Backed by government agencies and major European contractors, French concrete practitioners are advancing commercial application of high performance products fabricated without the use of passive steel reinforcement. The most recent application involves prestressed girders for what most construction oberservers would view as an otherwise ordinary motorway crossing.

Over the summer, Eiffage Group, a contractor based outside of Paris, completed the Bourg Lès Valence, a two-lane bridge in the Drôme region of central France. The structure is built with š-shaped girders, cast with a self-compacting mix dubbed BSI (Béton Spécial Industriel, note box). Steel content of the 22-meter-long girders is limited to prestressing strand and fibers, dosed in excess of 3 percent and acting, according to Eiffage staff, like a “fourth aggregate.” The girders were produced by Eiffage's Dutch partner, Hurks Beton, and shipped by rail from Eindhoven, The Netherlands.

The contract came about as the result of a 1999 design and build competition sponsored by the French Ministry of Transport. The agency has actively promoted construction innovations and technology transfer and, for Bourg Lès Valence, provided additional funding to offset the cost of using a product prior to its commercialization.

Eiffage landed the Bourg Lès Valence project as a means of showcasing BSI's engineering and production qualities. On the former point, notes Eiffage Technical Director Thierry Thibaux, “The material enables us to build lighter, more durable structures with ease. We can fabricate structural members with thin geometries that have no passive steel reinforcement or concrete cover requirements.” Production-wise, he adds, BSI is formulated for self compaction and rapid strength development with minimal curing measures.

Exporting ideas

Thibaux first reflected on the Bourg Lès Valence for North American prestressed interests during a presentation, “Use of Very High Performance Concrete on Site,” at the October 2000 PCI/FHWA/FIB International Symposium on High Performance Concrete in Orlando.

Site preparation at Valence and girder fabrication at Hurks Beton proceeded earlier this year after specimens cast with the proposed BSI formulation successfully performed in fire-resistance testing at Belgium's University of Ghent. Testing demonstrated the product's full resistance to 90 minutes of exposure to temperatures exceeding 1,800°F. While not commenting on specific dosages, Eiffage notes that the addition of polypropylene fiber augments BSI specimens' fire resistance. As steel fibers serve a reinforcing role, engineers contend, the polypropylene fibers — which melt when a matrix temperature surpasses 300°F — produce capillary voids that create a network allowing free release of steam produced.

Prior to his discussion of the Bourg Lès Valence contract at last year's symposium, Thibaux reviewed how BSI had been formulated to meet the requirements of France's national electricity provided by Electricité de France (EDF), and then used in high durability conditions at two nuclear power plants. The first BSI commercial application took place in 1997 and entailed production of precast beams for the renovation of four cooling towers at the Cattenom power station, located in the Moselle region of eastern France. Existing concrete structures had been subjected to a severe chemical environment, including brackish and slightly chlorinated cooling water, and a very aggressive thermal environment of freeze/thaw cycles during harsh winters. This resulted in premature wear on the suspension framework of the exchange bodies, leading to reduced cooling tower performance and ultimate loss of power.

In 1999, Eiffage and EDF tapped BSI technology for construction of a venting/cooling tower at the Civaux nuclear power station in the Vienne region of central France. The work called for on-site fabrication of four 20-ft. beams of approximately 10- × 22-in. cross section. Material for the beams, cast in plywood forms, was produced in a twin-shaft mixer-equipped ready mixed plant.

Government, industry initiatives

BSI was one of two high performance concrete formulations EDF has evaluated for nuclear power plant installations. A second, Ductal, is the product of a joint venture headed by Paris-based Lafarge Group. Also known as reactive powder concrete, Ductal is a fine aggregate material with super high strength and modulus charactertistics (Concrete Products, April 2001). Under a licensing arrangement, Lafarge North America has bugn promoting Ductal through its U.S. and Canadian concrete businesses.

BSI and Ductal have been spotlighted in two national research projects in France — Voies Nouvelles du Beton and BHP 2000. Concurrent with the projects, Eiffage, Lafarge and others behind new mix technology are developing standards for a family of high performance concretes, including formulations geared to 1) ultra high strength, lightweight members and structures; 2) high dosage steel and synthetic fiber reinforcement to replace conventional rebar; and 3) self-compacting placement in precast and cast-in-place operations.

Béton Spécial Industriel

At A Glance

Sample mix design

Constituent	Percent by weight
Cement	39.5
Silica fume	6
Aggregate (•7 mm)	38
Fiber (steel + synthetic)	8.5
Superplasticizer	1.4
Water	6.6

Batch sequence: Following 20 seconds of cementitious material and aggregate premixing, additional constituents are added at these points during an 11-minute cycle: water, 1 minute; superplasticizer, 3 minutes; steel and synthetic fiber, 4 minutes, 30 seconds.

Engineering properties

Characteristic compressive strength	175 MPa
Average tensile strength	10 MPa
Tensile-flexural strength	45 MPa
Compressive strength @ 48 hours	110 MPa
Young's modulus	63 GPa
Density	2.8 ton/yd.3
Source: Eiffage Group