Something new on the French menu--concrete.
BY SARAH DEEM
Photos courtesy of French Technology Press Office
Published on: June 1, 2002
Most people go to France for the wine and the scenery. There is also a more concrete reason to go. When we heard that France had moved to the forefront of concrete technology, we decided to see for ourselves. So, while others were off to tour museums, and to sample cheese and taste country sausage, we went to look at concrete. What we found were new approaches for moving the venerable building material into the 21st century. They include improvements in ultra high performance concrete, a new type of stronger, self-leveling concrete and a technique for engraving designs onto concrete.
Ultra high performance concretes (UHPC) may not seem exciting, but they have the potential to change the way structures are designed and built. They are more flexible than regular concrete, need no passive steel reinforcement and have a tensile strength approaching that of steel. Another new line of concrete products, called Agilia, promises to speed the lead time for projects and combat a shortage of skilled labor. Finally, a unique aesthetic process similar to silk-screening makes it possible to transfer subtle and complex artwork onto concrete with up to 1 millimeter of accuracy.
Ultra High Performance Concrete
The mechanical characteristics of the two leading UHPCs in France--BSI and Ductal--make them denser than regular concrete and give them a much higher tensile strength and flexural (bending) strength. As a result, up to three times less concrete is needed to produce thinner structures that can span greater distances. The high density also makes the material nearly impermeable to corrosion from abrasion and chemicals. This should result in longer-lasting structures that cost less to maintain.
Structures using BSI and Ductal can be built without conventional reinforcement or prestressing steel, enabling architects to design complicated, curved geometrical shapes that can be cast on site.
During our trip, we visited manufacturers of both BSI and Ductal. For a firsthand look at BSI, we visited the offices of Eiffage Construction in Neuilly sur Marne, outside Paris.
Originally developed as part of a project to reinforce existing cooling towers at the Cattenom nuclear power station in the Moselle region of France, BSI is a self-setting, fiber-reinforced concrete that uses premixed dry components that can be adjusted for specific applications, explained Thierry Thibaux, technical director for bridge construction for Eiffage. Like Ductal, BSI does not require heat treatment or vibration, and it can be poured into a mold to create modular structures. After 28 days it has a direct tensile strength of 8 to 10 MPa (megapascals), achieved by using fibers in the mix. While the characteristic compressive strength of BSI is 150 MPa, Thibaux says tests have shown a compressive strength of up to 175 MPa. Because the concrete no longer has to cover passive reinforcements, it is possible to construct BSI structural elements that are less than an inch thick.
Following the success at Cattenom, Eiffage used BSI, mixed with metallic fibers and superplasticizer, to build the Bourg-les-Valence Bridge, in southeast France, as a test project. The 2 x 24-meter span beams used for this project were built in the factory and later joined with concrete that was poured in forms on site. The bridge proved to be a good test of the strength of BSI. Eiffage learned that the cost of the fibers and the mixing times--batching can take up to 11 minutes--need to be reduced. BSI has so impressed bridge designers that te material is being used to build moveable bridges in the Netherlands, replacing some steel and composities.
Ductal, developed by Bouygues Construction, Lafarge Concrete and Rhodia, is a finer, denser concrete than BSI, yet it can support even higher ranges of stress. Its strength derives from the use of steel or organic fibers are that are added.
Ductal characteristically has a compressive strength of as high as 230 MPa, without reinforcement, and a flexural (bending) strength of 30 to 60 MPa. As with BSI, Ductal requires only a fraction of the thickness of traditional concrete. A tunnel lining made from Ductal, for example, would be significantly thinner than a conventional lining made of textured concrete, says Marcel Cheyrezy, development director of Ductal for Bouygues Construction Co.
Thin But Tough
Cheyrezy anticipates that Ductal will be used for everything from long-span roofs, flooring, beams and facade panels, to tunnel liners, prefabricated medium-span bridge decks and nuclear containment structures. In some cases, Ductal can even be laid without a foundation. It also can be heat-treated to lessen shrinkage and creep.
The first structure made completely from Ductal was the Seonyu footbridge in Seoul, South Korea. It was inaugurated in April 2002. Also called The Footbridge Of Peace, the structure connects the city of Seoul to Seonyu Island on the Han River. The bridge's arch--which spans 393.7 ft. and is made up of six wedge-shaped blocks 32.8 ft. long and 4.27 ft. deep--supports a deck that is 1.18 in. thick. According to Bouygues Construction, the structure required only about half of the amount of material that would have been used with traditional concrete, yet provides equivalent load-bearing and strength properties.
Cheyrezy says that research is being conducted on other applications including prestressed Ductal for earthquake-resistant structures. Blast-resistant Ductal is being tested by the French army.
Although Ductal and BSI are more expensive than traditional concrete, Thibaux and Cheyrezy point out that savings on steel makes them competitive. And no steel means no corrosion and resulting costly repairs. Further savings result from the obvious fact that lighter structures require smaller foundations with less associated labor and equipment costs, says Thibaux. Because both Ductal and BSI can be pumped and placed without vibrating them, there are added labor savings.
The low porosity for UHPCs--less than 3 percent porosity for BSI, and between 1 to 2 percent for Ductal--makes them nearly impermeable, thus better able to withstand damage from water or chemicals. In tests, both BSI and Ductal have even proven to be resistant to salt water, the chief nemesis of roadways and bridge decks. The durability should translate into structures that last longer and require less maintenance during their lifetime.
Ductal has its artistic side too. The first use of Ductal for a sculpture was the Arbre Martel. This replication of a cubist sculpture from 1925 was built by Jean and Joel Martel and stands in Boulogne-Billancourt outside Paris. The 27.5-ft.-high tree, built without any steel reinforcement, is a prestressed structure that has fiber in the concrete.
The Skill Factor
The French concrete makers have also taken aim at two of the construction industry's most pressing problems, labor costs and lead time, explains Gilles Renier, whose company, Lafarge Concrete, developed Agilia. Like UHPC, Agilia concretes use superplasticizers to make them more fluid than regular concrete, while also using less water. Pumped with a hydraulic hose, the self-leveling cement's fluidity lets gravity do the work. For supporting columns and main panels, no vibrating is required. The result is a very smooth and high quality surface with up to a 22 percent reduction in leveling and finishing times. Renier estimates that a 4-hour deck-pouring and finishing job would take about 1 hour with Agilia.
As with UHPCs, Agilia costs more than traditional concrete, about 50 percent more in vertical applications and about 30 percent more in horizontal. In spite of the premium prices, Renier says it has been accepted surprisingly quickly.
Designs On Concrete
French concrete research is also tackling the biggest hurdle to the broader acceptance of concrete in nonindustrial application. While some of the world's most beautiful structures are made of concrete, the typical concrete home, office or shopping center presents a face, that, to put it kindly, only an accountant could like. And so the industry is developing ways to make ordinary structures more attractive. At the Centre for Nature and Wild Animals, in the Jura region of France, an area famous for its Comte cheese, clockmaking and diamond cutting, we found a unique example of a novel approach to designing on concrete.
The images of deer, wart hogs and other woodland creatures we saw were produced from animal drawings by the French architect Christophe Brisé and applied to prefabricated concrete panels 10 ft. high x 33 ft. wide using a technique developed by the French concrete company Pieri.
The process is a variation of photoengraving and silk-screening. First, an image is engraved onto a polystyrene sheet. Next, workmen apply a layer of a special additive--developed by Pieri--to the sheet. This additive, which acts as a surface retarder, keeps concrete that comes in contact with the image area of the plate from hardening. The sheet containing the image to be transferred to the concrete is placed in a mold, and the concrete is cast on top of it. Twenty-four hours later, the mold is removed, the sheet is peeled off and the concrete block is washed with a pressurized spray. A layer of the concrete covering the image area washes off. The result is a smooth image on top of a rougher background, with detail accuracy of up to 1 millimeter.
Pieri also has developed a number of beauty products for concrete, including pigments and coloring agents, concrete additives (with the Grace Corp.), cement protection agents for facades, surface treating agents, paving products for indoor and outdoor applications and a new waxed covering that resembles a glaze.
When America was first developing its first grand buildings, it looked to French architecture for inspiration. From our visit to France and a look at what government and industry are doing to improve technology, it seems certain that their influence upon our shores is about to be felt again.