AFTER BUS DRIVERS in the southern Paris area of Thais drop off their last passengers, they return to a central depot that appears at first glace to be the coolest Lego fort that can exist outside of a child's imagination. This new regional bus center, designed by local firm Emmanuel Combarel Dominique Marrec Architectes, is clad with gray cast-concrete panels with a pattern of raised circles that resemble the knobs on the famous toy blocks. The panels are a near-identical color match (and in fact a deliberate homage to) the surrounding pavement, from which they rise in an inward curve. They then arc over the roofline, creating a 35-squaremeter, two-story volume with no right angles on its perimeter. The architects introduce vividly colored glazing in cutout sections, allowing light to filter deep into the building.
These glazed cutouts also reveal the secret of the building's concrete skin: the custom-designed panels' total thickness is a svelte 3 cm (less than 1¼ inches), and there is no additional backing, meaning that these panels are structural in and of themselves. Ideal for maximizing the interior space on a small site, the nominal width of these panels allows the square footage that normally would have been taken up by façade systems to be turned into usable space.
The science of being thin The material used to make the panels is Ductal, a fiber-reinforced concrete made by French concrete company La Farge. Whereas traditional concrete requires a system of metal rebar to be structurally stable, Ductal uses tiny fibers to give the necessary tension. These fibers can be steel or organic, depending on the end-use application of the material, and they typically constitute 2 to 4 percent of the overall concrete mixture. In most other commercially available fiber reinforced concretes, fibers make up 1 percent of the mix. The higher concentration of fibers in Ductal means a much higher tensile strength, allowing it to be used in applications that call for extremely thin profiles or complex forms.
According to Vic Perry, vice president and general manager of Ductal for La Farge North America, the tensile strength of the material comes down to the ratio of aggregate to structure and the distribution of those structural fibers in the mix. In Ductal, the ratio of aggregate to fiber is the same as that of standard concrete to rebar, just on a much smaller scale. The Ductal aggregate is very fine sand, with grains averaging 400 microns in diameter. “All [of the ingredients are] commercially available materials,” says Perry. “There's no magic dust or anything.” The self-consolidating material has a very smooth finish, and can be sanded or polished to a shine—or like the Thais bus center panels, molded to achieve fine patterns. Mixed in batches using the standard machinery, when wet Ductal looks, if anything, thinner than the traditional material. A smooth consistency and high ductility allow the material to be easily poured.
On the cutting edge and just catching up Introduced in 1997, Ductal is not a new product by any sense of the word. In Europe, the material is used regularly to create complex curving forms and skins. The Thais bus center, for instance, was not the architect's first foray into Ductal skin, nor was it for La Farge, which does a lot of cladding business in Europe.
Even in Canada, Ductal has been used in projects ranging from sunshades and roofs to pedestrian bridges. An undulating canopy at an LRT transit station was completed in Calgary in 2003. But in the United States, use of this material has not caught on as widely.
This has partly to do with codes, and partly to do with the unique fabrication requirements associated with Ductal. The material itself can achieve complex forms readily, but to do so, complex formwork is required. “You need to have fabricators who are willing to take on a challenge,” says Perry. Another challenge: Thin Ductal panels are too thin to be acceptable under existing U.S. codes for safe structural concrete. “We are working to get codes rewritten so that architects can design with Ductal and still be compliant,” says Perry.
And some of the struggles with Ductal are because the wonder concrete cannot always deliver on its long list of promises. Guy Nordenson, founder of Guy Nordenson and Associates Structural Engineers in New York, is using the material for the floor panels of two pedestrian bridges at Yale University. “It is a great material,” he says, “but because it is proprietary, there are limits to the ease of procurement. Many claims are made, but come crunch time, not all come through.” Still, Nordenson intends to learn from the challenges and use Ductal again.
In the domestic market, La Farge has worked with the Federal Highway Administration to build stronger and more durable highway bridges using Ductal. And the material works well in the context of boutique urban furniture and accessories such as sinks and countertops because the lack of rebar allows for complex shapes.
The company hopes to see the product catch on more in the U.S.—and not just for monetary reasons, according to Perry. “We work with a lot of architects, and a lot of them are early adopters,” he says. “But they are restricted in the projects they are working on. We would like to see more projects with interestingly shaped shells and canopies.”