Concrete: The Once & Future Liquid Stone

At an estimated rate of 5 billion cubic yards per year, concrete is the second most widely consumed substance on earth, surpassed only by water. For that reason, we tend to take it for granted. We generally assume that concrete has changed little since Classical Romans used it in the construction of architectural marvels that ranged from aqueducts to the Pantheon.But in reality, concrete is a dynamic technology that has evolved greatly over the millennia and continues to evolve today, as evidenced by “Liquid Stone: New Architecture in Concrete,” on display at the NationalBuildingMuseum through April 2006.The exhibition recognizes architects and engineers who have made innovative use of concrete’s versatility and strength over the years, and explores its future potential in light of three recent innovations: translucent, self-reinforcing, and self-consolidating concretes.“Liquid Stone is the latest exhibition in a series of shows we have done over the past six years on various building materials and their role in architecture and design and construction,” says Martin Moeller, the museum’s senior vice president for special projects and curator of the Liquid Stone exhibit. “In this case,” Moeller says, “we wanted to focus on contemporary architecture – very recent projects that use concrete in innovative ways – in order to help people understand that this is a material that has always been associated with experimentation and innovation from its earliest uses.” To take the exhibit one step further, he added, “We also wanted to talk about specific technologies that are now on the horizon – not really used very broadly in specific architectural applications, but that have the potential to rethink the fundamental way we think about buildings.” In some cases, the companies and people who developed these new products had existing designs and materials on hand for the museum to tap; in other cases, the museum worked proactively to produce samples specifically for the exhibition. That included commissioning a few architects to take on some of these new products and explore what their implications might be for the future of architectural design.“Translucent concrete was an obvious one from the start. Everyone reacted with equal excitement to that,” Moeller says. Three different varieties were included: Pixel Panels, LiTraCon (light transmitting concrete), and Translucent Panels.Bill Price, a professor at the University of Houston, developed Pixel Panels by embedding a fiber optic grid in concrete panels to carry light from one side to the other. Price first conceived of the idea of translucent concrete when he saw a model of a concert hall made using translucent materials, so that the model’s structure could be seen more easily. He started to wonder if the actual concert hall in question could be built so that it resembled the translucent model. For the exhibit, he used the panels to create a temple design he called the Pixel Chapel. He first rendered the simple space on a computer, then built a model to demonstrate what that space might look like by day, when sunlight squeezes in through tiny fibers in the concrete, and then at night, when artificial light from inside can be seen from the exterior.Another transparent concrete displayed in the Liquid Stone exhibit is called LiTraCon, an acronym for light transmitting concrete. Invented in 2001 by a Hungarian architect, Aron Losonczi, LiTraCon transmits light via thousands of embedded glass fibers that run parallel to each other. Shadows on the lighter side appear in sharp outline on the darker side, and even the colors survive, diminishing the sense of thickness and weight of a concrete wall. Because the fibers do not significantly reduce the compressive strength of the material, LiTraCon offers great potential for a variety of architectural applications.The third translucent concrete is the Translucent Panel, developed by Will Wittig, an assistant professor at the University of Detroit Mercy. Wittig’s process produces concrete panels that are one-tenth of an inch thick at their centers, thin enough to be translucent under direct light. His recipe calls for Portland cement and sand, reinforced with a small amount of chopped fiberglass. In addition to light, Liquid Stone explores innovations in the structural properties of concrete. A self-reinforcing concrete called Ductal is an ultra-high performance material manufactured by Lafarge North America. It contains organic or metallic fibers that make the finished product very dense and resistant to cracking and chipping. As a result, Ductal can be used to create very thin structural members without conventional steel reinforcement.Moeller says Ductal “fascinated me, largely because it defies several preconceptions of concrete.”“First of all, you think of concrete as being a rigid, hard, brittle substance,” Moeller says. “But Ductal – whose name itself is a play on the word ‘ductile’ – actually maintains a level of malleability. You can still bend this stuff even after it is fully set, and yet it doesn’t break because it is extremely rigid. It is so strong, in fact, that it can span great distances without any traditional steel reinforcement, which for 100 years has been a given of concrete construction.  It also has a finish that looks like Corian; it is absolutely smooth, seemingly perfect, and it doesn’t look like anything you’d expect from concrete.” Vic Perry, vice president and general manager of Lafarge’s North American Ductal division, observes, “It’s not exactly concrete; it’s not ceramic, metal or steel or anything else.”  And though Lafarge categorizes it as a high-performance, fiber-reinforced concrete,” Perry readily concedes, “It really is a new material.” Ductal has been deployed in a number of varying applications, including light-rail train stations, bridges, stairways and street furniture. It is also being used for architectural cladding, particularly retrofits, because it is so lightweight. “Architects are always trying to be creative in their solutions with slender, different-looking structures that are complex shapes, light, and low on maintenance,” Perry says. “When you present a material like this, you can mold into any shape and make it very thin because you don’t need the reinforcing steel in the structure. For instance, instead of making a shell four inches thick, with Ductal we can make it three-quarters of an inch thick and curve it into complex shapes. Once it’s finished, there’s no corrosion, and requires very little maintenance.” As for its looks, Perry adds that it is elastic enough to be cast in a mold to control surface texture. In addition, he says, “You can easily color it, plus it is fluid and self-placing like self-consolidating concrete. It has this unique combination of superior technical characteristics that makes it unlike anything else.” Of all the cutting-edge technologies on display in Liquid Stone, Moeller said, self-consolidating concrete was the trickiest to demonstrate. “I thought about that one long and hard,” he says, “because it’s a difficult concept for people to understand quickly, this idea of a substance that maintains great flowability, yet actually hardens to a similar strength and durability that you would expect from normal high-strength concrete.  I wanted to pursue it nonetheless, because I think it’s going to become increasingly important and increasingly common.” Self-consolidating concrete is composed of optimized aggregates, cements, and admixtures, including superplasticizer, which keeps the mix highly fluid during the pouring process without compromising the cured material’s ultimate strength. It requires no vibration, and can therefore be used for difficult or constrained pours, such as those involving unusually dense reinforcing steel or narrow channels through which the concrete must flow. It can also be poured into intricate molds to produce finished concrete with a very fine surface texture, and it requires little or no rubbing or patching. Such advantages not only improve a finished appearance; they can also reduce costs for the contractor. “Self-consolidating concrete allows contractors to do what they’ve been trying to do for years,” says Jack Holley, vice president of QA & New Product Development at Lafarge North America, which produces a self-consolidating product called Agilia. “With Agilia, placements of very large volumes can require two people, whereas the same pour for traditional concrete would require eight to ten people with vibrators and shovels and rakes.”  Holley said it was not just the latest generation of superplasticizers that gave the product its unique properties. “The selection and characterization of all the raw materials are critical, so they’re all compatible,” he says. “If they’re not compatible, you have to find ways to offset the incompatibility of either the mechanical gradings, the chemistries of the cements interacting with the plasticizers, or the whole matrix.” Currently, the most popular applications for self-consolidating concrete are in heavily reinforced sections of both new and rehabilitated structures, as well as for architectural surfaces requiring superior finishes. By one estimate, self-consolidating concrete will one day have 50% of the overall concrete market share. That day is a ways off though. The cost is still about 25% higher than standard mixes, and although the strength is similar, the industry is faced with the challenge of convincing design engineers and architects.“What’s interesting about the exhibit throughout,” says Moeller, “and not just in the ‘Future of Concrete’ section, is that architects have found ways to use concrete in these incredibly inventive forms and surfaces – things that really defy most people’s conception of the material. In almost every respect, they are doing things that are changing our attitudes about a material that is very nearly taken for granted.” Choosing which new technologies to include was one of the most difficult parts, he acknowledges. “In this case I tried to pick the ones where there were particularly interesting implications, ones that I thought could get architects excited about experimenting.”