“Is that concrete all around, or is it in my head?” asked Ian Hunter in “All the Young Dudes,” the song David Bowie wrote for Mott the Hoople in 1972. Concrete is all around us, and we haven’t quite wrapped our heads around it. It’s one of the indispensable materials of modernity; as we try to decarbonize the built environment, it’s part of the problem, and innovations in its composition may become part of the solution. Understanding its history more clearly, the Skyscraper Museum’s new exhibition in Manhattan implies, just might help us employ it better. Concrete is “the second most used substance in the world, after water,” the museum’s founder/director/curator Carol Willis told AN during a recent visit. For plasticity, versatility, and compressive strength, reinforced concrete is hard to beat, though its performance is more problematic when assessed by the metric of embodied and operational carbon, a consideration the exhibition acknowledges up front. In tall construction, concrete has become nearly hegemonic, yet its central role, contend Willis and co-curator Thomas Leslie, formerly of Foster + Partners and now a professor at the University of Illinois, Urbana-Champaign, is underrecognized by the public and by mainstream architectural history. The current exhibition aims to change that perception. The Skyscraper Museum in Lower Manhattan features an exhibition, The Modern Concrete Skyscraper, which examines the history of material choices in building tall towers. (Courtesy the Skyscraper Museum) The Modern Concrete Skyscraper examines the history of tall towers’ structural material choices, describing a transition from the early dominance of steel frames to the contemporary condition, in which most large buildings rely on concrete. This change did not happen instantly or for any single reason but through a combination of technical and economic factors, including innovations by various specialists, well-recognized and otherwise; the availability of high-quality limestone deposits near Chicago; and the differential development of materials industries in nations whose architecture grew prominent in recent decades. As supertalls reach ever higher—in the global race for official height rankings by the Council on Tall Buildings and Urban Habitat (CTBUH) and national, corporate, or professional bragging rights—concrete’s dominance may not be permanent in that sector, given the challenge of pumping the material beyond a certain height. (The 2,717-foot Burj Khalifa, formerly Burj Dubai, uses concrete up to 1,987 and steel above that point; Willis quotes SOM’s William Baker describing it as “the tallest steel building with a concrete foundation of 156 stories.”) For the moment, however, concrete is ahead of its chief competitors, steel and (on a smaller scale) timber. Regardless of possible promotional inferences, Willis said, “we did not work with the industry in any way for this exhibition.” “The invention of steel and the grid of steel and the skeleton frame is only the first chapter of the history of the skyscraper,” Willis explained. “The second chapter, and the one that we’re in now, is concrete. Surprisingly, no one had ever told that story of the skyscraper today with a continuous narrative.” The exhibition traces the use of concrete back to the ancient Roman combination of aggregate and pozzolana—the chemical formula for which was “largely lost with the fall of the Roman Empire,” though some Byzantine and medieval structures approximated it. From there, the show explores comparable materials’ revival in 18th-century England, the patenting of Portland cement by Leeds builder Joseph Aspdin in 1824, the proof-of-concept concrete house by François Coignet in 1856, and the pivotal development of rebar in the mid-19th century, with overdue attention to Ernest Ransome’s 1903 Ingalls Building in Cincinnati, then the world’s tallest concrete building at 15 stories and arguably the first concrete skyscraper. The exhibition includes a timeline that depicts concrete’s origins in Rome to its contemporary use in skyscraper construction. (Courtesy the Skyscraper Museum) Baker’s lectures, Willis reported, sometimes pose a deceptively simple question: “‘What is a skyscraper?’ In 1974, when the World Trade Center and Sears Tower are just finished, you would say it’s a very tall building that is built of steel, an office building in North America. But if you ask that same question today, the answer is: It’s a building that is mixed-use, constructed of concrete, and [located] in Asia or the Middle East.” The exhibition organizes the history of concrete towers by eras of engineering innovation, devoting special attention to the 19th- and early-20th-century “patent era” of Claude Allen Porter Turner (pioneer in flat-slab flooring and mushroom columns) and Henry Chandlee Turner (founder of Turner Construction), Ransome (who patented twisted-iron rebar), and François Hennebique (known for the re-inforced concrete system exemplified by Liverpool’s Royal Liver Building, the world’s tallest concrete office building when completed in 1911). In the postwar era, “concrete comes out onto the surface [as] both a structural material and aesthetic.” Brutalism, perhaps to some observers’ surprise, “does not figure very large in high-rise design,” Willis said, except for Paul Rudolph’s Tracey Towers in the Bronx. The exhibition, however, devotes considerable attention to the work of Pier Luigi Nervi, Bertrand Goldberg (particularly Marina City), and SOM’s Fazlur Khan, pioneer of the structural tube system in the 1960s and 1970s—followed by the postmodernist 1980s, when concrete could express either engineering values or ornamentation. The exhibition highlights a number of concrete towers, including Paul Rudolph’s Tracey Towers in the Bronx. (Courtesy the Skyscraper Museum) “In the ’90s, there were material advances in engineering analysis and computerization that helped to predict performance, and so buildings can get taller and taller,” Willis said. The current era, if one looks to CTBUH rankings, is dominated by the supertalls seen in Dubai, Shanghai, and Kuala Lumpur, after the Petronas Towers (1998) “took the title of world’s tallest building from North America for the first time and traumatized everybody about that.” The previous record holder, Chicago’s Sears (now Willis) Tower, comprised steel structural tubes on concrete caissons; with Petronas, headquarters of Malaysia’s national petroleum company of that name, a strong concrete industry was represented but a strong national steel industry was lacking, and as Willis frequently says, form follows finances. In any event, by the ’90s concrete was already becoming the standard material for supertalls, particularly on soft-soiled sites like Shanghai, where its water resistance and compressive strength are well suited to foundation construction. Its plasticity is also well suited to complex forms like the triangular Burj, Kuala Lumpur’s Merdeka 118, and (if eventually completed) the even taller Jeddah Tower, designed to “confuse the wind,” shed vortices, and manage wind forces. Posing the same question Louis Kahn asked about the intentions of a brick, Willis said, with concrete “the answer is: anything you want.” The exhibition is front-loaded with scholarly material, presenting eight succinct yet informative wall texts on the timeline of concrete construction. The explanatory material is accompanied by ample photographs as well as structural models on loan from SOM, Pelli Clarke & Partners, and other firms. Some materials are repurposed from the museum’s previous shows, particularly Supertall! (2011–12) and Sky High and the Logic of Luxury (2013–14). The models allow close examination of the Burj Khalifa, Petronas Towers, Jin Mao Tower, Merdeka 118, and others, including two unbuilt Chicago projects that would have exceeded 2,000 feet: the Miglin-Beitler Skyneedle (Cesar Pelli/Thornton Tomasetti) and 7 South Dearborn (SOM). The Burj, Willis noted, was all structure and no facade for a time: When its curtain-wall manufacturer, Schmidlin, went bankrupt in 2006, it “ended up going to 100 stories without having a stitch of glass on it,” temporarily becoming a “1:1 scale model of the structural system up to 100 stories.” Its prominence justifies its appearance here in two models, including one from RWDI’s wind-tunnel studies. Eero Saarinen’s only skyscraper, built for CBS in 1965 and also known as “Black Rock,” under construction in New York City. (Courtesy Eero Saarinen Collection, Manuscripts, and Archives, Yale University Library) The exhibition opened in March, with plans to stay up at least through October (Willis prefers to keep the date flexible), with accompanying lectures and panels to be announced on the museum’s website (skyscraper.org). Though the exhibition’s full textual and graphic content is available online, the physical models alone are worth a trip to the Battery Park City headquarters. Intriguing questions arise from the exhibition without easy answers, setting the table for lively discussion and debate. One is whether the patenting of innovations like Ransome bar and the Système Hennebique incentivized technological progress or hindered useful technology transfer. Willis speculated, “Did the fact that there were inventions and patents mean that competition was discouraged, that the competition was only in the realm of business, rather than advancing the material?” A critical question is whether research into the chemistry of concrete, including MIT’s 2023 report on the self-healing properties of Roman pozzolana and proliferating claims about “green concrete” using alternatives to Portland cement, can lead to new types of the material with improved durability and lower emissions footprints. This exhibition provides a firm foundation in concrete’s fascinating history, opening space for informed speculation about its future. Bill Millard is a regular contributor to AN.

Molecular Rebar Design, a nanomaterials company based in Austin, Texas, has patented a new additive manufacturing (AM) composition that utilizes oxidized discrete carbon nanotubes (CNTs) with bonded dispersing agents to enhance 3D printing resins. The patent, published under US20210237509A1, outlines methods to improve resin properties for applications such as vat photopolymerization, sintering, and thermoplastic fusion. The inventors, Clive P. Bosnyak, Kurt W. Swogger, Steven Lowder, and Olga Ivanova, propose formulations that improve electrical conductivity, thermal stability, and mechanical strength, while overcoming dispersion challenges common with CNTs in composite materials. Image shows a schematic diagram of functionalized carbon nanotubes. Image via Molecular Rebar Design. Functionalized CNTs for additive manufacturing At the core of the invention is the chemical functionalization of CNTs with dispersing agents bonded to their sidewalls, enabling higher aspect ratios and more homogeneous dispersions. These dispersions integrate into UV-curable acrylates, thermoplastics, and elastomers, yielding improved green strength, sinterability, and faster cure rates. The patent emphasizes the benefit of using bimodal or trimodal distributions of CNT diameters (single-, double-, or multi-wall) to tune material performance. Additional fillers such as carbon black, silica, and metallic powders can also be incorporated for applications ranging from electronic encapsulation to impact-resistant parts. Experimental validation To validate the invention, the applicants oxidized carbon nanotubes using nitric acid and covalently bonded them with polyether dispersing agents such as Jeffamine M2005. These modified CNTs were incorporated into photopolymer resin formulations. In tensile testing, specimens produced with the dispersions demonstrated enhanced mechanical performance, with yield strengths exceeding 50 MPa and Young’s modulus values above 2.8 GPa. Impact strength improved by up to 90% in certain formulations compared to control samples without CNTs. These performance gains suggest suitability for applications demanding high strength-to-weight ratios, such as aerospace, electronics, and structural components. Nanotube innovations in AM Carbon nanotubes (CNTs) have long been explored for additive manufacturing (AM) due to their exceptional mechanical and electrical properties. However, challenges such as poor dispersion and inconsistent aspect ratios have hindered their widespread adoption in AM processes. Recent advancements aim to overcome these barriers by integrating oxidation and dispersion techniques into scalable production methods. For instance, researchers at Rice University have developed a novel acid-based solvent that prevents the common “spaghetti effect” of CNTs tangling together. This innovation simplifies the processing of CNTs, potentially enabling their scale-up for industrial 3D printing applications. Similarly, a research team led by the University of Glasgow has created a 3D printable CNT-based plastic material capable of sensing its own structural health. This material, inspired by natural porous structures, offers enhanced toughness and strength, with potential applications in medicine, prosthetics, automotive, and aerospace design. Subscribe to the 3D Printing Industry newsletter to keep up with the latest 3D printing news. You can also follow us onLinkedIn and subscribe to the 3D Printing Industry YouTube channel to access more exclusive content. At 3DPI, our mission is to deliver high-quality journalism, technical insight, and industry intelligence to professionals across the AM ecosystem.Help us shape the future of 3D printing industry news with our2025 reader survey. Feature image shows schematic diagram of functionalized carbon nanotubes. Image via Molecular Rebar Design.

Every couple of days in early 2024, Leonardo Hummel would free-dive into the shallow waters surrounding Koh Tao, Thailand. Amidst the growing communities of reef fish, Hummel liked to document the progress of the first artificial reefs he'd created and deposited two years earlier while at nearby New Heaven Dive School. The original 9mm rebar had grown multiple times in size, with the accretion of calcium carbonate.Sandra Rubio, one of Leo's colleagues at Black Turtle Dive, remembers the passion with which Leo would speak about his work, and the pride that Hummel took in his creations. "They were like his babies," she says. From an early age, growing up in the Pacific Northwest, Hummel found himself perpetually fascinated by the natural world, origami, and art. These fascinations drove him across the globe - from growing up singing with Seattle's Northwest Boychoir and Vocalpoint! Seattle, to a B.A. in East Asian Studies at Carleton College (and post-college years teaching in Beijing), to a master's degree in Industrial Design at Georgia Tech. Hummel found himself in Koh Tao in pursuit of a dream that managed to fuse all of these passions, crafting beautiful artificial reefs in a project that he dubbed "SeaWeaver." These few handfuls of woven reefs still sitting off the coast of Koh Tao, however, now also serve as a gentle memorial; in March of last year, Leo Hummel, 34, passed away. His parents, professors, and colleagues kindly offered their memories of Leo for this article, in the hopes that it might help keep his work alive.—Hummel detailed his project in a paper, "SeaWeaver: Integrating Cultural Craft and Materials Innovation for Artificial Reef Conservation Strategies," which debuted at the Design Research Conference (DRS) in Boston, in June 2024. Leo's former professor, Georgia State's Lisa Marks, presented the research in his stead."Leo was the kind of person that, if something interested him, he would be in school 24 hours a day, ignoring all his other classes just to do that," Marks says. "I'd come back from Christmas break, and my lab was just covered in these insane laser-cut patterns. And I'd say, what's all this? But that was just his brain."Hummel first met Marks after taking her course on the intersection of industrial design and folk craft. Marks, in her work, combines parametric modeling with endangered traditional handcrafts (a field overwhelmingly derived from women's labor, and largely unconsidered in male-dominated industrial design). Leo soon became Mark's research assistant, and she his thesis advisor. Leo went on to complete his thesis on origami that possesses thickness - such as solar-powered marine lights that could fold and sink beneath the ocean waves, to shelter from coming storms."Because he would obsessively go into these deep dives, he would get burnt out," Marks recalls. "So I'd say: 'Leo. Take three days. Go do some other experiment.' And one of those was this hexagonal structure that theoretically could make kelp farms. And that little pet project got University of Washington interested." In June 2020, the University of Washington and the Nippon Foundation launched the "Ocean Nexus Center," whose stated mission is "to establish social equity at the center of ocean governance;" Leo's mother, who worked at the University, put him in contact with them.Ocean Nexus, as it turned out, suddenly had a travel budget going largely unused due to the COVID-19 pandemic, and could repurpose some of those funds for Leo to develop his work. Amidst a largely academic consortium of anthropologists and economists who study the complexities of human behavior, Hummel's work in fabrication served as a beloved addition.All along the West Coast, forests of kelp - "the sequoia of the sea," both organism and habitat - once grew in underwater canopies so tall and thick that they could be seen from space. But along with another underwater vegetation - eelgrass, a flowering marine plant in more tidal regions - these vast underwater landscapes have seen their once-Olympian numbers decline by as much as ninety percent in just the past few decades. Researchers at University of Washington hoped Hummel's structures of woven hemp could provide a means to anchor the vegetation and allow them a chance to regenerate.Leo's parents, Jeff Hummel and Beatrice Gandara, can't recall exactly how Leo's fascination turned to coral. But in 2021, Leo traveled to Koh Tao to take diving classes with New Heaven Reef Conservation. After countless hours bending rebar in his parents' basement, New Heaven helped him deploy his first full-sized design in 2022. Hummel hoped the reef's shape - a hyberboloid - provided an ideal combination: structural stability, interlacing strands that rendered it a single conductive object, and a woven structure that could be easily modified to accommodate different forms of marine wildlife."Because it was algorithmically driven, he could theoretically change these structures to have the openings be bigger or smaller," Marks explains. By customizing these openings to the local biome, the reef "would attract different types of fish and wildlife that need a certain amount of space, or hiding, or nesting."Although Leo started his deployments at New Heaven, he would later forge a relationship with another local diving organization, Black Turtle, which invited him to weave his reefs and teach classes on their construction in the beginning of 2024. His colleague, Sandra Rubio, described him as an amazing teacher whose passion inspired his students."Whenever we work in coral restoration, one of our main problems is getting the tools we need," Rubio explains. "Sometimes we have to weld, or cut metal, or cast concrete. And we don't really know how to do all these things! So he created this specific design for us to simplify this process, and to be able to create really complex structures without spending a lot of time or having a lot of knowledge about it."Much of Hummel's original design files, in Rhino or Grasshopper, remain on his as-yet-inaccessible computer at his parents' house. But Hummel would also document his creations with posts to his Instagram, @seaweaverreefs, which allowed him to add his own geometric color commentary."It broke my heart to do it but I have mostly switched from 12-symmetry to 10-symmetry weaves," he once explained in a post from January 2024, alongside a photo of a thin metal Star of Goliath nestled inside a decagram. "For a variety of reasons, but mostly because when working at scale, that change ends up saving a lot of material. The structure's stability is still many times overkill so that's unaffected, and only total weirdos who count rotational symmetry will even notice. Good thing I don't know any of those ??."One of the driving forces behind Hummel's dedication to SeaWeaver, meanwhile, was not just to perfect its design, but to ensure its accessibility to the low-resource coastal communities most at risk. Compared to other established artificial reef companies that have patented their designs, Hummel's designs could be woven by anyone, in just a few hours, and with everyday construction material."A lot of motivation for the paper [presented at DRS]," his father says, "was to leave a trail that made it clear that this was his intellectual property, and not something that could be patented in some predatory manner." In the wake of Hummel's passing, his parents maintained his online presence so that future researchers could learn from his work - which they described as "more than a technique: it's a philosophy of ecological intention and social equity." But amongst those who remember Leo, one of the most consistent themes was not his capacity to weave beautiful patterns, but the genuinely decent nature of his character."It's really rare to meet someone as talented and, for lack of a better word, almost obsessive [as Leo]. You meet people that do these deep dives into their work, and they're oftentimes not the best people in terms of how they treat other folks," Marks recalls. "But Leo was just a really, really good person."Last year, while in town for his memorial, Hummel's college a cappella group made sure to visit the Seattle Aquarium. There, unmarked, is one of Hummel's original experiments from three years prior: a nondescript patch of eelgrass, anchored by a weave of hemp. It's the only bundle of eelgrass that has survived, for years, in the entire aquarium."I had the feeling that Leo is like these artists that, when they die, their work gains value," Rubio says. "Sometimes, when someone passes away, they become a legend."Learn more about Leo and Seaweaver at Leohummel.com