2 + 2 = 10?
Without an ounce of steel, engineers believe these fiber structures will reinforce
a concrete bridge deck.
Courtesy Lawrence Bank, University of Wisconsin-Madison
A tailor's shop? No. These rolls of glass fiber are used to design composite bridges in the University of Wisconsin-Madison department of civil and environmental engineering.
High-tech composites are used in high-flyin' airplanes and fast-rollin' bikes. And they're appearing in, of all places, highway bridges. Carbon-fiber wrappings are protecting against earthquakes on supports for California highways.
But the major use of composites in construction would be to replace an earlier composite -- reinforced concrete. The key advantage, says Lawrence Bank, an engineering professor, is not strength or weight so much as speed of construction and, critically, immunity to rust. Bank should know: He edits the journal Composites in Construction.
Moisture and salt corrode steel, and Bank says most problems in roads, bridges and highways are from "corrosion in steel-reinforced concrete. The steel increases in volume, cracks the concrete, and you get potholes."
Because weight is not important in highway construction, glass fibers can be used instead of costly carbon. For example, Bank is helping design a concrete deck for a 2-lane bridge near Fond du Lac, Wis. The roadway will be 200 feet long and 43 feet wide, without a single piece of steel, he says. "All the reinforcing will be non-metallic, glass fibers."
Pultrusion is used in a wide variety of composite
products, including non-conducting fiberglass ladders.
Courtesy Creative Pultrusions, Inc..
The bridge will also have novel, stay-in-place forms that should cut construction time. The 43-foot by 18-inch forms -- like many composite products -- will be produced by pultrusion, a continuous, economical process that makes continuous parts by pulling fiber through a die. After the forms are laid cross-wise on conventional bridge beams, concrete will be poured into them. The forms, and a grid near the top of the road, will both supply tensile strength -- steel's usual role.
Rust goes to sleep
One
benefit of composites -- speed -- should arise during construction. First,
the forms are light enough to handle without a crane. Second, they are
the reinforcements, so they will remain in place, eliminating the
form-removal chore of conventional construction. "If you can build it
quicker, there's a benefit in labor savings, and also a societal savings"
from reducing the economic dislocations of road construction, Bank says.
The other advantages won't appear until many years down the line, he adds. Since there's nothing to oxidize in the roadway, it's safe to say that rust can finally sleep on this bridge, and the bridge deck may last far longer than the reinforced-steel twin being built for comparison on the same site.
Still, composites are expensive, and it's unclear whether the reduced construction labor and the longer lifespan will compensate for the higher material cost.
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