Additive Manufacturing: Technology, Applications, Markets, and T Opportunities for the Built Environment ⁎ ⁎ Ans Al Rashid , Shoukat Alim Khan, Sami G

Automation in Construction 118 (2020) 103268

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Automation in Construction

journal homepage: www.elsevier.com/locate/autcon

Review Additive manufacturing: Technology, applications, markets, and T opportunities for the built environment ⁎ ⁎ Ans Al Rashid , Shoukat Alim Khan, Sami G. Al-Ghamdi, Muammer Koç

Division of Sustainable Development, College of Science and Engineering, Hamad Bin Khalifa University, Qatar Foundation, Doha, Qatar

ARTICLE INFO ABSTRACT

Keywords: Additive manufacturing (AM) technologies (also known as 3D printing - 3DP) have been rapidly advancing into Additive manufacturing various industrial sectors, including aerospace, automotive, medical, architecture, arts and design, food, and 3D printing construction. Transitioning from the visualization and prototyping stages into functional and actual part re- Construction placement opens further design possibilities. Among the diverse applications of AM, construction applications Infrastructure have not yet seen a commercially available and widely used product in the market. Nevertheless, it has been a Construction materials subject of interest to researchers in recent decades. There are evident challenges and risks for the integration of AM towards large-scale construction. Therefore, progress in their commercialization seems to proceed at a slow pace, as only a few 3DP trials for large-scale construction can be found in the literature. This paper aims to provide a comprehensive and evidence-based baseline along with progresses in relevant disciplines related to 3DP in construction, which will provide an opportunity to experts in all domains to understand the multi- dimensional constraints and to specify the future research directions in these sectors. The distinct merit of this article is that it provides, for the first time, a diverse review on literature in the field of construction 3Dprinting. It offers up-to-date and in-depth information of hindrances (from processes, materials, structural designand standards) which add up towards low pace of automation in construction sector, identify deficiencies in the current research and provides future research trends for researchers. The findings from intensive literature review will guide engineers, designers and investors from construction sector to grab research gaps and business opportunities. First of all, the development of different 3DP processes in built environment are presented to highlight the process constraints along with the commercial applications of these processes for industrialists and investors to identify the business opportunities. Secondly, process parameters and difficulties in optimization of material mixtures are presented as a guide to civil engineers following the discussion on materials urging the need for development of eco-efficient and environment friendly materials. Conclusions drawn from discussion in individual sections along with issues/constraints and challenges involved are explained separately.

1. Introduction developed for more than three decades, the progress in the degree of automation for construction applications has been slow [4]. This is AM (a.k.a. 3D Printing) technologies have been considered pro- because AM processes, in their present form, cannot be directly utilized mising to take a significant role in design and construction of built in large-scale construction applications [5]. environment owing to their multiple benefits over conventional Despite recent advancements in AM processes and technologies, methods, including rapid production, custom products, reduction of there are still pressing needs and research issues to develop sustainable waste materials and labor costs, precision, and accuracy [1]. AM new materials for construction applications, obtain optimal process technologies can provide rapid housing solutions in cases of emergen- parameters and conditions, develop computational modeling techni- cies, and can be used to build exotic structures, which are not possible ques, introduce reinforcements and enhancements to AM structures, with conventional construction strategies [2,3]. Developed countries and develop new technologies [6]. Some researchers have tried to have been investing significantly in automation of the construction utilize conventional construction materials to print structures using AM sector to cope with shortages of skilled labor and to avoid vulnerable processes [7], whereas others have used some novel materials [8], in- situations at construction sites. Although AM processes have been cluding geopolymer concrete, natural fiber-reinforced materials, waste

⁎ Corresponding authors. E-mail addresses: [email protected] (A. Al Rashid), [email protected] (M. Koç). https://doi.org/10.1016/j.autcon.2020.103268 Received 17 February 2020; Received in revised form 11 May 2020; Accepted 14 May 2020 Available online 27 June 2020 0926-5805/ © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/). A. Al Rashid, et al. Automation in Construction 118 (2020) 103268 materials-reinforced construction materials, and recycled materials. research. Three main gaps were identified between academic, con- Although AM processes have been improved to adopt to construction struction industry and robotics industry along with the recommenda- requirements, there remains a need to develop materials that can re- tions to bridge these gaps. A. Darko et al. [22] performed a sciento- place conventional materials from the market, and that are integrable metric analysis and visualization of literature related to artificial to the above processes. intelligence in architecture, engineering and construction. Rakha and Researchers face difficulties in optimizing process parameters asAM Gorodetsky [23] provided a comprehensive review on utilization of processes require the material to be fluidic enough to pass a delivery system, unmanned aerial systems for built environment applications in auto- whereas rapid curing is required to ensure the structural integrity. Research mated buildings inspection followed by a case study to validate the efforts have been made regarding extrudability, buildability, printer nozzle proposed framework. G. D. Schutter et al. [24] published an article very design, printing speed and direction, aggregate sizes, optimum mixes, and specific to technical issues related to concrete 3D printing addingup the addition of reinforcements to the material. Nevertheless, the progress in economic and environmental potential related with technology. V. Li the standardization of AM processes in the construction sector is not ac- et al. [25] recently presented the study of process characteristics and ceptable for commercialization [6]. properties during fresh and hardened state of 3D printable cement- In this article, authors build upon and improve the previous reviews, based composites. V. Mechtcherine et al. [26] presented almost similar as summarized in the next paragraphs, by a systematic, thorough, and study for extrusion based processes, rather in this study the mechanical comparative analysis of the published literature. Review papers and properties of cementitious composite materials were discussed more in original research articles from well reputed journals were considered details. D. Asprone et al. [27] published a review article with special for this study to highlight the limitations and hindrances for utilization focus on reinforcement for 3D printed concrete structures. Review ar- of AM processes in construction sectors, as well as the guidelines to ticle presented by N. Roussel [28] accumulated literature to identify the future research directions for researchers. rheological requirements required for concrete printing. Although several review articles are present in this field, many of In the view of shortcomings of current literature, still there is a need to them fall short in bringing a multi-dimensional aspect and discussion. identify and address multi-disciplinary issues for proper utilization of AM For instance, the articles presented by R. Buswell et al. [12], S. Ghaffar processes. For implementation of AM processes in the construction sector, a et al. [13], D. Delgado et al. [14], J. Zhang et al. [15], G. D. Schutter current state-of-the-art needs to be established to highlight the constraints. et al. [24], V. Li et al. [25] and V. Mechtcherine et al. [26] discussed the AM processes, construction materials for AM, AM process constraints and material science involved in the process without extensive information commercialization of AM processes in built environment are the four target about development of the processes. On the other end, the processes sectors for this review, which would assist researchers from several back- were presented in detail with limited or no consideration to the issues grounds (including manufacturing, material science, designers and sus- related with implementation of materials to 3DP processes by F. Cra- tainability) to identify the future research directions. veiro et al. [17] and A. Paolini et al. [18]. Finally, I. Perkins et al. [9], F. This paper presents the development and adaptation of AM pro- Bos et al. [10] and Y. Tay et al. [11] are considered outdated reviews as cesses in the construction sector, with a more specific focus on built significant advancements in processes and materials have been wit- environment and infrastructure applications. The advancements and nessed in the field over past few years. research in process technologies, materials, modeling, and auxiliary I. Perkins et al. [9] presented a review on three most renowned AM systems involved in 3D printing (3DP) construction are thoroughly re- processes (contour crafting, concrete printing and D-Shape) in construction viewed and presented along with commercial projects revealed around sector. Potential applications, cost analysis and other benefits over con- the globe to date. Different databases (Web of Science, Scopus, Springer ventional construction strategies were reported. F. Bos et al. [10] presented and Wiley) were used to gather research articles from well reputed 3D concrete printing facility of the Eindhoven University of Technology journals. To define the boundaries of this search mostly used thekey- with other available printing processes. Issues related to design, geometry words as follows: built environment, digital construction, additive and process parameters were addressed along with potential research di- manufacturing in construction, 3D printing in construction. Articles rections. Y. Tay et al. [11] reviewed the articles published during containing these terms in title, abstract and keywords were considered. 1997–2016 with main focus on the development of 3D concrete printing Gathered articles were then filtered to classify most relevant ones for processes at Singapore Centre for 3D Printing. R. Buswell et al. [12] pre- review. The first section of the paper presents a general overview ofthe sented a review on fresh and hardened properties of concrete and mortar, AM techniques, along with their areas of application. The following along with their effect on geometrical parameters of the structures and sections are focused on the development of AM processes for built en- mechanical properties. S. Ghaffar et al. [13] reported a review on ce- vironment applications, specifically construction. AM techniques for mentitious and polymeric construction materials, as well as, socioeconomic built environment applications, along with factors affecting the print and environmental impacts of AM processes in construction. D. Delgado quality and challenges being faced by researchers, are discussed in- et al. [14] reported a paper which provided review of AM in construction dividually in the following two sections, to provide a framework for industry, identified the trend of AM processes and materials being used,and further improvements. The next section focuses on the mechanical potential advancements in applications of AM. J. Zhang et al. [15] presented properties of traditional materials and impacts of introducing re- a paper which extensively reviewed details on material mix designs and inforcement, along with the need to develop new materials. The last mechanical properties of 3D printable materials. Lu et al. [16] reported a two sections of the paper discuss sustainability aspects of using AM paper which aimed to systematically bridge the gap between the require- processes, along with conclusions and future recommendations. ment and research and development of 3D printable cementitious materials. F. Craveiro et al. [17] presented the review of AM applications in construction sector, with description of processes involved in digital construc- 2. Additive manufacturing (AM) processes and applications tion as well as the available and perspective materials. A. Paolini et al. [18] recently reviewed AM processes and systems in construction sector. C. Bu- AM or 3DP processes manufacture 3D parts and complex geometries chanan et al. [19] presented a broader view literature review to the appli- by adding layer after layer of a material. Generally, AM technologies cation of metal AM technologies with some discussion on construction ap- use CAD models. Cross-sections of the geometries are defined from plications. S. Goessens et al. [20] performed a feasibility study on different these CAD models, and material is added to each layer to make a solid drone based systems for masonry construction of practical structures. and intricate part. AM processes can be broadly classified into seven S. Cai et al. [21] recently presented a systematic study to review categories, depending upon the process and materials. Schematic dia- development of automation in construction sector for high-rise build- grams and materials that can be processed using these processes are ings, as well as, the market analysis when compared with academic reported in Table 1.

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Table 1 Materials and process descriptions of additive manufacturing (AM) processes.

Process Materials Process diagram

Vat polymerization Photopolymers bio-resins resins

Powder bed fusion Titanium alloys ceramics, metal, polymers, glass, sugar and starch powder

Material extrusion Hydrogel, thermoplastics, ceramic powder, bio-inks polycarbonate (PC), acrylonitrile butadiene styrene (ABS), medical-grade polycarbonate, wax, metal powder, chocolate syrup, dough

Material jetting Photopolymers, bio-inks, organic & inorganic electronic inks

(continued on next page)

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Table 1 (continued)

Process Materials Process diagram

Binder jetting Polymeric materials, sand, metal powder, metal alloys, ceramics, metal- ceramic composites

Directed energy Metal, nylon deposition

Sheet lamination Paper, ceramics, metals, plastics, fabrics, synthetic materials, composites

2.1. Vat Photopolymerization (VP) exposure to radiation. A variety of radiation sources can be used, depending on the chemical composition of the material. A single laser A photopolymerization process, also known as stereolithography, source, digital micrometer device, and two laser sources are possible uses photo-sensitive materials as primary materials for creating solid radiation sources for curing the photo-sensitive materials in cases of parts. These materials undergo chemical reactions, and solidify upon vector scans, mask projection, and two-photon processes, respectively.

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The applications of these processes are limited to materials which un- 2.7. Sheet lamination processes (SL) dergo photopolymerization [29]. Sheet lamination processes involve the shaping and bonding of material sheets. Laminated object manufacturing is conducted by 2.2. Powder bed fusion processes (PBF) trimming the material sheets into the required dimensions, and gluing them together to produce an object. Ultrasonic additive manufacturing In a powder bed fusion processes, the powder material is contained produces metal objects by making use of ultrasonic welding [36]. in a bed, and a heat source is utilized to produce fusion between powder particles. After the fusion of one complete layer in the bed, a new powder layer is added to the bed for the subsequent layer. Depending 2.8. AM applications in built environment upon the heat source, these processes can be classified as: A) direct metal laser sintering (DMLS) or selective laser melting (SLM), which AM processes have been adopted by the construction industry since make use of a laser source to melt the target material for the fusion of 1997 [37]. The integration of this technology in construction has been molecules; B) selective laser sintering (SLS), which is the same as SLM observed at a slow pace, even though it is superior to traditional pro- but does not involve full melting; C) electron beam melting, which uses cesses owing to lower ris7ks on sites, rapid production, lower emissions, an electron beam as an energy source to melt the powder material in a and reduced material waste [38]. To date, AM techniques have been vacuum; and D) selective heat sintering, which utilizes a thermal print- adopted for the construction of complex structures, simple wall struc- head to selectively fuse a thin layer of powder material [30]. tures, curved surfaces, concrete beams, architectural models, bridge structures, and houses and villas. The development of AM processes in 2.3. Material extrusion based systems (ME) built environment applications is discussed in the following sections, along with discussions regarding the processes and materials used for Extrusion-based systems make use of nozzles to extrude a semi-solid construction. material contained in a reservoir, which solidifies in the extruded shape and bonds to previously extruded material. The most common methods 3. Development of AM processes for built environment using extrusion-based AM technology are: A) fused deposition modeling applications (FDM), which utilizes a polymer material fed into the system in the form of filaments to liquify the polymer in a heating chamber, whichis To date, the processes that are being used for built environment then extruded through a nozzle with pressure [31]; and B) contour applications include extrusion-based processes and binder jetting. In crafting (CC) which utilizes a scrapping tool to smoothen the exterior this section, a review of the developments made in these processes and profile, thereby enhancing the surface finish of the object beingex- their applications are presented (Fig. 1). Table 2 summarizes different truded [32]. Although extrusion-based systems are some of the more processes reported in literature along with brief description of materials widely used AM processes, there remain strong limitations on the usage and results obtained. of pure metallic filaments, and metals with very low melting temperatures have utilized in this technology to date.

2.4. Material jetting (MJ)

Material jetting works on the same principle as inkjet printers, i.e., liquid radiation-sensitive material is dispensed from a printhead. Multiple printheads can be used to dispense one layer of material at once, which is then exposed to radiation to solidify the layer as it is being deposited [33].

2.5. Binder jetting (BJ)

Binder Jetting processes are very similar to powder bed fusion processes, but instead of using a heat source. A print head is utilized to deliver a binder at a spot where the fusion of the material is needed. After completing one layer, a new layer of powder material is added for a subsequent layer [34].

2.6. Directed energy deposition processes (DED)

Directed energy deposition processes utilize a narrow, focused beam Fig. 1. AM processes in construction. of energy to melt a material being deposited. These processes differ from powder bed fusion techniques, as the material is not pre-laid in a powder bed [35].

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Table 2 AM processes in infrastructure applications.

Process Material Description Ref.

Contour crafting Concrete, ceramics Extruded layer thickness 19 mm [5,41] • Extruded layer width 13 mm • Straight wall - 1.5 m (L) × 0.6 m (H) × 0.15 m (W) • Compressive strength of 18.9 MPa Freeform Concrete, thermoplastic • Bioinspired Freeform structure 1.2 m high and 24 cm wide [53] Freeform Concrete • 2.5 m × 2.5 m complex freeform interlocking structure [52] Concrete printing Concrete, steel • 3.0 m long steel reinforced beam [48] • 4.0 m long curved beam with variable cross sections Concrete printing Mortar, steel • 6.5 m long bridge, produced in segments and assembled [67] Concrete printing Concrete, polypropylene fibers • 3:2 sand-binder ratio [68] • Binder consisted of 70% cement, 20% fly ash, 10% silica fume • Reinforced with polypropylene fibers, 1.2 kg/m3 • 28-day Compressive strength of 110 MPa Contour Crafting Concrete • Aggregate-cement ratio 1.28, Aggregate-sand ratio 2.0 [69] • Compressive strength up to 42 MPa Concrete printing Concrete, fibers (carbon, glass, • Aligned fibers reinforcement to mortar cement [70] basalt) • Flexural strength of 30 MPa with 1% vol of Carbon fibers • Compressive strength up to 80 MPa Concrete printing Concrete • Cement 30–40%, crystalline Silica 40–50%, Silica Fume 10%, Limestone filler 10% with water- [47] • cement/sand ratio 0.1 • Wall dimensions 1360 × 1500 × 170 mm Concrete printing Concrete • Reproduction of historical buildings ornamental components [49] • Maximum Compressive strength of 19.8 MPa Smart dynamic casting Concrete • Numerically controlled formwork to support the structure [54,55] • Fabrication of 1800 mm high elliptical column • 4-m long canoe Pegna Sand, cement • 16 × 16 × 15 mm specimens produced by binder jetting [37] • Compressive Strength up to 33.8 MPa (E = 969 MPa) Powder bed Metal oxide aggregate, salt mixture • Novel technique for lunar construction applications [65] • Specimens exhibited compressive strength 20.3 MPa • Young's Modulus = 2350 MPa Concrete printing Blocks, mortar • Hybrid system for blocks and concrete printing [71] • Fabrication of mortar truss inside polystyrene blocks

3.1. Extrusion-based AM the design of its nozzle, which is able to deposit material at a higher extrusion rate [42]. Practical demonstration of the process was made by 3.1.1. Contour crafting 3DP of "Wonder Bench" (Fig. 2 (b)). Extrusion-based processes are the most widely used processes in In one study, type II hydraulic Portland cement was utilized to building and construction applications. CC was the first ever AM pro- produce a new mortar mixture with an extruded layer thickness of cess, and was proposed for use in the construction sector by Khoshnevis 19 mm, a width of 13 mm, and dual trowels, and the authors were able [39] at the University of Southern California (USC). The process takes to print a straight wall of 1.5 m (L) × 0.6 m (H) × 0.15 m (W) at a place in two steps: the formation of outer surface of the object/structure printing speed of 20 mm/s [41]. B. Khoshnevis [2,3] proposed the use using an extrusion process, and the subsequent filling of the object of a gantry-based robot, RoboCrane, or multiple mobile robots for the through manual pouring or extrusion. CC has the capability to produce fabrication of structures, embedding reinforcements in structures, fin- flexible and fancy structures that would be difficult to construct using ishing flooring and walls, electrical and mechanical utilities, andau- traditional construction approaches, can utilize various materials to tomated painting of buildings. facilitate in-situ fabrication, and is able to construct ready-to-paint The application of CC to large structures requires large gantries, surfaces, owing to its smooth textures [40]. Trowels guide the material which could be practically impossible [6]. To the contrary, P. Bosscher flow; a top trowel smoothens the extruded material to provide asmooth et al. [43] presented a cable-assisted robot for CC applications, and surface for aiding adhesion for the next layer, and a side trowel claimed that implementation of cable-suspended CC construction (C4) smoothens the external surface [41]. CC requires lesser build time as could be utilized for realizing “mega structures” that would carry lesser compared to other processes (concrete printing and D-shape), owing to system components and make this technology more flexible at a lower

6 A. Al Rashid, et al. Automation in Construction 118 (2020) 103268 cost. material extrusion and stereolithography. An extruder was used to de- As the reinforcement of structures is desired for large-scale build- ploy photosensitive materials that were rapidly cured using UV radia- ings, researchers have been able to successfully reinforce mortar using tion. The authors claimed that this innovation could lead to 3DP of free- CC technology, without causing and voids or discontinuities within the form and complex structures, owing to the additional axes of freedom structure [5]. Researchers are also putting in efforts to optimize ma- and avoiding the need for supports, thereby eliminating the waste chine operations for complicated large-scale building construction, materials produced during traditional melt extrusion (ME) or VP pro- which will help at construction time by reducing idle motions, ulti- cesses [51]. mately improving the efficiency of the process. One study proposed algorithms for the determination and implementation of ab optimum 3.1.3. Freeform construction tool path when using single and multiple nozzles [44]. Researchers at Traditional extrusion-based processes do not offer flexibility in USC are also aiming to optimize CC parameters, including the surface constructing freeform structures. Therefore, efforts have been made to finish, printing speed, printed part size, and design of the nozzles[41]. modify traditional ME processes to offer the flexibility and ability to construct complex structures. C. Borg et al. [52] successfully 3D-printed 3.1.2. Concrete printing a complex freeform interlocking structure using a four degree-of- Concrete printing was developed by researchers at the University of freedom extruder, with an acceptable surface finish, lower fabrication Loughborough for AM of buildings and components, and is similar to time, and less waste material (Fig. 2 (e)). B. Felbrich et al. [53] pre- CC [45]. Although the working principles of CC and concrete printing sented a 6-axis extrusion-based process inspired by the shell formation are the same, the latter offers a higher dimensional accuracy; however, of snails to fabricate freeform architectural models, by extruding ther- it provides a coarser surface finish, owing to the absence of side trowels moplastic shells through a flat nozzle as molds, and subsequently filling [38]. The concept was validated for large-scale fabrication applications, them with concrete mixtures to obtain double-curved structures. Using as a curved bench (2 m × 0.9 m × 0.8 m) was printed using this the prototype, the authors were able to print a 1.2 m high and 24 cm process [17,46]. wide double-curved composite shell, shown in Fig. 2 (d). Although the Although the CC and concrete printing processes provided ground- concept was novel, further research is required to utilize its potential at breaking progress towards automation in the construction sector, these a commercial scale. processes have limitations in terms of material utilization and flex- Researchers at ETH Zurich presented a novel technique called ibility. CC offers 2.5D fabrication and concrete printing offers 3Dfab- “Smart Dynamic Casting” that integrated conventional casting techni- rication, but loses its dimensional accuracy when printing out a hor- ques and digital fabrication [54]. The process consists of pouring fresh izontal plane [17]. Gosselin et al. [47] developed a 6-axis robotic arm- concrete into a formwork of the required shape, which is numerically based extrusion process capable of printing complex 3D structures controlled to rise along the fabrication direction as soon as the lower without supports with better dimensional accuracy, and successfully concrete layers harden sufficiently to withstand the loads of subsequent fabricated multi-functional elements. The authors were also able to layers. The process was able to produce an 1800 mm-long elliptical print concrete layers with varying thicknesses, as shown in Fig. 2 (c). column twisted 180 degrees along the length. A D. Asprone et al. [48] presented a novel technique that used steel 125 mm × 80 mm × 60 mm formwork was utilized to realize the rebar reinforcements to 3D print a 3 m-long concrete beam. An ex- structure. Another demonstration was presented by the fabrication of a perimental analysis was conducted to evaluate the flexural properties of 4 m-long canoe with a wall thickness of 1.8 cm [55]. the reinforced concrete beam. The authors were also able to print a curved beam of variable cross-section using the same technology (Fig. 2 (a)), and proposed that this technique could help address problems in 3.1.4. Powder melt extrusion (PME) 3D-printed reinforced structures. B.M. Boyle et al. [56] developed a powder melt extrusion (PME) 3D Xu et al. [49] demonstrated the feasibility of 3DP technology for the printer head to fabricate parts from a powder polymer material. Poly- reproduction of historical ornamental components. A historical cup- lactic acid (PLA), high impact polystyrene (HIPS), and acrylonitrile shaped plinth at the Huazhong University of Science and Technology butadiene styrene (ABS) specimens were printed using conventional (HUST) in China, was scanned, modeled, and successfully printed using filaments used in the extrusion process, as well as using powder ma- concrete printing. The fabricated component exhibited compressive terial in a newly developed prototype. The PME-printed objects, when strengths of 19.8 MPa and 15.6 MPa in the vertical and horizontal di- compared to fused filament fabrication (also known as FDM), exhibited rections, respectively. comparable viscoelastic properties. However, they demonstrated lower Keating et al. [50] presented a 6-axis multi-arm robot inspired by print quality and inferior mechanical properties, owing to process the human arm and shoulder, with a larger arm for assisting gross constraints. The authors concluded that PME could lead to significant fabrication and a smaller one for assisting precise fabrication while progress towards the rapid production of polymer-based 3D objects, using a polyurethane material for rapid curing. M. Asif et al. [51] and has the potential to improve in terms of print quality and me- presented a 5-axis photopolymer extruder which combined principles of chanical properties [56].

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Fig. 2. Concrete printing applications: (a) 3D-printed reinforced concrete beam with variable cross sections [48], (b) Wonder bench [42], (c) Multi-functional concrete wall by Gosselin [47]. Freeform construction applications (d) Doubly curved specimen extruded using 6-axis extruder [53], (e) Complex geometrical shell structure with interlocking mechanism, 3D-printed at TU Delft [52].

3.2. Particle bed-based AM typically water and hardener in cases of cement and epoxy binders, respectively. The particle bed processes hold an advantage over extru- Particle bed-based AM for fabrication of concrete components can sion-based processes, as support material is not required to print be performed using different strategies, including: (a) selective binder overhanging features. The powder material in the bed acts as a support activation (addition of binder activation agent to the aggregate and to structures involving the printing of voids and over hangings, which binder particle bed); (b) selective paste intrusion (jetting of binder and can be removed and reutilized for later use [11]. In addition, a higher binder activator into aggregate bed); and (c) binder jetting (aggregate resolution of products (up to 0.1 mm) is possible by using finer ag- and activator are placed in bed, and binder is selectively injected) [57]. gregates [57]. The processes developed based on powder bed tech- Classification of these processes is presented in Fig. 3. The activators are nology are presented below.

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Fig. 3. Particle bed processes for construction applications [57].

3.2.1. Pegna Fig. 4. Powder bed applications (a) Radiolaria Pavilion [58], (b) underwater “MOMA” [58]. J. Pegna [37] was the first to adopt the particle bed principle for free-form construction, and derived a new process using a selective aggregation of material. The process consists of the selective deposition of cement in a required pattern over a layer of sand, followed by the filament. This process is a novel technique for the reutilization of exposure to steam to all deposited layers of sand and cement, for the construction material, as the rocks can be disassembled as needed. The rapid curing of the mixture. The specimens (16 × 16 × 15 mm) ex- feasibility of the process was demonstrated by building a rigid 13-ft hibited anisotropic behavior, and when tested after six months of fab- column, which was later disassembled without producing any waste, rication, a maximum compressive strength of 33.8 MPa was observed Fig. 5. perpendicular to the printing plane. 3.4. Lunar construction 3.2.2. D-shape “D-shape” is a binder jetting fabrication technique introduced by Applications of AM are not limited to the construction of infra- Enrico Dini (Monolite UK Ltd.) for the 3DP of buildings and sculptures. structure on this planet, as space scientists are aiming to build habitats This process is capable of producing complex structures with internal on the Moon and Mars. AM technologies can be utilized to build com- cavities, voids, and conduits [58]. The initial application was demon- munities on other planets using in-situ resources [62]. B. Khoshnevis strated by the fabrication of a “Radiolaria Pavilion” measuring et al. [5] proposed a collapsible gantry-based mobile robot and different 3 × 3 × 3 m and "MOMA" (Fig. 4)[58]. Components printed using D- methodologies for producing concrete and using in-situ resources for Shape can exhibit a compressive strength of 235–242 MPa [38]. lunar construction. D-shape exhibits some limitations in practical applications; as it can Sulfur is abundantly available on Mars; this abundance has urged be affected by bad weather owing to the nature of base material, it researchers to explore the potential use of sulfur as an alternative cannot be used for in-situ fabrication of structures, and is limited to construction material, as it does not require as much water as tradi- available materials [11]. These processes therefore have a limitation, as tional concrete [6]. B. Khoshnevis et al. [63] produced sulfur concrete the size of the product is limited by the printing space [57]. Thus, samples varying in sulfur proportions and concrete mixture tempera- construction of a full-scale building would require a massive printer tures, and concluded that samples extruded at higher temperatures size. depicted a lower porosity, whereas lower sulfur contents improved the In addition to the above-mentioned processes, some innovative and final surface finish and shape. A novel mixing and extrusion system unique processes can be found in the literature for the fabrication of an were found stable and reliable, even after 500 h of utilization. A Mohr- architectural model. In addition the use of a single technology and a Columb plasticity model was used in a finite element simulation to single material, several efforts have been made to integrate different validate the experimental results and prove the potential application of processes and deploy multi-materials for the fabrication of complex the tool for further improvements in this field [64]. structures comprised of several materials [59,60]. The European Space Research and Technology Center (ESA) made use of D-shape technology for ongoing research in developing a habitat 3.3. Rock Printer on the moon [65]. A granular aggregate mixed with metal oxide was selectively bonded using a magnesium chloride mixture, and the re- ETH Zurich, in collaboration with MIT, introduced a new process for sulting material exhibited a compressive strength of 20.3 MPa. Re- AM of structural components using a rock and textile filament [61]. cently, a “Moon Village” was proposed for building a habitat beyond This process is similar to the powder bed process (D-Shape), but sand Earth [66]. Powder bed fusion technology was proposed as best suited and binder are replaced by rock and textile filament. A 3D robotic arm for lunar construction utilizing an in-situ lunar regolith, rather than was programmed to bind the rock in the required manner using the carrying material from earth (which requires significant resources).

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Fig. 5. Rock Printer [61] (a) Binding mechanism (b) 13-ft tall structure 3D-printed by Rock Printer.

3.5. 3D printing projects with the municipality of Dubai was successful in construction of the world's largest 3D-printed double story building fabricated on-site, as The applicability of 3D-printed structures seems to be advancing shown in Fig. 6 (g) [80]. Using gypsum-based material and pre-casted towards commercialization. In 2013, Hansmeyer and Dillenburger slabs, the 3D printer was able to successfully build a 9.5 m tall and 640 presented “Digital Grotesque”, a 16 sqm-large, 3.5 m high room, as sqm structure under harsh conditions. Cybe [81,82] Arup collaborated shown in Fig. 6 (i). The main aim of utilizing binder jetting technology to produce a 100 sqm 3D-printed house in Europe Fig. 6 (h), printed on was to discover the potential of this process for producing finer re- site within 48 h at a busy square in Milan [83]. solutions [72]. Xtree, French company, is currently playing an important role in In 2014, a Chinese Company (Winsun) successfully fabricated 10 developing new solution to 3D printing and have displayed a wide houses (200 sqm each) in less than 24 h (shown in Fig. 6 (b)) by as- range of project around the globe, from protypes to real practical ap- sembling pre-casted 3D-printed components, using a mixture of high- plications, as shown in Fig. 6 (k) & (l). grade cement, construction waste, steel, hardening agents, and glass fibers [17,73]. 4. Process parameters and conditions for AM In 2015, the same company unveiled a multi-story apartment building and a villa, constructed as a pair, shown in Fig. 6 (c) & (d) The print quality and structural integrity of 3D-printed building [74]. Using the same technology, recycled construction material and components depends upon several inter-linked parameters, and re- fast hardening cement were utilized for the project. Pre-casted com- searchers are putting forth efforts to optimize the material mixtures and ponents were assembled on-site along with reinforcements, and both process parameters. The mechanical properties of 3D-printed concrete buildings complied with the national building standards. elements depend upon a number of factors including the material de- Researchers at the University of California, Berkeley presented a position rate, material characteristics, printing speed, printing or- novel powder-based technique, and were able to print a 2.7 m high ientation, and material orientation. An overview of the process-related “Bloom Pavilion” [75]. The process involved the printing of iron oxide- constraints is provided in Fig. 7. free dry Portland cement, and then spraying the cement with water to Mortar is the most widely used 3DP material for construction ap- harden the structure. The structure shown in Fig. 6 (a) was obtained by plications, consisting of Portland cement and plasticizing materials, assembling 840 unique blocks fabricated by the above-mentioned along with some constituents to improve the workability, durability, process. water retention, and setting time of the mixture. The overall 3DP pro- In 2016, the potential for 3DP to fabricate curvilinear practical cess of cementitious materials can be divided into three phases: pre- structures was observed, as Dubai's first 3D-printed office was unveiled paration of the mixture, delivery of the mixture to the extruding system, (Fig. 6 (e)) [76]. The floor area of the office is 120 × 40 ft andthe and finally, extrusion. During the delivery phase, a perfect mixture is office stand 20 ft high. The structure totaled 2700 sq-ft andcosts expected to present good extrudability or flowability; however, the $140,000. material needs to stabilize rapidly after extrusion, to sustain the loads of In 2017, a 6.5 m-long bicycle bridge was fabricated by researchers subsequent layers. These two phases explain the thixotropy of the at the Eindhoven University of Technology, Netherlands. It was as- mixture. Progress in concrete printing processes has also led researchers sembled on site, and successfully complied with Dutch building stan- to define the properties of extrusion materials. Workability, extrud- dards [67]. In same year, the Institute of Advanced Architecture of ability, flowability, buildability, pumpability, and open time are de- Catalonia designed a pedestrian bridge to be installed in Spain, Fig. 6 fined as the parameters of concern [6,15,85,86]. Workability is defined (f). The 12 m-long and 1.75 m-wide bridge was fabricated in eight as the ability of an extruded material (which ensures proper adhesion components, using micro-reinforced concrete and D-shape technology and strength) to construct a structure without failure [15]. Buildability, [77]. Construction Building on Demand (COBOD) used four gantry- also termed as constructability, defines the ease of fabricating a struc- based printers to produce Europe's first 3D-printed house (“The BOD”, ture or how efficiently a structure can be produced [68]. Flowability is Fig. 6) (j) [78]. A concrete-based mixture was utilized in the process, evaluated by measuring the spread diameter of the mortar, which is a along with recycled tiles and sand as aggregates. quantitative parameter representing the workability of the mortars Recently, Apis Cor [79] (a 3D-printing company), in collaboration [15]. The extrudability of the mixture is the pass-through capacity of

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(a)

(b) (c) (d)

(e) (f) (g)

(h) (i) (j)

(k) (l)

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11 A. Al Rashid, et al. Automation in Construction 118 (2020) 103268

Fig. 6. 3D Printed Structures (a) Bloom Pavilion, 2.7 m high, 3.6 × 3.6 m footprint [75], (b)3D-Printed Homes in China, Winsun [73], (c) 3D-Printed Multi-Story Apartment, Winsun [74]. (d) 3D-Printed Villa, Winsun [74], (e) First 3D-Printed Office in Dubai [76], (f) 3D-Printed Bridge in Spain [77], (g) Double-Story Administrative Building in Dubai [80], (h)3D Housing 05, Milan [82], (i) Digital Grotesque [72], (j) Europe's First 3D-Printed Building (The BOD) [78], (k) Woven Concrete Benches (XtreeE) [84], (l) Yrys Concept House (XtreeE) [84]. the delivery system in the concrete printing process, and pumpability the addition of superplasticizer to mixtures with a water-cement ratio reflects the ease of transporting material from the pump to theextru- less than 0.42 can improve the compressive strength, but has negative sion nozzle [68]. effects on the buildability. A shorter hardening time was achieved Efforts have been made by several researchers to optimize these owing to the introduction of nano-clay, by decreasing the flowability parameters, for an optimum mix with acceptable extrudability and [87]. Calcium aluminate provided an increased flowability and hard- buildability. These efforts are discussed below. ening rate [87]. Rushing et al. [92] recommended adding fine aggregates and fly ash to address flow-related problems, bentonite forthe improvement of shape stability, and the utilization of polycarboxylate- 4.1. Extrudability based plasticizers for better workability of the mixtures. The authors concluded that introduction of silica fume and nano-clay can improve Soltan et al. [87] studied the effect of introducing additives on the the shape stability of the printing mixtures, whereas the polypropylene extrudability and hardening time of the mixtures. It was found that the fibers can provide minor improvements to the mixtures [93]. Similar addition of hydroxypropyl methylcellulose can alter the flowability of outcome was achieved by addition of silica fume and blast furnace slag the fiber-reinforced mixtures. Malaeb et al.[69] also concluded that the in fly ash-based mortar. Additives provided the better shape stability addition of superplasticizer to the mixtures improves the extrudability. and early age strength of the printed material [94]. Malaeb et al. [69] Inozemtcev et al. [88] found that the introduction of micro silica im- designed an optimum mix for a CC process, and achieved a buildability proved not only the flowability of the mixture, but also the compressive of four layers. Panda et al. [95] found that the addition of 0.5% of nano- and flexural strengths. The extrudability and compressive strength can clay improved significantly the buildability of the mixture. Early age also be improved by fly ash, but it decreases if the fly ash content ex- (green) strength of the print material also plays significant role in ceeds 25% of the cement mass. Ma et al. [89] utilized copper tailing to buildability of the structure. Lower green strength could also results in enhance the extrudability of concrete, but an increase in copper content plastic collapse of the structure in old age [96]. In geopolymer mix, as reduces the buildability of the mixture. B. Panda et al. [90] recently base, nano-clay was used to improve the thixotropic properties and performed the investigation on fresh properties of alkali activated slag provided significant buildability to the geopolymer mortar [97]. Effect mixtures with addition of nanoclay and hydro-magnesite seeds. Addi- of nano-clay in high volume fly ash mortar was also studied [98]. tion of 0.4% nanoclay provided improved extrudability and thixotropy, Significant improvement was observed in thixotropic properties ofthe furthermore, with introduction 2% nucleation seeds authors were able mortar, thus improving the buildability of the mixture, in addition to print actual 3D structure proving the commercial application of the improved mechanical properties were also obtained. proposed mixture. Kazemian et al. [93] presented a framework for the laboratory testing of printable mixtures (Fig. 8), to iteratively improve the print 4.2. Buildability/workability quality, shape stability, and printability of the mixtures. The authors defined the print quality as the quality of surface and edges andthe Rheologically, better workability can be achieved by concrete with dimensional consistency, the shape stability as the ability to withstand high viscosity and low yield stress [91]. Inozemtcev et al. [88] con- the load of upcoming layers without failure, and the printability as the cluded that the introduction of shell rock flour to cement mortars im- time span within which material can be extruded. proved the workability of mixtures. Malaeb et al. [69] concluded that Extrudability and workability of printable mixtures are strongly dependent on thixotropic behavior of the working fluid. High thixotropy of mortar mixture means better shape stability [95].

4.3. Interlayer adhesion

Interlayer adhesion compliments the yield strength of the printed structure, however, bond strength between two subsequent layers depends upon material properties and process parameters. Higher thixotropy of the mixture and longer time between printing of two layers reduces the bond strength, however, lower standoff distance improves interlayer bonding [95].

4.4. Other parameters

In addition to above-mentioned process parameters, some other parameters related to all extrusion-based AM processes also affect the print quality, including the nozzle shape, aggregate size, optimum mixture, printing orientation, and introduction of reinforcements while printing.

4.4.1. Printer nozzle The design of the nozzle for the CC process affects the process performance, and some researchers have pointed out the need for its optimization [99]. Finite element simulations have been performed to study the effects of nozzle shape on print quality. It was concluded that Fig. 7. Process parameters for AM in construction. a square-shaped nozzle is best suited for both interlayer adhesion and

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Fig. 8. Proposed framework for laboratory testing of fresh printing mixtures by A. Kazemian et al. [93]. surface profile, and this hypothesis was validated through experi- 4.4.4. Optimum mix mentation. The nozzle standoff distance is another critical parameter Le et al. [68] performed an experimental study to find the optimum for the printed layers' shapes and properties. Compaction and interlayer extrusion mixture. It was concluded that the optimum mixture consists adhesion can be achieved by maintaining a nozzle stand-off distance of fine aggregate and a 3:2 sand-binder ratio, where the binderis slightly less than the length of the nozzle opening [10]. An increase in constituted of 70% cement, 20% fly ash, and 10% silica fume reinforced tensile strength of geo-polymers was observed with a decrease in the with 1.2 kg/m3 of polypropylene fibers. Malaeb et al. [69] found 1.28 nozzle stand-off distance [100]. Researchers solved the problem of a and 2 as the optimum aggregate-cement and aggregate-sand ratios, round filament obtained by a circular nozzle by replacing itwitha respectively. The optimum mixture for fiber-reinforced cement mortar rectangular (40 × 10 mm) nozzle [10]. Eindhoven University also was developed by Hambach and Volkmer [70]. The cement mortar was produced a novel nozzle design and successfully reinforced mortar with reinforced with aligned carbon, glass, and basalt fibers. The mixture thin steel filament, without facing mortar/filament and interlayer ad- was prepared as 61.5% Portland cement, 21% silica fume, 15% water, hesion problems [67]. Optimization of tool paths via using single and and 2.5% water-reducing agent. Panda et al. [97] found the activator- multiple nozzles to reduce the idle time of processes can also be found to-binder ratio of 0.35 and water-to-solid ratio of 0.30, for an addition in literature [44]. of 0.5% nanoclay in geopolymer mortar, to achieve significant rheological properties of the mixture. 4.4.2. Printing direction and speed Printing direction significantly affects the mechanical properties of 4.4.5. Reinforcement the mixtures, including the compressive and flexural strength. When The utilization of fibers to enhance the mechanical properties of compared to conventionally casted samples, a 15% increase in com- construction materials can be traced back to the Egyptian period, when pressive strength was observed when the build direction was perpen- straw and hairs were incorporated into mud for the fabrication of dicular to the loading direction [7]. The printing orientation effect on bricks. Later on, this reinforcing technique was implemented for con- the mechanical properties of printed elements has also been validated struction of houses as well, as it was adopted as a main component for by other researchers [101,102]. An increase in the tensile strength of load-bearing walls [105]. Hambach et al. [106] found that the addition geo-polymers was observed with a decrease in printing speed [100]. of 1% carbon fibers by volume in cement paste can improve the flexural strength by 18.5 MPa, but the compressive strength was not sig- 4.4.3. Aggregate size nificantly altered. The effects of aggregate size on the mechanical properties oface- Kwon [107] printed fresh layers of concrete over metallic coil re- mentitious mixture have been validated by several studies. Zareiyan inforcement, and achieved reasonable adhesion between the layers. and Khoshnevis [103] studied the effects of the aggregate size on the Khoshnevis [3] proposed a robotic mechanism to place steel re- strength of structures. Specimens with a smaller aggregate size inforcements while printing. Bos et al. [108] developed a method to (2.4 mm) displayed a 104% higher compressive strength than a mixture introduce steel reinforcement cable directly into printing filament. The with a 12.7 mm aggregate size after 28 days. The early-age compressive results revealed a lower bond strength between the two media as strength for finer aggregates was also found to be 141% higher than compared to casted samples. Asprone et al. [48] utilized a different that of the mixture with a large aggregate size. approach to reinforce a pre-printed structure, by using steel rebars. S. Chaves Figueiredo [104] presented an approach to develop 3D- Researchers are aiming to develop self-reinforcement materials for AM printable strain hardening composite materials. Experiments were processes [6]. performed to obtain the optimum mix of constituents for printability, Details on the types of fibers and materials utilized in the con- shape stability, and strain hardening behavior. The optimum mixture struction industry as reinforcements are discussed in the subsequent was 3D printed to evaluate the pumpability and extrudability of the section. Nevertheless, reinforcement of the base material, mortar in the material. Finally, an object was printed, and the mechanical behavior of case of concrete printing, is itself a challenging issue to be resolved, as the 3D-printed part was analyzed. there is a need to maintain the printing quality while embedding the

13 A. Al Rashid, et al. Automation in Construction 118 (2020) 103268

Fig. 9. Construction materials. reinforcement. durability, resistance towards physical and chemical impacts, and resistance to elevated temperatures, and also promises sustainable solu- 5. Construction materials & mechanical properties for AM tions [109]. Researchers are aiming to develop novel materials by introducing different fibers, thereby eliminating traditional steel Materials in construction vary geographically, as the readily avail- reinforcements [15]. able materials and their costs vary geographically. The composition of Synthetic reinforcements include carbon, glass, polymer, and steel construction materials affects not only the mechanical properties ofthe [109,110]. The introduction of different reinforcements in construction printed structure, but also limits the applicability of the materials. The materials has revealed significant outcomes. Steel fibers improve the selection of the material also plays a vital role in the print quality and flexural strength and load bearing capacity of concrete [111], me- durability of the structure. Several conventional and non-conventional chanical properties of soil [105], and stability of asphalt [112] and materials have been attempted in 3D printers [6]. Conventional mate- provide resistance against cracking, but are prone to erosion in the rials were not found suitable for conventional AM processes, owing to concrete mixture [113]. Different polymers are also utilized to enhance flow complications [92]. B. Khoshnevis et al. [5] explored the appli- the mechanical properties. Polypropylene provides resistance against cation of CC to construction applications by the 3DP of structures using crack formation and shrinkage [114], along with improved strength different construction materials, including ceramics such as clay, and toughness [115]. Nylon improves the tensile strength and tough- plaster, and concrete. This section presents the development of con- ness and reduces shrinkage [113], as well as resistance against cracks struction materials using fiber reinforcement and multi-materials, and and impacts [114]. High-performance polypropylene provides higher the challenges faced by researchers in producing sustainable materials strength, elongation to failure, toughness, resistance to abrasion, and (Fig. 9). fatigue resistance [116]. Reinforcement using polyethylene terephthalate improves early age strength, impact strength, and toughness, but reduces over time, owing to degradation [114]. Poly- 5.1. Fiber-reinforced materials acrylonitrile and polyester provide better adhesion and bonding in asphalt concrete [117]. The introduction of glass fibers to soil improves Concrete has been the main element in construction for ages. the resistance against failure, and reduces brittleness [105]. Concrete exhibits excellent compressive strength, but lower strength in The natural fibers category includes basalt, palm tree, cereal, and tension. Furthermore, it experiences cracking and shrinkage problems. the leaves of plants and crops [109]. The introduction of basalt fibers to Therefore, for over two decades, fibers have been used for concrete geopolymer concrete enhanced the energy absorption capability and reinforcement, owing to their high strength and capability to produce strength [118]. When added to asphalt, basalt fibers improved the re- lightweight structures. Beyond that, fiber reinforcement offers en- sistance to rutting and drain down [119]. Sisal fibers reduce the hanced mechanical properties in terms of ductility, toughness, rigidity,

14 A. Al Rashid, et al. Automation in Construction 118 (2020) 103268 shrinkage of blocks, and provide resistance to crack formation [120]. have aimed at proving these processes as sustainable alternatives to Kenaf fibers were found to be sustainable, and a better solution than conventional manufacturing processes. synthetic fibers for eliminating binder drainage in road applications The powder material in an SLS process becomes waste after going [121]. Straw fibers improve the tensile and compressive strength of through a couple of production cycles, as the material is not utilized to asphalt [122], and have been used conventionally as a block re- produce the product, and experiences thermodynamic degradation. The inforcement material [105]. Coconut fiber-reinforced brick could also cost of producing the powder material is a major cost in the SLS process. be a sustainable and eco-friendly solution [123]. A sustainable method for utilizing this waste in FDM is converting it to Some researchers have made efforts to reuse waste materials as feedstock filaments. These can even provide a better product finishin reinforcements to conventional concrete mixtures. Recycled glass FDM, as the powder material is a high-grade polymer used in SLS. [124], cigarette butts [125], waste rubber crumps [124], metal slag Considering all of the involved investments and the costs of operation, [126], human hair [127], and waste tires [128] have also been used in the proposed methodology provides a sustainable and economical so- some studies, and have revealed positive contributions towards the lution if SLS and FDM are integrated [60]. R. Singh et al. [135] pre- mechanical properties of construction materials. Details on the appli- sented the use of three different types of thermoplastics (ABS, PLA, and cations of different fibers reinforced with construction materials canbe HIPS) with slightly different mechanical and thermal properties to found in Table 3. produce a multi-material using FDM. The manufactured multi-materials offered better mechanical properties than single constituent/base 5.2. Multi-materials polymer materials. The authors claimed multi-materials to be the future of AM, and that multi-materials can provide sustainable solutions with Rather than using one type of fiber, multiple fibers can be usedto enhanced mechanical properties. Ma et al. [89] explored the applica- produce hybrid materials that are more effective and efficient than tion of non-degradable copper tailings in conventional concrete, and single-material reinforcement. Khan and Ali [129] fabricated concrete their utilization can provide environmental benefits. with hybrid reinforcement including glass and nylon for bridge deck In evaluating sustainability, one must consider not only the AM applications. The authors concluded that glass and nylon fiber re- process, but also the processes involved in the production of powder inforcement in concrete can be utilized to control the early age cracking and the post-processing of parts. Life cycle inventory data for AM of concrete bridge decks. F. Craveiro et al. [130] developed a numerical processes are not available. Some authors have made efforts to estimate tool to design functionally graded building components using multi- the additional energy required to atomize materials. In-depth and more materials. The authors claimed that this tool could provide better ma- generic LCA studies should be conducted for AM processes. No (or very terial utilization and savings for construction industry. little) data are available on the details of energy consumption in AM processes. The environmental impacts of AM processes must be prop- 5.3. Eco-efficient materials erly assessed [136].

The traditional cement production process is not sustainable, as it 7. Conclusions and future recommendations requires an enormous amount of resources and causes intense environmental impacts. Therefore, efforts have been made to produce The automation in construction requires synergic efforts from dif- sustainable materials, to provide an alternative to conventional con- ferent disciplines of engineering and design. Involvement of materials struction materials compatible with AM processes. In addition to the science, mechanical engineering, civil engineering, designing and au- use of traditional construction materials and fiber-reinforced materials, tomation needs to play a combined role to deal with multi-disciplinary geopolymer cement has been produced using an extrusion-based pro- issues hindered in utilization of these processes on commercial scale. cess [97], and experimentation has been carried out to compare the This paper aims to provide baseline along with progresses in relevant properties of specimens produced by AM. The specimens showed some disciplines related to 3D printing in construction, which will provide an limitations owing to the anisotropy introduced in the structure, but it opportunity to experts in all the domains to understand the multi-di- was concluded that a geopolymer-based cement could be an alternative mensional constraints and to specify the future research directions in to traditional construction materials for rapid and green construction these sectors. First of all, the development of different AM processes in [131]. built environment are presented comparatively to highlight the process Sulfur concrete could be a sustainable solution for construction constraints along with the commercial applications of these processes applications. The main advantages of using sulfur concrete are its re- for industrialists and investors to identify the business opportunities. duced dependency on water, high strength, and durability and re- Secondly, process parameters and difficulties in optimization of mate- sistance to harsh environments. Panda et al. [94] performed experi- rial mixtures are presented as a guide to civil engineers, following the ments on variety of fly-ash based mortar mixtures with varying contents discussion on materials urging the need for development of eco-efficient of blast furnace slag and silica fumes to identify most critical process and environment friendly materials. Conclusions drawn from discussion parameters and material properties for extrusion 3D printing. It was in individual sections along with issues/constraints and challenges in- observed that addition of ground granulated furnace slag improved volved are explained separately. Fig. 10 presents different routes of early age strength of printed structures, therefore complimenting the progress, concerns identified by the literature review and future re- buildability. On the other hand, silica fumes provided control on early search directions. age yield strength and viscosity of the mixture. Researchers at the The main challenges in the 3DP of buildings and construction ap- Singapore Center for 3DP (SC3DP) are aiming to introduce printable plications are the design, process constraints, process parameters, the materials such as geopolymers and lightweight and fiber-reinforced behavior of the materials, and development of analysis tools to in- mortars, as well as waste materials such as recycled glass and crushed tegrate all of above to enable accurate predictions for expanded design rock dust, to assist in producing sustainable infrastructure [11]. flexibilities. The conventional approach to building element andsys- tems design limits the introduction and integration of automation and 6. Sustainability considerations for AM processes AM tools. Designers need to adopt a different approach towards the design of construction elements; bio-inspired design could be an answer The commercial application and utilization of AM processes relies towards how buildings should be designed. Numerical simulation tools on promising sustainable solutions to industries. AM processes need to could provide a better understanding of design perspectives and could be sustainable in all aspects, including resource utilization, energy use topological optimization and functionally graded materials to pro- consumption, cleaner production, and efficient production. Researchers mote sustainable solutions. Building classification tool developed to

15 A. Al Rashid, et al. Automation in Construction 118 (2020) 103268

Table 3 Utilization of fibers in infrastructure applications.

Reinforcement Base material Infrastructure application Remarks

Synthetic fibers Steel fibers Concrete, asphalt, soil Pavements, on ground slabs, Improves load bearing capacity and flexural strength [111] pavements, roads Resistance to cracks [113] Prone to erosion in concrete environment [113] Increased flexural strength and load bearing capacity for concrete pavements [111] Introduction to asphalt improved stability [112] Can enhanced the mechanical properties of soil [105] Polypropylene Concrete, mortar, sand On ground slabs, soil walls, Reduced permanent shrinking and crack formation [114] roadway slopes, embankments Enhances strength, impact resistance and toughness [115] Can withstand alkaline environment [114] Reduced workability & mixing owing to floating issues [113] Introduction to soil reveled improved strength, higher resistance to degradation and swelling [105] Utilization in repairing of roadway slopes and embankments [105] Nylon Concrete Improves tensile strength, toughness and reduces shrinkage [113] Resistance against cracks and impacts [114] Degradation in alkaline environment [114] Barchip shogun Concrete Slabs Slab exhibited same punching shear as for steel fibers [132] Reduction in visible cracks formation [132] Limited applications owing to lower melting point [132] High performance Concrete Water tunnel linings, shotcrete Improved tensile strength, flexural strength, toughness and polypropylene (HPP) energy absorption [116] Higher strength, elongation to failure, toughness and resistance to abrasion & high fatigue resistance [116] Introduction to shotcrete benefits ductility, strength, improved capacity & energy absorption [132] Polyethylene terephthalate Concrete Early age strength, impact strength and toughness is (PET) increases, but reduces over time, owing to degradation [114] Polyester Asphalt concrete Roads Increased toughness [112] Improved adhesion and bonding [117] Polyacrylonitrile Asphalt concrete Roads Better adhesion and bonding [117] Glass Soil Resistance against failure and reduced brittleness [105] Natural fibers Basalt Concrete, cement mortar, Roads Degrades in alkaline environment [133] geopolymer concrete, Resistance to degradation in alkaline environment can be polymer, asphalt enhanced through treatments and modifications [118] Introduction to geopolymer concrete enhanced the energy absorption capability and strength [118] Reinforcement of fibers to polymer bars can be usedto replace traditional steel bars [118] Improved resistance to rutting and drain down of asphalt [119] Sisal Soil Blocks Reduction in shrinkage of blocks and resistance to crack formation [134] Date palm Clay Bricks Improved thermal properties for hot climates [120] Kenaf Asphalt concrete Roads Sustainable and better solution than synthetic fibers to eliminate binder drainage [121] Straw Asphalt, soil Pavements, blocks Improved tensile and compressive strength of asphalt [122] Conventionally used block reinforcement material [105] Coconut Soil, clay Blocks, bricks Marginal improvement in compressive strength but considerable increase in ductility [134] Sustainable and eco-friendly solution to fabrication of bricks [123] Jute Soil Filtration, drainage Improved mechanical strength [105] Waste material fibers Recycled glass Asphalt Pavements Lower time to drying after exposure to water [124] Cigarette butts Asphalt, clay Pavements, bricks Can be utilized even under heavy traffic, exhibited superior mechanical and thermal properties [124] Acceptable properties in load bearing bricks [125] Nylon Concrete Improves tensile strength, toughness and reduces shrinkage [113] Resistance against cracks and impacts [114] Degradation in alkaline environment [114] Human hair Soil Embankments, slopes Enhanced strength and stability [127] Steel slag Asphalt Pavements Enhanced strength and resistance to skidding but limitations owing to conductive properties of the mix [126] Waste tire Asphalt Pavements Improved strength and toughness, down drain was eliminated, stability of the mixture enhanced [128] As binder, Enhanced mechanical properties including durability and resistance against cracks [126] Waste rubber crumbs Concrete Slabs Improved resistance against fire [124]

16 A. Al Rashid, et al. Automation in Construction 118 (2020) 103268

design processes, which would provide solutions in emergencies.

Abbreviations

AM Additive Manufacturing DMLS Direct Metal Laser Sintering SLM Selective Laser Melting SLS Selective Laser Sintering EBM Electron Beam Melting SHS Selective Heat Sintering FDM Fused Deposition Modeling MJ Material Jetting BJ Binder Jetting DED Directed energy deposition C4 Cable-suspended Contour Crafting Construction LCI Life Cycle Inventory FFF Fused Filament Fabrication PME Powder Melt Extrusion ABS Acrylonitrile Butadiene Styrene PLA Polylactic Acid HIPS High Impact Polystyrene DOF Degree of Freedom

Declaration of competing interest

Fig. 10. Future research directions. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influ- identify the complexity of the process and building components in- ence the work reported in this paper. volved could be a promising tool for designers [137]. The traditional materials used in the construction sector are not Acknowledgements feasible for AM processes without modifications. An in-depth study on the behavior of construction materials is needed. Material mixtures Authors would like to acknowledge Qatar National Library (QNL) require two contradictory properties; the mixture needs to be fluidic for supporting open access publication. This publication was made enough to exhibit good extrudability, and conversely, requires rapid possible by the TÜBİTAK - QNRF Joint Funding Program grant hardening to withstand subsequent layers. It is crucial to map the (AICC02-0429-190014) from the Scientific and Technological Research mixture characteristics to the mechanical properties of the structures. Council of Turkey and Qatar National Research Fund (QNRF). Any The reinforcement of conventional materials is still a challenge when it opinions, findings, and conclusions or recommendations expressed in comes to AM. Although AM processes have been improved to adopt to this material are those of the author(s) and do not necessarily reflect the construction requirements, there remains a need to develop materials views of TÜBİTAK or QNRF. Authors also would like to acknowledge that can replace conventional materials from the market, and that are Arslan Salim Dar (Wind Engineering and Renewable Energy integrable to the above processes [138,139]. Efforts should be made to Laboratory, EPFL, Switzerland) and Ramsha Imran (Department of design processes to accommodate the reinforcement of materials. The Mechanical Engineering, Institute of Space Technology, Pakistan) for development of novel materials with superior mechanical properties, providing useful remarks on the preparation of the article. specifically designed for AM processes, could eliminate the needfor reinforcement. AM techniques also hold the potential to fabricate sti- References muli-responsive structures. Traditionally applied construction materials are not sustainable, as they cause severe environmental impacts. Thus, [1] K. Rane, M. Strano, A comprehensive review of extrusion-based additive manu- it is currently necessary to develop sustainable materials compatible facturing processes for rapid production of metallic and ceramic parts, Advances in Manufacturing 7 (2019) 155–173, https://doi.org/10.1007/s40436-019-00253-6. with advanced manufacturing processes [140]. [2] B. 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