PSI - Issue 64

Isabella Mazzatura et al. / Procedia Structural Integrity 64 (2024) 114–121 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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economic/construction boom period that took place after the II World War, when the awareness about the physiological degradation of materials, especially concrete, was not yet fully matured. The invention of a technology as simple as sophisticated and economically advantageous, because of the possibility of using containing quantities of materials to gain considerable spans lengths, did not contemplate the eventuality of future inspections for a degradation assessment. The system core are the post-tensioned cables made of strands, wires, or bars. While very severe damage conditions are generally perceived by visible degradation phenomena, such as cracking of the concrete section, on the other hand, some unexpected collapses have occurred. The inspection of embedded cables in a post-tensioned structure is a complex issue if there are no severe and visible degradation phenomena. The complexity stems essentially from the intrinsic conformation of the post-tensioning (PT) system: the cables are inside grouted plastic or metal ducts, which in turn are located inside the concrete matrix and protected by a grouting material. Therefore, Non-Destructive Techniques (NDTs) represent the most suitable solution. In this context, t he individuation of the cables’ trace of a PT beam represents the fundamental starting point for any other operations involved in the assessment of PT structures. The determination of the cables’ l ayout along the structural component is necessary for any subsequent in-situ investigations, allowing to perform the tests in a smarter way (only on the tendons) and collecting information about the execution of the structure and, therefore, about potential differences with the design. The use of low-impact techniques is strongly encouraged due to practical and safety reasons, but issues still exist about the most efficient methodology to adopt: the presence of a thick concrete cover, the surrounding reinforcing bars, and the ducts’ material can negatively affect the tests’ results. Ground Penetration Radar (GPR) is considered the reference technique for tracing cables (Giannopoulos et al., 2002; Liu et al., 2013; Pasculli et al., 2018; Pollock et al., 2008; Sławski et al., 2016; Wai -Lok Lai et al., 2018). It is a radar imaging technique using electromagnetic pulses transmitted to the element at very high frequencies: the echoed signals are elaborated individuating the reflections generated by the metallic elements included in the concrete. GPR was widely used (Terzioglu et al., 2018) for achieving information about reinforcements’ layout (Beben et al., 2012; dos Santos et al., 2014; Kohl & Streicher, 2006; Pasculli et al., 2018; Shaw et al., 2005; Soldovieri et al., 2006), the position of prestressing strands and metallic ducts (Clem et al., 2015; dos Santos et al., 2014; Kohl & Streicher, 2006; Terzioglu et al., 2018), and the presence of voids in concrete. The present research aims to determine the accuracy of GPR in locating PT ducts and individuate potential influencing parameters in the tests’ execution. An experimental campaign was carried out on six specimens presenting ducts in the laboratory, mainly studying the influence of the modalities of execution of the test and the concrete cover thickness, as detailed in the following. 2. The experimental campaign The objective was to quantify the accuracy of the GPR in the location of tendons in PT structures. The main influencing parameter is hypothesized to be the execution modality of the test; indeed, four configurations are considered and carried out. The other parameter investigated is the concrete cover depth. The 2.0 GHz GPR ( C-thrue model) consists of a box containing both the sensors and the results display screen, being a very handy, fast, and easy to-use tool for a single operator (Fig. 1). In particular, the instrument incorporates two antennas perpendicular and spaced 10 cm apart. The dual polarization of the system allows for multi-level detection. In detail, the longitudinal polarization antenna can detect deeper objects than the transverse polarized one (the two channels are respectively represented in Fig. 1a).

a) c) Fig. 1. a) Plan upside-down view of the GPR with the location of the two antennas; b) Plan view of the GPR; c) Actual view of the GPR. b)