PSI - Issue 62

Isabella Mazzatura et al. / Procedia Structural Integrity 62 (2024) 369–376 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The assessment of existing structures has always been a difficult issue to tackle. The more complex the structural typology, the more uncertain and difficult the assessment procedure. Among all the different structural typologies, post-tensioned prestressed concrete (PT) ones present several critical issues requiring a special focus right from the knowledge phase. The presence of hidden defects, that cannot be visually recognized, needs to be deeply analysed: tendons are made up of high-strength steel wires and strands embedded within grouted ducts, and this makes not possible to recognize criticisms like corrosion of steel components caused by the defects in the grout (voids, water, etc.) (Carsana & Bertolini, 2015; Menga et al., 2022; Wang et al., 2014). The general tendency is to search for non destructive methodologies able to provide information about the maintenance condition without being invasive. The case of prestressed concrete bridges with post-tensioned cables raised attention, particularly in Italy, in recent years after some serious collapse events, culminating with the Polcevera viaduct in 2018. Many researchers (Krause et al., 1997; Muldoon et al., 2007; Shickert, 2002; Terzioglu et al., 2018) focused on detecting corrosion defects by relating their presence to inefficient grouting. Other research (Brauss et al., 1998; Carfagno et al., 1995; Morelli et al., 2021; Zanini et al., 2022) aimed to define the stress level in the tendons, denoting the poor/good maintenance condition of the cable around the observation point. Regarding the latter aspect, the X-ray diffraction technique is a possible non destructive methodology. For its application, it is necessary to directly access the tendons removing the concrete cover, the duct and the grouting. Although these operations are quite invasive, they are commonly adopted during the inspection process to detect corrosion occurrence. Many laboratory and in-situ applications were performed; however, it is nowadays lacking a study focused on the determination of its repeatability and the quantification of the errors affecting the X-ray diffraction technique in the specific application field. The present research aims therefore to investigate the reliability of the X-ray diffraction technique in the estimation of the actual stress; the method is applied to various samples and in mainly two conditions, i.e., loaded/unloaded. 2. The X-ray diffraction method: quick review The X-ray diffraction technique investigates the near-surface sample region (about 10 ), estimating the stress as a function of the interplanar spacing between the crystalline lattice planes, assuming linear elastic distortions. Several methods exist to estimate the residual stresses in metallic materials, such as the single-angle, the two-angle, sin 2 ψ, etc.) (P. S. Prevey, 1986). In the present campaign, the sin 2 (ψ) method was used to evaluate the acting stress. Equation (1) can be adopted for the stress measurement: − 0 0 =( 1+ ) 2 ( ) − ( ) ( 1 + 2 ) (1) being E the Young’s modulus, ν the Poisson’s ratio normal to the (hkl) orientation of the material (i.e. the values for the crystallographic direction normal to the lattice planes in which the strain is measures as specified by the Miller indices (hkl) (P. S. Prevey, 1986), 0 the interplanar distance at stress-free condition (inherent property of each crystalline material (British Standards Institution, 2008)) and the interplane stress with respect to the two principal stress components 1 and 2 . Under certain assumptions, Equation (1) can be rewritten as (2): = (1 + ) ℎ 1 0 ( 2 ) (2) The main assumption of the method is that plain stress is applied. By the Bragg law, it is possible to estimate , i.e. the interplanar distance in the stress condition. Then, these strain values can be determined by measuring the sin 2 ψ values at different orientations, i.e. varying . The strain values are therefore used to calculate the stress by fitting the experimental data to the sin 2 ψ equation (Luo & Jones, 2010). In the present campaign, for a single determination of a lattice spacing, i.e. a single stress measure, 9 measures were performed by varying angle (in the range of ±35°), with a counting time of 45 s; the stress was defined by a least square minimization of the regression equation. A portable Spider TM X GNR diffractometer was employed, working at 30 kV and 90 µA with a radiation source of chromium. The X-ray diffraction technique investigates the near-surface region of the sample, affected by the presence of residual stresses generated by the manufacturing process. Even if the final step of the production process applied