A Critical Review of Construction Using 3D Printing Technology

Different raw materials are used in 3D printing (or additive manufacturing) technology to construct heavy structures. Owing to the potential use of 3D printing technology in construction projects because of advantages such as improved construction speed, reduced human resources, building complex designs, and waste material removal, several 3D printing devices and techniques are being researched. In 2018, the revenue obtained from 3D construction was USD 22.7 million, and it is expected to reach USD 226.0 million by 2025 [1]. However, the COVID-19 pandemic adversely affected the results of this study. A recent study shows that the market size of 3D printing construction in 2021 was USD 11.268 million and is expected to have a compound annual growth rate of 100.7% from 2022 to 2030 [2].

3D printing technology can provide modern house designs at low costs for middle-class families. Therefore, housing associations in many countries can benefit from 3D printing technology by building multiple homes at low cost. For example, Don McQuaid (a global agency for solving homelessness) used 3D printing technology to construct 50 homes for homeless people in Tabasco, Mexico.

The 3D printing technique was first applied in 1981 through the additive process by Hideo Kodama, a Japanese inventor [3]. In 2004, Khoshnevis [4] developed the contour crafting extrusion of cement for the automatic construction of houses, as shown in Fig. 1. In this method, the printing nozzle was installed with an overhead crane to begin construction directly on site [5].

In 2011, Lim et al. [6] developed concrete 3D printing to improve printing performance. In the first experiment, a small print nozzle and chair were built successfully. In 2014, Cesaretti et al. developed the D-Shape technology that uses sand as support, and complex structures are built over the sand to strengthen it. However, preparing and strengthening the sand requires significant time, and the dimensions of the building are limited [7].

Since 2016, the Institute for Advanced Architecture of Catalonia in Barcelona has been working on improving 3D printing methods to develop different architectural designs using robots. One of their projects is a 3D-printed bridge in Madrid, Spain (Fig. 2) [5]. Currently, they are using metals and plastics to develop architectural designs [5].

While 3D printing technology has entered the global construction market, it is also used in other fields. This study provides a comprehensive review of experimental 3D printing technology in construction, identifies the recent trends in 3D printing processes and the materials used in construction, discusses potential applications and case studies of various companies and research groups, and identifies research needs to further adopt widespread use of 3D printing technology. This study will serve as a guiding point for researchers interested in 3D printing technology in construction to understand the research trends and future prospects.

3D printing in construction is used to fabricate buildings by extruding layers of concrete mixture containing mortar (cement, sand, and water) with different particle sizes through a nozzle using a computer-controlled a robotic arm or a gantry [8,9]. The concrete extrusion speed ranges from 50 mm/s to 500 mm/s and is controlled by applying different rates of pressure, which help form the desired shape. The concrete is arranged layer-by-layer, and the shape varies between circular, rectangular, and linear [10]. Additionally, several 3D printing techniques used for industrial construction (Figs. 3(a) and 3(b)) depict the on-site vertical and horizontal printing methods.

In 3D printing, the structure is first designed using computer-aided design (cad) software [11]. Then, the design is transferred to a slicing software, called the computer-aided manufacturing (cam) software, which translates computer codes to create an in-site structure that uses mechanisms such as robotic arms, construction cranes, or gantries. cad and cam are the two main software that cover specified steps in the printing process, which begins with building information modeling (BIM) and ends with the final object, as shown in Fig. 4. The CAD and CAM processes have been discussed in detail in Refs. [11–13].

CAD involves the following steps: BIM modeling, standard tessellation language (STL) file, and slicing.

• BIM Modeling

  BIM modeling is an integrated method for designing and managing new structures, such as a house or a bridge [14]. Figure 5 shows examples of SketchUp and self-cad 3D construction software [14,15].

• STL file

  The SLT file is used to transform the designed model into triangles. The more complex the design, the more triangles it produces, as shown in Fig. 6. Because triangulation recognizes 3D printing, the designed file generated using 3D software is converted into an STL file [16,17]. However, a more accurate and smooth design format (higher number of triangles) produces a larger STL file size. Additionally, considering 3D printing accepts a specific file size only, it is crucial to modify the size of the STL file (to ensure the required surface accuracy is achieved). Moreover, the STL format considers the surface geometry of the model without gathering relative information, such as the model color [18–21].

• Slicing

  In 3D printing construction, slicing is used to intersect the geometric model into parallel planes, called layers. Figures 7(a) and 7(b) show the triangles in the STL file and the slicing step, respectively [22–24]. All the above steps are considered as the G-code; finally, the designed model is ready for printing [25].

CAM involves the following steps: material components, 3D printing, and finalized object.

• Material components

  In CAM, the first step is to determine the appropriate mixture to obtain the required properties such as extrudability, buildability, and contact between layers. Herein, the materials used for printing are prepared and tested. The composition ratio of materials in 3D printing technology is calculated accurately to prevent the layers from collapsing [26]. However, one of the challenges faced by the automated construction of expeditionary structures (ACES) team in its first project (first full-scale military group bridge) was calculating the quantity of materials to be mixed, given that the materials are local and mixed on-site. Consequently, automation was implemented for mixing materials and calculating the quantity to obtain accurate results. Therefore, water was pumped automatically using an m-tec duo mix pump.

• 3D Printing

  In general, 3D printing involves the following steps:

  1. Fill the tank with the designed mixture.

  2. Pump the designed mixture through the nozzle at a specific speed to prevent the layers from collapsing.

In addition, 3D printing uses several types of devices and methods, such as gantry cranes or robotic arms, which use linear and continuous movement to extrude the mortar layer-by-layer, as shown in Figs. 8(a) and 8(b), respectively.

As shown in Fig. 8(a), the gantry system uses three axes of the building based on three dimensions and comprises three motors, each responsible for one axis. However, the robotic arm (Fig. 8(b)) uses more than three dimensions to build the layers. In addition, owing to its freedom of movement, the robotic arm can reach complex areas [28]. Table 1 lists the types and sizes of printers used by different 3D printing companies, and the location of each house built for each printer.

Concrete is the standard material used in 3D printing construction. However, the concrete mixture must initially be sticky and harden once it adheres to the surface. To avoid collapse, the concrete should be characterized by a specific viscosity and fast curing time. This section discusses the characteristics and tests of the 3D printing materials.

As mentioned earlier, most 3D printing construction projects use concrete, and the design is limited based on several influencing factors, such as weather. Although it is a new technology, there are significant challenges in preparing feedstock materials for 3D printing. The mixture comprises cement, aggregate, water, and admixture, and should exhibit high extrudability and buildability, as well as good contact between layers.

Extrudability, which is the ability of concrete to move from the mixer to the nozzle without changing its physical properties, is highly essential for successful concrete extrusion. Indeed, after mixing the components, the mixture acquires a plastic state, which is flexible and easy to form before it hardens. The plastic state time can be increased or decreased by adding chemicals and adjusting the printing speed [13].

To avoid a poor surface finish, the extrusion should correspond to the printing speed to ensure the layers are correctly placed on top of each other without excessive deposition. Therefore, the extrusion rate must be calculated when adjusting the printing speed [29].

Buildability is defined as the ability of concrete to harden before the next layer is extruded and placed on top of it. However, because the size of coarse aggregates affects the buildability of the concrete mix, a higher mixture volume is required for a higher rate of core aggregates (preferred size is typically 4–16 mm) [13]. Furthermore, using a coarse concrete mixture can negatively impact the printing process because coarse aggregates can cause the mixture to stick to the nozzle. However, this does not affect the density of the concrete [13]. The buildability of concrete can be controlled by factors such as chemical admixtures, temperature, printing speed, and by using less gypsum cement.

Figure 9 shows three samples with three different past ages and the thickness of the layers becomes thicker with increasing past age. The past age is the time of concrete from pumping the mixture until it sets. Therefore, the buildability can be improved by increasing the stiffness according to the paste age [31,32].

According to ASTM C 494, there are various types of chemical admixtures, each having a specific role in altering the properties of concrete and its compliance standard. Some chemical additives can be accelerators, inhibitors, and water reducers, which affect the durability and workability of fresh concrete [33].

In 3D printing, a chemical admixture is used to obtain high-performance concrete from the nozzle and harden it before the next layer is applied. To avoid mix failure, the recommended amount of chemical additives is 0–2% of the total cement volume [34].

Inhibitors are used to reduce interactions between the concrete components to maintain consistency while being transported from the factory to the construction site. At the construction site, accelerators are added to the mixture to harden the concrete before the next layer is applied. Table 2 summarizes some examples of chemical admixtures.

The adhesion between the deposited layers is crucial for increasing contact between layers. To increase the bonding ratio of the layers, the concrete mixture must be wet when placed on a layer and not in a complex state. The layer shape can be controlled based on the shape of the print nozzle (circular, rectangular, or square), as shown in Fig. 10 [13]. However, printing with a circular nozzle is a challenge owing to a possible collapse of layers, which can cause accidents. Therefore, rectangular and square nozzles are commonly used for higher accuracy and less construction failure [13].

Before the actual printing, the print is tested for nozzle size, print speed, and rotation angle. The rotation should be 90 deg to obtain accurate construction. Figure 11(a) shows the slippage of layers owing to bad nozzle size and high print speeds [40]. Additionally, considering the 3D printing process is based on a layer-by-layer deposition method, it could result in poor adhesion between layers and voids, as shown in Fig. 11(b) [10]. While this issue can be minimized by decreasing the thickness of the layers, it requires a longer process time considering the number of layers increases [1,10].

Aggregate is a component of coarse to medium grained particulate material used in construction. Sand, gravel, crushed stone, slag, recycled concrete, and geosynthetic are some examples of aggregates.

In addition, aggregates whose size typically ranges from 4 mm to 6 mm increases the durability and strength of the concrete, which in turn increases the strength of the printed structure. Considering the nozzle diameter ranges from 20 mm to 40 mm, the aggregate size should not exceed 4–6 mm to prevent nozzle blockage.

The water-to-cement ratio and concrete strength have an inverse relationship; when the ratio decreases, the concrete strength increases and vice versa, as shown in Fig. 12. The water-to-cement ratio typically ranges from 0.4 to 0.6 and 0.25 to 0.44 in conventional construction and 3D printing, respectively. However, in 3D printing, plasticizers are added to the mixture to obtain buildable properties. Additionally, when this ratio becomes less than 0.4, the concrete workability decreases [41].

To obtain a consistent mixture, the water-to-cement ratio is varied depending on the size and shape of the nozzle, given that the ratio affects the hydration of the concrete, which is crucial in 3D printing. Therefore, using the least amount of water that is free of salt and impurities can help obtain improved concrete cohesion and hydration, and avoid corrosion of steel reinforcement. In some designs, steel reinforcement is used to increase the strength of the building.

Additionally, polyvinyl alcohol (PVA) fibers, which are high-performance reinforcement fibers for concrete, can be used to increase the molecular bond, crack resistance, strength, and workability. The fiber length and diameter play an important role in the strength and workability of the concrete, respectively. The percentage of the PVA fibers and the length and diameter of the fibers depend on the required properties in the design of the concrete mixture. PVA fibers are 300% better than other fibers in terms of increasing the molecular bonds between components.

Slump flow tests is a concrete drop test that is performed using a special hose and adding concrete sample inside the slump test cone to determine the effective viscosity of materials depending on the prevalence of stagnation flow [41]. The hole was raised as shown in Fig. 13, and the concrete slump flow was measured between 50–100 mm and 190–200 mm for construction buildings and 3D printing construction, respectively. Additionally, this test determines the strength of the concrete and the percentage of water in the concrete [42,43]. For example, a test was performed with four samples, and the water-to-cement ratio and slump flow spread was calculated, as shown in Table 3. We can conclude that, the water-to-cement ratio ranges from 0.25 to 0.44, while the slump flow spread ranges from 190 mm to 200 mm.

Cement materials used in 3D printing construction differ from regular cement composition depending on the raw materials, and the casting or extrusion processes, given that the characteristics of conventional concrete are not suitable for 3D printing. 3D printing cement mixture should be more fluid and have low cement-water ratio to obtain a suitable viscosity for the concrete flow from the nozzle. Additionally, several research groups have studied different concrete mixtures, as presented in Table 4.

Le et al. [44] tested a concrete mixture comprising fine aggregates to verify its buildability and extrusion efficiency. The nozzle diameter was set to 9 mm for high accuracy. Table 4 summarizes the properties of this mixture, wherein the compressive and shear strengths were addressed for the printed specimens and workability of concrete, respectively. The optimum compressive strength of this mixture was 110 MPa after curing for 28 days [44].

Malaeb et al. [43] tested the compressive strength by printing straight lines with different mixing ratios for mixture No. 3, which has a water-to-cement ratio of 0.4. The compressive strength of the mixture was calculated as 42 MPa. Tay et al. [45] developed a low-cost mixture by adding 1.05% of water-weight superplasticizer to cement and is considered an excellent mixture for 3D printing (Table 4). Gosselin et al. [46] studied a concrete mixture with different components (Table 4). A polymer resin was added to the mix to develop a fineness interface between the layers. Furthermore, activators were added to obtain suitable rheology. This mixture is a high-performance mortar paste [48]. Hambach et al. [49] studied different proportions of concrete mix and found the optimum mix proportion for their experiment (Table 4). Concrete mixture samples were prepared and printed using a WASP Clay Extruder Kit machine with a nozzle diameter of 2 mm. Short fibers, such as carbon, glass, and basalt fibers, were added to the mixture by placing them in random directions to test the strength, density, and porosity. The optimum compressive strength for a concrete mix with a short carbon fiber volume of 1% was calculated as 80 MPa [49]. Kazemian et al. [50] studied and improved different concrete mixtures, such as Portland cement: SFPM is Portland cement with silica fume, FRPM is Portland cement with fiber, and NCPM is Portland cement with nanoclay, used in 3D printing (Table 4). The cylinder stability and layer settlement methods were used to test the capability of the fresh mixture. The results showed that the silica fume and nanoclay improved the capability of the fresh concrete to resist weathering action, chemical attack, and abrasion.

Figure 14 compares 3D printing to conventional building processes. While human resources are essential in traditional construction, 3D printing requires fewer human resources as it automatically builds structures from computer-aided design model [51,52]. Additionally, 3D printing does not require formwork, fixtures, or tooling, thereby reducing material waste, labor, and manual processes [1,45].

The formworks for conventional construction account for 25–35% of all project costs [1]. In addition, producing formwork could negatively affect complex designs that require specialized labor. Therefore, construction companies are using more robots owing to the shortage of skilled laborers and to improve the safety side and building efficiency [53].

Placing the reinforcement on the base structure is an issue in 3D printing. Furthermore, the rebar and post rebar are placed manually, which could affect the architectural design [1]. Nonetheless, building the reinforcement structure using machines can be useful in 3D printing technology.

Unlike conventional methods where the framework is considered waste unless it is reused, 3D printing reduces damage to the environment. Indeed, 3D printing construction generates lesser material waste because the architects know exactly what they require with the help of 3D printing software. Furthermore, 3D printing machines are operated using electrical energy and consume less energy, resulting in no carbon dioxide emissions [54].

3D printing construction can be affected by extreme weather conditions, considering temperature affects concrete. Therefore, most construction 3D printing is performed at night in high-temperature cities. Indeed, the reaction and hardening rates of concrete components vary depending on whether it is a high- or low-temperature area. Therefore, inhibitors and activators are used to avoid the influence of high or low air temperatures on concrete.

Additionally, APIS COR, an American construction technology company, devolved its equipment for extreme weather conditions, such as those in Dubai. Building materials such as concrete should be improved by adding admixtures for suitability to extreme cold or hot weather conditions [24]. Finally, all types of weather effects (e.g., temperature, humidity, windy, and precipitation) should be investigated for better processability and performance.

The deposition process involves adding a mixture to build a 3D project to avoid the deformation of layers. The layers compress under self-weight, and the hydrostatic pressure increases as the height of the building increases, which increases the distance between the nozzle and working surface, thereby changing the shape of the deposited material.

In addition, accelerators, which help fast-track the hardening process of the lower layer to withstand an increasing load, should be added to avoid structural deformation. For example, in the ERDC-CERL project, the structure collapsed under self-weight during printing owing to its large size and the need for more layers [13,55]. Therefore, to prevent any effect on the layers, 8–10 layers were printed in succession, as shown in Fig. 15 [13].

It is necessary to consider the aggregate size, which is typically one-third of the nozzle diameter to avoid clogging. For example, the aggregate and nozzle diameters used in the ERDC-CERL project were 1 cm and 3.2 cm, respectively [30]. Mixing aggregates with polyolefin monofilament fibers could close the nozzle and hose. Hose blockage can be treated by dividing the suspended concrete into several parts and pushing them out using water and a foam ball. If pushing is inadequate, the hose should be hit several times with a moderate force to stop concrete flow. If the hose blockage is treated for less than a few minutes, a fresh batch can be used. Furthermore, this issue can be avoided by increasing the nozzle diameter, as shown in Fig. 16 [56].

These problems can be avoided by modifying the G-code, reducing the aggregate and fiber size, or increasing the nozzle size. However, while reducing the nozzle size can reduce the printing time, it also reduces the accuracy owing to nozzle clogging [40].

Compared with conventional methods, construction 3D printing reduces labor costs; however, it exhibits increased material costs [54]. Table 5 shows the cost of one dragon tooth (ERDC-CERL second project) constructed using 3D printing and conventional methods. The labor and material costs in construction 3D printing were two-thirds and thrice of that of the conventional construction method, respectively [30]. For example, the cost of one dragon tooth is 750$ and 500$ using 3D printing and conventional methods, respectively; hence, the cost remains an obstacle in 3D printing construction. Therefore, the material cost of the construction 3D printing method should be reduced to obtain the highest cost efficiency. One way to reduce this is by using recycled materials.

Moreover, the cost of building with 3D printing is more expensive than a traditional building in terms of the device and material costs. The device cost is a one-time cost only, which turns into cycle maintenance. Therefore, real estate investors must be supported and motivated to adopt this idea to benefit from this technology.

The 3D printing material mixture requires a constant flowrate, which can be achieved by controlling the setting time for good extrusion. As mentioned earlier, inhibitors and accelerators (chemical admixtures) are added to the mixture to control the setting time and speed rate of the hydration of the concrete. Furthermore, tartaric acid and sodium gluconate showed favorable retarding effects.

However, printable materials still require a brief setting time to achieve strength sufficiently early after being deposited. Therefore, setting accelerators, a type of admixture, are commonly used in concrete for instant setting. Accelerating admixtures are chemicals that improve cement hydration, reduce setting time, and improve early stiffness development. Paglia et al. [57] studied the setting behavior of cementitious mixtures containing various types of accelerators, and the results showed that the setting time of concrete containing 4.5% alkaline accelerators was approximately 57% concrete containing 8.0% alkaline-free accelerators. Furthermore, Maltese et al. [58] found that a 2–7% dosage of alkali-free accelerator can reduce the setting time of the cement from 360 min to 150 min.

Shrinkage is a major concern associated with the cementitious material printing performance and affects the dimensional stability and accuracy of the printed structure. Increasing the sand-to-cement ratio and decreasing the water-to-cement ratio can help prevent shrinkage. Mixture materials for 3D printing should exhibit high water content to obtain optimum workability and extrudability. However, more water than required is commonly added for hydration purposes. The extra water vaporizes from the cement, resulting in drying shrinkage deformation during the setting and hardening processes. Furthermore, 3D-printed components are directly exposed to a surrounding condition that is larger than conventionally casted constructions using formworks and molds. Additionally, finer aggregates can also help reduce shrinkage deformation [59].

Shrinkage-reducing admixtures, such as fly ash and calcium sulfoaluminate concrete, can be used to minimize shrinkage by over 80% under drying conditions by reducing the surface tension through the evaporation of water [60]. Furthermore, the fibers can affect the level of shrinkage crack. According to the experimental test results, the crack region in the mix with structural nano-synthetic fiber (0.26 vol%) was reduced by 36% compared to normal concrete with no mixing fibers [61].

The materials used in this technology are not widely available because factories do not produce them often due to lack of orders. That is why they are expensive and rare. However, research and development is being conducted to develop new advanced materials for this technology in the future.

The design used in 3D printing is different from traditional construction because 3D printing technology depends on building horizontal layers. This obstacle can be overcome by providing training courses and supporting universities in teaching how to design buildings using this technology. Additionally, practical experiments on the design and construction of layers using 3D printing should be increased.

There is no fixed standard for 3D printing construction technology for several reasons, which are as follows:

• Lack of similar elements for the different 3D printing construction technologies. Each technology has its way of working and has strengths and weaknesses in different fields.

• Lack of information and studies, which affects the dependence on the safety and efficiency of the printed structure.

• Lack of a building permit for 3D printing in some countries, which reduces the development of this technology, especially in these countries.

This section discusses various promising activities of 3D printing technology in the construction industry. The advancements in 3D printing technology have led to competition in the market. As a result, each company aims to improve its printing methods. Table 6 provides a brief description of each company and their comparison.

The CyBe company uses a mobile cart with a robotic arm called CyBe RC 3Dp, as shown in Fig. 17 [30]. The advantage of a mobile cart is that it can build on-site, and the ease of mobility helps reduce transportation costs. The robotic arm is characterized by building at different angles, and hence, can build complex designs [1]. It is used in different fields, such as 3D printing concrete for flow-profile sewer pits. Additionally, this and most 3D printing companies consider placing a gap between the internal and external walls of the pits for high thermal and sound isolation, as shown in Fig. 17 [30]. Furthermore, the company developed a concrete product comprising 3D printable mortar with high hardening speed and efficiency. This product can be molded within 1 h.

This company uses 3D printing technology to build houses that will be sold in the business market, as shown in Fig. 18 [62]. The first house built by the company is called House Zero.

icon^® uses a gantry printer in a square area (2000 Sq ft) that can be transported to the construction site [62]. Furthermore, they build the house as one piece, exhibiting high thermal insulation and resistance to stress caused by natural calamities. Although a gantry printer is limited to a specific area, it can be used to build floor beams and automatic roofs, considering they can hold heavy particles [62].

Win-Sun is a China-based company that considered building a large building in a short time using a 3D printer. They started by building ten houses in 24 h using a robotic arm for printing on the standard home dimensions, 120 × 40 × 20 ft. Furthermore, they built houses in factories and transported them on-site [63]. Moreover, this company improved its business in building multistory apartments in countries like the United Arab Emirates, as shown in Fig. 19 [63].

The 3D printer is located in China so that the structures can be built and transported to other countries as prefabricated concrete pieces. Table 7 shows the building components and construction methods for the construction of a multistory building in the United Arab Emirates [1,63].

APIS COR is a San Francisco-based company, uniquely designed for mobile 3D printers that print structural parts, such as walls and beams, in cylindrical coordinates (Fig. 20) [62].

The 3D printer is beaded on a lightweight robotic arm that prints the structure component on-site. The 3D printing crane and robotic arm can be transported easily to construction sites [1]. This design contains gaps between the interior and exterior walls to place the fiberglass reinforcements and add a polyurethane mixture for insulation, as shown in Fig. 20 [62]. The company uses geopolymers to increase the reliability and strength of the building components. Therefore, their material mix comprises cement, sand, geopolymers, mixed-size aggregates, and fibers.

TOTAL KUSTOM uses a gantry crane and is one of the first companies to engage in 3D printing houses. It has produced three 3D printers: the LAByrinth, Architect, and StroyBot concrete printers. The LAByrinth printer, with dimensions 5 × 5 × 3 m, is designed for education and laboratory purposes. The Architect printer, with dimensions 0.5 × 0.5 × 0.5 m, is a 3D plastic printer designed for creating plastic models of houses. The StroyBot printer, with dimensions 10 × 20 × 6 m, is a 3D printer created for house construction.

The StroyBot concrete printer is a lightweight mobile printer that can be moved easily without using a crane. This printer is also characterized by its low cost, considering it can print a 150 m² house for 2000–5000 USD. It has been used to build houses in the Philippines. Furthermore, it is based on an unusual design wherein columns and doors are shaped spirally, as shown in Fig. 21 [13]. The spiral columns comprise steel and concrete with sufficient structural strength. Additionally, support materials are used to design door frames (Fig. 21) [13]. Therefore, the concrete framework was supported by a wooden bridge built between the walls [13,30].

WASP is an Italy-based company that aims to provide 3D printing at minimum costs using natural materials, resulting in a new eco-sustainable house for 3D printing. This company is the first to use a material mix comprising environmental materials (e.g., geopolymer): 25% soil (30% clay, 40% silt, 30% sand), 40% straw chopped rice, 25% rice husk, and 10% hydraulic lime. The materials are mixed in a wet pan mill to achieve appropriate workability [1]. The 3D printed house for WASP is shown in Fig. 22 [62].

Tu Eindhoven has significantly contributed to improving 3D printing technology. It uses a gantry crane and a unique nozzle. Therefore, a thin layer is extruded from the nozzle to achieve good resolution and reduce construction finishing work. Indeed, the university managed to build several houses in different architectural 3D printing designs in the same neighborhood, as shown in Fig. 23 [62].

The University of Nantes in France uses polyurethane 3D printing instead of concrete 3D printing to build houses, considering it is more suitable for thermal insulation properties and exhibits a faster build rate. The Social Housing Building in France, which was created in the YHNOVA BatiPrint3D project, is one of their projects. As shown in Fig. 24, it uses a robotic arm with a mobile cart and polyurethane nozzle, which decreases the building time [62]. The idea is to create a polyurethane mold and fill it with concrete [62], which helps achieve sufficient strength and good thermal and sound isolation. In addition, the build time decreases and the thermal insulation increases owing to the polyurethane material [62].

In 2018, the US Marines, Navy, Army Corps, and Air Force decided to use 3D printing to build a bridge, as shown in Fig. 25. The Marines started by designing the bridge and beams, while the Army Corps completed the structural design and reinforcement base. The mixed concrete was measured and mixed using a skid steer and volumetric mixer and was placed into the nozzle [56].

The military group collaborated with ERDC-CERL to print the bridge. ERDC-CERL formed the ACES program, a 3D printing machine using local materials. The main goal of ACES is to decrease material usage and number of workers and build a more robust structure [56]. The ACES group started its first full-scale project, which was the military group bridge. Additionally, they developed ACES lite to build and test 3D printing materials, as shown in Fig. 26 [64]. The ACES is transportable, easy to prepare, and requires fewer workers.

The 3D printing process for this project was based on four steps: transportation and construction, programming and structural design, preparation of the material, and printing and finishing.

As mentioned earlier, ACES is transportable and requires fewer workers. ERDC-CERL transported the printer from Champaign to Camp Pendleton with the help of four workers within 2 h.

The Marines designed the beam and piers using autocad and sent the design to ERDC-CERL. ERDC-CERL then determined the reinforcement location and converted it to G-code. Additionally, because the design considers a temporary bridge, the piers were based on holding vertical loads. Therefore, the bridge was reinforced every five layers.

The concrete mixture comprises cement, coarse and fine aggregates, rheology-controlling admixtures, water, and short polyolefin monofilament fibers. In contrast, concrete-incorporated polyolefin monofilament fibers were added to increase the properties of the material, such as resisting temperature changes and shrinkage and increasing the toughness.

The concrete mixing process was the same as that of the previous ERDC-CERL project, and the mix was measured manually in batches. While the process for material measurement is labor and time-consuming [65,66], it is appropriate and incurs less cost compared to conventional construction methods. In military projects, the process of mixing the concrete is improved with CreteMobile [66], a volumetric mixer that provides accurate mixing measurements, as shown in Fig. 27 [64].

Therefore, CreteMobile is used only to measure the requirement of each component (fibers, cement, and coarse aggregate) and mix them with a small concrete batch mixer connected to a skid steer [56]. Water and fine aggregates were manually measured and gathered into the batch mixer. Post-mixing, each batch was collected into a variable frequency drive pump and extruded through the nozzle.

First, a plastic tarp was used to separate the concrete layers from the pavement. Then, four beams were printed over 2 days and transported on-site, where they were sheeted with plastic tarps and cured for 5 days. Then, the bridge deck was printed, as shown in Fig. 28 [64]. Finally, the two-support girder structure that transfers loads to piers and supports the bridge deck was printed.

Considering ACES Lite cannot place the reinforcement, this step was performed manually by stopping the printing process and setting the rebar or wire reinforcement (Fig. 29; [64]) before continuing with the printing.

Finally, despite the various challenges faced by the US military, such as weather, materials, equipment, and energy, the bridge construction was completed using 3D printing technology. This bridge was the first of its kind to be built on a field ground within 12 h of printing. Furthermore, the US military achieved its goal of cooperating with the ERDC-CERL company and training while using this modern technology. Conversely, ACES Program created the idea of Exercise Burgeon Strike, a project in collaboration with the US military that aims to expand training on this technology, owing to the successful construction of the bridge using 3D printing.

As discussed in Sec. 5.9, ERDC-CERL created the ACES program in 2015, which continued to develop through the Exercise Burgeon Strike project. Further, ERDC-CERL owns six 3D printers, one of which they developed in-house [56]. In 2016, the ACES team built its first large printer, which was a military printer capable of building an area of 1.8 m × 1.8 m × 2.4 m, as shown in Fig. 30 [30].

Furthermore, in 2017, the ACES team built a 9.75 × 4.9 × 2.4 m concrete hut (B-hut) using 3D printing in Champaign. In the following year, ACES cooperated with the Marines to build a second version of the concrete hut (B-hut), as shown in Fig. 31 [30].

The Burgeon Strike exercise, which focuses on the printing materials and methods, was divided into two sessions: the G-code and the printing process. On the first day of the course, the G-code was introduced, and a large percentage of the attendees had no previous experience with the G-code. Table 8 summarizes some G-code terminology; four of the commands were defined by students within 3 h of learning. After 2 days, the students designed a new sample that was ready for print [55].

The ACES team also designed three dragon teeth as a working example of the course. The exercise began using CAM for G-code simulators for 3D printing, as shown in Fig. 32. Each dragon tooth was spiral printed, clockwise from outside to inside and vice versa, and made up of 48 layers to ensure the printing did not stop. The first 24 layers were the bases for the body, whereas the remaining layers were manually hollowed out for reinforcement. The nature of concrete differs between 3D printing and traditional construction methods [67].

In this course, a concrete material comprising Portland cement-based plaster with a blend of admixtures and fine aggregate was used from Gulf Concrete Technology and only required water to be added. This mixture was characterized by high adhesion and resistance to cracks. The mixture was then mixed in specific proportions and transferred to the printer using an m-tec pump [64].

Additionally, ACES lite 2 was used to print the dragon teeth, as shown in Fig. 33 [64]. This printer, initially created with an agreement between ERDC-CERL and Caterpillar, can be easily transported, disassembled, and assembled again by four workers within 30 min. Furthermore, only two workers were required for the operation [68]. The nozzle position, flow speed, and nozzle path were set by CNC control before printing.

Finally, the US military was able to construct three Dragon Teeth using 3D printing construction [30]. This achievement opened doors to teaching 3D printing construction and proving its ease of use.

Further research in 3D printing construction through collaboration with universities and traditional construction companies is needed to enhance this technology. Both the material characteristics (e.g., mechanical properties such as bondability) and the process (e.g., minimizing human intervention and increasing automation level during the plumbing and electrical connection for 3D printing) should be enhanced, which will contribute to the automatic organization of mechanical and electrical networks. Additionally, the projects and capital in this technology should be raised owing to its increasing popularity, using recycled materials to ensure this technology exhibits environment-friendly constructors.

Conversely, the future of this technology is growing fast and is promising as it reduces the construction period, construction waste, and use of human resources. As a result, 3D printing technology can become more affordable and may potentially be introduced in different industrial fields. In addition, the increase in research on materials suitable for construction in 3D printing may help discover new and available materials; this may address the problem of the high cost of materials.

We herein discuss several topics in 3D printing technology: the history of the technology, the printing process method, characteristics of materials used, the advantages and challenges, and some of the applications in the construction industry. Furthermore, this study simplifies the idea of 3D printing in construction and urges more research and studies of this technology by the government and society. We also demonstrate the impact of this technology on the future of modern construction.

Construction 3D printing is the future of construction industry, considering it can develop the housing market by building structures in less time while using fewer workers. The analysis of construction 3D printing conducted in this study can facilitate the education of this technology worldwide. The development of this technology can achieve the following three advantages:

• Possibility of teaching 3D printing construction design. For example, in the ERDC-CERL project, eight people without any G-code background were taught how to design the 3D printing construction within 3 h.

• Possibility of building any design using a limited number of workers (one worker each assigned to monitor the computer, pump, concrete mixture, flowrate of pumping, and printing quality).

• Possibility of increasing the level of safety in construction sites during work considering it needs a limited number of workers, thereby reducing the incidence of work injuries.

There are no conflicts of interest.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.