# Experimental Study of 3D Concrete Printing Configurations Based on the Buildability Evaluation
## 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 printing technology has been used in many fields such as aerospace, medical equipment, metal parts, etc., with its high accuracy, short production cycle, and diverse materials [1, 2]. The research on and application of 3D concrete printing (3 DCP) technique (3 DCPT) have been accelerating, from Pegna using steam injection to cement powder dating back to 1997 [3], to contour craft technology [4] printing curved walls in 2004, and Shanghai Winsun Ltd. printing various types of houses as well as many recent applications all around the world [5]. Compared with traditional building construction, 3 DCP technology has the following potential advantages: (1) high degree of mechanical automation with short construction times, safety, cleaning, and accuracy; (2) no frame required, and resource consumption is relatively reduced; (3) labor-saving; (4) light weight, high strength and multi-functionality can be achieved through structural design; and (5) high customization can realize both standardization and personalization in construction products. Printable material needs to possess the required properties, including print-ability, pump-ability, build-ability, and open time [6], with which the printing equipment can pump the mortar to the printing mechanism, print/extrude the material to the platform, and build a layer-by-layer structure with proper bonding strength between layers in an open time. The material should possess the following characteristics: (1) have good fluidityto be pumped from a remote container, (2) can be easily extruded from a nozzle to the printing bed, (3) keep its finished shape with neither obvious fractures nor cracks [7], (4) gain early strength as fast as possible to bear the continuous load from the following layers, and (5) good bonding with adjacent filaments/layers. Because of the advancement of printing devices, many material proportions have been formulated with acceptable printing results. The mixture in the extruding technique is pre-mixed, and a paste is formed before printing, which remains in paste form on the printing platform. The paste passes through the nozzle to be \"printed\" in the form of thin filaments, under the operations of pumping, squeezing, and vibrating. Several studies on rheology have been carried out to help design printable materials [8; 9; 10]. [PERSON] et al. [11] stated that the material should have high compressive yield stress to bear the load from upper layers during the process of hydration; however, [PERSON] et al. argued that the high yield stress can make the extrusion operations difficult [12]. Many studies evaluated buildability simply based on the number of layers printed or height achieved in a single printing process without failure [13; 14; 15]. It is worth noting that buildability also depends on the workability and mix proportions. [PERSON] et al. [16] reported that a high buildability of 30 layers can be achieved using calcium sulfoaluminate cement. [PERSON] improved their printing from 13 to 20 layers by using high-volume fly ash mixtures [17]. [PERSON] et al. [18] included limestone filler, additives, rheology modifiers, and a small number of polypropylene fibers to the printing materials and managed to print 30 layers without collapse. On the other hand, [PERSON] et al. [7] and [PERSON] et al. [19] evaluated the buildability by measuring the height reduction of the bottom layer from a two-layer printed specimen. To quantitatively characterize the buildability, [PERSON] et al. [20] determined the factor as the ratio of vertical deformation of 20 stacked layers to the optimal height. [PERSON] et al. [8] proposed the buildability as the printed height. They established a linear relationship between the static yield stress and the buildability, but factors such as material density, gravity, and the geometry of the tested specimen were kept constant. [PERSON] et al.