GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 3.2 465 Results and interpretation. The georadar and ultrasound tomography investigations were carried out with the aim of verifying the cables position and locating any injection defects along the viaduct beams. For the ultrasound tomography sections, the prestressed cables were marked with dashed yellow lines (Fig. 2). A single chromatic scale was chosen for these sections in order to favour a direct comparison between all the data acquired on the entire viaduct and, therefore, to uniquely identify any areas where further investigations are recommended to verify the presence of injection defects. In general, the ultrasonic survey identifies the variations in amplitude once a velocity has been defined (calibration phase): the instrument has to give the position of some elements of known thickness which the real depth is known. All the minimum colour variations in blue and maximum in red, represent variations in the amplitude of the ultrasonic pulse; a variation in amplitude corresponds to a variation in the properties of the medium which can be mainly correlated to density (acoustic impedance). A well-injected prestressed cable incorporated in the concrete presents a low variation in amplitude while the presence of voids, generating a strong amplitude decay (the air does not support the shear stresses), will give result in a high variation; the injected cables are identifiable by anomalies with colour up to yellow, the cables associated with anomalies with shades ranging from orange to red are attributable to possible cables not injected. More specifically, these anomalies can represent: • non-injected cables; • cables with very small voids not identifying with endoscopies; • cables affected by oxidation and corrosion processes which can lead to the cracks’ formation in the surrounding concrete; • cables injected but embedded in damaged concrete. After identifying all suspected areas, video endoscopies were set up with the aim of validating (confirming or denying) the results obtained. The video endoscopies, carried out one for each area, confirmed the presence of less injected or corroded cables, or moderately porous concrete for all the areas marked as “to be observed”. An example in fig. 2c. Fig. 2 - a) acquisition scheme; b) B-scan of the cable; c) video endoscopy image; d) 3D UT volume.