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 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. 60