PSI - Issue 78

Dalila Rossi et al. / Procedia Structural Integrity 78 (2026) 98–104

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highlighted this issue, such as the Fossano Viaduct in 2017 and the Polcevera Viaduct in 2018 (Rymsza, 2021), both within the Italian context, and other tragic events worldwide (Menga et al., 2023). There are two methods for prestressing concrete: pre-tensioning, where steel wires are stressed before casting the concrete, and post-tensioning (PT), where steel wires are stressed after the concrete has cured (Gupta, 2006). In PT systems, steel strands are placed inside ducts filled with cementitious grout to prevent corrosion. However, inadequate grouting, such as incomplete injection or water infiltration, can lead to corrosion of the steel strands, causing tension losses. Substantial tension losses can significantly affect the bridge’s behaviour under service loads, increasing its deformability and reducing its ultimate strength. These effects can have serious consequences for safety and infrastructure resilience. Residual tension in cables represents the key parameter of the actual condition of the PT system, making it an essential information for existing bridges assessment and management. However, the deep knowledge of the aforementioned phenomena which lead to post-tension loss is not sufficient, nor does it allow to determine the exact value of the residual tension as of today. To quantify the residual post-tension in existing bridges, Non-Destructive Tests (NDTs) with a negligible impact on the structure need to be performed (Quaglini et al., 2023, Pettorruso et al., 2025). The techniques that are gaining interest in the scientific community are the Saw-Cut Method (Kukay, 2008), the X Ray Diffraction (XRD) (Morelli et al., 2021) and the Flat-Jack Test (Proverbio et al., 2021). The Saw-Cut Method is a stress release technique. This is done by isolating a portion of concrete with saw cuts and measuring the stress relieved through strain measurements. Bellini and Mazzotti (2024) reported an accuracy of 7% to 8% for the Saw-Cut method under controlled conditions. Similarly, Rossini et al. (2024) confirmed its reliability through laboratory testing, with results showing good agreement with analytical and numerical predictions: the method underestimated the actual prestress by about 10%, with a Coefficient of Variation ( CoV ) of 21%. The XRD operates on a different principle. X-Rays are shot at the steel wire/strand to measure the strain within the crystal lattice and estimate the stress, knowing the material composition and its elastic properties. Thus, the steel wire/strand must be exposed. Morelli et al. (2021) state that the methodology they proposed estimates the prestressing force in the wire or strand with an accuracy of ±10% for typical working loads (60% of the yielding strength) and ±30% for low working loads. Mazzatura et al. (2024) added that the technique overestimates the pre-stressing force for strands and underestimates it for wires. While the accuracy for the Saw-Cut and XRD methods can be found in the literature, such information is not available for the Flat-Jack Test. The Flat-Jack Test is a well-known and established technique for masonry structures, for which both American standards, such as ASTM C1196-14 (2014), and RILEM recommendations (1999) are available. By contrast, its application to reinforced concrete structures is limited (Fedele and Maier, 2007), and only a few references exist for post-tensioned concrete (Proverbio et al., 2021). Nevertheless, it is a promising technique. It is relatively easy and practical to perform, and it is minimally invasive as confirmed by its use on buildings of historical and cultural significance (Carpinteri et al., 2005; Binda and Tiraboschi, 1999; Forcellini et al., 2019). Most importantly, it directly provides the stress value, unlike the other two tests which require the knowledge of the material properties. For these reasons, it is of interest to investigate the application of the Flat-Jack Test to post-tensioned concrete, with the aim of determining its residual stress. This paper builds upon a previous preliminary investigation (Rossi et al., 2025) and aims to further assess the accuracy and implementation of the Flat-Jack method under controlled conditions. To this end, an experimental campaign was carried out in the laboratory, where concrete samples were subjected to a known state of compression. The Flat-Jack Test was performed under load-controlled conditions, enabling a direct comparison between the stress measured by the flat-jack and the actual stress applied to the samples. The remainder of the paper is structured as follows. In Section 2, the Flat-Jack Test method and the calculation process used to estimate the stress in concrete based on the internal pressure in the flat-jack are described. The section also presents the details of the experimental campaign, including the materials used and the procedure followed for conducting the tests. Section 3 illustrates the results of the Flat-Jack Test carried out during the experimental campaign. Finally, Section 4 discusses the conclusions drawn from this preliminary study.