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¹ For example, the concrete can contain reinforcement fibers that are laid down at the same time as the concrete or the printer can extrude continuous metal chains along with the concrete.

appropriate-size printer for a project that could be as large as a stadium. After a successful proof-of-concept, SkyBAAM was scaled to its current size of 25 × 25 feet.

SkyBAAM is fixed via an overhead lift device (to lift the 1,000-pound print-head) that enables precise positioning over the entire workspace. Real-time laser feedback systems track work volume accuracy and calibrate the system. SkyBAAM is rugged enough for site transport, and the equipment and its automatic calibration systems take just an hour to set up. Base station elevators and macro-micro manipulator configurations allow it to scale to large platforms. The biggest technical hurdle, Post said, was getting the material composition right, because concrete presents many more challenges than plastics.

Post said that an exciting future possibility for large-scale, robotic, deployable AM construction is multi-agent printing, where thousands of drones, each pouring small amounts of concrete, work collaboratively to quickly construct a large structure. He noted that other ideas and innovations could come from architects who want to build novel, complex forms, and from the military’s need to build in remote areas with limited labor.

Discussion

Zavattieri asked how durable and reliable SkyBAAM’s cables were. Post answered that the laser tracker’s continuous feedback provides data on cable age and changes, and enables tension adjustments to reduce calibration error. In addition, the models incorporate this feedback and resulting calibrations to continuously compensate for cable stretch.

Asked about structural integrity, Post described how reinforcement can be incorporated at every layer, vertical reinforcement can be added at the end, or more traditional reinforcements can be used with SkyBAAM. More advanced techniques could print reinforcements simultaneously or use in situ monitoring. As the technology progresses, he speculated that new qualification and certification techniques for structural integrity will emerge—perhaps ones that are even more accurate than those used for traditional construction.

As to whether a digital twin could be recorded for inspection, Post answered that this approach works well with metal printing, but said that it will require additional work to develop accurate models and gain the trust of standards committees for using digital twins in concrete construction. One challenge with model validation, for example, is that if the concrete cures too quickly, structural integrity could be affected.

Site-specific challenges can pose other hurdles for SkyBAAM. However, Post noted that wind load is not an issue, because of the constrained and stiff positioning of the cables. If there were extreme wind disturbance, the laser tracker and manipulator would still be able to make the necessary adjustments, he said.

government incentives, have led to strong construction innovation, such as 3D-printed bathroom pods. Australia, the United Kingdom, and areas of the United States are also at the forefront.

Marks asserted that all the products and workflows and their potential interconnections are not yet fully understood. Once workflows are productized, standardized, and automated, it will be necessary to adjust other influential factors such as contracts, supply chain, risk management, and insurance for full implementation.

Asked about generational barriers, Marks said that leaders benefit from a diversity of ages, genders, and perspectives. Noting that technology is more fully integrated into younger workers’ daily lives, she said that reverse mentorship can be helpful for leaders to learn from more tech-savvy employees. Given the overall importance of education and understanding, she said that her employer will soon launch an industrialized construction educational master’s level course available to a global audience.

As to building waste, Marks noted that waste avoidance is more important than waste reduction. She suggested that buildings should be designed not as single entities, but as a set of decoupled products where obsolete systems can be replaced or removed and then reused elsewhere without damaging the entire structure. She asserted that some prefabricated innovations that are not designed for or connected to the larger ecosystem will not become successful products, no matter how on-trend they are. She said that a connected ecosystem, enabling many technologies that act as digital glue, with an underlying connected platform is necessary to achieve this new possible.

Transitioning into the next workshop session, Zavattieri introduced a panel discussion that included Peter Stynoski, U.S. Army Engineer Research and Development Center (ERDC), Construction Engineering Research Laboratory (CERL); Scott Jones, National Institute of Standards and Technology (NIST); and Jan Olek, Purdue University. Following their remarks, Kurfess moderated an open discussion.

REMOTE 3D PRINTING AND DEPLOYMENT

Peter Stynoski, ERDC

Stynoski is the materials subject matter expert on the ERDC Additive Construction team, whose goal is to build capability to print customized structures in a forward environment on demand, with locally available materials and minimal loss of life. While preprint capabilities are important, DoD requirements emphasize the need for a printer that can be deployed as far forward as possible.

Identifying the right materials is critical to these efforts, and Stynoski said that aggregates are judiciously tested to ensure a scalable, locally sourced, printable

concrete. The lightweight gantry system in use by ERDC has high tolerance for mounting surface and print envelope irregularities. Pumping limitations remain a challenge, as mixing and pumping cannot take place at the print head owing to mass and stiffness constraints of the lightweight gantry. The team is also examining turning plastic waste into filaments, elastomers, and foams for interior applications and printed building insulation.

Two example structures demonstrate the capabilities of the equipment and the local material sourcing approach. The first, a barracks hut printed at the CERL in Champaign, Illinois, uses less concrete, time, and labor, while offering better structural properties than a comparable masonry unit structure. The second, a bridge in California, was printed adjacent to the emplacement site with local materials and emplaced after only 3 days of curing. The design was validated by shipping the decks to CERL for structural load tests.

CERL’s thorough in-house testing ensures that materials meet existing building codes and accreditation to speed up industry acceptance and utilization, Stynoski said. Solving the pumping limitations, validating structural engineering, and ensuring code compliance will be important steps for further advancing the technology and its utilization in the field.

METRICS

Scott Jones, NIST

Jones heads NIST’s Infrastructure Materials Group’s Concrete AM project, which is examining methods to measure the properties of 3D-printed concrete. This entails understanding how AM processes affect materials, as well as implications for quality control, assurance, and the ability to meet specifications and standards.

Understanding the material is critically important, Jones said. NIST has created methods to measure material properties at relevant times and scales, testing materials on a wide range of attributes from rheology to electrical conductivity to microstructural changes. A better understanding of how material changes on a microstructural level affect material performance, which in turn affects structural performance, can be used to create tools to simulate or predict 3D-printed materials’ structural performance. In addition, Jones noted that combining data from multiple tests such as electrical conductivity and other tests that are nondestructive can help to eliminate the need for destructive testing.

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² J.G. Sanjayan, A. Nazari, and B. Nematollahi, 2019, 3D Concrete Printing Technology: Construction and Building Applications, Oxford, UK: Butterworth-Heinemann, https://doi.org/10.1016/C2017-0-02407-2.

³ M.S. Khan, F. Sanchez, and H. Zhou, 2020, 3-D Printing of Concrete: Beyond Horizons, Cement and Concrete Research 133:106070, https://doi.org/10.1016/j.cemconres.2020.106070.

⁴ D. Avrutis, 2019, Industrial Adoption of 3D Concrete Printing in the Australian Market: Potentials and Challenges, pp. 389–409 in A. Nazari and J.G. Sanjayan (eds.), 3D Concrete Printing Technology: Construction and Building Applications, Oxford, UK: Butterworth-Heinemann, https://doi.org/10.1016/B978-0-12-815481-6.00019-1.

⁵ M. Moini, J. Olek, J.P. Youngblood, B. Magee, and P.D. Zavattieri, 2018, Additive Manufacturing and Performance of Architectured Cement-Based Materials, Advanced Materials 30(43):1802123, https://doi.org/10.1002/adma.201802123.

the printed elements.⁶ Many researchers continue to examine alternative materials for new opportunities.⁷

Olek stressed that the effective use of 3D printing in construction requires collaboration with experts from fields outside the traditional construction industry, such as materials science, rheology, chemistry, robotics, computer science, and more, bridging the gap between basic research and applied technology. Realizing the full benefits of 3D printing in the construction field will require scaling up of the technology, increased level of funding, and buy-in from the industry so that what is now a niche market can become a widely adopted practice, he said.⁸

PANEL DISCUSSION

In an open discussion period following panelists’ presentations, attendees touched on the importance of materials, performance-based standards, workflows, and site conditions.

The Importance of Materials

Panelists began with a discussion of the important role of materials in determining the feasibility and reliability of 3D-printed construction. Olek noted that concrete is very durable in most applications, but that the layering in 3D printing may impact structural integrity or reveal material or mixture flaws, which could be especially damaging in large-scale structures. Several methods are being tested to improve durability by preventing interface cracks, including incorporating known crack-controlling inclusions (e.g., fibers or superabsorbent polymers) to a structure’s advantage. Stynoski added that CERL has not seen interlayer bonding issues, but noted that other groups are testing the use of reinforcements between layers or adding fiber reinforcement in the mixes. He suggested that a metrology method for identifying layering issues during printing would be helpful. Post added that material shrinking can create cracking, and rebar can be susceptible to corrosion,

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⁶ R. Buswell, P. Kinnell, J. Xu, N. Hack, H. Kloft, J. Maboudi, M. Gerke, et al., 2020, Inspection Methods for 3D Concrete Printing (Version 1), Second RILEM International Conference on Concrete and Digital Fabrication, Loughborough University, https://hdl.handle.net/2134/12233735.v1.

⁷ A. Juan-Valdés, D. Rodríguez-Robles, J. García-González, M. Isabel Sánchez de Rojas Gómez, M. Ignacio Guerra-Romeroa, N. De Belie, and J.M. Morán-del Pozo, 2021, Mechanical and microstructural properties of recycled concretes mixed with ceramic recycled cement and secondary recycled aggregates: A viable option for future concrete, Construction and Building Materials 270:121455, https://doi.org/10.1016/j.conbuildmat.2020.121455.

⁸ R. de Laubier, M. Wunder, S. Witthöft, and C. Rothballer, 2018, Will 3D Printing Remodel the Construction Industry? BCG Global, January 23, https://www.bcg.com/publications/2018/will-3dprinting-remodel-construction-industry.

especially if the site is near seawater. Asked how grain size affects compaction, Stynoski replied that topology optimization targets the right materials for the right places in the structure in response to environmental stressors such as seawater, and in the case of concrete, vibration is typically used to reduce voids, but vibration would be difficult to apply to layer-by-layer additive construction processes.

John Koszewnik, Achates Power (retired), asked about materials variability, which was essential to improving quality in the automotive industry. Olek said that because many mixes are proprietary, there is a lack of available information on variability that can be applied to 3D printing. He pointed to the need to control weather-related or other site conditions that affect variability, not necessarily via strict codification, but perhaps through identifying strategies to minimize the impacts of variability. Jones added that the ability to measure material properties in-line can improve understanding of variability and adjust processes accordingly to minimize the impacts.

Kurfess asked how materials affect the energy efficiency of 3D-printed structures. Olek answered that researchers are examining multiple ways to enhance a building’s thermal performance, such as the ability to print a building’s insulation layer and exterior shell simultaneously. There is also interest in developing cementitious binders or printing methods with reduced CO₂ emissions.

Highlighting the relationship between materials design and construction, Jones said that implementing multiple materials systems to create blast resistance or thermal efficiency is an area of high interest that demonstrates the advantages of 3D printing, because multi-material, multi-component buildings are not possible with traditional concrete construction. Olek added that multi-functional materials can also create targeted material applications.

Creating Performance-Based Standards

Kurfess asked what policies or standards are needed to enable 3D printing for construction. Jones replied that NIST and other standards bodies are developing performance-based standards. He said that identifying performance levels and factors that influence structural safety will be key to this effort and noted that measuring performance instead of requiring certain materials or processing will help create appropriately flexible standards.

Building on this point, Kurfess suggested that embedding sensors during printing could be useful, such as to confirm building health after an earthquake. Stynoski added that new clauses in the Unified Facilities Criteria and Unified Facilities Guide Specifications, compatible with existing codes, could help printed structures meet performance-based criteria, but stressed that inspectors and professional engineers will need the right data to validate a structure properly. Jones added that it is crucial to understand how in situ measurements relate to material properties, processing procedures, and the resulting structural performance that is being

tested. Simulation techniques that incorporate the quirks inherent to 3D printing, perhaps via real-time data collection during construction, will also be a necessary step to performance-based standards, he said.

Olek, a member of the American Concrete Institute (ACI), added that ACI is working on performance-based standards and on identifying better tests to learn more about the fundamental properties of the materials. One potential avenue is to use the international design and quality control building codes with a codified evaluation process for materials and durability. A 3D-printed building that meets the criteria would signify that the process is safe. Developing these tests will take time, but considering the potential for failure, there has to be a proven way to verify performance. Modeling and simulation can also help by reducing the number of trials needed, he added. Jones, also an ACI member, confirmed that the organization is developing guidance for these issues, incorporating best practices, materials assessment, and other aspects. Kurfess noted that for any new material, standards development is, rightly, a slow and iterative process that is often outpaced by technology advancement.

Workflows

Angus Kingon, Brown University, asked what a remote printing workflow would look like and how close it was to being a reality. Stynoski answered that communication and training are crucial foundations. A typical ERDC workflow starts with site reconnaissance, sampling local materials, and slowly enlarging trials until the right mix of both buildable and pumpable concrete is identified for on-site mixing and printing. The goal, where service members run projects themselves via low logistics within the local economy, is close to reality, he said, but requires more industry-academic efforts. He also noted that other binding agents are being tested in case the cement supply worsens.

Site Conditions

Haydn Wadley, University of Virginia, asked how 3D-printing systems and buildings would fare in extreme climates like the Arctic. Stynoski replied that concrete is not always the best material for cold environments, and newer materials such as those using supercooled water are under study. Explosions can also affect durability, and Stynoski noted that printed structure response to blast and impact loads is an active area of research for DoD.

Wadley asked if the concrete structures’ visibly rough edges made them vulnerable to site conditions. Olek acknowledged that they were, and said that there are ways to smooth them or prevent their exposure, especially important in more extreme environments. Stynoski noted that his team has been able to address some of the problems posed by rough surfaces using conventional coating techniques.

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⁹ See NASA, “STMD: Centennial Challenges,” updated January 12, 2021, https://www.nasa.gov/directorates/spacetech/centennial_challenges/3DPHab/index.html.

prototype, bioSEA, is made of a consortia of wild microorganisms and grows slowly, a process that helps it mitigate failure and build strength over time, requiring less maintenance than conventional concrete. The material self-bonds outward from the center, drawing on the ample carbon and calcium in seawater to heal and strengthen itself. Native ecologies recognize it as a living substance. Dosier said BioSEA is now being tested to create a more efficient living shoreline.

In another DARPA project, BioMASON is creating lighter-weight concrete landing strips out of local materials in forward-operating bases where soil conditions and dust impact landing ability. While it currently requires scientific expertise to implement this process in the field, Dosier said that the company is working toward a streamlined process that would allow military personnel to build the landing strips with available military hardware and without specialized expertise.

Discussion

Zavattieri moderated a brief discussion following Dosier’s remarks. Asked about material lifespan and recyclability, Dosier said that bioLITH has a similar lifespan to mined limestone used in traditional concrete. He added that the ability for biological systems to disassemble material also has interesting applications that could be explored further.

With regard to key hurdles, Dosier said that using biotechnology to produce a well-known commodity is quite novel, and to become ubiquitous, the product must be engineered so that it can be mixed and used without any specialized expertise. There are also regulatory and supply chain challenges to replacing a widely used material like concrete, and getting across regulatory hurdles can take years. In addition to passing tremendous proof-of-performance testing, Dosier noted that a new material has to slot easily into existing supply chains and building processes.

Corrosion of reinforcement materials such as steel rebar is another important consideration. Dosier said that BioMASON engineers have found ways to minimize corrosion by tweaking the feedstock and processes for creating their bioLITH materials, but it remains an issue. Newer materials currently under development are at less risk of corrosion themselves, but could have corrosive effects on rebar, which is under study. Kingon asked if liquid sewage was a useful raw material, and Dosier answered that the bacteria content of sewage could be an asset, but it comes with handling challenges.

Wadley asked if BioMASON’s biological cement could be used across different aspects of the built environment. Dosier said that their binder works well across many different materials. He noted that biological systems can handle impurities better than chemical ones, which means that biotechnologies can be suitable for more varied material parameters.

concrete production emits high amounts of CO₂, with ~90 percent of these emissions attributable to OPC production. OPC is heavily used because it is economical and produces high-quality concrete, and the raw materials needed to manufacture it are abundant worldwide, but its production accounts for ~8 percent of annual global CO₂ emissions. Hun discussed efforts to minimize carbon emissions related to concrete production and use.

During OPC production, ~81 percent of the energy needed and ~86 percent of the CO₂ emitted occur at the kiln and preheater/precalcinator, which are necessary to produce Portland clinker, a major component of OPC.¹⁰ Worldwide efforts to identify Portland clinker substitutes or minimize the amount needed have been able to reduce the clinker-to-cement ratio, but more needs to be done.¹¹

Beyond production, transportation and amount of concrete used incurs costs and emissions as well. In collaboration with the Precast/Prestressed Concrete Institute (PCI) and the University of Tennessee, Chattanooga, ORNL researchers developed a method to produce precast insulated concrete panels with a 58 percent reduction in weight, saving transportation, erection, and structural costs. This involved developing a concrete mix with lower density, high early flexural strength, and self-consolidation. The new design reduces the clinker-to-cement ratio, uses less concrete, and is more likely to be adopted because of PCI’s involvement. Unfortunately, there could be availability constraints to some clinker substitutes, such as fly ash, and Hun said that more alternatives are needed.¹²

Worldwide approaches to reduce carbon in concrete include using alternative binders, adding CO₂ during concrete mixing, using CO₂ in concrete curing, and sequestering CO₂ while producing aggregate. Combining these approaches could attain carbon-negative concrete, Hun said.

ORNL developed a high-strength binder system for AM.¹³ Similar principles as were used in AM of strong silica sand structures enabled by polyethylenimine binders could be used to develop a polymer cement that could be potentially carbon-neutral and has superior mechanical properties compared with OPC. The

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¹⁰ T. Czigler, S. Reiter, P. Schulze, and K. Somers, 2020, Laying the foundation for zero-carbon cement, McKinsey & Company, https://www.mckinsey.com/industries/chemicals/our-insights/layingthe-foundation-for-zero-carbon-cement#.

¹¹ F. Preston and J. Lehne, 2018, Making Concrete Change: Innovation in Low-Carbon Cement and Concrete, London, UK: Chatham House, https://www.chathamhouse.org/2018/06/making-concretechange-innovation-low-carbon-cement-and-concrete.

¹² T.D. Kelly and G.R. Matos, 2014, Historical Statistics for Mineral and Material Commodities in the United States, U.S. Geological Survey, https://www.usgs.gov/centers/nmic/historical-statisticsmineral-and-material-commodities-united-states.

¹³ D.B. Gilmer, L. Han, M.L. Lehmann, D.H. Siddel, G. Yang, A.U. Chowdhury, B. Doughty, et al., 2021, Additive manufacturing of strong silica sand structures enabled by polyethylenimine binders, Nature Communications 12:5144.

team aims to enhance future adoption and commercialization by tailoring the novel cement to be aggregate-agnostic and easy to integrate into any ready-mix plant procedures.

During the discussion, Antonio Aldykiewicz, Hun’s collaborator at ORNL, commented that to use fly ash in concrete, it needs to meet or comply with ASTM standard C618. In addition, Tomonori Saito, also a collaborator, said that the team was researching which polymer binder was the most promising to be readily adopted by the concrete industry.

GENERAL DISCUSSION

In the day’s final discussion session, attendees covered issues related to sustainability, lifespan, workforce, and scaling up for novel concrete alternatives in 3D printing and beyond.

Sustainability

Kurfess asked what policies were needed to drive innovations and technology adoption to improve sustainability in construction. Dosier remarked that carbon taxes have been successful at motivating companies to create low-carbon technologies in Europe and Canada, but that implementing a similar approach in the United States would require more government support than currently exists. In terms of driving the use of recycled materials, Tian commented that Quikrete has mainly focused on industrial waste and plastics, because both are so plentiful.

Lifespan and Extreme Environments

Kurfess asked how the speakers address lifespan concerns for alternative concretes. Dosier commented that lifespan properties can be more easily predicted when producing a known material, while novel composite materials must be thoroughly tested for failures that would impact lifespan. Hun noted that there are established test standards to evaluate the durability of concrete.

Asked about extra-terrestrial applications, Dosier noted that he had previously worked with simulations to study binding with Martian and lunar regolith, and found the lack of easily accessible water a challenge.¹⁴

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¹⁴ Concrete, for example, is fabricated by mixing cement powder, some form of agglomerate (regolith), and large quantities of water.