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Federal Highway Administration > Publications > Focus > April 1997 > Ground-Penetrating Radar Hits the Streets-and the Bridge Decks
April 1997Publication Number: FHWA-SA-97-022

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Ground-Penetrating Radar Hits the Streets-and the Bridge Decks

It's slower than a speeding bullet, and it's not able to leap tall buildings, but ground-penetrating radar (GPR) does share one of Superman's powers-the ability to see through bridge decks, pavements, and other solid objects. New software, developed under the Strategic Highway Research Program (SHRP) and fine-tuned by the Federal Highway Administration (FHWA), now makes this powerful technology an easy-to-use tool for bridge and pavement management.

The GPR system evaluated and enhanced by SHRP consists of radar antennas mounted on the bumper of a specially equipped van. The radar antennas emit very short, precisely timed pulses of radiofrequency energy into a pavement or bridge deck. When the pulses transition from one material to another-such as from an asphalt overlay to a concrete bridge deck-part of the energy echoes back to the surface. A personal computer on board the van records how "loud" this echo is and how long it took to return to the antenna. The raw data are then analyzed by a software program, which generates easy-to-understand reports.

When used to survey a pavement, the system can determine the thickness of the pavement's layers. This information helps highway engineers determine the pavement's load capacity and decide how much material to remove when rehabilitating a pavement.

When the GPR system is used to survey a concrete bridge deck, the software can determine the following factors:

  • Location and quantity of delaminated concrete.
  • Depth of the reinforcing steel.
  • Thickness of the bridge deck and the asphalt overlay.

This information is used to locate sections of a bridge deck in need of repair, thus ensuring that repairs are done as cost-effectively as possible.

GPR technology is also extremely useful for bridge and pavement management, providing highway agencies with the means to quickly collect inventory data on all bridges and pavements they maintain.

Although GPR technology is not new, until recently it has been very difficult to use. "People used to joke that it took a Ph.D. and 20 years' experience to use," says Bill Scannell of CONCORR, Inc., the contractor conducting FHWA's Physical Assessment of Concrete Structures Showcase Workshop. "Thanks to the software developed under SHRP, that's no longer true." Now that FHWA has upgraded the software program to run under Microsoft Windows, the software is even easier to use.

GPR has many advantages over traditional evaluation methods, such as removing pavement cores for laboratory analysis, dragging a chain across a concrete bridge deck or pavement, or striking the concrete with a steel rod to locate delaminated areas. Because the van can travel as fast as 89 km/h (55 mi/h) while collecting data, there's no need to stop traffic. The test is nondestructive, so there's no follow-on repair work. And unlike other methods, GPR works on bridge decks covered by asphalt overlays.

These advantages come at a price, however: a fully outfitted GPR van can cost $200,000 or more. But FHWA's Donald Jackson says, "The benefits outweigh the costs, making GPR cheaper than almost any other test method." Jackson predicts the price will drop dramatically as a result of falling prices for computer hardware.

To help State highway agencies learn more about GPR, FHWA recently purchased two radar vans from Penetradar Corp., which had developed the GPR software for SHRP. The vans are equipped with four radar antennas (each able to scan a 41-cm-wide [16-in] section of bridge deck or pavement); a console for controlling the radar antennas; a personal computer for recording and processing the radar data; and extra passenger seats, to facilitate hands-on training.

An 8.5-m (28-ft) van with room for 10 people is being demonstrated as part of the Physical Assessment of Concrete Structures Showcase Workshop. The 2-day workshop includes sessions on the GPR system's capabilities, equipment operation, and how the system can be used in bridge management.

A smaller radar van with room for five people will soon be available for loan to State highway agencies that have participated in the showcase workshop. During the loan period, an instructor will train highway agency staff to use the equipment. The training sessions will take place on the State's own bridge decks and pavements. "We already have requests from 15 States. We are in the process of scheduling those States now," Jackson says.

Dave Hall of the Nebraska Department of Roads is one of those who has already signed up to borrow the radar van. Hall plans to examine a half-dozen bridge decks and several stretches of asphalt and portland cement concrete pavements. The results will be compared with information collected from core samples and sounding.

Among other applications, Hall says, Nebraska could use the radar van to determine how much asphalt concrete can be reclaimed from a pavement scheduled for rehabilitation. "If it does what they say it will do, it will be incredible," Hall says.

The Texas Department of Transportation (DOT), which has been using GPR since the early 1990s, is also impressed with FHWA's unit. Last fall, one of FHWA's new vans was driven to Amarillo, where it was tested side-by-side against the radar van used by Texas DOT.

Randy Cox of Texas DOT says FHWA's radar system is much faster than his agency's system: it is able to collect as much information in one pass at about 72 km/h (45 mi/h) as Texas DOT's system collects in four passes at about 16 km/h (10 mi/h).

For more information on using GPR on concrete bridge decks or on borrowing the GPR van, contact Donald Jackson at FHWA (telephone: 202-366-6770; fax: 202-366-7909; email: donald.jackson@fhwa.dot.gov).

For more information on the use of GPR technology to measure pavement layer thickness, contact Sonya Hill at FHWA (telephone: 202-366-1337; fax: 202-366-3713; email: sonya.hill@fhwa.dot.gov).

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