Ultra-high performance, fiber reinforced concrete is changing the definition of concrete.
By Michael A. Chusid, R.A. FCSI
Progress is born in dreams. Those who can imagine the world improved draw the map to the future. Highway engineers dream of concrete bridges with no steel reinforcement bars to corrode and fail. Architects envision billowing, vaulted roofs that appear weightless, yet can bear the loads of wind and snow. Industrial designers fantasize about corrosion-resistant vessels that are light, strong and at a fraction of the cost of conventional materials. Artists and decorative designers dream of rich colors and sensual finishes that will endure the wear-and-tear of daily living, the ravages of time and exposure to the elements.
Lately, these dreams are becoming concrete reality.
The limitations that have defined our concept of concrete for the past century are now breaking down as designers, contractors and precasters begin to explore the potential of Ultra-High Performance, Fiber Reinforced Concrete (UHPFRC) manufactured by Lafarge under the trade name Ductal. According to LaFarge it has five to 10 times the compressive strength of conventional concrete – up to 30,000 psi – at virtually the same weight. More importantly, it has sufficient tensile strength to be used as an engineering feature, delivering flexural strengths up to 6,000 psi without steel reinforcing bars. (Conventional concrete without rebar has virtually no flexural strength.) This is closely related to the material’s ability to bend (within limits) without breaking – a property called “ductility” that inspired its name. Unlike conventional concrete, Ductal will deflect as it reaches its load limit, providing an important margin of safety.
It is also durable, impermeable, corrosion resistant, stain resistant, and so free-flowing and self-leveling that it can be placed without vibration or other forms of consolidation. It is so finely textured it will perfectly replicate any casting surface, even a mirror finish. It is concrete that achieves things “you can’t do with concrete.” It is concrete to dream with.
Ductal products contain many of the same materials as ordinary concrete – portland cement, silica fume, fine aggregates and superplasticizers – but each ingredient is tweaked to supercharge their collective performance. Fine aggregates are supplemented with very fine silica particles in the form of ground quartz that fill gaps between the other materials to increase density and cementation. To increase its strength and resiliency even further, fibers are blended into the mix: high carbon metallic fibers for structural applications or poly-vinyl alcohol (PVA) fibers for architectural uses. The type of fiber and the Ductal formulation are customized to best fit a project’s requirements. When selecting the type of fiber to be used, determining factors include strength and rheology requirements, exposure to corrosive agents, desired aesthetics and exposure to human contact.
The new material renders obsolete most preconceptions of what concrete can and cannot do. For technological dreams to become realities, however, the existence of a new technology is not enough. Pioneering practitioners must have the courage and imagination to break through the barriers of conventional thinking and explore new potentials.
In rural Iowa, there is a bridge that leads to highway engineers’ dreams. The uninformed eye might not notice anything unusual, but the Mars Hill Bridge in Wapello County is actually the first UHPFRC highway bridge in North America. It is a single span, 110 feet long, supported by three precast prestressed UHPFRC girders that contain no steel rebar stirrups. It is a first step toward the goal of building a bridge with no rebar at all.
The U.S. Federal Highway Administration (FHWA) has been researching UHPFRC for several years. “New and innovative materials that can make our infrastructure last longer and allow us to build things faster are of large interest to the nation,” says Benjamin Graybeal, Ph.D., P.E., research structural engineer with FHWA. The FHWA was ready to provide information, support and funding, but they were waiting for someone to step up to the challenge.
In a state with heavy snowfalls and literally tons of corrosive deicing salt on the roads, a common goal of forward-thinking bridge designers is to eliminate rebar. “Regular concrete, even when designed properly, cracks,” says Dean Bierwagen, MSCE, a methods engineer for the Office of Bridges and Structures, Iowa Department of Transportation (IDOT). “Then salt gets in and the rebar rusts out. Ordinary concrete decks tend to fall apart.” Bierwagen wanted a more durable bridge; Ductal’s density and impermeability would minimize moisture intrusion, and its strength and superior freeze/thaw characteristics would make it more resistant to cracking. Salt wouldn’t hurt it at all, he reasoned, because the salt won’t get into it.
No one in the U.S. had ever built such a bridge, but IDOT was ready to learn about using the material. They needed a ‘beginner’ project.
Wapello County engineer Brian Moore, P.E., was intrigued, and he had a bridge that needed replacing. His vision revolved around a material so strong that it would need no steel rebar and therefore have nothing to corrode. With Iowa State University and IDOT, he applied to the FHWA’s “Innovative Bridge Research and Construction (IBRC)” Program and received preliminary funding to cover the extra costs involved in building an innovative project.
The width of the girders is reduced, too, making the web 30 percent narrower than the standard “ Iowa 45 inch bulb T” beam. The girders each have 49 prestressing tendons but no rebar stirrups to resist shear, relying entirely on the strength of the fiber-reinforced material.
In August 2005, the three 110 foot girders were cast by Lafarge Canada Inc. in Winnipeg and shipped to the Iowa site in September. The bridge was completed and opened to traffic in February 2006 and has since won a Portland Cement Association 2006 Concrete Bridge Award.
The structure in Wapello County not only links two pieces of road over a creek, it bridges the past and future of concrete bridge construction, a first step toward the ultimate dream. “We would like to cast decks integrally with girders at the precast plant,” says Graybeal. “The entire bridge would be modular components, which we could then assemble on site. Normal concrete would be too heavy. You can use less UHPFRC because it is stronger and more durable. We could replace a bridge very quickly and have traffic on it sooner.”
In fact, Iowa’s Buchanan County is currently designing a modular UHPFRC bridge. According to Buchanan County engineer Brian Keierleber, there will be modules of a modified pi-beam construction, 8 feet by 50 feet with girders and deck integrated into a single precast unit.
The design is evolving. “The deck at this time is roughly 3 inches thick,” Keierleber says, “but I think we have to do a little more stiffening on that. I’m a conservative engineer.” The beam-and-deck is just under 33 inches deep and has prestressing cables in the bulbs of the beams, but the only other steel is the fibers. They hope to have a bridge in place by summer 2008.
Ductal already has Keierleber fantasizing about the possibilities. “Look at skyscraper construction, for example. You can make your columns thinner, yielding more usable room. Think of earthquake design issues: this concrete is not brittle, it’s ductile. Technology is changing,” he says.
Change in the Air
Change is clearly manifest in the gravity-defying canopies of the Shawnessy Light Rail Station in Calgary, Alberta, an intersection of solid structure and architect’s fantasy. The functional intent was to provide waiting passengers with lighting and shelter, but the owner – the City of Calgary – and the community wanted to make a positive architectural contribution, too. Together with the designer, Calgary-based Stantec Architecture Ltd., they agreed on a light, airy concept with tree-like forms that complement the surrounding neighborhood.
Despite the desired visual effect, the canopies needed to be able to withstand heavy snow loads and strong winds. The ultra-thin, arched shells were originally designed in steel, although the rest of the station’s superstructure was to be precast in conventional and high-performance concrete.
Precaster Lafarge Canada Inc. of Calgary, Alberta, suggested that it would be possible to achieve the light, floating effect in concrete using the ductility and high strength of Ductal. At 16 feet long and spanning 20 feet between columns, they are a mere ¾-inch thick. Nonetheless, under testing, they exceeded design loading by a comfortable margin. The use of Ductal also improved the durability and aesthetics in comparison with steel.
The effortless appearance of the canopies belies extraordinary efforts made to create and install them. The precaster instituted an internal R&D program to determine the best methods of casting such complex geometrical shapes. They built a rotatable steel form to facilitate casting in one orientation and curing in another. They discovered that the location of injection ports in the mold influenced orientation of steel fibers during casting and could be strategically maximized for strength. They also fabricated special handling frames to transport and erect the shells. The station opened in 2004.
Their process demonstrates how new possibilities require new thinking and how architects, precasters and contractors can rise to an exciting challenge. Their efforts have been acknowledged with numerous honors within the concrete industry.
Sometimes the dream is simply finding a more cost-effective solution to a known problem. The creativity of it is to envision this new material performing in a way that most people presume concrete can’t.
“I think Ductal has tremendous potential as a stainless steel replacement where corrosion-resistance is a factor,” says Don Zakariasen, director of Marketing-Concrete Products, Lafarge North America. As an example, Zakariasen points to the City of Edmonton’s Gold Bar Wastewater Treatment Plant.
Lafarge was involved in construction of the plant’s overall structure and heard there was a supply problem for stainless steel effluent troughs, a key element of the treatment chain. “There was a steel shortage,” Zakariasen says. “Availability involved a long wait, and the price had jumped substantially. We met with the consultant and suggested using Ductal.”
The troughs, 2 feet deep by 2 feet wide by 20 feet long, have to be light enough not to damage the thin stainless steel “plate settlers” directly underneath them, yet strong enough to withstand fluid pressure and very resistant to corrosion. “The sewage is pretty aggressive,” comments Zakariasen.
The troughs were cast in Ductal with steel fibers, with a wall thickness of only 11/16 inch. The dense, impermeable finish resists water infiltration and contamination, and the smooth, durable surface remains easy to clean.
“There was a very strong price advantage to using Ductal,” Zakariasen points out. “The troughs ended up being half the price of stainless steel. Even without the steel shortage, Ductal would still have a price advantage.”
Zakariasen sees many possibilities for Ductal in uses where concrete would never previously have been considered. While granite countertops tend to absorb stains due to the nature of stone being highly porous, high-density Ductal concrete can repel those same compounds that discolor granite surfaces.
Form Beautifies Function
For some dreamers, the potential of UHPFRC is in the realm of a sensual, emotional experience. Solus Decor, a manufacturer of decorative concrete products with operations in Vancouver, British Columbia, and Calgary, Alberta., has recently adopted Ductal for a smooth, lustrous finish in a lightweight material that can stand up to the loads of living.
“In terms of the finish and the effect,” says Solus partner Brad Carpenter, “it’s quite spectacular. We can get very deep, beautiful, organic colors, and we can cast very, very large panels, very thin.”
Solus hand polishes its products to yield an appearance comparable to stone. The brand name of their first line of Ductal products, “Litho” vases and planters, immediately underscores the stone connection. Even their largest planter, 36 inches by 16 inches by 28 inches high, has thin, 3/4-inch walls, making it light enough to move even when fully planted.
“The real beauty of Ductal is the ability to take it well beyond what we’re capable of with normal concrete,” Carpenter says. “We’re a design- and architecture-driven shop. Ductal opens up avenues that we never imagined.”
Solus has also begun producing large-format panels, 4 feet long by 9-inch, 12-inch, or 16-inch widths, and only _-inch thick. “People are using it as flooring,” explains Carpenter, “imitating planks of stone.” It’s also finding use as a decorative wall element.
Solus has made thin concrete exteriors tiles, too, taking advantage of the excellent freeze-thaw characteristics of UHPFRC. “ Calgary experiences extreme weather conditions,” Carpenter says. “It can go from -4 F to 50 F in a single day, even twice in a day. Ductal allows us to put this material outside.”
Carpenter relishes the effect of the material on his designer clients. “It’s pretty riveting stuff. When they see it, they go ‘Wow.’”
Dewulf Concrete, Culver City, Calif., is a boutique concrete precaster that has moved all its production into UHPFRC. Owner James DeWulf, who admits, “Concrete is my life,” discovered Ductal while still in his 20s, and sees it as the concrete of his future. “The sexiness of concrete,” he says, “is its earthy feel. I bring that out and enhance it.” He is awed by the precise textural detail captured by Ductal. “It mimics exactly whatever I pour on. If I cast it on melamine, it comes out looking like melamine.”
UHPFRC has revolutionized his business. Where a concrete countertop job used to require several job site trips with a two-week turnaround, he can now do the fabrication completely in the shop and then deliver the finished lightweight countertop. He’s doing four jobs per week, and there’s no end of work in sight.
Pointing to one of his sleek, modern bathroom countertops, Dewulf comments ironically, “It’s an inch and a half thick just for appearance. In terms of strength, I could easily have made it 1/2 inch.”
The new concrete has clearly won Dewulf’s love. He grabs the edge of his 8-foot diameter, 3/4-inch thick Ductal table and says with a kind of parental pride, “This will be here 100 years from now and still look beautiful.”
In Calgary, Alberta, huge trout are jumping, and they will stay that way for a long time. How huge? 14 feet. How long? Perhaps centuries.
These exuberant leviathans, swimming on a highway retaining wall at the Glenmore/Elbow/5th Street interchange, are the vision of artists Violet Costello and Bob Thomasson. When completed, there will be 144 of them.
The construction budget for the subgrade interchange included an art set-aside, so the city instituted a design competition. “The retaining wall seemed to me obviously like a river,” recalls Costello. She dreamed up an homage to the nearby Bow River and its famous trout, with undulating waves and gargantuan fish both swimming below the surface and leaping above, into the sunshine.
The original concept called for bas-relief fish cast integrally into the precast retaining wall panels. The fish were so long they would have been divided over three panels, with joints visible. Moreover, Costello wanted the above-surface fish to be brightly colored using integral concrete pigmenting.
The project precaster suggested casting the fish as single, separate units. Using Ductal, they could make the fish very thin – two inches at the center tapering to one inch at the edges – and thereby very lightweight.
Costello and Thomasson refined the design, then had their drawings transferred to CAD. Using a CNC router, mother molds were cut from Medium Density Fiberboard by Laser Spec, Inc., Calgary, Alberta, and then made into rubber molds for casting.
The fish experience has Costello hooked on the possibilities of UHPFRC. “I’m hoping to be able to use Ductal concrete again. There are a lot of applications for it in architectural work. As an artist, I see a huge, huge future for the material.”
Exploration of UHPFRC’s potential is just starting. The difference between the properties and abilities of conventional concrete vs. Ductal is not unlike the leap in technology from the Bronze Age to the Iron Age. The new material is stronger, maintains its integrity under more extreme conditions, and is capable of producing a finer, more durable surface finish. That historic change forced practitioners to adapt their skills and learn new techniques, but it made possible the civilizations of ancient Greece and Rome. Ductal may not have that broad an impact, but it is already altering our concept of one of the basic materials of our civilization. With the learning curve comes inspiration. Invention of a new technology is only the beginning; practitioners with the inventiveness to use its full potential are the people who dream the future.