## 2 Existing Concepts for Reinforcement Integration
### Review of Existing Methods  
The methods (a)-(h) show the broad range of possible solutions for reinforcement integration which differ considerably in terms of both the degree of automation and the practical maturity. Contour crafting with post-installation of reinforcement (a), (b) can be understood just as a short-term transition solution with regard to the degree of automation. Post-installation of (vertical) reinforcement within the narrow contour of a component is circumstantial and extremely labor-intensive due to the limited lateral working space (Figure 1a,b). In this context, encasing of reinforcement with multi-nozzles (c) or spraying shotcrete (d) to a manually pre-installed reinforcement is a much more promising alternative from a process engineering point of view. An example of the implementation of (c) is the two-nozzle concrete printer developed for wall panels by the Chinese manufacturer HuaShang Tengda Ltd. [20], which encases reinforcement fabrics with a fork-shaped nozzle from two sides (Figure 1c). The disadvantage of this technology is that the producible wall height is limited by the fork length of the printing head and intensive manual finishing is required to achieve an adequate surface quality. On the other hand, a greater spatial design flexibility is offered by enclosing the reinforcement with shotcrete (Figure 1d), which was developed at the University of Braunschweig [21; 22]. A challenge in concrete  
Figure 1: Concepts for integration of reinforcement (**a–h**) into 3D concrete printing processes. deposition is to clasp around the pre-installed reinforcement in such a way that shadowing effects are avoided, which can lead to the formation of defects in the vicinity of the reinforcement. Within both methods (c), (d) the reinforcement structure has to be conventionally (manually) pre-installed before placing the concrete which is unfavorable compared to fully digital production processes. In contrast to encasing pre-installed reinforcement, [PERSON] [11; 23] proposes additive manufacturing of reinforcement (Figure 1e). The so-called wire arc additive manufacturing (WAAM) based on a metal arc gas welding process (MAG) allows the reinforcement to be built-up by incremental weld points. Comparable approaches of shape welding have been used in a research project on pin connectors for steel-concrete composite structures [27; 28; 29] and are also already being used in the printing of pure steel structures [30]. The overall process proposed in [23] is called parallel \"multi-arm printing of concrete and reinforcement\". However, the proposed concept neither specifies how the printing processes for steel and concrete will interact nor which methods will be used to place concrete around the printed reinforcement. It is also important to note that the incremental build-up of the reinforcement from individual weld points requires considerable energy for the melting of steel. Furthermore, heat accumulation occurring within the printed rebar will significantly affect the material properties of reinforcing steel. However, presumably the biggest obstacle for a real-scale implementation of this process is the limited melting rate of the welding process (approx. 1-4 kg/h) [31], which would massively inhibit and slow down the concrete extrusion process. The Mesh Mold process [24] (f) allows a faster production of complex-shaped reinforcing elements using a resistance welding robot (Figure 1f). In this process, the reinforcement bars are installed at small spacing. Thus, the dense steel mesh combines the functions of reinforcement and formwork and allows for conventional pouring of concrete. However, mesh mold structures have small concrete cover, for which critical aspects like protection of reinforcement against corrosion as well as a sufficient bond between steel and concrete have not been resolved, so far.