily been used for non-load-bearing elements, such as lost formwork to cast complex geometries, or for elements with low structural demand comparable to masonry. This unsatisfactory situation was primarily due to (i) the absence of mechanical models and design codes accounting for the anisotropic properties of the 3D-printed concrete caused by its layering and (ii) the lack of proven concepts for reinforcement integration as well as models for the reinforcement-3D-printed concrete interaction. 

Intensive research was carried out to address these issues and fully harness the potential of digital fabrication with concrete for load-bearing elements. On the one hand, a structural concept and mechanical models for 3D-printed reinforced concrete were developed and experimentally validated on reduced and full-scale reinforced 3D-printed concrete columns. Furthermore, the reinforcement-3D-printed concrete interaction was investigated on two series of reinforced 3D-printed concrete ties. Finally, a new material test for 3D-printed concrete, the so-called Modified Slant Shear Test, was established. This relatively simple test enables quantifying the influence of 3D-printed concrete layer interfaces, including dry joints and cold joints, i.e., layers printed with a certain time gap, caused by interruptions during printing. 

All this has made it possible to reinforce the 3D-printed concrete of the White Tower with steel reinforcement and prestressing in a way that ensures a mechanical behaviour analogous to conventional structural concrete, ensuring structural integrity and building code compliance. Accordingly, the White Tower is the world’s first multi-story building to use fully load-bearing reinforced 3D-printed concrete.

Automatically Integrated Reinforcement – Two Robots Collaborating in Tandem

The steel reinforcement was integrated into the 3D printed columns during the printing process. In the newly developed 3D concrete printing method, two robots collaborate: one extrudes concrete layer by layer into complex free-form elements, while the other inserts reinforcement between these layers. This makes the 3D-printed structure fully load-bearing. Since concrete is only used where structurally needed, this formwork-free method significantly reduces material consumption compared to traditional casting techniques. 

The key innovation of this process lies in the automated integration of reinforcement during the 3D printing process. Two robots work in tandem: one applies fresh concrete continuously, while the other places reinforcement between the layers of concrete. After printing the thin-walled hollow elements, longitudinal reinforcement is placed into vertical channels, which are then grouted. This robotic manufacturing process allows the use of 3D-printed concrete in a fully structural and load-bearing manner – a world first.

3D Concrete Printing: Formwork-Free and Material-Efficient

In 3D concrete printing, a robotic arm successively applies thin layers of soft concrete through a nozzle. The material is soft enough to bond and form continuous, homogeneous components, yet it hardens quickly enough to support the successive layers. The printed material is based on a multi-component technology that combines white concrete with a stabilizer and an accelerator for rapid hardening. This enables the creation of freeform elements with large overhangs. The 3D concrete filament used in the process is applied in layers, that are 8 mm high & 25 mm wide, forming a continuous print path of approximately 5000 meters per column

By employing robot-assisted concrete extrusion and eliminating the need for molds, concrete is precisely deposited only where needed, resulting in a 40% reduction in material consumption compared to conventional casting methods.

Each column cross-section is composed of three filaments: the outer filament features the aforementioned ornamental texture, the middle layer contains the encasing reinforcement, and the inner filament forms hollow channels for the main longitudinal reinforcement.

Carbon Capture and Monitoring

The CO₂ footprint of 3D-printed concrete is often higher than in conventional concretes due to 3D printing’s unique processing requirements. The 3D printing process itself, however, can offer unique avenues to balance this through design practice, particularly by utilizing less concrete in the overall structure in designs that are uneconomical to fabricate using conventional methods. Another unique research opportunity in the Tor Alva is exploring how design can be exploited in reducing a structure’s CO₂ footprint through carbon capture. During its lifetime, concrete is capable of capturing back some of the CO₂ that was released during its production, a process that can be very slow and is dependent primarily on the thickness of the concrete in the structure – thinner concrete elements absorb CO₂ exponentially faster than thicker ones. Samples of printed elements are placed near the Tor Alva to monitor how quickly carbon (re)capture is occurring in the tower over time. To avoid durability issues that the carbonation would cause to ordinary concrete, Tor Alva employs stainless steel, making for a new paradigm in sustainable design using 3D printing, and opening the conversation on how tradeoffs in design and fabrication must be balanced in order to reach sustainability and economic goals.

Computational Design

The White Tower’s design is entirely code-generated, with no manual drawing or modeling. Every detail is parametrically scripted, enabling easy adjustments, immersive visualization, fabrication simulation, and compliance with robotic 3D printer constraints. All project data is stored in a digital twin, which enables the coordination, simulation, evaluation and realisation of the tower without the need for conventional construction plans. The semantic digital model allows for optimizing material use and structural performance while integrating high-resolution technical details like electrical and lighting systems to minimize onsite concrete work. Augmented reality and virtual reality are used extensively in both design and realisation.

The parametric 3D-printed column design comprises multiple complex thin-shell layers, including surface, structural, and rebar pocket layers, for enhanced functionality. A novel fabrication-informed design workflow automates the generation of robotic print-path data in high precision for those layers, incorporating rebar data into the digital workflow. The computational design workflow supports mass customization, with each column featuring unique algorithmic ornamental patterns.

The entire structure of the tower is programmed and designed using customised software that allows precise definition of the geometry and can send the required data directly to the printing robots. This technology also enables the efficient production of customised elements.

Disassembly and Reuse

Designed for circularity, the White Tower features detachable connections, allowing it to be dismantled after its five-year use in Mulegns and reassembled at a new location.

Each load-bearing column is made up of three components — the central column, the base, and the capital. The central column is 3D printed, while the capital and base are conventionally cast using 3D-printed formwork. The parts are assembled in a local prefabrication facility 10 km from the construction site, transported by truck, and then assembled on-site with a crane. 

3D printed formwork and casting for horizontal elements

The horizontal components, which are not suitable for concrete printing, are produced using a novel process in which 3D-printed formwork is combined with castings made from an innovative, sustainable concrete.

Lightweight façade solution

The tower’s weather protection is achieved via an ultra-light, innovative membrane construction that can be removed in the summer months.