3D concrete printing
3D concrete printing, or simply concrete printing, refers to digital fabrication processes for cementitious materials based on one of several different 3D printing technologies. 3D-printed concrete eliminates the need for formwork, reducing material waste and allowing for greater geometric freedom in complex structures. With recent developments in mix design and 3D printing technology over the last decade, 3D concrete printing has grown exponentially since its emergence in the 1990s. Architectural and structural applications of 3D-printed concrete include the production of building blocks, building modules, street furniture, pedestrian bridges, and low-rise residential structures. A comprehensive overview on the progress in 3D concrete printing is available at.
History
Automating building processes has been an area of research in architecture and civil engineering since the 20th century. The earliest approaches focused on automating masonry. In 1904, a patent for a brick-laying machine was granted to John Thomas in the US. By the 1960s, the technology developed significantly and functional equipment, such as the Motor-Mason, were in use on building sites.At the same time, automating concrete construction processes was also being developed. Slip forming, a widely used technique today for building vertical concrete cores for high-rise buildings, was developed in the early 20th century for building silos and grain elevators. The concept was pioneered by James MacDonald, of MacDonald Engineering Chicago, and published by Milko S. Ketchum in an illustrated book: The Design of Walls, Bins, and Grain Elevators in 1907. Later, MacDonald published a scientific paper: Moving Forms for Reinforced Concrete Storage Bins in 1911. Finally, on 24 May 1917, MacDonald was granted a US patent for a device to move and elevate a concrete form in a vertical plane.
Innovations in the automation of concreting processes continued throughout the 20th century. 3D printing processes were first developed in the 1980s for photopolymers and thermoplastics. For some time, 3D printing technology was limited to high-value-adding sectors such as aerospace and biomedical industries due to the high cost of materials. However, as the knowledge base for 3D printing grew, new additive manufacturing processes were developed for other materials, including for concrete. 3D printed concrete technology originated from Rensselaer Polytechnic Institute in New York when Joseph Pegna first applied additive manufacturing to concrete in 1997. This experiment was just a proof of concept, but Pegna recognized the developing robotics industry and saw it as an opportunity to automate the construction process, while also decreasing costs and waste production. Pegna's research would later become the basis for binder jetting, or powder based 3D concrete printing.
In 1998, Behrokh Khoshnevis at the University of Southern California developed Contour Crafting, which was the first layered extrusion device for concrete. The system used a computer-controlled crane to automate the pouring process and was capable of creating smooth contour surfaces. Khoshnevis initially designed this system to serve as rapid home construction for natural disaster recovery, and he claimed that the system could complete a home in a single day. With innovations in materials, mix design, and printing technology, researchers and engineers have since expanded on these two printing techniques, which will be discussed further in the following section.
Construction methods
A number of different approaches have been demonstrated to date, which include on-site and off-site fabrication of building elements or entire buildings, using industrial robots, gantry systems, and tethered autonomous vehicles. Demonstrations of construction 3D printing technologies have included fabrication of housing, building elements, bridges, civil infrastructure, artificial reefs, follies, and sculptures. Three different construction methods are currently used in 3D concrete printing: binder jetting, robotic shotcrete, and layered material extrusion.Binder jetting
Binder jet 3D printing, also known as powder bed and binder 3D printing, was originally developed at the Massachusetts Institute of Technology for activating starch or gypsum powder with water as a binder, before Joseph Pegna applied the system to concrete. In binder jetting, a print head selectively deposits a liquid binder on a powdered substrate, layer by layer. The layer height typically varies between 0.2 and 2 mm and determines both the speed and the level of detail in the finished part. Post-processing steps are necessary in binder-jetting once the layered fabrication is complete. First, the unconsolidated powder needs to be removed mechanically, using brushes and vacuum tubes. Additional curing steps may also be necessary in ovens with controlled humidity and temperature or microwaves. Finally, coatings may also be applied on the surface to consolidate small surface features or to improve the surface quality of the part. Typical materials used for coatings are polyester or epoxy resin.3D concrete printing with binder jetting technologies has been demonstrated at large scale by Enrico Dini with D-Shape. D-Shape relies on a non-hydraulic Sorel cement that is based on sand activated with magnesium oxide in the powder bed and a liquid magnesium chloride solution as binder. The technology has mainly been used to create furniture, such as a coffee table and the Root Chair designed by KOL/MAC LLC Architecture + Design in 2009. Furthermore, D-Shape produced large architectural parts, such as the 3 × 3 × 3 m Radiolaria pavilion designed by Shiro Studio in 2008, the Ferreri House for the Triennale di Milano in 2010, and a twelve-metre-long footbridge designed by Acciona in Madrid, in 2017.
Another exponent of binder-jet 3D concrete printing is California-based firm Emerging Objects. For their Bloom pavilion built in 2015, the company used an iron oxide-free cement and organic binder. While it is unclear if there is any cement hydration involved in the process, the project is often cited among other binder-jet 3D concrete printing projects due to the use of cement in the powder bed. Unlike the structures of D-Shape, which were fabricated in one piece, Emerging Objects fabricated 840 small building blocks that were stacked to create the 3.6 × 3.6 × 2.7 m structure.
Advantages and limitations
Compared to other 3D printing methods for architectural applications, binder jetting allows for a higher degree of geometric freedom, including the possibility of creating unsupported cantilevers or overhangs and hollow parts. Unlike other 3D printing processes that require auxiliary support structures, binder jetting relies on the bed of unbonded powder to ensure continuous support for consecutive layers during fabrication.Typically, in binder jet 3D printing, the leftover powder can be reused for future parts. However, the recyclability of the cement and aggregate powder is problematic due to the exposure to ambient humidity, which can trigger the hydration process. Therefore, binder jet 3D printing is not suitable for on-site construction.
Layered extrusion 3D printing
Concrete layered extrusion 3D printing involves a numerically controlled nozzle that precisely extrudes a cementitious paste layer by layer. Layers are generally between 5 mm and a few centimeters in thickness. The extrusion nozzle may be accompanied by an automatic troweling tool that flattens the 3D-printed layers and covers the grooves at the interlayer interfaces, resulting in a smooth concrete surface. Additional automation steps have been proposed for the integration in one fabrication step of modular steel reinforcement bars or integrated building services, such as plumbing or electrical conduits. For this process, process planning and deposition speed are critical parameters that influence the material's stiffening and hardening rate.Layered extrusion 3D concrete printing is most commonly used in on-site construction and is accompanied by large-scale 3D printers. The technology has seen a growing interest recently, with numerous universities, start-ups, and prominent established construction companies developing dedicated hardware, concrete mixes, and automation setups for concrete extrusion 3D printing. Applications include bridges, columns, walls, floor slabs, street furniture, water tanks, and entire buildings, both in prefabrication or on-site setups.
Advantages and limitations
Unlike conventional concrete casting and spraying, layered extrusion 3D printing needs no formworks. This is a significant advantage considering the fact that formworks in concrete construction can account for 50-80% of the resources, more than raw materials, reinforcement, and labour combined. The main challenges of layered concrete extrusion are the set on demand rheology of concrete, the integration of reinforcement, and the formation of cold joints at the interface between consecutive layers.Slip forming
Robotic slip-forming, a process developed at ETH Zürich under the name Smart Dynamic Casting, is sometimes included in the family of concrete 3D printing processes, together with layered extrusion and binder-jetting. The process loosely fits the definition of 3D printing, due to its additive nature, with material being slowly extruded through an actuated mould that can vary its section. However, unlike the other 3D printing processes, slip forming is a continuous process, and not discrete or layer-based, and therefore it is more closely related to formative processes such as casting and extrusion.Technology
3D printers for concrete (concrenters)
There are a few main categories of robots that are used for 3D concrete printing, which depend on the application, scale of the project, and printing technique. All construction 3D printers generally consist of a support structure and a printer head with a nozzle that extrudes the concrete. Printers are usually used in tandem with modelling software that uploads the building plans directly to the printer.- Gantry robots: Gantry robots are the most common in 3D concrete printing, and consist of a mobile gantry system with mixing and deposition systems. They can range from small lab models to large-scale printers for printing full components or structures. These printers are typically limited to vertical extrusions but have the benefit of high stability and easy scalability for larger projects. Gantry robots must be larger than the assembled structure, which can add cost to transportation and set-up costs. However, they are the easiest to control of all 3D printers.
- Cable-driven system: In a cable-driven system, the print head is suspended between several fixed points within a frame. It has more geometric freedom than a gantry system and is more lightweight and transportable. However, it requires a wide area for equipment and planning is essential so that the cables do not overlap with the printed structure.
- Robotic arm: This is similar to the robotic arms seen in assembly lines, which have six-axis movement and the most freedom of 3D printing systems. These are also capable of depositing concrete, embedding components like rebar, and performing any post-processing that may be required after the concrete sets. Robotic arms are the most compact system but are most commonly used for small-scale applications. However, large scale robotic arms based on heavy duty construction equipment are now available, combining the print size of large gantry systems and the transportability of any standard construction equipment.