As one may know, construction is one of the largest industries in the world, which contributes globally to around 13 percent of the global gross domestic product (GDP). The usability of resources in the construction industry is astoundingly high and itself devours fifty percent of the world’s overall resources. Moreover, the construction industry has traditionally been extremely averse to change and strongly adheres to traditional values, weakness in innovative construction, and lowliness in productivity. However, nowadays, companies are turning towards modern technology and boosting innovation that is taking place in design, engineering, maintenance, and operations as well as infrastructure, architecture, urban furniture, industrial molds, artificial intelligence, and sculpturing.
Additive manufacturing represents a new horizon in the field of concrete and cement-based materials. The research activity on additive manufacturing in the construction sector involves the development of two types of technologies: powder-based and extrusion-based.
In powder-based techniques (also called binder jetting), a binder solution is selectively deposited onto the ceramic powder bed (about 5–10 mm thickness) through a print nozzle, bonding these areas together to form the pre-designed solid part one layer at a time. The final object is removed after a specific drying time and excess powders are eliminated by an air jet [ 1 2 ].
These techniques are suitable for the production of complex-shaped construction components with a high print resolution, a high degree of geometric freedom, and reasonable manufacturing speeds in line with industrial demand [ 1 5 ]. However, the process is an emerging strategy and therefore still being optimized. The main criticalities of the technique concern the limited amount of cement materials on the market that can be used in powder-based printers, the difficulty in introducing structural reinforcements, and the need to perform several post-manufacturing operations (such as infiltration of binder solution or additional curing steps) that can adversely affect production times [ 1 5 ].
Over the last twenty years, many research teams (both industrial and academic) have based their studies on the potential of extrusion-based additive production for construction applications. The main aspect that emerges from the predicted works is the analogy regarding the steps that lead to the final print product. Usually, the printing process involves a software part and a hardware part ( Figure 2 ). The first is related to the use of 3D software, such as AutoCAD or SolidWorks, to model the object. The 3D design of the prototype is sliced (with the help of specific software) to define the size of each layer and subsequently converted to G-code format, which represents the machine language recognized by the printing device. The hardware part consists of an extrusion system (which deposits the material layer by layer), a material delivery system (which sends the material to the print head through a pumping system), and a controller (monitors the printer and pump according to the design of the final object) [ 7 ].
Most concrete printing apparatuses are based on a robotic arm connected with the material storage system and moved through the appropriate software system. The print nozzle is attached to the robotic arm and is connected to the concrete mixer through a hose pipe. A pumping system allows the mix to be transported from the mixer to the deposition head. Some examples of this type of technology are Apis Cor and Singapore Centre for 3D Printing (SC3DP). The difference among these manufacturing systems is related to the apparatus design and the category of use.
2 and the dimensions of the machine (i.e., “Apis Cor 3D printer” (Apis Cor [ 15 ], one of the creators of the first 3D-printed houses, describes in detail the technical features for site printing. The size of a standard cross-section of a printed layer needed is 2.5 × 2.5 cm, the current version of the construction 3D printer covers an area of 132 mand the dimensions of the machine (i.e., “Apis Cor 3D printer” ( Figure 3 a)) in folded state is 4 × 1, 6 × 1, and 5 m and 2 tons of weight. As for accuracy, if the printing process complies with all the technical specifications, precision is up to 0.5 mm. The printer is capable of extruding at speeds up to 16 cm/s. Printer speed is automatically calculated using embedded software, and it depends on the printing path. Singapore Centre for 3D Printing (SC3DP) has developed two types of printing devices: a four-axis gantry and a six-axis robot ( Figure 3 b). Their use depends on the complexity of the product to be made: the first is mainly intended for large-scale prints while the other is for the creation of more complex shapes due to the fact of its six-axis rotational ability. The SC3DP printing devices grant greater degrees of freedom in the manufacturing process than Apis Cor technology but allow small-scale additive construction and, thus, better suited to the development of building components [ 16 ].
In addition to robotic arm-based systems, there are two other types of technologies: a gantry system (e.g., Contour Crafting and ICON Vulcan II) and a delta system (e.g., WASP Big Delta).
Gantry systems are crane-like manufacturing apparatuses that can be transported in specific trailers and allows the pre-designed structure to be developed directly at the construction site. The printer is equipped with a rotating print head (single- or multi-nozzle) combined with a hose connected to a mixer pump. The printer head is fixed on a vertical arm that is controlled by a four degree-of-freedom mainframe system. The manufacturing process is based on two approaches: formwork additive manufacturing and walls additive manufacturing.
The first approach (typical of the Contour Crafting printing system) involves combining two types of processes: extrusion and filling. The extrusion process allows for depositing two layers of cementitious materials to generate a formwork [ 14 ]. Printed formwork is simpler than the traditional design. A traditional concrete wall form typically consists of sheathing, studs, wales, ties, and bracing. The fresh concrete is confined to the sheathing and places a lateral pressure on the sheathing until the concrete is cured. Contour Crafting formwork is built using a printable mortar and secured with U-shaped tie rods. Compared to the conventional system, formwork developed through additive manufacturing involves the use of only two components: sheathing and tie. The sheathing is created in position by adding mortar continuously according to the pre-defined material deposition sequence; the ties are inserted at the sheathing locations. The advantage is that the formwork, made by additive manufacturing, can be built without using separate formwork materials resulting in economic benefits (lower production time and costs than traditional construction) and architectural advantages (greater design freedom) [ 17 ]. The filling process can take place through pouring or extrusion to achieve the core of the structure. Additional interventions are performed to improve the surface finish and mechanical integrity of the printed artifact. The result of the manufacturing process is a hollow wall filled with cementitious structural material. Research activities in the field of Contour Crafting technology led to the development of a vertical wall prototype 1.5 m long and 0.6 m high [ 14 ].
A walls additive manufacturing approach is more sophisticated and faster than the Contour Crafting process, as the printing apparatus allows one to develop the pre-designed building directly without the need for formworks, then through a single-step deposition process [ 18 ]. Typical examples of construction processes based on a walls additive manufacturing approach are ICON technology and WASP technology.
2 surface) was built in 48 h (for at 9000 Euro) and consists of a living room, a bathroom, and a bedroom. The vertical elements of the building were made by the overlap of printable concrete layers that form a double wall divided by an interspace with a reinforced structure. The roof, windows, and doors are the only items not printed but installed later. Future perspectives are related to the introduction of technological improvements to reduce costs and production time. The ICON company’s main purpose is to provide a viable strategy for housing construction in the poorest regions (e.g., South America) for the homeless [The ICON Vulcan II ( Figure 4 a) is a printing system designed and developed by a construction technology company located in Austin (USA). The machine is 3.45 m high and can print surfaces up to 8.5 m wide. Linear printing speed is about 12–17 cm/s. The printer is associated with an integrated tablet-based operating system to control every aspect of printing operations [ 9 ]. This technology was responsible for the construction of one of the first low-cost 3D-printed housings with requirements in line with local building standards. The house (32 msurface) was built in 48 h (for at 9000 Euro) and consists of a living room, a bathroom, and a bedroom. The vertical elements of the building were made by the overlap of printable concrete layers that form a double wall divided by an interspace with a reinforced structure. The roof, windows, and doors are the only items not printed but installed later. Future perspectives are related to the introduction of technological improvements to reduce costs and production time. The ICON company’s main purpose is to provide a viable strategy for housing construction in the poorest regions (e.g., South America) for the homeless [ 19 ].
Delta
” systems are a set of printing plants developed by the Italian company WASP. The purpose of WASP is to develop eco-sustainable buildings and structures using natural materials such as soil or agricultural waste. Research on this technology has led to the design and implementation of two types of apparatuses: the WASP Big Delta Printer and the WASP Crane (” systems are a set of printing plants developed by the Italian company WASP. The purpose of WASP is to develop eco-sustainable buildings and structures using natural materials such as soil or agricultural waste. Research on this technology has led to the design and implementation of two types of apparatuses: the WASP Big Delta Printer and the WASP Crane ( Figure 4 b). The Big Delta configuration is 12 m high and 7 m wide, assembled with 6 m modular arms. All the machine-components have a maximum length of 3 m so that they can be easily loaded on a trailer and transported. The engine and electronic parts have been designed to be powered by solar panels, allowing to minimize energy consumption (approximately a 60 V power voltage). The printer can work at a maximum speed of 40 cm/s, but the printing rate depends on the amount of material inside the extruder. The extruder can handle large amounts of material (up to 200 kg) but, to minimize the effect of mechanical vibrations during deposition, the weight is reduced to 40–50 kg. The design of the print nozzle is suitable for the deposition of mixtures containing long-fiber materials following the targets of WASP technology: the extrusion of construction materials based on raw terrain and straw optimized with natural or synthetic fillers. Using 40 tons of straw/clay mixture, the apparatus was able to print, in 20 min, a circular wall 2.7 m in length and 5 m in diameter for a total cost of about 50 Euro [ 20 ]. The WASP Crane is an evolution of the Big Delta system.
GAIA
”. The house was printed utilizing a natural mud blend produced using soil taken from a natural site, as well as waste materials from rice production, for example, chopped straw and rice husks. This project is the consequence of a restricted and upgraded utilization of agricultural assets, which through innovation has been transformed into a highly functional raw material. The mixture was printed layer by layer using the WASP Crane system, creating walls with vertical cavities inside, where these cavities then need to be filled with rice husks for thermal insulation. It took 10 days for the realization of the external casing (designed with the aim of integrating natural ventilation systems and thermo-acoustic insulation systems in only one solution), for a total of 30 m2 of wall whose thickness is 40 cm and the total cost of the materials used in the wall structure was 900 Euro [This is a collaborative modular manufacturing system consisting of the main printer unit that can be assembled in various setups depending on the print area and then the size of the architectural product to be built. The single module is made of a diameter 6.60 m and 3 m height, can be extended by adding traverses and printer arms generating an “infinite” digital manufacturing system. This construction strategy implies a potentially infinite printing area, as the individual modules can be reconfigured and can advance with a generative attitude depending on the growth and shape of the artifact [ 21 ]. One of WASP’s main projects is “”. The house was printed utilizing a natural mud blend produced using soil taken from a natural site, as well as waste materials from rice production, for example, chopped straw and rice husks. This project is the consequence of a restricted and upgraded utilization of agricultural assets, which through innovation has been transformed into a highly functional raw material. The mixture was printed layer by layer using the WASP Crane system, creating walls with vertical cavities inside, where these cavities then need to be filled with rice husks for thermal insulation. It took 10 days for the realization of the external casing (designed with the aim of integrating natural ventilation systems and thermo-acoustic insulation systems in only one solution), for a total of 30 mof wall whose thickness is 40 cm and the total cost of the materials used in the wall structure was 900 Euro [ 22 ].
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