Ductal-Stretching the Performance and Durability of Concrete
By Brent Ehrlich
Concrete has great compressive strength but poor tensile strength. It is brittle, cracks, and doesn’t flex, which is why it is reinforced with steel—but steel corrosion is a primary source of concrete failure. Lafarge’s Ductal line of specialty ultra-high-performance concretes (UHPC) offers significant improvements in these areas, with a thin, lightweight, extremely durable concrete avoiding standard steel reinforcement.
Ductal is made from portland cement, silica fume (a byproduct of electric arc furnaces), silica flour, silica sand, water, and polycarboxolate high-range water reducers (superplasticizers) that help cement flow; Lafarge adds metal or polyvinyl alcohol fibers to provide structure. “All the materials in Ductal are standard products,” said Vic Perry, vice president and general manager at Lafarge North America. “It’s how we select them and put them together that makes Ductal work.” Lafarge controls the size, geometry, and orientation of all the materials “at the micro level,” in Perry’s words, to create a very dense matrix that can be formed into intricate shapes while maintaining its performance.
According to Perry, Ductal’s compressive strength is ten times higher than that of normal concrete, and its flexural strength is three to four times higher; he also says it has lower permeability and the same abrasion resistance as the highest-quality granite. With such low permeability, chlorides and water that degrade normal concrete have almost no effect on Ductal. At a test site along the Maine coast, samples of Ductal have been inundated with seawater twice a day and exposed to freeze-thaw cycles since 2006. A normal 3,000 psi concrete lasts only one year in these conditions, according to Perry, but the Ductal sample still has the same square edges, with virtually no chloride penetration or degradation. Based on the company’s calculations, he estimates Ductal’s lifespan to be a staggering 1,000 years.
For the past ten years, Lafarge North America has marketed Ductal primarily for bridges, where its long lifespan, low permeability, and ductility are ideal for handling heavy traffic, freeze-thaw-cycles, and road salt. But in France, Ductal has been marketed toward the architecture community, largely because of its ability to be cast into different shapes, and three years ago Lafarge began slowly promoting those uses in North America.
For structural applications, Ductal could be a good option, especially in marine environments or earthquake zones, but price is a barrier. Ductal costs 20 times more than normal concrete per cubic yard. “If normal or high-performance concrete will do the job, that is what you should use,” concedes Perry. But unit cost is different from installed cost, he insists, and for infrastructure work, reduced maintenance costs alone can make Ductal a good investment.
For the building market, Ductal has been used primarily as a “signature” cladding material. While it will never compete with low-cost claddings, it can be formed into large panels less than ¾" thick and perforated and shaped as desired by the architect. According to Perry, the panels are light, strong, and easy to install, so the installed square-foot price is competitive with that of other high-end claddings. D’Ambrosio Architecture + Urbanism used Ductal on the Atrium, a 204,000 ft2 (19,000 m2) LEED Gold commercial/retail building in Victoria, British Columbia. The firm chose Ductal as a spandrel material because it is thin and light enough for a curtainwall system. Its light weight meant that the structure requirements of the building could be reduced while still meeting Victoria’s seismic requirements. Many of the panels are curved, and the original striated pattern came from a mold hand-carved by the firm’s principal, Franc D’Ambrosio, MRAIC.
Is Ductal green? The greenhouse gas emissions associated with portland cement are its main environmental drawback. Perry argues that Ductal requires significantly less raw material, less transportation energy to produce, and less steel reinforcement than standard concrete. It will never need replacement and can be reused at the end of the building’s service life. Ductal has even caught the eye of security agencies, which are beginning to use it to protect buildings from possible terrorist attacks—without as much bulk as conventional concrete requires.
Ductal is a unique product whose potential is just starting to be realized. Whether or not it can carve out more of a niche for itself will depend largely on lowering costs—but by making buildings more durable, flexible, and resilient, Ductal has the potential to reduce the consumption of resources over the long term.
For more information:
Lafarge North America