# Sustainable Support Material for Overhang Printing in 3D Concrete Printing Technology
## 1 Introduction
This method omits the need for support by using an alternative structural design for the 3D printing application. Though this method is simple and easily adopted by print path designing, it is still limited by the  
Figure 1: Support for anchoring of floating object [4]. Figure 2: Support structure requirement for inclined angle [4]. slope angle, which is highly material-dependent. Ref. [6] studied the different material formulations of concrete for 3D printing applications and demonstrated that the maximum slope angle for corbelling is 17.5 degrees from the vertical axis. Any angle greater than this will result in the failure of the specimens. Another method of printing overhangs in 3 DCP is the use of temporary partial formworks to support the material in the overhanging sections. For instance, it was reported that the house printed by [PERSON] used scaffolding and wooden planks to temporarily support the overhang sections during the printing process, which were removed after the concrete had cured (Figure 3b) [7]. A similar method was also used by [3], where a temporary external support for the overhang sections was placed during the printing process (Figure 3c). However, the placement of the support is disruptive to the printing process which increases the time gap between the concrete layers. This increase in the time gap causes weaker bonding of the layers and results in a vulnerable point in the structure due to poor interlayer strength [8]. The printing of supports using the same material as the primary material in 3 DCP was first proposed by [3]. This method is accomplished by increasing the nozzle speed at the areas where the support structure is located while maintaining the same flow rate. This creates an underflow of material in that area, inducing discontinuities in the filaments which act as support for the overhang structures that are to be printed above it (Figure 4a). The discontinuities allow the support to be removed, although additional mechanical post-processing may also be required to achieve a better finish (Figure 4b). The mechanical removal process may also pose a risk of damaging the printed part. Figure 4: Printing of supports with primary material: (**a**) before removal and (**b**) after removal of supports [3]. Figure 3: (**a**) Corbelling design for overhang [3]; (**b**) Use of scaffolding and wooden plank as temporary support [7]; and (**c**) Temporary external support for overhang [3]. D-shape is a powder bed fusion additive manufacturing process that was developed for architectural applications [9]. Unlike extrusion-based 3 DCP, the granular materials and fibers are first laid out before an \"ink binder\" is selectively deposited on the powder bed to form the structure. This process is repeated for each layer until the printing is completed. Similar to the SLS and SLM processes, the excess powder that is not bound also acts as the support for the overhangs and is removed after the printing process is completed. With the excess powder support, complex architectural designs, as seen in Figure 5, that are unachievable by extrusion-based concrete printing, become achievable in this process. The printing of overhang structures has been extensively studied in various types of 3D printing technologies [10; 11]. Nonetheless, the printing of overhangs in extrusion-based 3 DCP can only be accomplished with the use of external supports, such as wooden planks. Though the use of corbelling has been proven to be feasible, the solution is limited by the slope angle and the structure design. The advantage of 3 DCP technologies is the ability to fabricate free-form structures. The method, however, is restricted by the limitation of creating a free-form opening in the printed structure. Although a solution was proposed to print the support with the primary material and reduce the filament quality at the support area, by varying the travel speed of the nozzle to allow ease of removal, it has been shown that a mechanical removal process still may pose a risk of damaging the printed part.