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 14 th . 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 NonDestructive 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. 60

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Page 1: 18737.00 Hallowell Millikens Bridge Ground Penetrating ... · Ground Penetrating Radar data was collected on the deck of Millikens Bridge in Hallowell on Thursday, April 14th. This

 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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