Journal of Critical Reviews

ISSN- 2394-5125 Vol 7, Issue 12, 2020

APPLICATION AND RESEARCH ON BUILDING

WENJIE LUO1*, XINCHEN MA1 ,JIAN YIN2

1 Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong, 999077, China

2 School of Civil Engineering, Henan Polytechnic University, Henan, 454003, China Corresponding author’s e-mail: [email protected]

Received: 11.03.2020 Revised: 12.04.2020 Accepted: 13.05.2020

Abstract As a novel category of green construction method, Building 3D Printing has the advantages of material saving, energy-saving, time-saving and customization. Nevertheless, this technology still cannot suffice the practical requirements of large-scale production in the construction industry, and it requires further exploration and research in printing equipment and materials, supportive software, evaluation standards and other aspects. Hence, this paper starts with the introduction of 3D Printing in industrial manufacturing, expounds the classification of 3D Printing in the construction field and the research status, introduces the technical process of Printing within the Building 3D Printing, printing equipment and software concerned, eventually analyze the existing problems and look forward to the prospective development. Keywords: Additive Manufacturing; 3D Printing; green construction; Concrete Printing; research status

© 2020 by Advance Scientific Research. This is an open-access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

DOI: http://dx.doi.org/10.31838/jcr.07.12.103

INTRODUCTION German Photonics Research Plan, which included financial The manufacturing technology employing Computer support for the research and development of 3D Printing Numerical Control (CNC) is also known as Subtractive technology [3]. In August 2012, the US government spent $30 Manufacturing, which is a way to remove excess parts of raw million to establish the National Center for Defense materials for a final product through cutting, grinding, Manufacturing and Machining in Ohio [4]. corrosion and melting. In contrast, 3D Printing, called Additive The Chinese government also attaches great importance to the Manufacturing as well [1], is an advanced technology that research and application of 3D Printing technology. In 2012, combines layers of printed material into three-dimensional the Ministry of Science and Technology released the National entities using a computerized numerical control model. In High-tech Research and Development Plan (863 plan), which short, 3D Printing is the process of layering and superimposing highlighted the field of 3D Printing. In 2015, the Ministry of to produce three-dimensional objects without the need for Industry and Information Technology of China officially moulds. launched the Promotion Plan of the Development of National The notion of 3D Printing was first patented in 1989 by Additive Manufacturing Industry, which is the first unique Emanual Sachs et al. from Massachusetts Institute of promotion plan for 3D Printing technology in China. In 2016, Technology. In the 1990s, Charles W.Hull from the USA China's 13th five-year plan put forward five development developed a system which is capable of concepts of "innovation, coordination, green, development and producing manufactured parts, which is a significant milestone sharing", which also requires that the construction industry in the development of 3D Printing technology [2]. should emphasize the concept of green development, take new-type construction industrialization as the core, deeply Since the 21st century, governments around the world have integrate information technology, and update and upgrade the gradually carried out plans and policies to promote the industrial chain of construction industry[5]. In the same year, development of 3D Printing technology. In May 2011, the Ministry of Housing and Urban-Rural Development Bundesministeriums für Bildung und Forschung released the outlined the 2016-2020 Construction Industry Informatization

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Development Outline: We should actively research 3D Printing technology-intensive industry is becoming more urgent. equipment and materials in the construction industry, and Intending to solve this significant problem, 3D Printing explore 3D Printing technology in conjunction with Building provides practical solutions and technical support. Traditional Information Modeling (BIM) Application in construction construction methods have high labour intensity, high safety production. risks, high material waste, and lengthy construction cycles. In comparison, the application of 3D Printing in the construction With the gradual weakening of China's advantage of industry has significant advantages, as shown in table 1. demographic dividend, the demand for the construction industry to change from a labour-intensive industry to a

Table 1: Strengths of 3D Printing technology in construction industry

Advantages Descriptions

(1) In terms of labour, most of the 3D Printing of buildings uses computer control and mechanized operations, which saves human resources to a large extent; (2) In light of materials, 3D printed buildings form at one time, without the need for mould support and mould removal, which reduces the consumption and waste of material resources; Resource-saving (3) Given the construction period, 3D Printing technology greatly retrofits production efficiency compared

with traditional construction techniques, and can achieve a reduction of more than 50% of the construction period [6], shortening the investment recovery period; (4) In point of cost, the cost of 3D printers to print complex components is generally no more than the cost of making simple components, i.e., there is almost no cost increase.

In the process of 3D Printing construction, operators only need to manipulate the computer to complete Safety most of the work, without having to enter the construction building to work at any time, significantly reducing the probability of accidents.

Environmental There is almost no environmental pollution such as noise, dust and intense vibration during the printing protection process.

The shape of the product constrains traditional construction techniques and the tools used, but 3D Printing Flexible design and technology can dispense these limitations to meet the needs of the builder and also allow designers to give customization full play to their creativity. It can easily print out building curves that are difficult to achieve in other ways, making the building function and shape more innovative.

By following the preset computer program and constructing layer by layer, artificial errors can be largely Accuracy avoided, and the quality of the finished product can be guaranteed.

3D Printing can be carried out according to the actual demand, thereby reducing inventory, and at the same Immediacy time, it can be formed at one time without assembly, which can realize the immediacy of Printing to delivery.

Extreme conditions refer to severe cold, high temperature, and weightlessness, which lead to severe Applicability of difficulties in human activities, and where traditional manufacturing processes cannot be implemented any extreme conditions more. At this time, 3D Printing technology can fully replace the manual operation and execute the construction of building entities under extreme conditions.

Nonetheless, the application of 3D Printing technology in the optimization that need to be resolved, inevitably restricting the construction industry still confronts a series of difficulties. E.g., promotion of 3D Printing building technology and the a complete and adequate evaluation standard and index formation of an industrial chain. Hence, it is particularly crucial system have not been set up yet. There are still numerous to establish a unified standard and evaluation system and problems in the process flow, printing materials, equipment develop more advanced and applicable 3D Printing equipment and supporting software, design methods, and model and materials.

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printing materials to classify 3D Printing technology into five Classification of 3D Printing categories (Table 2) : At present, 3D Printing technology has not formed a unified (1) FDM using thermoplastic as a raw material; classification standard. Generally speaking, according to the (2) LOM using thin sheet materials (paper, metal film, plastic size of printing equipment, it can be divided into a desktop, film) as raw materials; medium and large 3D printer. In accordance with the usage (3) 3DP with gypsum powder and powder as raw classification, it can be divided into aerospace, medical and materials and adhesive bonding moulding; health care, and construction 3D printer. Most commonly, it (4) Photopolymerization moulding technology using can be divided according to their moulding principles, namely photosensitive resin as raw material and light source for Stereo Lithography Appearance, Selective Laser [7], curing, including SLA, Poly Jet and DLP; , Fused Deposition Modeling [8], (5) Laser powder moulding technology, that is, the technology Laminated Object Manufacturing, Three Dimensional Printing using powder materials (metal, alloy, ceramic and paraffin) as and Gluing [9], Digital Light Processing, Poly Jet, Direct Metal raw materials for laser sintering/melting molding, including Laser Sintering and Electron Beam Melting. SLS, DMLS, SLM and EBM. Li Xuan [10] combined the printing molding principles and

Table 2: The classification of 3D Printing techniques

Representative enterprise or Type name 3D Printing Basic material Application range institution

Medical and health, Fused USA OPM, Utrecht Institute of Thermoplastics, rubber, automobile manufacturing, Deposition FDM Pharmacy and Nanyang syrup and chocolate aerospace engineering and Molding Technological University food processing

Layered Paper, metal film and Car lights, shoes and cast Helisys, Japan Kira, Sweden Direct LOM plastic film iron handles Sparx and Kinergy Molding

Aerospace engineering, 3D Printing Gypsum, ceramic, metal, automobile manufacturing, Bonding 3DP composite powder and TPM3D and Stratasys medical and health and civil Molding quartz engineering

SLA, DLP Photopolym-e Liquid photosensitive Jewelry, dental molds, and Z Corporation, 3D Systems and Poly rization resin small / fine parts and OBJET Geometries Jet

Powders for metals, SLS, DMLS, Laser Powder alloys, , Sensors, human tissues and DTM and Sowell 3D Printing SLM and Molding paraffin and moulds Technology EBM materials

Compared with other 3D Printing technologies, 3D Printing developing technology. Bonding Molding Technology has a significant advantage, that is, it does not need to use a laser system or a melting heating Research status of Building 3D Printing system so that the costs of equipment purchase and The application of 3D Printing technology in the field of maintenance are low, and the operation is simple. architecture can be mainly divided into two stages: the first Consequently, 3DP technology has become a leader in the 3D stage is the planning and design stage, in which designers can Printing industry and is now the most dynamic and rapidly establish 3D models and use 3D printers to make physical

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building models and urban planning models; the second stage which only took 60min[11]. is the construction phase, in which designers directly use the In a real sense, automated 3D Printing technology was first large 3D printers to build full-size buildings that meet used in the field of architecture in 1997 by Joseph Pegna, an structural safety and functional requirements. The latter is American scholar. He layered accumulation of described below. materials and selectively solidified them, and finally made Due to the late start in China, the related research on the free-form building components [12]. application of 3D Printing technology in the construction field In 2001, Professor Behrokh Khoshnevis of the University of is still relatively lagging. In contrast, the United States and South California proposed , which is a novel some developed European countries have been exploring the 3D Printing technology for architecture. It realized the layered concept of "automated building manufacturing" as early as the Printing of concrete through the nozzle with a spatula and the 1970s. large extrusion equipment (Fig. 1). In 2012, Behrokh Italian architect Dante Bini came up with the creative idea of Khoshnevis used CC to print hollow walls that could reduce the "building houses in layers of concrete". In 1965, he succeeded building's dead weight and optimize its spatial arrangement. in his vision by building a square structure out of cement in Bologna. The building has a height of 6m and an area of 12m2,

(a)

(b)

Fig. 1: (a) Contour Crafting technique (b) The trowel[13]

In 2007, the British company Monolite introduced D-shape. concrete, and (3) 3D construction technology based on Different from the CC, this technology uses a cementing agent mechanical arm drive. to cement each layer of sand and gravel powder selectively. In 2008, Richard Buswell of Loughborough University in the Layered bonding technology based on sand and stone UK proposed a Printing process called Concrete Printing. In powder — D-shape this process, the semi-flowing concrete is sprayed, layered and Italian engineer Enrico Dini (Monolite, a British company) superimposed. The most apparent difference from the CC designed the world's first architectural 3D Printing conceptual process is that there is no trowel on the nozzle for auxiliary machine in 2004. In 2007, he came up with the D-shape levelling. technology, a method of stacking sand and gravel powder Subsequently, by coding in a computer, the Swiss Federal selectively solidified by spraying adhesives (Fig. 2 for the Institute of Technology Zu rich (ETH Zu rich) [14] controlled the process flow and Fig. 3 for the printing device), which led to six-axis robot, an automation device commonly used in the the successful Printing of a 1.6-meter-tall sculpture in 2009. automobile industry, to build a facade with light and shade The first building printed was called a "radiological library", effects. over 4m high. Later, he completed the world's first green In the course of more than 20 years' development, the experts utopia, Masdar City, which was leap progress for 3D Printing in and scholars in the academic circle have put their heads the construction industry. together to explore the development of 3D Printing from As Printing starts at the bottom of the building structure, the multiple angles and through various methods and approaches. device moves back and forth along the horizontal axis of the Based on the differences in materials and processes used, this beam and four vertical columns. Thousands of nozzles spray paper divides architectural 3D Printing technology into three grit and adhesive (magnesium-based glue) at the same time, categories :(1) layered bonding technology based on sand and layer by layer upward. Magnesium-based glue firmly binds the stone powder, (2) layered shotcrete technology based on sand into a solid (texture similar to marble), and eventually

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form a prescribed shape. Due to the solid microcrystalline compactness. Besides, the thickness of each printing layer of structure, the new material has excellent tensile strength and D-shape technology is relatively small, only 5 ~ 10mm[15].

Fig. 2: The flow chart of D-shape

Fig. 3: The printer for D-shape Also, D-shape has the potential to print buildings on other extraterrestrial space. planets. Enrico Dini is now collaborating with the European Space Agency, Foster + Partners and Alta Space to explore In 2016, the world's first 3D printed footbridge was construction techniques that can print by using . In successfully erected in Alcobendas, Spain[17]. The footbridge, 2014, the preliminary results were obtained. Cesaretti et al. which is 12m long and 1.75m wide, consists of eight separate [16] used lunar soil as raw material. They explored the sections and is designed by the Institute for Advanced feasibility of in-situ printing residential buildings on the lunar Architecture for Catalan(IAAC). Allegedly, the bridge was surface based on D-shape process, so as to facilitate the future utterly printed from concrete and reinforced with application of this technology in the development of polypropylene thermoplastic plastic (Fig. 4).

Fig. 4: The footbridge manufactured by 3D Printing

The substantial advantages of the D-shape printing technology reduces carbon emissions. The D-shape is especially suitable are as follows: for buildings with the characteristics of hollow holes and (1) the construction speed is faster than the traditional cantilevering, which requires a high degree of precision. method; Whereas, D-shape printing technology also has some (2) the printed component has high resolution and smooth disadvantages: (1) compared with other 3D construction surface; technologies, the moulding process is relatively slow; (2) (3) good strength and integrity; printing equipment covers a large area; (3) the size of (4) no pipeline blockage problem; components is limited by the size of printing equipment; (4) (5) the materials used are only 1/3~1/2 of those used by high cost [18]; (5) due to the need for outdoor Printing, it is traditional technologies, and almost no waste is generated. greatly affected by bad weather; (6) it takes time and energy to The selection of non-cement printing materials also greatly spread printing materials and remove the remaining materials

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on site [19]. 20min, and the entire printing process lasted a total of 41h. Layered shotcrete technology based on concrete Then, to test the mechanical properties such as the Concrete Printing compressive strength of the cube, the chair was peeled in situ In 2008, in the Innovation and Architecture Research Center of [20,21]. Loughborough University, Lim et al. proposed the architectural Lim et al. [22] used Grasshopper software to optimize the 3D 3D Printing technology, later known as "concrete printing", Printing path, which was transformed from a plane layered which is a process of layered stacking moulding based on path to a curved layered path, and then successfully made a concrete shotcrete (Fig. 5 for the flow and Fig. 6(a) for the hyperboloid sandwich concrete slab (Fig. 6 (b)) with a size of printing equipment). In 2009, the team used independently 1.5m × 1.5m × 0.1m. He pointed out that compared with the developed polypropylene fibre concrete suitable for CP, divided traditional plane path, the surface path had more advantages them into 128 layers and successfully printed a concrete in surface strength, apparent mass and printing time. backrest chair (2m × 0.9m × 0.8m). Each layer consumed

Fig. 5: The flow chart of Concrete Printing (a) (b)

Fig. 6: (a) The printer for Concrete Printing (b) The hyperboloid sandwich concrete slab

Since 2002, Yingchuang Construction Technology (Shanghai) resistance and are prone to cracking, which to a large extent Co., Ltd. Had spent 12 years independently developing a giant restrains the application of 3D Printing in high-rise buildings. 3D Printing equipment (150m×10m×6.6m), printing ink Then, to explore the safety of 3D Printing construction (made from building waste mixed with cement, fibre and technology, in January 2015, Yingchuang Construction binder) and continuous printing technology. It is worth Technology (Shanghai) Co., Ltd., Tongji University, and China mentioning that the company first used 3D Printing technology Construction Eighth Engineering Division Co., Ltd. Built the to print building walls in 2008, and printed 10 physical first multi-storey solid building in Suzhou Industrial Park (Fig. buildings in Qingpu District, Shanghai in 2014 (Fig. 7(a)) [23], 7©), which used 3D printed reinforced masonry structure, and the whole printing process lasted the only 24h. The key to with an area of 1100m2 and an actual printing height of 15.3m. the technology was to ensure that the test body did not All the vertical and horizontal walls were printed at the factory collapse when the ink was layered and stacked and to ensure and then transported to the construction site for assembly. that the growth of ink strength process matched the printing Bai Jie et al. [24] analyzed the rationality of the application of process. The printing process was that it firstly completed the reinforced masonry structure in the above projects and construction of the building wall (Fig. 7(b)), and then expounded the critical points of its construction. Wang Liping assembled and shaped. et al. [23] analyzed the critical technical problems in the However, the flexural strength of concrete used for 3D Printing construction process of this project and pointed out that the is still low, so the brittle walls are not conducive to earthquake combination of 3D Printing technology and reinforced

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masonry shear wall structure had solved the adaptability the seismic performance of the experimental building had problem of the structural system to a certain extent. However, yet to be verified.

(a) (b) (c)

Fig. 7: (a) 3D Printing buildings in Qingpu District, Shanghai (b) Printed walls (c) The testing building in Suzhou

The concrete printing technology is relatively straightforward, printing equipment can sketch the outline of the 3D model on the printing efficiency is relatively high, and it has high the site. At the same time, the printer automatically extends three-dimensional freedom and a small stacking resolution.[14] two trowels from its nozzle to flatten the shape of the concrete However, its shortcoming is that the surface of the printed strip. However, the gantry hanger is too bulky, which is not product is rough and has layered ripples. The size is limited by conducive to equipment disassembly. After improvement, the size of the printing equipment [18], which is suitable for some scholars have replaced the gantry frame with a buildings with large volume and relatively simple structure. lightweight rigid frame, which makes the printing equipment more flexible and portable, which makes it more feasible to use Contour Crafting the contour crafting process to print buildings on the Professor Behrokh Khoshnevis from the University of Southern construction site [25]. California (USC) proposed an architectural 3D Printing The key technology of the Contour Crafting is to control the technology that used a computer-controlled nozzle to stack lateral pressure on the concrete contour reasonably. In other concrete in layers to form structures [22] (Fig. 8). Contour words, if the lateral pressure exceeds the limit of the bearing Crafting technology has become a mainstream 3D Printing capacity of the contour, the Printing fails. Therefore, the lateral technology for architecture [19]. pressure is the decisive factor that restricts the filling speed of The printing equipment of the contour crafting process the contour kernel and the printing height of the (generally XYZ triaxial printer) is supported on the gantry structure/components [19]. Only by coordinating the frame, which can complete the plane movement on the guide relationship between the squeezing speed, kernel filling speed, rail, and squeeze the semi-fluid concrete through the nozzle concrete condensation and hardening and the speed of which can be vertically retractable to achieve the architectural strength development can the best printing effect be achieved. Printing. If the pre-designed 3D model is imported, the

Fig. 8: The flow chart for Contour Crafting

After many years, the contour crafting technology has Ltd. successfully printed a villa of 400m2 within 45 days using developed rapidly, and it can print large and multi-storey architectural 3D Printing technology based on contour buildings. For example, Beijing Huashang Luhai Technology Co., moulding technology; ApisCor from Russia successfully printed

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a single-storey building of 37m2 within 1day using its circular printing process, which is composed of a truss robot, control 3D printer; China Construction Second Engineering Bureau Ltd. unit, stirring device and pumping device [26-28]. In 2017, the used the Contour Crafting to accomplish the first in-situ 3D world's first bicycle bridge made by 3D printed prestressed printed double-storey building in Longchuan County, concrete (Fig. 10) was successfully erected by TU / e. The Guangdong Province. The building is 7.2m high and has a design life of the bridge was 30 years, and the bridge was 8m construction area of approximately 230m2, and the printing long and 3.5m wide [29, 30]. The printing material of the process took only 3 days. bridge was made of special fibre reinforced concrete (FRC), The research team from Eindhoven University of Technology and high-performance steel wire was added to increase the (TU/e) successfully developed a 3D printer with four degrees bearing capacity when spraying concrete layer by layer. of freedom (9m × 4.5m × 2.8m) based on the Contour Crafting

(a) (b)

(c) (d)

Fig. 9: (a) The printer (b) The printing process (c) Prestress tension (d) Bridge erection

Like the D-shape, Contour Crafting has the potential to print moon surface [31]. The 3D Printing technology of Contour buildings on an extraterrestrial planet (Fig. 10). Currently, the Crafting technology makes it possible to print space bases and National Aeronautics and Space Administration (NASA) is highways directly using in-situ materials from the actively assisting in the research of Contour Crafting extraterrestrial space. technology for the future construction of space bases on the

Fig. 10: The schematic diagram of Contour Crafting in space

Compared with the Concrete Printing process, its main single/hyperboloid form of architecture. Therefore, it is sought advantage lies in the use of a trowel to the surface of the after by architects who pursue free-form. printed member for levelling, so the surface of the finished There are several deficiencies in the use of contour crafting product is relatively smooth. Besides, steel bars can be printing yet. E.g., its efficiency is low (the wall printing rate is configured in the cavity structure generated by the printing about 3min / m2), and the size of the finished product is process, which realizes the strengthening of the structure and limited by the size of the printing device [18], and the makes the Printing of high-rise buildings easier [32]. In interlayer adhesion strength is low as well, and the printing addition, the contour crafting process can easily print the accuracy depends on post-processing (trowel). When printing

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architectural entities, it is prerequisite to print out the cavity other operations by robotic arms equipped with end-effector. with high-performance requirements, and then cast the core The metal grid can play the role of porous permanent concrete in layers, which is prone to form a construction joint formwork, and at the same time, it can be integrated with the with reduced long-term service performance. concrete during the construction process of concrete pouring D construction technology based on robot arm drive to function as structural reinforcement [33-36]. In other words, Mesh Mold the mesh mould has the dual role of reinforcing steel and In recent years, the Swiss Federal Institute of Technology in moulds. In addition to its steel wire, it can also use Zu rich (ETH Zu rich) has created a hyperbolic metal mesh thermoplastic , such as PE plastic and ABS plastic. template in open space (Fig. 11) by welding steel wire into a [33,34]. three-dimensional mesh structure by cutting, bending and

Fig. 11: The hyperboloid metal mesh mold

In 2017, Branch Technology in the United States built a machine setup, structural strength and concrete performance. large-scale sophisticated building wall, which was precisely the 3.3.2 Brick Stacking use of Mesh Mold technology. Compared with traditional craft Since 2006, Fabio at the Swiss Federal Institute of Technology concrete walls, this wall had nearly the same structural in Zu rich (ETH Zu rich) has begun to research strength and lighter weight [37]. In the same year, MIT three-dimensional construction with a giant as the developed the Digital Construction Platform based on Mesh lead. Among them, the more typical technology is called brick Mold technology, which could print an open dome with a stacking, which is a construction process that uses bricks as 3D functional gradient within 13.5 h [38]. Printing materials and a CNC program to manipulate a The core technology of the Mesh Mold is to use the concrete 3m×3m×8m robot to grab and stack bricks, and uses epoxy with suitable rheological property to fill the 3D grid to prevent adhesive to connect the upper and lower bricks which are the lateral outflow of concrete. Therefore, it is necessary to connected by an agent to build a brick wall with a façade area design and test the mesh density and concrete viscosity before of more than 300m2 (Fig. 12) [39]. Benefiting from the construction [19]. accuracy of computer operation, traditional masonry materials Mesh Mold technology can significantly reduce labour, material have demonstrated unprecedented architectural charm under consumption and transportation costs, much refine the the guidance of new technology. After that, the researchers construction efficiency, and be suitable for the site further proposed a new technology that used small flying construction. However, the construction process of Mesh Mold robots to grab and superimpose bricks, which perfected work technology is limited by many factors, such as mesh topology, efficiency and degree of freedom [40].

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Fig. 12: The brick wall fabricated by ETH

Because brick and masonry are applied as printing materials, buildings. The following takes this technology as an example to the printing strength of Brick Stacking technology is relatively illustrate its process, supporting software and equipment low, which is suitable for buildings with low strength composition. requirements. Process flow Building 3D Printing process, equipment and supporting The concrete printing process of a building can be divided into software four stages: model preparation stage, concrete preparation Concrete Printing has become a widely used technology in stage, building entity printing stage and post-processing stage scientific research and the field of physical Printing of (Fig. 13).

Fig. 13: The flow chart of 3D Printing technology

3D models are essential for 3D Printing. In the stage of model Supporting software preparation, there are two paths for a model generation: firstly, Building 3D Printing software is divided into modelling (or 3D data can be obtained through manual computer modelling; scanning) software, slicing software and three-dimensional secondly, the entity can be scanned based on reverse travel control software (Fig. 15 in 4.3 for the printing engineering and 3D scanning technology [41], and then the sequence). contour model of the entity can be reproduced in the computer after data analysis. During entity scanning, the scanning Modeling software technique has a huge impact on the accuracy of model In the field of construction, 3D Printing models are generally generation. After the 3D model is generated, it needs to be acquired by computer software, that is, Revit, Auto CAD, layered to obtain the thickness and shape data of each layer. Inventor, Tekla, 3D MAX and other software are used for model Besides, the optimization of the printing path can not only save rendering. At the same time, there are some situations where the time of the printing process for complex buildings or you need to duplicate some buildings, components, or existing components but also maintain the fresh mixing performance of terrains (Fig. 14). In this case, 3D scanning technology is the concrete material (fluidity, cohesion, water retention, needed to obtain the model of the original object. Nevertheless, pumpability) [32]. no matter how the model is obtained, it must be converted to a printable format (usually STL format).

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(a) (b)

Fig. 14: (a)An model of a existing terrain(b) A finished product of 3D Printing

Water tightness is a required property of a 3D model, meaning difference is the degree of personalization, slicing accuracy and that the model should not have free one-dimensional edges slicing speed. and two-dimensional surfaces, and the model does not have singularities, and two surfaces should share its edges. The Three-dimensional travel control software purpose of most of the 3D modelling software today is The function of the three-dimensional travel control software rendering and visual management, and the established models is to decipher G code and control the printing device to sometimes cannot meet the requirements of water tightness. complete Printing. Travel control is the most crucial part of the The STL format uses a triangular grid to represent the 3D 3D Printing process. Similar software such as Printrun, Printing model. The connection of the triangular grid can repetier-host and replicatorG is used only in their respective reflect whether the model is watertight, and the smaller fields. For concrete 3D Printing technology, there is currently triangular grid can reflect complex surface shapes and subtle no suitable open source software on the market. If concrete 3D surface changes. It is worth noting that to avoid printing failure Printing is carried out, the travel control software shall be under gravity, every effort should be made to reduce the sharp developed independently. and slender parts of the 3D model [42]. In addition, the point cloud data obtained after the three-dimensional scanning is Printing equipment too large, and it is difficult for conventional computers to run. Concrete 3D printer is another foundation for concrete 3D You need to input these data into Cyclone for block processing Printing. The concrete printer for 3D Printing of buildings is and thinning (screening) processing, and then import them presently still not in mass production in the market. Thereby, into Auto CAD software to convert to the required STL format. the implementation of 3D Printing technology of concrete requires r&d strength of 3D Printing equipment for Slicing software universities and enterprises. As the name suggests, the slicing software divides the 3D Generally speaking, concrete 3D printer consists of the model into layers for subsequent Printing, namely, converts the following systems :(1) control system; (2) jet extrusion system; exported STL format file into G code (3D Printing stroke coding (3) mechanical motion system. The printer system relationship language), such as Cura, Slic3r, Skinforge, etc. The primary is shown in Fig. 15. functions of the above software are roughly the same, but the

Fig. 15: The relationship diagram of printer systems

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Control system For the time being, most 3D Printing adopts the combined The control system shall have the following modules to drive transmission mechanism of toothed belt and polished rod, the mechanical motion system and the extrusion system: whose disadvantage is that the toothed belt lacks the bearing (1) External data input interface — usually USB universal capacity and is easy to wear, which leads to the decrease of serial port bus, TF cardholder, and TF card slot; transmission accuracy and control sensitivity. In contrast, the (2) Central microprocessor — the control system centre, ball screw pair has the advantages of high transmission which determines the printing speed and accuracy; efficiency, fast dynamic response, high positioning accuracy (3) Human-computer interaction display interface (such as and long service life, while the rolling linear guide has the LCD touch screen) — the operator obtains various working apparent advantages of simple mechanical structure, low parameters in the display interface and completes the printing friction resistance, high stiffness and no lateral clearance operation through the interface; movement [43]. In consequence, professional-grade printers (4) Stepper motor drive m that have high requirements for accuracy generally use a odule — which controls the operation of a mechanical precision ball screw pair with a rolling linear guide movement system; transmission mechanism on the market. (5) Insurance and alarm modules (including switching power The functionality of the drive component is to convert the supply, air switch, overload protector and solid-state relay) electrical impulses transmitted by the control system into displacement. Nowadays, the motor is the most commonly Extrusion system used drive component, among which the stepping motor can The extrusion system consists of three components: the directly convert the electrical impulse signals into linear and material storage device (the material barrel), the material angular displacement, with high precision, sensitive response, feeding device (such as the motor screw extruder) and the simple structure, low cost, long life, wide speed adjustment nozzle. The extrusion system should suffice several basic range and other advantages. Hence, the stepping motor is a requirements: commonly used open-loop control motor in the field of CNC (1) it should not leak during the printing process and is not [44]. easy to block; (2) its printing speed should be controllable; CONCLUSION AND PROSPECT (3) it should be easy to disassemble and clean; By summarizing the 3D Printing technology and its application (4) its nozzle should be determined according to the printing and research status in the territory of building construction, characteristics and accuracy requirements. the following conclusions can be drawn: Generally, it should not be too small. A nozzle that is too small (1) Printing method. Albeit the D-shape, Contour Crafting, and reduces the printing efficiency significantly, and its fineness Concrete Printing mentioned above have been gradually put requirements of the aggregate are too high. In other words, the into actual production, they are currently mostly used in the proportion of concrete aggregate should also match the nozzle production of building components, and the components are diameter, which is generally 8-12mm. assembled after Printing. Whereas, the development goal of building 3D Printing is one-time full-size Printing other than Mechanical motion system printing prefabricated components, but the limitation is still Mechanical motion system includes equipment support conspicuous: the larger the building size, the larger the size of components, transmission mechanism and drive components. the printing equipment required; and the larger the device size, The equipment support member plays an important the higher the control difficulty and the worse the printing supporting role for the entire printing equipment, and its accuracy. But for the time to come, full-size printing load-bearing capacity and working stability have a massive technology will also become mature. In addition, using robot impact on the forming accuracy of the finished product. groups to print from multiple directions and multiple angles at Therefore, the supporting member must have sufficient the same time can significantly surmount the constraints of strength and stiffness, and no vibration occurs during the building size, which is also a possibility for the prospective printing process. development of building 3D Printing.

Journal of critical reviews 575

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(2) Printing materials. Due to the characteristics of concrete jointly promote the vigorous development of 3D Printing in such as high strength and excellent formability, it is broadly building construction. used in the research and application of 3D Printing of buildings. However, the rheological properties of traditional concrete REFERENCES materials cannot meet the requirements of 3D Printing. 1. Wamke P H, Seitz H, Warnke F, et al. Ceramic scaffolds Moreover, the earthquake resistance of houses made of pure produced by computer-assisted 3D Printing and sintering: concrete printing is insufficient, which has imposed Characterization and biocampatibility investigations. restrictions on the application and promotion of building 3D Journal of Biomedical Materials Research Part B Applied Printing. Therefore, the use of new admixtures and additives in Biomaterials 2010; 93(1): 212-217. 3D Printing concrete materials can improve their overall 2. Dickens P M. Research development in rapid prototyping. performance, and even the use of smart materials, fibre Proceedings of the Institution of Mechanical Engineers, materials, composite materials instead of concrete to complete Part B:Journal of Engineering Manufacture 1995; 209(4): Printing is a research trend on the 3D Printing materials. 261-266. (3) Supporting software. The accuracy of the supporting 3. Scheithauer U , Schwarzer E , Richter H J, et al. software determines the accuracy of 3D printed building Thermoplastic 3D Printing—An Additive Manufacturing entities to a certain extent. Therefore, the development of Method for Producing Dense Ceramics. International more efficient and accurate supporting software and the Journal of Applied Ceramic Technology 2015; 12(1): effective conversion of 3D models into computer languages are 26-31. also topics worth discussing. 4. Armstrong R, Gregory A. Layers of complexity. The (4) Test methods and evaluation indicators. Compared with the Medical Journal of Australia 2012; 197(6): 311−357. traditional concrete, the concrete used in building 3D Printing 5. Zhang Tiankai. Analysis of the general situation of have considerably different working performance, mechanical development of building construction technology in properties and durability, such that its performance test China . Science and Technology Innovation and methods and evaluation indicators specified in the previous Application 2012; 10: 224. codes are no longer applicable. To better characterize the 6. Cheng Bihua, Wang Xiao, Pan Ting. Application of 3D performance of 3D printed concrete, it is of particular Printing technology in the field of construction and significance to establish a valid and applicable calculation analysis of problems . Science and Technology Research model and the prediction model of its service life cycle and to Management 2018; 7: 172-177. promote unified code standards. 7. Li Zhichao, Gan Xinpeng, Fei Guoxia, et al. Research (5) Architectural functionality. Up to now, the successful cases progress of selective laser sintering 3D printed polymers of 3D Printing are still staying in satisfying structural safety and their composites . Polymer Materials Science and and simple use functions, and there is no approach to fulfil the Engineering 2017; 33(10): 170-174. requirements of residents through 3D Printing directly. To this 8. Du Yulei, Sun Feifei, Yuan Guang. Development status of end, future 3D Printing equipment is likely to possess more 3D Printing materials . Journal of Xuzhou Institute of complete functions, such as installing pipe networks, spraying Technology: Natural Science Edition 2014; 29 (1): 20-24. waterproof materials and printing finishes. 9. Gao Yu, Yang Jianfei, Qiu Xin, et al. Research on the Building 3D Printing is the fruit of multi-disciplinary collision realization method of 3D Printing of metal powder based of architecture, civil engineering, mechanics, material science, on digital micro-spraying . Journal of Nanjing Normal computer science, automatic control and robotics. It makes use University: Engineering Technology Edition 2017; 17 (3): of the most advanced scientific achievements, with a view to 1-6. revolutionizing the traditional construction. But meanwhile, 10. Li Xuan, Mo Hong, Li Shuangshuang, et al. Research owing to the intersection of multiple disciplines, there is an progress of 3D Printing technology process control inevitable problem of acting blindly. Only by strengthening the problems . Journal of Automation 2016; 42 (7): 983-1003. exchanges and cooperation of various disciplines can they 11. Yang Jianjiang, Chen Xiang. Application trend of 3D Printing building technology . Construction Technology

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