PSI - Issue 62

Alessandro Bellini et al. / Procedia Structural Integrity 62 (2024) 315–322

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A. Bellini et al. / Structural Integrity Procedia 00 (2019) 000 – 000

To make the critical situation worst, today’s traffic loads and intensity are much higher than in the past and not considered during the original design phase. This contributes to further accelerating structural degradation (Bencivenga et al., 2022). In this framework, the need to evaluate the conservation status and assess the overall deterioration level of elements composing bridges and viaducts becomes of paramount importance, in order to have an estimation of their structural safety during both operational conditions and exceptional events. Specific guidelines recently approved (MIMS, 2021) provide administrators and technicians of the Italian road and motorway network with an innovative tool for assessing the safety of existing bridges, in relation to different types of risks, and consciously planning the following monitoring and intervention strategies. This renewed sensibility pushed a number of new scientific studies and recalled many existing others concerning the assessment techniques of existing concrete (Liu et al., 2023; Fox et al., 2023; Santarsiero et al., 2023) and masonry bridges (Liu et al., 2023b; Biscarini et al., 2020; Ferretti et al., 2019; Sassoni and Mazzotti, 2013). Among the different technologies, the present paper focuses on prestressed concrete bridges, which are a large share of the built heritage. Unfortunately, it should be noted that degradation often involves the internal part of the structural elements and is not visible from the surface, such as in the case of steel strands present in such type of bridges and viaducts. Prestressed reinforced concrete elements can be subjected to relevant stress variations over time because of steel stress relaxation and concrete time-dependent deformations: these phenomena, anyway, are generally predictable and taken into account during the design phase, but often not properly. In addition, the effectiveness of the prestressing system may be largely reduced or impaired by the possible corrosion of steel strands and their subsequent partial failure, due to progressive material degradation, with a significant reduction of the prestressing force acting on concrete. In this framework, special inspections are needed (Latte Bovio et al., 2022; MIMS, 2021), in particular for all the bridges with post-tensioned cables, with the aim of investigating possible defects which can promote corrosion and safety reduction. Techniques such as endoscopic tests or sampling of portions of the injection material, even if effective, are invasive and provide only local information. For this reason, a number of non-destructive and partially destructive techniques have been developed along the decades, with particular interest and efforts during recent years. Among them (Parivallal et al., 2011; McGinnis and Pessiki, 2016), prestress release tests are becoming more and more widespread, because they can be carried out as preliminary tests and can provide more general results (useful in terms of cross-sectional analysis). This technique consists in the evaluation of the residual pre-compression force inside the concrete, in correspondence of some specific elements cross-section, by releasing the prestressing strain acting on this material due to tendons and strands. The deformation can be measured by means of strain gauges, glued on the surface of the concrete in correspondence of the area identified for the extraction of the sample. If the elastic modulus of the material is known, through simple mechanical tests on concrete, strain can then be easily converted into stress. Different procedures for performing prestress release tests are described in the literature (Kesavan et al., 2005; Lofrano, et al., 2018; Kral’ovanec et al., 2021; Martinello, 2021). However, although this technique is widely used in professional practice, there isn’t yet any robust validation of this diagnostic method in scientific literature, in terms of reliability and accuracy (Lupoi and De Benedetti, 2021). The present paper describes the results of an experimental campaign focused on prestress release tests carried out on reinforced concrete elements, with the aim of obtaining a scientific validation of the method in a controlled laboratory environment, by estimating its accuracy and the expected residual error that should be reliably taken into account during its application. Tests were performed on RC columns subjected to a preassigned and constant compressive axial load, considering different parameters, such as the geometry of the extracted block and the order of the saw cuts, with the purpose of proposing a robust and easily applicable procedure. 2. Experimental program and materials mechanical properties Prestress release tests were performed on RC columns subjected to a preassigned compressive axial load. Columns were built adopting a square cross-section (40×40 cm 2 ) and a total height of 150 cm (see Fig. 1a), using only the minimum reinforcement required for moving and placing them within the experimental set-up. The choice was made in order to avoid positioning longitudinal bars and stirrups close to the areas subjected to prestress release tests. Columns were made with C32/40 concrete and B450C steel bars. Concrete was mechanically characterized in order to obtain its cubic compressive strength and elastic modulus obtaining, respectively, 55.3 MPa and 28188 MPa. The experimental plan is summarized in Table 1, where, for each type of test, the number of tests performed and the most important parameters used during the experimental campaign are reported. In detail, after a description of the test type (prestress release test or test for evaluating the drying shrinkage release), also the applied vertical stress σ v , the order of making the saw-cuts for extracting the samples, their orientation (P = parallel, O = orthogonal to the applied vertical stress – see Fig. 1b) and the inclination of them with respect to the concrete surface (90° or 45°) are indicated. The exact order of making each saw-cut is indicated with a letter (from A to D), according to the scheme reported in Fig.1b. Further details on these parameters will be presented and discussed in the following.