Three dimensional printing has made its advent in construction in the recent past. With recent advancements in technologies, materials and further inclusion of building information modelling into the construction process, 3D printing is proving to be advantageous in streamlining and improving the scheduling requirements of a project digitally. The precision, intricate designs, repeatability, quality control and other specific advantages of 3D printing makes it more suitable with respect to construction. However, the challenges with respect to 3D printing model, cost and materials etc has to be addressed to make the technology more suitable and prevalent in construction.

M. Surya, Scientist, S. K. Singh, Senior Principal Scientist and S. K. Kirthika, PhD. Student, AcSIR, CSIR-Central Building Research Institute, Roorkee

Introduction
3-D Printing in Construction - Lowke et al - 2018Lowke et al (2018)
Lowke, D., Dini, E., Perrot, A., Weger, D., Gehlen, C., & Dillenburger, B. (2018). Particle-bed 3D printing in concrete construction–possibilities and challenges. Cement and Concrete Research, 112, 50-65.
Three-dimensional printing has been widely used in manufacturing of automobile parts, surgical equipments and other products over the past 25 years. The technology originated in 1983 and extended as selective laser sintering, extrusion, fused deposition modelling, sprinkler jet, electronic beam melting, laminated object manufacturing, direct energy deposition, powder bed fusion, etc. (Delgado Camacho et al., 2018; Ngo et al., 2018). However, its advent in construction industry has been in the recent past. The inhibitions in the use of 3D printing in construction comes from technical, economic and social point of view (De Schutter et al., 2018). The technical inhibitions involve the assembly and production of 3D printer and development of materials with suitable properties. The 3D printing is presently in demo/ research scale whereas the establishment cost of printer is an expensive affair. However on a large scale considering the returns, 3D printing becomes cost effective than conventional construction. The construction industry being one of the major sectors for manpower employment, the advent of 3D printers is not widely accepted in developing countries as it leads to fear of unemployment. However, with recent advancements in technologies, materials and further inclusion of building information modelling into the construction process, 3D printing is proving to be advantageous in streamlining and improving the scheduling requirements of a project digitally. The precision, intricate designs, repeatability, quality control and other specific advantages of 3D printing makes it more suitable with respect to construction (Bos et al., 2016; Xia and Sanjayan, 2016). Also, the lack of skilled labour and the demands for faster construction are making it need for the hour to further probe research in 3D printing.

3-D Printing in Construction
The basic steps involved in 3D printing construction are as shown in Fig.1. 3D CAD model with building information / BIM model is developed. This model is sliced layer by layer with all the components to required thickness. This layered arrangement also known as stereo lithography files is given as an input to the 3D printer assembled as per requirement. After this step, the actual printing process is carried out and the building is printed in layers by any one of the chosen methods. Duballet, Baverel and Dirrenberger, (2017) classifies the 3D printable building systems based on the following parameters viz. Object scale, extrusion scale, printing environment, printing support and assembly parameter.

Various steps in construction 3 D printingFigure 1: Various steps in construction 3 D printing

Types of 3 D Printing
3-D Printing in Construction - Types of 3 D PrintingFigure 2: (a) D-shape; (b) contour crafting; (c) concrete printing
There are various types of 3 D printing currently existing and certain types are more suitable for construction than the others. The major types of concrete printing suitable for construction are Contour crafting, concrete printing and D Shape (Perkins and Skitmore, 2015).

Contour crafting
Contour crafting is fabrication technique in which the concrete is crafted layer by layer and was developed by Khoshnevis (2004) in California (Perkins and Skitmore, 2015). This is the most promising 3D printing technology and allows the fabrication of the complete house in situ (Hager, Golonka and Putanowicz, 2016). Two or more nozzles move simultaneously along the gantry and simultaneously print various components of the structures. This method also enables the printing of accessories and conduits along with the buildings. The Potential areas of applications for contour crafting are i) low income housing or emergency sheltered housing and ii) architectural buildings involving complex shapes (Perkins and Skitmore, 2015).

Concrete printing
Concrete printing is an extrusion based printing process in which the nozzle extrudes material while moving in a predetermined path in a continuous process. There are three categories of concrete printers presently available and widely used in real time construction viz. gantry, robotic and crane system (Paul, van Zijl and Gibson, 2018) along with a printing nozzle with continuous supply of materials (Buswell et al., 2018). Gantry has a fixed height of operation whereas the cranes are adjustable in vertical direction. Robots typically have a fixed dimension and are difficult to scale up however they accommodate all operational direction and enable continuous and complete in-situ printing.

Selective binding and Binder jetting / D- shape
D shape printing uses layers of powders and adhesive sprinkled on it at desired locations. In D shape printing the powder material is laid and compacted to required thickness (Perkins and Skitmore, 2015). Then the binder is injected at required places using a nozzle. Once the hardening is complete, the loose powder can be removed and the hardened model can be taken separately. Additive manufacturing techniques including ink jet printing and laminated object printing are also binder jetting and are used in construction (Ngo et al., 2018). These methods are more suitable for polymers, metals and ceramics. Fig. 2 shows an example of buildings each printing can achieve.

Printable Materials
3-D Printing in Construction - Hager et al - 2016Hager et al (2016)
Hager, I., Golonka, A. and Putanowicz, R. (2016) ‘3D Printing of Buildings and Building Components as the Future of Sustainable Construction?’, Procedia Engineering. 151, pp. 292–299. doi: 10.1016/j.proeng.2016.07.357
In recent years, various construction materials for 3D concrete printing materials have been developed. These materials can primarily be classified into 3D printable plain concrete, 3D printable geopolymer, 3D printable fibre reinforcement concrete, 3D printable rapid hardening materials and 3D printable earth-based materials (Zhang et al., 2018). For successful 3D printing, high-quality final properties such as compressive strength, flexural strength, have to be targeted. However, considering that the material needs to continuously flow through the print head and has to gain strength immediately to carry its self-weight and weight of the added layers, the fresh properties and the strength gain properties should also be targeted. (De Schutter et al., 2018). Since the printing process requires a continuous, high degree of control of the material during printing, high performance building materials with properties such as low to zero slump concrete, high static viscosity within the nozzle and low viscosity after extrusion, are preferred in the fresh properties. Various studies have been carried out around the world to improve the fresh properties, printability and mechanical properties of the materials used.

Feng et al., (2015) and Xia and Sanjayan, (2018) studied the possibility of Powder 3D Printing using plaster powder and additives. A calcium aluminate cement with OPC blend was developed to enable powder printing feasible (Shakor et al., 2017). Interlayer stress strain relationship and failure criterion of the 3D printed elements were proposed and it is reported that the behaviour of 3D printed elements depends mainly on direction of 3D printing and direction of loading (Feng et al., 2015). Christ et al., (2015) studied the use of fibers in powder printing using dental gypsum along with polypropylene fibers to attain material of required flexural strength and interlayer bonding. Improvement of interlayer bonding in 3D printed concrete using FRP wrapping was attempted and strength improvement by 80% was acquired. It was also observed that the failure mode shifted from brittle to ductile with use of FRP (Feng et al., 2015). Sanjayan et al., (2018) studied the effect of interlayer water on strength of the printed elements and it was seen that the elements printed with 30 minutes delay had better properties than the elements with a delay of 20 minutes or less. Bong et al studied the printability of 3D printable geopolymer concrete using properties such as shape retention ability, open time, extrudability and compressive strength (Wangler and Flatt, 2018). A high strength fiber reinforced concrete of 107 MPa strength was developed and 3D printed by extrusion. It was also observed in the study that the density increase from 2250 kg/m3 to 2350 kg/m3 during printing (Le et al., 2012). Vaitkevičius, Šerelis and Kerševičius, (2018) developed a binder of final setting time 5 – 20 minutes and also the hydration process was accelerated by ultrasonic dispersion.

Major Structures by 3D Printing
Various lab scale and small trials have been carried out in research laboratories and institutes for implementation of 3D printing. The various showcase examples available in three dimensional printing for construction are by Smart Dynamic Concrete, XTree, TotalKustom, WinSun (De Schutter et al., 2018) and are described in subsequent paras.
  • In 2014 Dutch designing company Dus Architects built a canal house in Amsterdam completely by 3 D printing. The duration of the project was 3 years and this was the first building recognised to be built completely by 3 D printing. A giant 3D printer called Kamer Maker, which is an extrusion type printer, prints components of the house. (Hager, Golonka and Putanowicz, 2016)
  • WinSun Decoration Design Engineering Co is a Chinese enterprise, working on material similar to concrete that will be suitable to use in 3D printing technology and built an apartment building of five storeys. (Hager, Golonka and Putanowicz, 2016). They also built a villa of 1100 m2 at Suzhou, China. WinSun Co also built a series of 10 houses using a giant printer of size 150 × 10 × 6.6 m (Bos et al., 2016).
  • An Office building in Dubai, UAE of area 250 m2 was built in 2016, by Chinese construction company Winsun Co. The building was printed using a 120 × 40 × 20 feet 3D printer (approximately 36.6 × 12.2 × 6.1 m), featuring an automated robotic arm (Bos et al., 2016).
  • Andy Rudenko built a castle in his garden with the help of contour crafting technology and software from RepRap 3D printing open source project. A cement sand mortar was used and the whole building was printed in single casting. (Hager, Golonka and Putanowicz, 2016)
  • A Two-storeyed house in China, measuring 400 m2 was built by Beijing-based HuaShang Tengda in 2016 (Bos et al., 2016).
  • Interiors of a hotel Suite measuring 12.5 × 10.5 × 4 m, in Philippines was done by Total Kustom using 3 D Printing in 2015.
  • A Children’s Castle in Minnesota, USA, was built by Total Kustom using 3 D printing in 2014.
Current Challenges
3D printing in construction is a recent approach and possesses many limitations. Albeit the steadily growing number of researches and efforts in the field, 3D printing is still in its infancy. For construction 3D printing methods to be successful in the future, there must be features for all the construction processes to be able to be performed onsite with little to no effect of the everyday outdoor conditions of building sites. The following challenges have been speculated in diversified adoption of 3 D printing in construction (Wu, Wang and Wang, 2016), (Ngo et al., 2018), (Hager, Golonka and Putanowicz, 2016).
  • Printing of large-scale buildings: Gibson (2002) and Berman (2012) advocated that the 3 D printing technology is suitable for small-scale buildings and small elements. The size of the building to be printed was widely and predominantly dependant on the size of the printer. Also, with the currently available materials and technology, only a few researchers were able to acquire all the properties required
  • Properties of the materials: The materials used for 3D printing inspite of the use of fibers and other means to improve its properties seemed to be more brittle, preventing its usage in horizontally spanning structures such as staircase and slabs. In addition, attainment of properties such as rheology, open time and strength specific to the requirements are still on trial and error basis.
  • Digital Model: 3D printing requires a BIM model, which includes geometry information along with material performance (i.e. yield strength, tensile strength, shear modulus, thermal conductivity, etc.), spatial relationships, and manufacture information. However, with the present sizes of printer 3 D printing is suitable for small-scale printing or for components, which are printed first and assembled later. However, this design for assembly approach in synergy with BIM is not predominant at present.
  • Cost: The major issue in 3D printing is it is cost sensitive; however, whether it will lead to a cost higher than or lower than conventional construction is unpredictable. According to Le et al. (2012), the integration of mechanical and electrical services in the 3-D printing process could optimize materials usage and site work, thus leading to reduced likelihood of costly remedial works. However, the cost of 3-D printing should also include the cost of 3-D printers. In summary, although short-term potential cost reduction can be achieved by 3-D printing, empirical studies are needed to investigate the financial performance of the printed construction product or project over its life cycle.
Conclusion
The application of 3D printing in construction may shorten the project duration, reduce the project cost, and improve productivity. It has the potential of reducing the number of site workers, speeding up the construction process, quality control and reducing risks during construction. A 3DP model can also be very conveniently linked to a building information model (BIM) making the whole process from design, construction, management, maintenance, digital. Thus, 3D printing can be considered as an inevitable technology for future. Also, India being one of the developing countries has a large shortage of urban housing and according to PMAY (Housing for All mission by Indian government) total housing shortage of 20 million is to be completed by 2022. Under this Mission a Technology Sub-Mission has been set up to facilitate the adoption of modern, innovative and green technologies for faster and quality construction of houses. Therefore, 3D printing shall be considered as one of the viable options for the Sub Mission. With more research towards materials, modelling technologies and more versatile printers 3 D printing shall be considered as a technology for future construction.

Acknowledgment
This paper is submitted with the kind permission of Director, CSIR-CBRI, Roorkee.

References
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Christ, S. et al. (2015) ‘Fiber reinforcement during 3D printing’, Materials Letters. Elsevier, 139, pp. 165–168. doi: 10.1016/j.matlet.2014.10.065.

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Hager, I., Golonka, A. and Putanowicz, R. (2016) ‘3D Printing of Buildings and Building Components as the Future of Sustainable Construction?’, Procedia Engineering. The Author(s), 151, pp. 292–299. doi: 10.1016/j.proeng.2016.07.357.

Le, T. T. et al. (2012) ‘Hardened properties of high-performance printing concrete’, Cement and Concrete Research, 42(3), pp. 558–566. doi: 10.1016/j.cemconres.2011.12.003.

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