18737.00 hallowell millikens bridge ground penetrating ... · ground penetrating radar data was...
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18737.00 Hallowell – Millikens Bridge
Ground Penetrating Radar (GPR) Summary
Introduction/Testing Methodology
Ground Penetrating Radar data was collected on the deck of Millikens Bridge in Hallowell on Thursday, April 14th. This
evaluation was completed in an effort to evaluate the existing condition of the bridge deck, which requires GPR readings
(amplitudes) be collected from the top layer of rebar in the deck itself. This deck presented a unique challenge to the
Departments Non‐Destructive Testing Unit because of the reinforced concrete wearing surface. In addition to this issue,
the top layer of rebar in the wearing surface runs longitudinally along the deck which also presents an additional
challenge because this rebar could potentially block the GPR signal from penetrating down to the desired top layer of
rebar in the deck. Because of these issues, it was determined that the longitudinal rebar in the wearing surface would
have to be identified and physically located on the deck before GPR data could be collected. To accomplish this, the
Departments personnel utilized a small hand held radar device known as a “Structure Scan Mini”. This device is shown in
Photo I below. Longitudinal rebar was located transversely every 30 feet along the deck. This rebar was spaced at
roughly 1 foot intervals. Lines were then painted at approximately two foot intervals between the longitudinal rebar as
the “desired target area” for GPR collection. GPR data was collected with the Departments 1600 ground coupled
antenna utilizing a scan rate of 24 scans per foot. This antenna is deployed using a three wheel cart. Photo II shows the
data collection process.
Photo I: Structure Scan Mini Photo II: Data Collection with the 1600 Antenna
Department personnel worked closely with Roger Roberts from Geophysical Survey Systems, Inc. (GSSI) to assure GPR
data was collected and evaluated properly. GSSI is the manufacturer of the Departments GPR equipment.
Data Analysis/Summary
After the data collection process was complete, GPR data was returned to the office and the analysis process was
completed. Generally, when the desired rebar layer is the first layer encountered by the GPR scans (for example: in the
case of a bituminious wearing surface), the processing software (RADAN7) will automatically select the rebar and
accompanying amplitudes. Because the desired rebar layer was located below the wearing surface reinforcing steel, all
of the deck rebar had to be manually selected. Photo III (below) shows one of the processed files.
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Photo III: Processed GPR Data File with Transverse Deck Rebar Selected
After this selection process was completed, comma delimited files (.csv) were created and the rebar amplitude values
were plotted using Dplot. Color coding was then created based on the rebar amplitude range.
A copy of the plotted results is shown below.
All data is reported from north to south and is referenced transversly from the right curb. In addition, repaired areas are
labeled on the plotted results and correspond to the attached photos below.
Transverse Rebar in the
Wearing Surface Selected Transverse
Rebar in the Bridge Deck
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Repair #1 ‐ North End of Bridge SB Lane Repair #2 ‐ North End of Bridge NB Lane
Repair #3 ‐ 34 Feet from North End of Bridge NB Lane Repair #4 ‐ 39 Feet from North End of Bridge SB Lane
Repair #5 ‐ 39 Feet from North End of Bridge SB Lane Repair #6 ‐ 63 Feet from North End of Bridge SB Lane
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Exposed Rebar #1 ‐ 112 Feet from North End of Bridge Repair #7 ‐ 314 Feet from North End of Bridge NB Lane
SB Lane
Conclusions
The conditions (reinforced concrete wearing surface and longitudinal rebar orientation) of this deck presented a unique
set of challenges to the non‐destructive testing group. With the data collection process and analysis proving successful;
the group validated that additional bridges with similar construction characteristics can be evaluated using GPR
technology.
Based on the plotted amplitude values, the areas of greatest deterioration appear to be at the north end and along each
side of the bridge. Areas with amplitude values less than (‐42 dB) should be the primary focus for further investigation.
Prepared by:
Stephen Colson – Senior Technician
Maine Dept of Transportation ‐ Materials Testing and Exploration
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