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Road Management & Engineering Journal |
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December 7, 1998 TranSafety, Inc. 1-800-777-2338 (U.S. and Canada) (360) 683-6276 Fax: (360) 335-6402 |
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:
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).
FHWA's ground-penetrating radar van can survey bridge decks and pavements while
traveling at speeds up to 89 km/h (55 mi/h).