UIC ENGINEERS HELP BUILD A BETTER BRIDGE
A full-scale mock-up of a bridge constructed of prefabricated concrete supported on steel beams survived a simulated truck loading test designed to make it fail, withstanding hundreds of tons of pressure with only hairline cracks. Connections between the concrete and steel beams also passed the test intact.
The findings from the June 22 research test led by University of Illinois at Chicago civil engineering professor Mohsen Issa may mean a new era of cheaper, faster and safer bridge construction and replacement.
The UIC experimental evaluation of full-depth, precast, prestressed concrete bridge deck panels is part of an ongoing research project funded by the Illinois Transportation Research Center/Illinois Department of Transportation.
IDOT's maintenance yard in Elk Grove Village was the test site as the UIC team subjected the bridge deck to "destructive" pressures, applying massive loads at various points and configurations. Sensors along the bridge were used to determine bridge stability, allowing inspectors to examine the structure after each successive load increase.
The testing concluded at 590,000 pounds, or 295 tons. At that point, the steel beams showed signs of deflection. Yet, only hairline cracks were detected in the concrete, mainly around the points where hydraulic pumps were placed to apply pressure.
After more than a decade of research into this type of concrete bridge deck replacement system, Issa was pleased by the results. "The concrete used in casting the panels is easily controlled, since it is precast," he said. "This system provides better durability. The cost effectiveness, in terms of reduced construction time, is significant, especially for long-span bridges or big jobs."
Precast concrete can be cured prior to placement on a bridge. Concrete that is poured at a bridge site must be allowed to sit and cure without any vehicle passage, sometimes for weeks or longer, before it is considered safe to use.
But weather conditions can affect the curing rate. Concrete that does not cure properly breaks up quicker, so it is subject to costly repairs or replacement.
While the full-depth, precast, prestressed high-strength concrete used in the UIC test may prove to be both quicker to install and more durable, public highway officials also hope the new method will reduce costs by minimizing the extra wear and tear of traffic detours and reduce the irritation motorists face with the added congestion of being rerouted.
Two Illinois highway bridges-one near Springfield, the other near St. Louis-are now using this type of concrete bridge deck panel. The UIC stress test results will be used in setting exact standards for manufacturers to follow in the future.
The American Society of Civil Engineer's latest survey of Illinois bridges finds 21 percent are "structurally deficient or functionally obsolete." The society says roads and bridges are the state's top infrastructure concern.
If such prefabricated bridge decks become commonplace in highway construction, experts say bridge replacement could be completed in a matter of days or weeks rather than months, as is now typically the case. Issa says this is particularly appealing for metropolitan areas that suffer from heavy traffic congestion.
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