GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.2 ___ GNGTS 2023 Methodology of investigation and modelling of geophysical data The GPR methodology is very widespread in various fields: underground service research, structural analysis and infrastructure maintenance. Georadar uses the reflection of electromagnetic waves to explore the surface layers of the subsoil. A transmitting antenna introduces energy pulses into the subsoil electromagnetic waves of very short duration (nanoseconds). When these impulses meet a contact between materials of different composition, a part of the energy is reflected towards the surface where it is picked up by a receiving antenna, amplified and recorded. The GPR system can usually be equipped with antennas with a central frequency from 40 to 2500 MHz and acquisition band from 10 to 5000 MHz, reaching variable depths depending on the type of soil investigated and the specific used antenna. The position transducer (optical encoder) allows precise locations on the ground (or vehicle in general) of the detected anomalies. The penetration of the electromagnetic wave is a factor closely related to attenuation; therefore, it is strictly correlated with the propagation velocity of the wave. The estimated propagation velocity is essential to correctly measure the targets depth; it is function of the dielectric constant of the medium, and can change nearly 10 times as a function of the soil nature. the propagation velocity is expressed, in the MKS system, in m/s; the propagation velocity in air is approximately 3∙108 m/s (300.000 km/s). Thus, the correct GPR data interpretation is based on the correct determination of velocity; this process is called “calibration”. The calibration consists in modifying the velocity value in order to fit the depth obtained from GPR data interpretation and the depth obtained from direct tests. To set the right velocity value, some direct investigations are essential. For each set of precast segments, the depth of the concrete cover has been identified and, basing on this value, the right velocity is been chosen. More specifically, n° 109 direct tests are being conducted to verify the real rebars’ depth. The calibration process provided a velocity value equal to 12.45 cm/ns; therefore, it was decided to use a relative dielectric constant equal to a value of 5.8, which corresponds precisely to the velocity of 12.45 cm/ns obtained from the 109 direct tests. In this work, the GPR Proceq GP8100 S/N: GP81-002 0043 was used (Fig.1a). This type of GPR, characterized by a modulated frequency range from 400 to 4000 MHz, allows to obtain a concrete inspection of large areas in all buildings or bridge decks, an object detection and damage prevention before drilling, coring or cutting into concrete and allows to superimpose the 3D view into the volume below scan. The GPR investigation is composed by a series of steps: ● The first phase, carried out in the precast segments’ construction plant, consisted in testing this methodology and verifying its accuracy. A grid with guidelines spacing 10x10cm was used. The GPR measurements along this grid allows to verify the percentage of data dispersion, of instrumental error, and to verify the number of swipes necessary to obtain the information requested by the client. For this reason, three swipes are been necessaries (Fig.1b).