GNGTS 2022 - Atti del 40° Convegno Nazionale

464 GNGTS 2022 Sessione 3.2 Fig.1 - a) MIRA A1040 tomograph; b) ray paths involved during a measurement; c) Scan sections acquired by the MIRA A1040. for more precise reflectors’ localization. A potential reflector is observed from different angles and with different time delays t and for each point of the image all signals are added to the respective delay times t. In this work, the SCAN methodology was used; it allows to collect a complete data set using measurements made on a grid of predefined positions on the survey object surface. This mode automatically saves the B-scan data related to each measurement position, having previously allowed the display of the B-scan image that correspond to the sections parallel to the major axis of the instrument (Fig. 1c). The system also stores the settings used and a bitmap image of the MIRA screen display at each point of the grid. Bitmap images are transferred to a folder on a laptop which contains the IDealviewer 3D visualization software that merge the 2D images to create a 3D model of the survey object. The user can manipulate the 3D model by rotating it and looking at different orthogonal planes that cross the model itself. The views on the three orthogonal planes have formal names: the C-scan section appears to be perpendicular to the Z axis; the D-scan section appears to be parallel to the Y axis. In this work, we investigated some area for each span suggested by the client. For each area, size 0.4 x 1.5 m, the first step is to identify the prestressed cables and define their path using GPR and sign it over the area. More specifically, the GPR survey consisted of transversal line surveys along the whole beams’ height (from top to bottom). The GPR strips pitch was 0.25 m in the first and last quarter of the length of the beam and 0.5 m in the remainder. Ultrasound tomography investigations were performed in specific areas characterized by cables’ portions performing a variable number of UT scans with a constant pitch of 0.1 m, positioning the instrument transversely investigated cable development. For the ultrasonic tomography survey, a pulse velocity propagation of 2650 m/s was set, obtained following a calibration test performed on the known thickness of the beam. The ultrasonic pulses frequency was chosen following a series of tests at a variable frequency between 20 and 50 KHz; a 35 KHz frequency was chosen, which returned the best result in terms of resolution and noise.