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1、 Overview

3D printing technology is a new rapid prototyping manufacturing technology that began to rise in the 1980s. It is based on digital model files and uses computer automatic control technology to construct objects by printing the printing materials layer by layer, * eventually turning the blueprints on the computer into real objects. It has the characteristics of digitalization, intelligence and mechanical automation. The essence of architectural 3D printing is to comprehensively use the specific combination of engineering technologies such as management, materials, computers and machinery to complete engineering construction. Under the requirements of building functionalization, structural modeling complexity and scale expansion, traditional construction techniques and methods are difficult to meet and realize. Compared with the traditional construction industry, 3D printing technology can adopt an industrialized production mode, reduce labor input, improve efficiency, shorten the production and construction cycle, and prevent large-scale environmental damage during the construction process.

The Ministry of Housing and Urban Rural Development issued the Outline for the Development of Building Industry Informatization in 2016-2020, which stipulates that "actively carry out research on 3D printing equipment and materials in the building industry. In combination with the application of BIM technology, explore the application of 3D printing technology to the production of building articles and components, and carry out demonstration applications". At present, 3D printed concrete buildings are still in the initial stage in China. Domestic colleges and universities, research institutes and joint construction units are developing and developing 3D printed building technology. Compared with traditional construction methods in terms of construction logic, structural form and construction technology, the feasibility and application trend of 3D printed building technology are theoretically analyzed and proved; According to the application status of 3D printing technology in the construction industry, corresponding countermeasures are proposed based on the problems and limitations, and the application trend is analyzed with its technical advantages.

2、 Application Progress of 3D Printing Technology in China

In recent years, 3D printing technology has been widely used in machinery manufacturing, aviation, medical and other fields, and gradually expanded to the construction field. 3D printing technology can effectively solve the problems existing in the traditional construction of buildings, such as more manual work, large amount of templates, difficult to achieve complex modeling, and has significant advantages in personalized design and intelligent construction of buildings.

In actual construction, buildings have a large scale compared with other industrial products. Compared with traditional construction technology, 3D printing technology is expensive, which makes 3D printing technology limited to the production of building models. Therefore, the key to the application threshold of 3D printing technology in the construction field is cost, but it is believed that it is only a matter of time before this threshold is crossed. Building 3D printing technology and existing construction technology are complementary and interdependent. The future development trend is not to replace the existing construction technology with 3D printing technology, but a powerful supplement to the existing construction technology. 3D printing technology will undoubtedly promote the digital development of manufacturing industry. It is believed that in the world of 3D printing, architects, engineers and construction workers will use technology to combine architecture and 3D printing to complete the reconstruction of existing building technology.

The principle of concrete 3D printing technology is to convert the graphic design model of the building into a 3D printing path, and use the printing system to accurately distribute the concrete with short setting time and fast strength development layer by layer, and achieve the formwork free construction by layer superposition and accumulation. The printing system is composed of concrete conveying system, concrete distribution system and printing path control system. The three systems work together under the computer control to realize the intellectualization of the construction process.

Concrete 3D printing is a template free construction process. The concrete extruded by the printing head is not only the main structure, but also equivalent to a layer of concrete template. If the printed structure is hollow, ordinary concrete or mortar and other materials can be filled after the printed concrete is hardened, and the printed concrete on the outer layer becomes a non removable permanent template.

In terms of application, in addition to printing the use of general buildings, another potential application way of concrete 3D printing technology is to print special shaped municipal public facilities, landscape components, and even large concrete sculpture outlines designed by designers. These special applications can also reflect the characteristics and advantages of concrete 3D printing. At the same time, municipal landscape components and specially designed components generally require a certain color and decorative effect. Generally, the color of cement is monotonous, while the color 3D printed concrete can meet this requirement.

1. 3D printing house

Printing buildings through 3D printing is an important application direction of building 3D printing technology. At present, building 3D printing generally prints the outer contour of the wall according to the 3D model of the building design. The middle is usually hollow. The printing shell of the wall can be used as a template, and the middle is filled with concrete or thermal insulation materials.

In recent years, with the continuous deepening of 3D printing technology research, the application demonstration projects of 3D printing buildings have gradually increased. Printing projects of two storey buildings in situ have begun to appear at home and abroad, and prefabricated 3D printing buildings have also increased. Figure 4 - Figure 8 is a typical 3D printing construction project in China. The application demonstration of these projects has promoted the development and improvement of 3D printing technology.

Figure 4 and Figure 5 are the * 2-floor office projects completed in November 2019 based on "contour process" in situ 3D printing in China. They use a frame type 3D printer, 3D printing of the main wall, prefabrication of beam and slab structures, and the main wall printing time is about 60 hours. The printing efficiency is good, reflecting the advantages of 3D printing formless construction technology.

Figure 6 shows a double-layer villa printed by slip form technology, which is printed from concrete with 15mm coarse aggregate. The printed wall is solid. During the printing process, vertical reinforcement is directly embedded into the wall through the design of the print head, which solves the problem of 3D printing vertical reinforcement preparation. Figure 7 and Figure 8 show the printed wall components, which are assembled to form a Chinese courtyard and multi-layer 3D printing apartment project.

2. 3D printing of bridge main structure and auxiliary components

Concrete 3D printing is not only used in building construction, but also suitable for rail bridge engineering, especially for printing pier column templates in bridge engineering and municipal rail engineering.

Figure 9 and Figure 10 show 3D printed pedestrian bridge components, and then a pedestrian wall is built by on-site installation and tensioning prestress. The printing effect is good and the design is beautiful.

Figure 11 shows an arched pedestrian bridge project designed and printed by Tsinghua University, and Figure 12 shows a scaled Zhaozhou Bridge printed by Hebei University of Technology. Both arched bridges use 3D printed component shells, then concrete is poured, and then assembly is completed.

3D printing can not only print the component shell of the bridge body, but also print the fence components of the bridge, which has good strength and durability.

Compared with the printing of the wall structure itself, 3D printing of concrete is more suitable for printing the formwork of special-shaped pier columns in some bridge projects and municipal rail projects, which can save a lot of mold costs and reduce the cost. As a permanent structure, the printed mold shell does not need to be removed, and can play a good role in protecting bridge columns.

3. 3D printing utilities

Public facility buildings in cities are generally characterized by miniaturization and diversification. Building 3D printing for such small buildings has great advantages over traditional mold making. It can not only meet the requirements of rapid production, but also can be customized according to the design. It has a good application prospect. Figures 19 to 26 show some examples of public buildings that can be printed with 3D printing technology.

These printed public designs are generally small in size and can be hoisted and moved, printed and prefabricated according to the needs of different locations in the city, and then directly transported to the site for use.

4. 3D printing of large landscape components

3D printing has unique advantages in the printing of irregular curve paths. Through concrete 3D printing, some landscape sculptures and sketches can be made, and then the surface treatment in the later stage can improve the apparent quality of works, which can make great achievements in beautiful cities and cultural cities today. 3D printing technology can also realize the architect's bold design, reduce the environmental pollution caused by traditional stone carving, and save energy and environmental protection. Figures 27 to 32 show some large landscape components suitable for 3D printing.

5. 3D printing decoration enclosure

Building 3D printing technology can print some custom pattern enclosure walls, multi-functional green plant walls, and even the revetment wall blocks in the river bank project, which is template free, fast and efficient. Figures 33 to 40 are decorative members and enclosures printed or designed for 3D printing.

6. 3D printing special-shaped sketch components

3D printing technology can be used to print leisure tables and chairs in the park, tree pools and flower beds, as well as some promotional art fonts. These components are generally difficult to complete or costly through customized templates, but through 3D printing technology, you only need to conduct 3D modeling according to the design drawings, and then import the designed drawings to the printer for printing. Figures 41 to 44 are printed and designed park functional structure sketches and promotional art fonts.

3、 Application Progress of International Architectural 3D Printing Technology

The United States and Europe took the lead in exploring the application of 3D printing technology for buildings. A large number of landscape components and single storey houses were successively printed using various types of 3D printing equipment. In recent two years, the development of 3D printing technology application in architecture has gradually been led by relevant research institutes and enterprises in China, and there are few reports on large-scale 3D printing construction projects in the world.

Apis Cor completed 3D printing of the wall structure of a two-story administrative building for the Dubai Municipal Government of the United Arab Emirates (Figure 57 - Figure 58). This office building is 9.5 meters high, with a total area of 640 square meters. The concrete wall is printed by a 3D printer. Since the total building area is larger than the printing area that can be reached when the 3D printer is in a static state, the printer needs to move on the construction site through a crane to print all walls. The whole construction process is open air, which means that the site temperature, humidity and other environmental factors cannot be controlled. The development and testing of 3D printing materials are carried out in bad weather conditions, which can meet the needs of open air construction.

Cobod International of Denmark has developed a large-scale 3D printing equipment, which can print buildings with a length of 27 meters, a width of 12 meters and a height of 9 meters. It hopes to use this equipment to achieve the in-situ printing of buildings. At present, the equipment has not yet implemented the actual building test of printing.

The Federal Institute of Technology of Zurich has designed and built a three storey DFAB House (Figure 59) using digital technology. The building covers an area of about 200 square meters and is located on the upper floor of a building called NEST. The living room is equipped with concrete ceilings cast with 3D printing templates and arc concrete walls created by construction robots. It is the first large-scale residential project combining smart home and 3D printing.

Although there are few large-scale application projects of architectural 3D printing in the world in recent two years, the basic research on 3D printing equipment and materials in the United States and Europe still has some advantages over China. 3D Printing Branch will use the association platform to organize member units to promote the rapid development of China's architectural 3D printing industry by participating in international academic conferences and going to international excellent equipment and application enterprises for research and exchange.

4、 Prospect of Architectural 3D Printing Technology

The application of 3D printing in the construction industry has many advantages:

Lower cost. Printing building components can effectively avoid the cost of material storage, and can also make full use of construction waste, forming resource recycling; The construction period of 3D printing is short, the labor required is small, and the labor cost is low.

Strong shaping ability. It is suitable for the construction of special-shaped concrete components, which can be hollow and hollow, and can achieve shapes that cannot be achieved by traditional technologies.

Strong environmental protection. The components are prefabricated in advance, and the construction waste and dust generated by the construction and installation project are less than those generated by traditional methods.

3D printing technology is a multi discipline cross-border integration technology system, including the digitalization of building models, structural design, concrete materials, intelligent printing systems that adapt to the characteristics of large building volumes, reinforcement in or out of concrete, overall printing or assembly construction technology of printed components, etc. Its complexity is self-evident. The promotion of research and application of 3D printing construction will accelerate the pace of modernization of the construction industry characterized by assembly, mechanization, intelligence and greening of China's engineering construction, and boost China's economic transformation and upgrading. Although there is a slight shortage in the relevant basic research and engineering application of physical construction, 3D printing construction has great development potential. We need to clarify its development direction, clarify its development ideas and formulate its development strategy. Although 3D printer technology is a new technology, it also has corresponding new requirements on the basis of inheriting the traditional 3D printer technology and process. Although building 3D printing has broad application prospects, it is still in the exploration stage. There are some problems to be solved in printing materials, printing methods, printing equipment, structural system, design methods, construction technology and standard system.

(1) The 3D printing building adopts concrete products that are stacked layer by layer. The surface roughness of the components is uneven and the perception is low, which reduces the aesthetic effect of the building. Moreover, the size requirements of building components are different, and the requirements for corresponding printers are also different. Based on the structure and working principle of printers, their application scope will be limited. On the other hand, 3D printing buildings are irreversible and have strict requirements on the printing process of printers, which increases the technical operation difficulty of printing buildings with high requirements on accuracy. Of course, technological limitations can be solved by means of small machines or prefabricated buildings. Compared with traditional methods, there are still relative advantages that can be adopted.

(2) There is no relevant enforceable industry standard for 3D printing architecture in China. Although 3D printed buildings can refer to the floor height, plane layout, component size, pipe harness layout and other specifications in the building code, there is no relative reference index when it comes to the actual requirements in the fields of material strength, safety and reliability, load strength, seismic mitigation and other fields. The emergence of new technologies and materials requires a certain period of development to form industry norms, which requires the gradual establishment of a relevant standard system for 3D printing architecture in terms of design methods, construction processes, acceptance standards, etc.

(3) 3D printing materials. At present, the materials used are all mixtures of cement, fiber, etc., which should not only meet the strength requirements of materials, but also have good workability, and also meet the performance requirements of rapid setting of materials in the printing process. Based on the principle of "stacking layer by layer" in the 3D printing process, the printing materials are stacked layer by layer. If the condensation is not timely, the lower layer materials do not reach sufficient strength and bearing capacity, which will cause the upper layer materials to be stacked and deformed, thus affecting the accuracy. It is a key step to develop 3D printing architecture technology to develop printing materials with high tensile property, strong toughness, good crack resistance, fast initial setting time and appropriate initial setting strength.

(4) Integration of 3D printing and BIM technology

With the gradual application of BIM (Building Information Model), it is believed that the building model in the computer can be fully connected with the 3D printer. Parametric design, special-shaped building shape or local complexity are difficult to accurately express with traditional design software, drawings and construction technology. With the development of 3D printing technology, printable materials have increased, printable volume has become larger and cost has decreased. Building decoration components can be directly printed, transported to the construction site for installation and splicing, and even structural components with sufficient strength can be printed. Manufacturing enterprises have a very clear production and manufacturing process. Although the construction process in the construction engineering industry is a bit chaotic, this confusion will be eliminated in the future, and a building will be built, just as manufacturing enterprises now produce a product. With 3D printing technology, the process will be more sophisticated and more perfect. In the face of 3D printing technology, the construction of a large-scale building in the future