FHWA NDE CENTER RESEARCH & DEVELOPMENT
The HERMES Bridge Inspector
The HERMES bridge inspector is shown below. The system is designed to launch high frequency electromagnetic pulses from an array of radar antennas into a bridge deck while moving over the surface. The radar system may be controlled from the towing vehicle using a laptop computer. The interior of the HERMES trailer is also illustrated which shows the computer workstation, radar electronics and antenna array.
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| The HERMES bridge inspector. |
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| The interior of the HERMES trailer. |
The system is comprised of a computer workstation and storage device, survey wheel, control electronics and an array of 64 antenna modules or transceivers.
The most important design feature of the HERMES is the antenna array. The array consists of eight banks of eight transceivers, totaling 64 channels, housed within a 2.1by 2.1 m frame. Each pair of transmit
and receive antennas is skewed at 45° relative to the axes of the frame. Therefore, the emitted polarized radiation gives rise to
reflections of equal magnitude from transverse and longitudinal features such as steel reinforcement bars. The arrangement of the
transceivers gives samples across a 1.9 m width of the deck at 3 cm intervals. In the direction of movement, sampling intervals as
small as 1.5 cm are attainable. The density of collected data enables synthetic aperture radar techniques to be used in the processing
of the data. As data is taken over a 1.9 m width, a significant proportion of a lane width is covered. 3D images can be produced by
reconstructing the collected data. This is preferable to recording data along one or more displaced survey lines which yields
2D profiles along the paths of system transceivers.
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On site with HERMES.
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Sampling can be done at 1.5 cm intervals in the direction of HERMES movement and can be performed at up to 20 mph. Increases in speed are possible with a reduction in the sample spacing of collected data. Operation at 60 mph is achievable with 6 cm sample spacing which is equivalent to reducing the number of equivalent-time samples by one fourth.
The LLNL GPR bridge deck inspection systems use ultra-wide band microwave sources which produce signals with a frequency content ranging from 0.5 to 5 GHz. Air-coupled transmit and receive horn antennas form the transceivers. A time range of around 6 ns is suitable to penetrate 30 cm of concrete, where material properties permit this depth of penetration.
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Surface Depth 25 cm |
8.6 meters
Figure A. Plan view of reconstructed data over
Bridge 71 eastbound in Nashville, TN. |
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Width 1.9 meters
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8.6 meters
Figure B. Cross-section of reconstructed data at depth 14.5 cm, Bridge 71 eastbound in Nashville, TN. |
Scans of bridge decks have been conducted towing the HERMES trailer at speeds
from 5 to 55 mph. An example image after applying wavefield backpropagation processing is presented above. The images relate
to a horizontal plane through an asphalt overlaid concrete bridge deck in Figure A and a vertical plane through the same bridge
deck in Figure B. The data indicates reinforcing steel in Figure B as focused points of high reflectance indicated in white and bright
orange colors. Reinforcing steel are indicated in Figure A by the high reflectance linear features that traverse the width of
the image at an angle. The joint where the bridge deck deck ends and road pavement begins can be observed near the right hand
side of both images. This feature is indicated where the reinforcing steel ceases to be imaged and the reflection magnitude increases
significantly.
Field testing of HERMES has been conducted and controlled testing is ongoing at Turner-Fairbank Highway Research Center.
For more information, please contact either by e-mail or telephone.
Frank Jalinoos (202) 493-3082
frank.jalinoos@fhwa.dot.gov
Advanced Ground-Penetrating Radar Inspection of Bridge Decks
The Problem of Deteriorating Bridge Decks
The PERES Bridge Inspector
Current Work on Hermes II
