PSI - Issue 84

Luca Ranedda et al. / Procedia Structural Integrity 84 (2026) 1191–1197

1193

The prestressing reinforcement of the suspended (drop-in) beam includes three cables per web; two of these, of type M5/20, are anchored at the beam ends, while the third of type M5/16, is anchored in the slab (Figure 4).

Figure 4 – Prestressing system of the suspended beam (Final accounting excerpt).

3. Survey of Prestressing Cable Layout Ground Penetrating Radar (GPR) is a geophysical investigation technique based on the analysis of the reflection of electromagnetic waves emitted at predetermined frequencies by an antenna positioned in close proximity (a few millimetres) to the scanned surface. The purpose of the survey is to investigate the internal structure of the analysed medium; when a change in the dielectric properties of the traversed element is present, part of the emitted wave is reflected and returns to the surface, where it is detected by the same antenna. It is important to highlight that, for electromagnetic waves, the reflection coefficient R is equal to 1 (total reflection) in the case of metallic materials and equal to −0.45 (partial reflection) in the case of air. This implies that the use of ground-penetrating radar makes it possible to: – investigate beyond voids; however, it may be difficult to detect voids of limited size and extent. – detect metallic elements; however, continuous or closely spaced metallic elements located near the surface may hinder deeper investigation. 3.1. Equipment Used The investigations were carried out by performing scans on the right side of the cantilever deck and the buffer beam, using the equipment listed below: • Proceq GPR Live GP8000 single antenna system, with stepped-frequency technology, frequency bandwidth: 0.2–4.0 GHz, maximum investigation depth: approximately 80 cm in dry concrete, capable of performing linear scans and area scans with an acquisition step of 10 cm. • Proceq GPR Live GP8100 six-antenna system, with stepped-frequency technology, frequency bandwidth: 0.4–4.0 GHz, maximum investigation depth: approximately 80 cm in dry concrete, capable of performing super-line scans (derived from the combination of the six scans acquired by each individual antenna) and area scans with an acquisition step of 30 cm. 3.2. Investigation Techniques The survey of the cable layout was carried out using two different scanning techniques in order to compare execution speed, technical–operational complexity, and the results obtained. With the first investigation technique, a series of vertical linear scans was performed on the side of the box girder, spaced at intervals ranging between 0.75 ÷ 1.5 m. For each section, a B-scan was acquired, through which it was possible to locate the cables. The position of each individual cable was recorded using X coordinates (longitudinal horizontal distance from the pier axis or from the outer edge of the beam) and Y coordinates (vertical distance from the top surface of the deck slab) (Figure 5). The intersection points of each individual cable across the different sections were then connected in order to visualize the cable profile along the beam.