Concrete is good at handling compression forces, but not particularly good at handling tension, so it needs to be reinforced with something strong against tension. Traditionally, that has meant metal rebar and/or plastic or metal fibers (fiber-reinforced concrete). In this report, a team from Technische Universität Dresden (TU Dresden) and the Shimizu Corporation’s Institute of Technology discussed and tested some ways to translate those methods into the realm of 3D-printed concrete.
“Significant progress has been made in non-reinforced concrete printing in the recent years with research and industrial organizations producing geometrically complex structures as well as innovative rapid mass-customized buildings,” the researchers note in their report. “However, the research on incorporating reinforcement into fully automated digital construction approaches is limited, with the suggested solutions still being rudimentary.…Since the use of reinforcement is mandatory in most structural applications, there is an urgent need to bring the technology of reinforcing 3d-printed structural elements forward.”
The proposed solutions for reinforcing 3D-printed concrete fall into two categories: continuous and discontinuous. In continuous techniques, the reinforcement is laid down at the same time as the concrete (i.e., printing concrete with reinforcing fibers already in it, or extruding continuous metal chains along with the concrete). In discontinuous techniques, the reinforcement is laid down either before or after the concrete is poured (i.e., using a robotic tool to lay down reinforcement that concrete is later poured around). Continuous methods are better, because they retainthe timesavings and convenience that 3D printing promises.
In the experimental part of their report, the researchers looked into two different continuous reinforcement methods: strain-hardening cement-based composites (SHCC) and 3D printing of steel reinforcement with gas-metal arc welding (GMAW).
In the first part of their experiment, the researchers created what they believe to be the first SHCC to be 3Dprintable. They used plastic fibers as their reinforcement, at a volume of 1 percent of the total material. They were able to print the mixture through a nozzle with consistent filaments to build up seven layers in a single printing session. After the 3D-printed mixture was allowed to dry for 21 days, there were no cracks due to drying shrinkage, and the material was as good at taking strain as non-3D-printed SHCCs.
Next, the researchers3D printed steel reinforcement with GMAW, where an electric arc is formed between the metal and a wire-electrode to melt the metal and the process is surrounded by a nonreactive gas shield to protect it. These 3D-printed bars had lower yield stress and tensile strength than conventional steel reinforcement, but higher ductility. The researchers hope to further refine the printing method and find a way to print it continuously with concrete.
The report concludes that “Various presented concepts and prototypes are encouraging; however, none of these approaches satisfies the entire spectrum of relevant requirements for wide-ranging implementation of DC and need to be developed further.”