Draft:Extrusion-based 3D concrete printing

Extrusion-based 3D concrete printing, or simply concrete printing, refers to digital fabrication processes for cementitious materials based on one of several different 3D printing technologies. 3D printed concrete eliminates the need for formwork, reducing material waste and allowing for greater geometric freedom in complex structures. With recent developments in mix design and 3D printing technology over the last decade, 3D concrete printing has grown exponentially since its emergence in the 1990s. Architectural and structural applications of 3D-printed concrete include the production of building blocks, building modules, street furniture, pedestrian bridges, and low-rise residential structures.

3D Printers for Concrete
There are a few main categories of robots that are used for 3D concrete printing, which depends on the application, scale of the project, and printing technique. All construction 3D printers generally consist of a support structure and a printer head with a nozzle that extrudes the concrete. Printers are usually used in tandem with modelling software that uploads the building plans directly to the printer.
 * Gantry Robots: Gantry robots are the most common in 3D concrete printing, which consist of a mobile gantry system with mixing and deposition systems. They can range from small lab models to large scale printers for printing full components or structures. These printers are typically limited to vertical extrusions but have the benefit of high stability and easy scalability for larger projects. Gantry robots must be larger than the assembled structure, which can add cost to transportation and set up costs. However, they are the easiest to control of all 3D printers.
 * Cable-driven systems: In a cable-driven system, the print head is suspended between several fixed points within a frame. It has more geometric freedom than a gantry system and is more lightweight and transportable. However, it requires a wide area for equipment and planning is essential so that the cables do not overlap with the printed structure.
 * Robotic arms: This is similar to the robotic arms seen in assembly lines, which have six-axis movement and the most freedom of 3D printing systems. These are also capable of depositing concrete, embedding components like rebar, and performing any post-processing that may be required after the concrete sets. Robotic arms are the most compact system, but are most commonly used for small-scale applications.

Printer Parameters
In addition to printer type, specific printer parameters significantly impact the final performance of 3D printed concrete and must be carefully selected when planning for 3D printing construction. These parameters can simply be broken down into print head design and print speed.

Print Head Design
The print head must be selected so that the concrete mix can smoothly pass through the nozzle and create the bonding effect between each layer, while also initiating the solidification process. Similar to printer selection, nozzle shapes and sizes vary depending on the application. 3D printed concrete samples from nozzles with rectangular holes typically have higher strength than those printed with circular nozzles, because there are fewer gaps between each printed layer. However, circular nozzles are more adept for printing complex geometries. For samples printed from the same nozzle type, mechanical properties are improved when a larger nozzle is used.

The height of the print head is the height of the nozzle relative to the printing platform. This parameter affects the surface quality between layers including bond strength, and must be precisely adjusted. A print head that is set too high will reduce the bond strength between layers, causing an unstable shape. A nozzle too close to the printing surface may interfere with the printing process and place additional loads on the concrete. Research proposes a print height equal to the width of the nozzle.

Print Speed
The speed at which the print head is set also influences the bonding strength. Increasing the nozzle speed generally decreases the adhesive strength, as the concrete has little time to set into place. However, taking too long to print successive layers reduces interlayer bonding, so a balance must be established that accounts for strength without premature collapse. Other factors that influence the quality of 3D printed concrete include the pumps and controls used to monitor the printer, as well the concrete mix design (See section on Mix Design).

3D Printer Suppliers
3D concrete printing technology has grown exponentially over the last decade, and is expected to continue to grow as researchers learn more about the software, hardware, and construction capabilities of these printers. Below are some notable companies and 3D printers that are used globally:

Critical Mix Properties
For 3D printed concrete, buildability and extrudability are two of the most critical design properties for a mix. Extrudability is the mixture's ability to pass through nozzles in the printing head, while buildability is the capacity to support additional layers. These properties are governed by the consistency, cohesiveness, and stability of the mixture, which stem from the mix design and selected materials. For both properties, a balance must be met between stiffness and workability. A stiff mix will increase strength, but decrease flow rate and print speed, potentially clogging the printer head. Conversely, decreasing the stiffness too much may increase workability and extrudability at the expense of strength and buildability.

Since concrete is printed in layers, layers must sufficiently bond to each other to allow for proper curing and full-strength capacity. Significant research has been conducted to create an optimal mix for 3D printing, although there are no current industry standards. However, the use of supplementary cementitious materials (SCMs) such as metakaolin, fly ash, silica fume, and superplasticizers are common in all 3D printed concrete mixtures (See section on Admixtures).

Cementitious Materials
Cementitious materials are integral to any concrete mix design. These materials serve as the binder that holds the mix together, as they chemically react with water to undergo the curing process. Portland cement is the most common material in construction for both 3D printed and traditional concrete applications due to its low cost and widespread availability. However, it's high setting time and low bonding ability are disadvantageous for 3D printed applications. Therefore, polymers and other admixtures are often added to reduce shrinkage and improve adhesion. Some of these polymers include rubber, mixed sand aggregates, carbon-sulfur polymers, and geopolymers, which also have added benefits of crack repair and resistance.

One alternative is sulfoaluminate cement which can be mixed with Portland Cement to quicken the hydration process and help develop early concrete strength after placement. While the setting time of Portland Cement is about half an hour, the setting time for sulfoaluminate cement is just six minutes. Therefore, higher strength can be achieved in a much shorter time period, increasing buildability.

Aggregates
Aggregate content and selection are just as important as the selected cementitious materials when it comes to concrete mix design. In particular, particle size has a significant effect on 3D printed concrete mixes. Particle sizes that are too large may block the nozzle of the 3D printer, while aggregates that are too small decrease the strength of the mix and can cause cracking. A rule of thumb for mix design is that the maximum aggregate particle size should be less than 1/10 of the nozzle diameter to ensure smooth extrusion.

Several studies have been conducted to examine the influence of aggregate size on mechanical properties for 3D printed concrete. It was found that increasing coarse aggregate improves volumetric stability of concrete and decreases hydration heat and shrinkage, which were common problems in early 3D printed concrete mixes. The use of coarse aggregate also increases concrete deposition rate and printhead speed, which can increase printing efficiency and productivity. Therefore, the printed structure achieves greater stability and strength, as observed by Ivanova and Mechtcherine. There is a limit to coarse aggregate content and size, as the challenge of controlling rheology become apparent. Natural aggregates such as sand and gravel are preferred as they require less energy to produce compared to artificial aggregates, but aggregate selection can be limited by regional deposits.

Admixtures
Admixtures include any materials outside of water, aggregates, and cementitious materials, that affect the concrete mix properties. Especially in 3D printed concrete, these admixtures are critical to balancing buildability, workability, and extrudability. Fly ash is the main admixture for high performance 3D printed concrete, as it improves working performance and durability. However, large amounts of flyash can lead to slower development of strength and buildability, which is why it is often mixed with other admixtures like clay, to retain shape stability.

Silica fume is another common admixture for 3D printed concrete mixes, as it increases the initial strength of printed concrete as well as flexural strength once the concrete cures. The main advantage of silica fume is that its small particles fill in the void spaces around the larger aggregates, which improves bonding performance with the cement binder. This also helps optimize the particle size distribution of the mix, which increases yield stress and buildability.

Mechanical Properties
As with standard concrete mixes, mixes for 3D printed concrete are typically tested for their compressive and flexural strength. These mechanical properties are highly dependent on the mix design, and can be improved by adding admixtures such as the ones described in the above section. For a mix containing ordinary Portland Cement, fly ash, silica fume, and fine glass aggregates, the compressive strength is around 36 to 57 MPa, which is comparable to the compressive strength of normal weight concrete. High performance concrete strengths of over 100 MPa have also been achieved by using superplasticizers and additional chemicals, but these mixes are more energy intensive to produce.

For 3D printed concrete, the structural properties are largely influenced by the interlayer bonding performance. Increasing the print speed and printhead height can reduce the interlayer bond strength, while adding a mortar between the layers can improve this strength. Particular, a resin mortar composed of black charcoal, sulfur, and sand has been found to be effective.

Concrete Suppliers for 3D Printing
Since there are no standards set for 3D printing concrete mix design, companies often pursue their own research and development if they decide to offer 3D printing as a construction service. Below are some notable companies that have successfully implemented 3D concrete printing into their scope of services.

Notable Projects and Applications
Due to challenges of reinforcement and limitations in printing technology, applications of 3D printed concrete have been mostly limited to small scale projects, including models and residential homes, as opposed to large commercial buildings. There are, however, some notable projects around the world that demonstrate the potential of 3D printed concrete.

ICON: 3D Printed Homes
ICON is creating a community of 100 3D printed homes in Georgetown, Texas. Reservations will begin in 2023 with starting prices in the mid $400,000. The fleet of Vulcan printers can produce eight different floor plans of 3 to 4 bedrooms and 2 to 3 baths. A concrete feeding system known as Magma supplies the Vulcan printer with Icon's developed concrete mix known as Lavacrete, which can adjust for site weather conditions and supply read-to-print concrete automatically. The 90 to 200m2 3D printed homes take around five to seven days to print, compared to a timber frame which would take up to 16 weeks in the same area.

ICON also completed a project in March 2020 for seven 3D printed homes in Austin, Texas. Each 400 ft2 home was printed in just 27 hours using ICON's Vulcan printer. The first residents moved into the homes in 2020 and are estimated to house 480 of the city's homeless, about 40% of the city's homeless population.



Habitat for Humanity: Affordable Homes Fast
In 2021, Habitat for Humanity, the world's largest non-profit home builder organization, built two 3D printed homes in Williamsburg, Virginia, and Tempe, Arizona. The Virginia home was 1,200 ft2 and printed in just 28 hours with a COBOD 3D printer, which was about four weeks faster than standard construction. The organization estimated that the 3D printed concrete walls saved about 15% per square foot in building costs. The 1,738 ft2 home in Arizona was constructed in the summer: a time where construction typically halts due to the extreme heat. 80% of the home was constructed using 3D printing including the interior and exterior walls, while the remainder, such as the roof, was constructing using traditional methods. Habitat for Humanity hopes that 3D printed homes can be a solution for affordable housing as well as labor shortages in extreme climates and environments.

PERI: Project Milestone
The first 3D printed residential building in Germany was constructed in September 2020 by PERI, using COBOD's BOD2 printer and Heidelberg Cement's concrete mixture. 24 concrete elements were printed at a facility and then transported to the site for assembly. The printer created 1 m2 of wall every 5 minutes, completing the 160m2 home by November 2020. Only two operators were required to print the walls, which included water placement, electricity, and pipe connections.

Nijmegen, Netherlands: Pedestrian Bridge
In 2021, the Dutch city of Nijmegen revealed the world's longest 3D printed concrete pedestrian bridge, spanning 29 meters. It was estimated that 3D printed saved about 50% in materials because concrete was only placed where structural strength was required. 3D printed bridge components were manufactured by BAM and Weber Beamix offsite, where it was then transported and assembled on-site. The previous record holder for the longest 3D printed concrete bridge was 26 meters, constructed by Tsinghua University in Shanghai.

Economic Impacts
In terms of cost and economics, one advantage of 3D printed concrete is that it does not require formwork, which is used to form the mold for conventional concrete pouring. Formwork can account up to 50% of total concrete construction due to material and labor costs. However, there are costs associated with machinery including the print head nozzles and supplemental monitoring devices. In addition, 3D printed concrete mixtures often differ from conventional concrete with additions of nano-clay, nano-silica, and other chemical admixtures that aid the extrusion process.

There are indirect economic benefits from 3D printed concrete in terms of productivity. The construction sector is often highly traditional and for the most part, processes have remained similar over the past decades. This is in large part because current processes are still effective in many construction applications. For example, a study by Garcia de Soto compared a robotically fabricated and conventionally constructed wall assembly with different degrees of complexity and found that that conventional construction outperformed robotic fabrication for simpler walls, while the robot was more productive as geometric complexity increased. There was no additional cost due to robotic fabrication and for both cases, material production was the driving factor for cost, as opposed to construction procedures.

Environmental Impacts
The environmental impact of 3D printed concrete is heavily dependent on the processes and materials used for a given project. 3D printed concrete has the potential to reduce material in the production of concrete due to the elimination of formwork, but the specialized admixtures and required technology may have just as much of an impact on the environment as conventional concrete construction. A cradle to grave life-cycle assessment (LCA) comparing the environmental impact of a conventionally constructed concrete wall with a 3D printed concrete wall revealed that the 3D printed alternative only reduced environmental effects when no reinforcement was used. The LCA impacts of global warming potential, acidification potential, eutrophication potential, and smog formation potential were used to measure environmental impacts. Once reinforcement was introduced to the 3D printed concrete structure, these impacts were greater than conventional construction methods, specifically for global warming and smog formation potential.

Another LCA conducted a similar study comparing conventional and 3D printed concrete walls but varied the complexity of the structure. It was found that as the complexity of the structure increased, the 3D printed method how a lower environmental impact. This was mostly due to the ability of 3D printed concrete to achieve complex forms while saving building materials in terms of formwork and concrete volume. Overall, the environmental impact of 3D printed concrete is influenced by the structure's design and how well the engineer can optimize material usage. On a material basis, the environmental impacts are similar to that of conventional concrete, as a cement binder is still required. However, the streamlined construction process that comes with 3D printing decreases material waste and onsite emissions.

Challenges and Limitations
There are several limitations that prevent 3D concrete printing from being widely adopted throughout the construction industry. First, the material palette that can be used for 3D printed concrete is limited, particularly due to nozzle extrusion and the deposition process of concrete layers, which introduces the challenge of premature collapsing. Therefore, research on material properties and developing high quality cementitious materials that comply with both structural concrete codes 3D printing applications is a current area of focus. Due to the sensitivity of a concrete mix, a change cement type, aggregate, or admixture will impact concrete properties and behavior.

Current building codes consider concrete has a homogenous material when in the reality, concrete is anisotropic. This anisotropy is further exposed with printed layers, so new methods for estimating deformations and cracking must be developed. In addition, current material testing for concrete consists of cylindrical specimens in accordance with ASTM C39. There is currently no systematic or theoretical basis for 3D printed concrete, especially when it comes to standard testing.

Current 3D printed projects have been limited to model prototyping and low-rise, large area buildings as opposed to high-rise commercial buildings because of restrictions in 3D printer technology. Printers need to be compatible with the height of the building, so additional research in 3D printer stability and design is required. There are also challenges with reinforcement in 3D concrete printing, which is required for taller structures. See reinforcement for 3D concrete printing for more details.