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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to wires and strands (i.e. the so- called ‘stabilizing’) consists of a thermo -mechanical treatment claiming to relieve the tensile residual stresses induced by the cold-drawing (Atienza et al., 2005, 2012; Atienza & Elices, 2003; Elices, 2004; Ruiz et al., 2003; Ruiz-Hervias et al., 2011), such stresses are not completely relieved. Several researchers focused on the latter theme (Caballero et al., 2011; Ruiz-Hervias et al., 2006) by comparing the residual stresses acting through both “as drawn” and “stabilized” wires ; a reliable estimation of the residual stresses coming from the stabilizing phase is very difficult due to the different conditions (load/temperature) applied by different producers. Carfagno et al. (Carfagno et al., 1995) performed some measures in-situ and observed that the variations achieved from measures executed on the same element could be due to friction losses that occurred while stressing the tendon. Moreover, the possible presence of corrosion and/or any isolated discontinuity coming from removing the grout also complicates the measure. In (Morelli et al., 2021) the X-ray diffraction method was applied, in laboratory and in-situ, providing a good correlation of achieved results, but no mention of the influence of residual stresses on the measurements is given. 3. Experimental campaign The experimental campaign is articulated in two main phases. The first one aims at stating the influence of residual stresses and grout removal operation by testing two samples, geometrically identical and with very close mechanical characteristics, one extracted from an existing structure and the other one new. The second phase has the objective of assessing the level of residual stresses acting on different samples and on different locations of the same sample. 3.1. First phase – execution of the tests Experimental tests were performed at the Laboratorio Ufficiale per le Esperienze sui Materiali da Costruzione of Pisa University, using the portable X-ray diffractometer of Padua University. The selected samples consist of a wire extracted from an existing structure and a new one, named respectively 'extracted wire ' and ' new wire ' in the following. Both samples have a diameter of 7 mm and a length of 600 mm. A portion of the extracted wire, which was originally longer, was tested by the tensile machine and achieved an ultimate tensile strength of 1739.8 MPa. The new wire has an ultimate tensile strength of 1769.5 MPa. A well-known stress level was imposed on the wires, checking then the reliability of the X-ray diffraction method in detecting the actual tension level on the specimens. To exclude the influence of the operator in achieved data, all the tests were performed in blind modality. The two wire specimens were tensioned using an INSTRON tensile machine. Five load levels were applied, and four measurements were performed for each stress level. The imposed tension was checked by measuring the strains through two linear strain gauges positioned on each specimen. The maximum load level imposed was 60% of the ultimate tensile strength. Measurements were also performed without any load applied. No preparation treatment was employed on the specimen surface since no corrosion/oxidation was present on the extracted wire. Besides, the application of superficial treatment could variate the stress state. Fig. 1 shows the adopted test setup.

Fig. 1. Test setup