# Sustainable Support Material for Overhang Printing in 3D Concrete Printing Technology
## 1 Introduction  
Openings are essential in the design of concrete structures for practical applications, such as windows and door access to buildings. In 3D concrete printing (3 DCP) technology, fresh concrete is deposited through a nozzle at designated locations to build a concrete structure in a layered manner. Similar to regular cast concrete structures, openings are also essential in the printed concrete structure for the post-processing of windows and doors. However, the printing of openings in concrete structures using this technology is limited due to the presence of overhanging sections [1]. Overhangs are the sections above the opening without any support or surface to rest upon. Without support, the fresh concrete filaments will collapse due to gravitational force. The printing of overhang structures in various types of 3D printing or additive manufacturing technologies has already been well established with the assistance of temporary supports [2]. The overhang sections in the 3D printed parts do not only include the bridging between two vertical structures but also the area underneath an inclined wall as well as the anchoring of a floating section (Figure 1). In general, the maximum inclination angle from the horizontal axis at which the part can be printed without support should be at least 45 degrees (Figure 2). If the inclination angle is more than 45 degrees, the support structure can be omitted. However, the requirement for support can vary with the different types of material and 3D printing technologies. Different solutions to overcome overhang printing in various 3D printing techniques, as well as 3 DCP, are discussed in this section. In the current state-of-the-art 3D printing technologies, various types of support structures are already widely used in different forms. In some extrusion-based polymer printers, the support material can be the same as the primary material but is printed at different printing parameters, which allows weaker bonds between the printed part and the support structure to facilitate ease of removal after the printing process is completed [3]. Such supports are typically removed through mechanical means by breaking away the material or with the aid of machining tools [4]. However, these means also impose a risk of damaging the printed part. On the other hand, soluble materials for support structures have also been developed for 3D printing [4]. Such soluble material can be easily removed with the usage of a cleaning solvent. These soluble supports also allow more complex geometries to be printed, as the supports that are inaccessible through mechanical removal can be easily removed. However, due to the multi-material configuration, a multi-nozzle setup is required for such operation, which increases the complexity of the printer setup, print path design, as well as overall printing time of the part. Furthermore, the soluble supports are non-reusable due to the materials being dissolved in the solvent. In the powder bed 3D printing techniques, such as selective laser sintering (SLS) or selective laser melting (SLM), the excess powder that is not bonded to form the part acts as a natural support and can be removed after the printing process is completed [4]. The removed excess powder can also be reused for the printing of another part, reducing the wastage of the raw materials. In 3 DCP technology, the printing of overhangs has been a major limitation for the further development of this application. The extrusion-based concrete printing technique prints a concrete structure by depositing fresh concrete in a layered manner. Due to the fluid state of the concrete, the technology is still incapable of printing overhangs as the filament will collapse without a physical surface to rest upon. Previous research has proposed methods to overcome the printing of overhangs in extrusion-based 3 DCP. The simplest and most common method is to design corbelling in the structure to gradually create overhangs (Figure 3a) [5].