The Shape of Concrete to Come
Consider for a moment a scenario in which safe, habitable structures must be built as quickly as possible: in the midst of a refugee crisis, for instance, or following a natural disaster like a flood or hurricane — or even when a human landing party arrives on a distant planet.
Now think about the material, equipment, and labor required to build such structures in the traditional way, and how the complexity of bringing all of these elements together affects the speed of a project. Wouldn’t it be great if there were a technology that offered shorter construction time, lower labor costs, the potential for 24-hour operation, and even the ability to create unique shapes that harmonize with the environment?
There is such a technology, and you may well be familiar with it already. Additive manufacturing (AM) started out as a fast, economical way to make product prototypes for testing and fine-tuning prior to full-scale production using more traditional methods. Now, however, AM has advanced to the point where in some industries, it is being used as a production technology, even in building homes. While progress is being made in areas such as printing curable stone-like composites, most applications for AM in construction currently revolve around concrete.
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Cementitious additive manufacturing (also known as three-dimensional concrete printing, or 3DCP) is still in a relatively early stage of development. Most projects so far involve smaller-scale houses and other structures like bridges and outdoor benches that are being used to test and evaluate construction processes and methods.
However, many experts in the field believe 3DCP use can grow, and that it has the potential to address housing issues around the world. Getting to that point will require new standards specific to the materials, printing parameters and processes, and 3DCP performance characteristics, as well as harmonization with current building codes and regulations. The ASTM International committees on concrete and concrete aggregates (C09) and additive manufacturing technologies (F42), among others, are working to make all of this happen.
A Promising Technology
The first widely recognized effort to employ AM for construction was when a University of South Carolina professor attempted to print a 3D wall in 2004. From that point, to the construction of a canal house in Amsterdam in 2014 and the completion of the “Office of the Future” in Dubai in 2016, progress has been slow but steady.
Details of the Dubai office building hint at the promise of 3DCP. According to Syska, an engineering firm that partnered on the Dubai project, a massive 3D printer — 20 feet tall, 120 feet long, and 40 feet wide (6 m, 36 m, and 12 m) — outfitted with a robotic arm was used to produce the floors, wall, and ceiling as two-dimensional modules off-site. The modules were then cut in half, shipped to Dubai, and assembled. The entire process, start to finish, took 17 days and required 18 workers.
Another structure located near Zurich, Switzerland, showcases more recent advances in the use of concrete printing technology for construction. Officially opened in February 2019, DFAB House was actually built on the top floor of the Next Evolution in Sustainable Building Technologies (NEST) modular research and innovation building. (DFAB is digital fabrication.)
DFAB House features a curved concrete wall created by a construction robot, a digitally planned floor slab that was structurally optimized to greatly reduce the amount of material used compared to a typical concrete slab, and a concrete ceiling cast in 3D-printed formwork. “3DCP was responsible for the ceiling slab fabrication. A large-format binder jetting 3D printer was used to create the molds used to cast the concrete slabs,” notes Alex Liu, Ph.D., head of AM programs in Asia for ASTM.
All these activities translate to an optimistic outlook for the growth of 3DCP. One estimate, from ResearchandMarkets.com, suggests the global market for the new technology will grow to $1.5 billion in U.S. dollars by 2024.
A Matter of Scale
Many wonder how 3DCP is even possible. After all, much of the activity in the AM space involves the use of powdered polymers or metals to make relatively small objects. How do you extrude concrete, much less build an entire house with it?
The key is a precisely calibrated mix that allows the material to flow freely without clogging the nozzle; to retain enough liquidity to bond successfully with each new layer; and to harden quickly enough to ensure structural integrity. Various additives, like superplasticizers that reduce the amount of water required for the mix, are used to achieve the proper consistency.
In the future, a concrete extruder may be as commonplace as today's cement mixer.
Despite the different scale of the end results, the actual process is quite similar to that used in polymer- and metal-based systems. Designs are created using CAD software and transmitted from computer to printer in G-code, a machine language that directs the print head to lay down layers of extruded material in the desired configuration.
The basic operation is the same whether you’re using titanium powder to make a small medical implant or a concrete mix to build a house. But, building a house is a much more complicated undertaking. The flexibility offered by 3DCP can make it much easier.
“Cementitious additive manufacturing is generally divided into two categories: 3D printing of construction components in a factory environment and 3D printing of structures on site,” says Shane Collins, vice president and general manager of Additive Industries North America and a chair of the applications subcommittee (F42.07), part of the AM technologies committee. “The former offers higher precision, more color options, and full control over the printing environment. On-site 3D printing requires no transport of the construction components nor any lifting of the building blocks into place.”
Labor Shortages and Affordable Housing
It’s clear that 3DCP offers valuable benefits whether the process is carried out at an offsite manufacturing facility or at the building site. Another upside to the technology relates to labor — or the lack thereof.
A 2019 survey by the Associated General Contractors of America and Autodesk highlights the problem, finding that 80% of construction firms reported difficulty filling the hourly skilled positions that comprise the bulk of their workforce.
Paul Bates is ASTM’s AM lead project engineer. His conversations with people in the construction industry often touch on the lack of experienced workers in the building trades, and how 3DCP might help to alleviate it. “As with any labor-intensive task, automating it can help not to eliminate jobs, but to manage a demand on the existing workforce that exceeds what is available,” he says.
Automated deposition of concrete layers helps builders sidestep the shortage of experienced workers. Design flexibility, another key attribute of 3DCP, also contributes to reduced labor requirements. For instance, programming ductwork and utility chases (channels) into the printing design eliminates the need for these openings to be created by a worker. Reinforcements can be incorporated into the wall in the same way.
The relative portability, automated operation, and speed of the concrete printing process make it especially exciting to those trying to ameliorate the pressing need for affordable housing globally, Bates explains.
“The most promising application of 3DCP is providing affordable housing and shelter for countries with space constraints and increasing demand,” Bates says. “This is especially crucial as urban population numbers are rising while the availability of land continues to dwindle, resulting in an increase in housing costs. In addition, 3DCP can be a potential solution for homelessness and provide fast accommodation solutions for those displaced by natural disasters and other calamities.”
A community of 50 printed concrete homes recently completed in Tabasco, Mexico, provides a glimpse at how 3DCP can help fulfill this potential. The 500-square-foot (46 m2) structures were built in an impoverished area where people lived in makeshift shelters. Each house took just 24 hours, spread over a couple days, to finish.
Bates points out that someday this technology might even be used to build shelters in more exotic locations. “Some theorists for space travel have talked a lot about using ‘local’ materials,” he says. “If you’ve got to ship just the binding agent for the type of dust or sand on certain planets, for example, that’s a lot less stuff you have to fly out there. Rather than ship all the components one by one, it would make more sense to use what’s already on a planet, if possible.”
Standards Drive Acceptance
“A variety of challenges have to be addressed in order to encourage wider adoption of 3DCP in the construction industry,” according to Liu. One of those challenges is aesthetic in nature.
“The ASTM term for the additive manufacturing technique is called material extrusion, and it produces a unique surface condition that will take time to become accepted in the way a stucco surface is today,” he says.
Other challenges are not surprising given the newness of the technology. “To date, there are very few established standards surrounding 3D concrete printing with regard to the materials being used, the printing parameters, and the printing processes. In addition, there is a need to draw parallels and tie in new standards to the building codes and regulations that exist today,” Liu says.
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Bates echoes Liu’s point about leveraging work that has already been done. “One of the things that’s important to understand is that, when it comes to 3DCP construction, a lot of existing standards still apply,” he says. One example is the widely accepted test methods that can be used to determine a 3D printed concrete structure’s fire resistance: the test method for surface burning characteristics of building materials (E84) and test methods for fire tests of building construction and materials (E119). Another is resistance to water penetration with the test method for water penetration of exterior windows, skylights, doors, and curtain walls by uniform static air pressure difference (E331) and the test method for resistance of concrete to rapid freezing and thawing (C666).
A number of other well-established ASTM test methods are available for evaluating the mechanical properties of 3DCP building components in areas like compressive strength, flexural performance, and bond strength.
As various ASTM groups — including the construction group (F42.07.07) in the additive manufacturing committee — identify how these and other existing standards can be applied to printed concrete, efforts are also under way to develop new standards that focus more specifically on unique aspects of this technology. ASTM’s International Conference on Additive Manufacturing (ICAM) provides a forum for stakeholders to discuss all aspects of AM, including standards related to concrete printing. ICAM 2021 is scheduled for Nov. 1-5.
ASTM also sponsored an international symposium last December that brought together — virtually, of course — a number of experts from the field to discuss where new standards are needed. Titled “Standards Development for Cement and Concrete for Use in Additive Construction,” Scott Jones, Ph.D., of the U.S. National Institute of Standards and Technology, was chair of the symposium.
“On the material side, new tests to characterize the rapid hydration of 3DCP materials will be required — ones that look at strength prior to initial setting and the rapidly changing rheology come to mind here,” Jones says. “On the structural side, we now have to contend with a structure that has a layered quality to it and likely new reinforcement mechanisms. We need to be sure the test we conduct probes these features.”
Activity is already underway in the ASTM subcommittee on fiber-reinforced concrete (C09.42), part of the concrete and concrete aggregates committee. The subcommittee is working on a standard related to direct tension testing that Liu believes will, when finalized, rely on proven test methods to help evaluate 3DCP structures built using this type of concrete.
Other initiatives to create new standards are collaborative in nature. For example, ASTM is part of a joint working group on 3D concrete printing with ISO, the International Organization for Standardization. The work of the F42 construction group, chaired by Sam Ruben, chief sustainability officer with Mighty Buildings Inc., is also working to create new standards in this area.
Still another new 3DCP evaluation methodology currently under development by global safety certification organization UL will complement the ongoing work of various ASTM committees. ASTM is on board as UL’s standards development partner for this effort: UL 3401, Outline of Investigation for 3D Printed Building Construction.
“The purpose of this methodology is to ensure that the equipment, systems, and materials used, as well as the fabrication processes employed, will result in building elements of acceptable standards being consistently produced. This will help builders identify equivalent code compliance with existing building and residential codes and gain approval for 3D-printed structures,”
He also emphasizes that issues of quality control and repeatability at larger scales must also be addressed. “Confidence is gained when high-quality and high-precision products can be produced consistently.”
Approval of new standards and the adaptation of existing ones will go a long way toward advancing the use of a new technology that is still unknown to many in the construction industry. “It’s no different than other additive technologies,” Bates concludes. “Whether you’re doing a titanium metal part or a printed concrete wall, the question is, ‘Is it going to perform the same as what we’re used to?’ New standards and regulations need to be in place and inspectors educated on them before 3D-printed concrete structures are more
widely accepted.” ■
Jack Maxwell is a freelance writer based in Westmont, New Jersey.