Bridge condition technologies are centered on decks and superstructures, and much less on substructures. “Cracks in steel and concrete, damage from corrosion, all are areas in which various technologies are available for assessment,” Chase said. “There’s a lot of work on bridge decks and superstructures. By themselves, cables have been the focus of a lot of attention.”
But except for scour, the condition technologies available for bridge substructures tend to be focused on evaluating the quality of the substructure as it’s constructed, not after it’s in-place, said Chase.
“Evaluating the integrity of piles, drilled shafts and other substructure elements while they’re being constructed has evolved to the point where technology such as cross hole sonic logging and other methods are routinely employed to assess whether substructure elements are constructed properly,” he said. “But once the bridge is placed in service, we tend not to be too concerned with the substructure, with the one exception of scour. That’s because we tend not to have large failures of substructure elements; most of the collapses or other failures have been things that happen in the superstructure.”
Ground Penetrating Radar
Like with pavements, ground penetrating radar (GPR) has great applicability for studying bridge deck condition. GPR systems use electromagnetic radiation at microwave frequencies, and the radiation penetrates and characterizes concrete, while reflecting off metallic material like rebar. It has the potential to replace manual chain drag testing of decks, in which experienced staff listen to the sound the chain makes; ringing indicates a sound spot, while a dull thud indicates subsurface delamination.
The FHWA pioneered GPR for bridge decks by underwriting the development of the HERMES (High-Speed Electromagnetic Roadway Measurement and Evaluation System) product at the Lawrence Livermore National Laboratories in California. HERMES was intended to provide a GPR system that can reliably detect, quantify and image delaminations in bridge decks, at normal highway speeds, and was delivered in 1998.
“HERMES was a radar system that was much, much smaller than anything at the time was commercially available,” said Chase. “It had a much higher frequency, and was based on new technology that had been developed for producing extremely short, precisely timed pulses, which was created for Livermore’s work on fusion reactors. The integration of all of that into a system that employed computer-aided tomography, with an array of antennas, was a first.”
HERMES included a computer workstation and storage device, survey wheel, and control electronics, in addition to the array of 64 antenna modules or transceivers mounted in a towable trailer. To investigate specific areas of a bridge deck that require more detailed study, a single-antenna scanning device called PERES (Precision Electromagnetic Roadway Evaluation System) was developed as an extension of HERMES.
In mid-decade they both were supplanted by HERMES II/PERES, new GPR technology using a single transmitter and receiver antenna pair configuration, developed under a participating states’ pooled fund. Work using HERMES II/PERES incorporating other technologies continued into 2009 under the auspices of the University of Vermont.
A variation of this technology is the digital synthesis arrayed radar. HERMES and most radar systems employ a very short pulse, radiated into the deck as a very broad-band signal. “The pulses are on the order in picoseconds [one trillionths of a second], and the frequency content are in gigahertz [billions of cycles per second],” Chase said. “But they are very low energy, spread out over a broad band. Short wavelengths give you more detail, but don’t propagate very far into a material. Lower frequencies give you less resolution, but go deeper.
“What the digital synthesis radar does is, rather than rely on a pulse, it synthesizes [creates] a particular frequency digitally, and radiates that specific frequency,” Chase said. “By hopping or changing frequencies rapidly you can ‘interrogate’ the bridge deck, and it makes it easier to comply with prohibitions on intentional radiation of electromagnetic energy in certain protected wavelengths.”
Acoustic Emission Technology
Acoustic emission technology is a process especially suited for steel bridges, by which engineers “listen” for characteristic signals associated with cracks forming and extending.
“It’s used primarily for detecting and locating fatigue cracks in steel highway structures, and it’s a technology used in a variety of other industries,” Chase said. “When energy is released as a crack propagates, a series of stress waves are generated that can be detected with sensitive accelerometers, combined with sophisticated signal processing that separates those signals from the rest of the noise of a highway bridge.”
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