# Overview of the Development of 3D-Printing Concrete: A Review
## 2 3D-Printing Concrete Technology  
3D-printing concrete technology is a new technology developed on the basis of 3D-printing technology and applied to concrete construction. Its main working principle is to pass the configured concrete slurry through the extrusion device, under the control ofthe 3D software, according to the preset settings A good printing program is extruded by a nozzle to print, and finally the designed concrete component is obtained. As a new type of concrete moldless molding technology [3], it can apply the computer-aided design tools to the construction process. Compared with the traditional construction forms, 3D-printing concrete technology consumes less energy when applied to complex structures, and 3D-printing concrete technology can optimize the structure according to the working conditions. With the large-scale application and promotion of 3D-printing concrete technology, it can effectively reduce the input of materials, personnel, and machinery in the building construction process, and promote the development of digital and intelligent building construction technology [4]. 3D-printing concrete technology originated from Rensselaer Polytechnic Institute in New York, USA. [PERSON] [5] first explored 3D-printing construction technology in 1997, which proved the feasibility and prospects of 3D-printing technology in the field of construction. In 1998, at the University of Southern California, [PERSON] et al. [6; 7] developed contour crafting(r) (CC), which adopts computer precise control to automatically complete the pouring process and realizes the smooth contour surface and complex feature model pouring by controlling the nozzle. In 2007, Dini demonstrated its D-shape(r) technology [8]. D-shape(r) technology uses hundreds of nozzles at the bottom of the printing device, which can spray magnesia binders. Fine sand containing magnesia powder is sprayed on the binders to gradually solidify to form a stone solid. The sand layers are glued together to form a stone building in the end. In 2009, [PERSON] et al. [8; 9] of Loughborough University in the United Kingdom developed concrete printing technology (concrete printing, Conprint 3D(r)). Compared with the contour process, the preparation of the equipment and computer control program used in the concrete printing technology is simpler. In 2012, ETH Zurich [10; 11] launched the mesh mold project to explore the possibility of digitally manufacturing concrete structures with high geometric complexity. This technology uses 3D-printing to build a polymer network. The mold selects the appropriate particle size aggregate according to the size of the mesh to design the concrete mix ratio, so as to realize the retention of the concrete by blocking the mesh. In 2015, ETH Zurich [PERSON] et al. [12] developed smart dynamic casting (SDC(r)) based on the fast sliding mold process. Compared with other 3D-printing building technologies, its significant advantage lies in the ability to form a sliding mold around the steel bar, which realizes the enhancement of the printing structure. In 2019, China Construction Industrial Technology Research Institute Co., Ltd. and China Construction Second Bureau South China Company jointly established a project to print \"In-situ 3D-printing double-layer demonstration buildings\", marking a breakthrough in In-situ 3D-printing technology in the field of construction [13]. Figure 1 shows the development history of 3D-printing construction technology based on cement-based materials [14]. Since the concept of 3D-printing concrete was put forward for more than 20 years, the research and application of 3D-printing concrete technology has developed exponentially, relying on the advantages of short construction period, topology optimization, high mechanization, non-modeling, meeting personalized customization, complex structure building construction and shape Unique architectural construction and other advantages have entered a period of rapid development.