PSI - Issue 64

Mercedes Solla et al. / Procedia Structural Integrity 64 (2024) 293–300 M. Solla et al. / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 4. Specimen III: steel bars (R1 to R16) spacings and coverings.

3. GPR surveys: data acquisition and processing For each of the three specimens, a longitudinal profile line (perpendicular to the direction of the bars) was acquired along their middle, using the common-offset mode. Each profile was acquired with both frequency-domain (SFCW) and time-domain (pulsed radar) GPR systems, as they may offer complimentary advantages and considerations regarding resolution and depth penetration. The purpose is to evaluate which system provides better resolution (both horizontal and vertical) and more well-defined rebar signature for further AI developments. This comparative evaluation will help inform decisions regarding the selection of GPR systems for future applications, ensuring optimal performance and accuracy in rebar detection and characterization using AI techniques. The GPR raw data collected in this work are openly available in Zenodo (Solla 2024). 3.1. Pulsed radar GPR In these systems, short-duration pulses of electromagnetic energy are transmitted into the ground or into a structure, and measure the time to reflect the signal from subsurface features (or dielectric discontinuities). A ProEx system was used, with a bistatic ground-coupled 2.3 GHz frequency antenna. The setting parameters used for data acquisition were trace-interval distance of 0.5 cm and total time window of 10.95 ns composed of 368 samples per trace. Data was acquired in common offset mode, with the antennas oriented perpendicular to the profile line direction (parallel to the longitudinal direction of the bars) this is also called “broadside or HH” polarization, and a constant separation distance of 4 cm between transmitter and receiver. To measure trace-interval distance, the GPR antenna was mounted on a survey cart with odometer. To improve the received signals, the following preprocessing was applied: Time-zero correction, subtract-mean (dewow filter), gain function (linear/exponential), and background removal. 3.2. SFCW GPR These systems operate differently, as opposed of using pulses centered at a single nominal frequency, they broadcast continuous electromagnetic signals of increasing frequencies across a modulation frequency range. The wider bandwidth provides increased resolution allowing for detailed analysis of subsurface structures. A Proceq GP8800 system was used, with a bistatic ground-coupled 400-6000 MHz frequency range antenna. The setting parameters used for data acquisition were trace-interval distance of 0.5 cm and total time window of 16 ns composed of 653 samples per trace. Although this system can be configured in cross-polarized mode, for consistent comparison, data was acquired in common offset mode, with the antennas oriented perpendicular to the profile line direction (parallel to the longitudinal direction of the bars), and a constant separation distance of 3.6 cm between transmitter and receiver. To measure trace-interval distance, an odometer wheel is attached to the rear of the GPR antenna.