## 1 Introduction  
Three-dimensional printing technology with digital, customized, and new material features is a promising emerging technology. After more than 20 years of development, 3D technology has been used in aerospace, medical equipment [1, 2], and metal parts [3], owing to its high accuracy, short production cycle, and diverse materials. The application and research of 3D concrete printing (3 DCP) have gradually increased [4], from Pegna using steam injection to cement powder in 1997 [5], to Contour Craft technology [6] printing curved walls in 2003, and Shanghai Winsun Ltd. Printing various types of houses and many printing applications worldwide in recent years. Among existing digital concrete fabrication methods, extrusion-based 3 DCP is the most widely used and developed technique. Besides the studies of shape retention and the mix formulation [7, 8, 9], the reinforcements play an important role in 3D printed components to meet structural safety requirements. The printed output has low toughness due to the layerwise fabrication process and lack of reinforcements. There are layers and filaments from the printed nozzle, stacking, and bonding to form a 3D object, between which there are interfaces during the stacking process of the 3 DCP, and the interfaces are the weakness zone [10, 11, 12] or, worse contact [13] area, requiring reinforcement. Several studies have been conducted to improve the layer adhesion by controlling the moisture, varying the printing speed and layer thickness, or brushing black carbon-sulfur or concrete glue. Increasingly, researchers have attempted to introduce fibers/rebar to reinforce printed components [14, 15, 16, 17] along with the printing procedure. Studies on reinforcing 3D-printed concrete have been carried out using fibers, wires, cables, chains, meshes, nails, rebar, or rods. These reinforcements could be applied inner, between, or outer filaments/layers, as demonstrated in Figure 1. Inner reinforcement means that the reinforcements stay only inside the extruded strips. Short reinforcements can mix with the cement powders at the dry stage, transmitted to the printing head, and then extruded out of the printing nozzle within the mixed mortar. These short fibers can be steel fibers [16; 18; 19], polyvinyl alcohol fiber [18], carbon fiber [17], bio-based fibers [19; 20], polymeric fibers [14; 16], basalt fibers [17], or glass fibers [21]. The short fibers with a high percentage [22] considerably reinforced the printed output. The compressive strengths of 3D-printed to fiber-reinforced concrete can be up to 107 Mpa [23]. Long reinforcements, such as steel chains [24], steel cables [13], fish lines [25], and wires [25; 26; 27; 28], can be embedded into extruded filaments during the printing process. The reinforcements can also be placed across the interfaces to lock the adjacent layers by inserting steel rebar [29; 30], nails [31], metal-printed rebar [16], or plastic-printed rebar [16]. Nails inserted into the layered composite improved the bending force by 50%. Wire meshes [32] and rebar [33], placed between the layers, were also considered across the interface reinforcement. The U nails [34] (6 mm in width and 22 mm in height) considerably improved the ultimate tensile strength and shear strength of 3D-printed concrete by 145.0% and 220.0%, respectively. There are still reinforcements outside the printed parts. Steel rods can be installed on the printed and cured components to enhance structural toughness [35]. Printed mesh [36], steel wire meshes [37; 38], and steel cages can be placed in the printed contour before mortar grouting. However, the previous techniques still have challenges, especially in engineering applications. Short reinforcements require thin and soft fibers to avoid blocking the extrusion; large and long reinforcements cannot be easily manipulated automatically. Cables need low stiffness to be easily redirected as the printing direction turns; otherwise, soft cables easily become knotted [13], thereby increasing the difficulty of automatic embedding.