Ultra-High Performance Concrete

Iowa Bridge Gives Glimpse Into the Future (Ascent, Designing With Precast, Summer 2006) describes the use of extraordinarily high-strength concrete in precast beams for the Mars Hill highway bridge in Wapello County, Iowa.

The "bulb tee" beams  (photo left) were cast with a proprietary concrete mix from Lafarge North America capable of producing concrete with compressive and tensile strengths as high as 30,000 psi and 7000 psi, respectively. (For comparison, conventional concrete is typically specified with compressive strength in the range of 2500 - 5000 psi, and taller buildings are being designed with concrete with compressive strengths as high as 10,000 - 20,000 psi.) Among the reported benefits are:

• A lighter, thinner structural member: The bulb tee beams used in the Mars Hill Bridge have webs 4-1/2 inches wide, compared to 6-1/2 inches for a conventional precast bulb tee, bottom flanges  5-1/2 inches deep, compared to 7-1/2 inches for a conventional tee, and top flanges 2-3/4 inches deep, compared to 3-3/4 inches for a conventional tee.
• The more dense concrete results in a structural member that is more resistant to the effects of weather and road salts, and that should be longer-laster.
• Reduction in the need for conventional steel reinforcing: Aside from the prestressing strands, the only other steel reinforcing in the Mars Hill Bridge beams are U-shaped bars used to develop bond between the precast beams and the cast-in-place concrete bridge deck. There are no stirrups or other conventional reinforcing steel.

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For more about Lafarge Company's proprietary Ductal concrete, see Iano's Backfill Flexible Concrete.