CHAMPAIGN, Ill. -- Easy to design and inexpensive to build, pretensioned deck-beam bridges account for more than 7,200 bridges on Illinois highways and county roads. Two beams from such bridges have fallen apart, initiating a crash course in bridge inspection and repair.
"Two years ago, a beam in a state highway bridge collapsed at Kankakee, and then a beam in another bridge broke apart near Lawrenceville, while being removed," said Neil Hawkins, a professor of civil and environmental engineering at the University of Illinois. "Intended to last for at least 50 years, several of the state's pretensioned deck-beam bridges have shown unacceptable levels of corrosion damage after only 20 to 30 years of service."
In a project funded by the Illinois Department of Transportation, Hawkins and colleagues at the UI are helping to ensure public safety by designing better standards for evaluating a bridge's condition. The researchers have developed visual inspection procedures to assess the corrosion damage in deck beams, the rate at which the damage is spreading, and the remaining service life of a bridge.
"Each pretensioned deck beam resembles a long, hollow box," Hawkins said. "To create a beam, high-strength steel cables are first stretched tightly, additional reinforcing bar is added, and then concrete is poured around the steel. After the concrete has cured, the cables are cut at each end of the beam, pulling the concrete into compression."
In building a bridge, the beams are laid side by side, bolted together and the joints grouted with cement. Over time, the joints can crack, allowing water and salt to infiltrate in winter. The salt corrodes the steel reinforcing cables within the concrete. As the strands break, the bridge weakens.
"By chipping away the concrete to expose the rusted steel, and by testing beams in the laboratory, we came up with a reasonably reliable scenario for visually determining how many strands have been lost to corrosion," Hawkins said. "We can therefore assess the hidden damage and evaluate the remaining capacity and life of the structure by inspecting the location and amount of rust on the surface."
As traffic flow increases and heavier truckloads are permitted, bridges can become obsolete, Hawkins said. "Bridges often have to be updated, and that means replacing the deck beams with stronger ones. We consider a successful bridge as one with a 50-year service life."
Down the road, the ultimate solution lies in building better bridges. "From the time these bridges were first built, the technology has improved considerably," Hawkins said. "For example, the state now incorporates corrosion inhibitors in the concrete to significantly reduce the likelihood of corrosion and increase the service life."
Bridges in Illinois are inspected every two years, Hawkins said. "Deck beams are fairly easy to replace once they have been identified as bad, so it's mainly a matter of being vigilant."
The new procedures will start going into effect as IDOT swings into its summer inspection period.