GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 3.2 463 GPR AND ULTRASONIC TOMOGRAPHY APPLIED TO PRESTRESSED CABLES FOR VERIFYING VIADUCT CONSERVATION STATE D. Di Massa, P. Luiso, D. Fiore Socotec Italia srl, Italy Introduction. The use of prestressed cables has spread in the major infrastructure projects such as bridges and viaducts, in order to improving the resistant behavior of the projects. The phenomenon of the prestressed cables’ corrosion is a serious concern; a loss of prestressed system functionality leads to the emergence of criticalities in the structural behavior, favouring of crisis cracking phenomena of the opera. In the post-strained structures, the cables are located in a sheath filled with cement grout which separates them from the concrete, endorsing better resistance to corrosion. Thus, corrosion problems often arise when grout injection is performed poorly, leading to the local defects’ formation; sometimes, the concrete deteriorates, generating small voids, pores or micro-cracks. The request for information and knowledge on the correct diagnosis of degradation in reinforced concrete and prestressed concrete structures is increasingly frequent. The main difficulty in assessing the degradation level is the inability to access the reinforcement directly, and in a minimally invasive way; thus, indirect investigation techniques are most frequently used (Ferrarella 2021). For this purpose, ultrasonic tomography (UT) developed in recent years has been used. The UT methodology was developed for the identification of voids and cracks within the concrete, in particular on walls and flooring surfaces (Taffe et al. , 2006; Hoegh et al. , 2011; White, et al. , 2014), for checking the concrete thickness inside tunnels (Gorzelanczyk et al. , 2012; Schabowicz 2015) and for the diagnostic study of cultural heritage (Capizzi et al. , 2011, Alberghina et al. , 2013). Recently, however, the ultrasonic methodology, supported with GPR survey, has been used for locate the prestressed cables and verify of their conservation state. More specifically, the georadar methodology provides a geometric description of the cables even though they are affected by disturbances such as the humidity; the ultrasonic methodology provides information regarding the density variations of the investigated material (Samokrutov et al. , 2006; Krause et al. , 2009). In this work, we present the potential of the combined methodology carried out above a viaduct in A1; for privacy we are not allowed to name the viaduct. Methodology of investigation and modelling of geophysical data. The UT methodology is based on the pulse-echo method using the analysis of elastic waves propagation (share waves) to locate areas characterized by structural inhomogeneity, decay, fractures or injuries within the investigated volume. In this work, we used the tomograph Mira A1040 (Fig. 1a) able to create a 3D representation (tomogram) of internal defects in the concrete. MIRA tomograph is based on the pitch-catch method of ultrasound because of the presence of an antenna composed of a matrix of spring-loaded contact transducers (DPC) to conform to an irregular surface, which emit shear waves. The first transducers’ row acts as the transmitter and the remaining transducers’ rows of act as the receiver. Thereafter, the next transducers’ row is energized and the other rows act as receivers (Fig. 2b). This process is repeated until each of the 11 transducers’ rows has acted as a transmitter. It takes less than 3 seconds to complete data acquisition and processing at each location; the measured values ​of transit times are processed by the computer in order to create a 2D image of the volume below the antenna (“synthetic aperture focusing technique”, SAFT). The received signal by a single emitter is affected by structural noise (diffraction due to aggregates, small defects) but the composition of 66 waveforms reduces noise and allows