PSI - Issue 84

D. Rossi et al. / Procedia Structural Integrity 84 (2026) 337–343

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1. Introduction In Post-Tensioned (PT) concrete systems, steel strands are typically housed within ducts and protected by cementitious grout to mitigate corrosion. Deficiencies in the grouting process, such as incomplete filling or moisture ingress, can compromise this protection and initiate corrosion of the steel, ultimately leading to a reduction in prestressing force. Significant losses of prestress may substantially alter the structural response under service conditions, resulting in increased deflections and reduced load-carrying capacity, with potentially serious consequences for safety and infrastructure resilience. The residual tension in post-tensioning tendons represents a key indicator of the actual condition of a PT system and is therefore essential for the assessment and management of existing bridges. Nevertheless, a thorough understanding of the physical mechanisms responsible for prestress losses is not sufficient, by itself, to determine the current value of the residual prestressing force in existing structures. This limitation primarily arises from the inaccessibility of the tendons and from the intrinsic difficulty in directly evaluating their present stress state. Methods based on stress release at the level of individual wires, although conceptually straightforward, are highly invasive, as they require direct access to the tendon and the cutting of at least one wire, which is generally unacceptable for in service structures. For this reason, reliable quantification of residual post-tensioning force in existing bridges necessitates the use of Non-Destructive Testing (NDT) techniques capable of providing meaningful information while minimizing the impact on the structural integrity (Pettorruso et al., 2025; Quaglini et al., 2023). Several methods have recently attracted growing interest within the research community for the evaluation of prestress in existing structures, including the Saw-Cut Method (Kralovanec et al, 2024), X-Ray Diffraction (XRD) techniques (Morelli et al., 2021), and the Flat-Jack Test (FJT) (Proverbio et al., 2021). The Saw-Cut Method relies on a stress-relief mechanism, in which a limited volume of concrete is mechanically isolated by means of controlled saw cuts, and the associated strain release is monitored to estimate the internal stress condition. By contrast, XRD-based approaches operate at the material microstructural level: X-rays are directed toward a steel wire or strand to quantify lattice strain, which is then converted into stress using the elastic properties of the steel. The necessity of directly exposing the steel component, however, significantly restricts the practical applicability of this technique. The FJT, on the other hand, represents a well-established methodology for masonry structures, supported by standardized procedures such as ASTM C1196-14 (2014) and RILEM recommendations (1999). Its transfer to reinforced concrete has been investigated only in a limited number of studies (Fedele and Maier, 2007), and applications specifically targeting post-tensioned concrete elements remain relatively scarce in the literature (Proverbio et al., 2021). For these reasons, further investigation into the applicability of the FJT for PT concrete structures is warranted, with particular emphasis on its ability to estimate residual prestress. This paper builds upon a previous preliminary study (Rossi et al., 2025) and aims to further evaluate the accuracy and practical implementation of the method under controlled laboratory conditions. To this purpose, an experimental campaign was conducted on concrete specimens subjected to known compressive stress states. FJTs were performed under load-controlled conditions, allowing a direct comparison between the stress estimated from flat-jack measurements and the actual stress applied to the specimens and analyzing two different sizes of flat-jack. The paper is organized as follows. Section 2 introduces the principle of FJT methodology. Section 3 describes the experimental program and the procedure adopted to estimate the stress in the concrete. Section 4 presents and interprets the experimental results. Finally, Section 5 summarizes the main conclusions of this preliminary investigation. 2. Principle of the Flat-Jack Test The FJT is employed to estimate the normal stress acting across a selected cross-section of a structural element. The method consists of executing a slot cut perpendicular to the external surface of the member. In the presence of compressive stress, the removal of material induces a partial stress release, causing a reduction in the distance between the faces of the cut. This deformation is recorded by displacement measuring devices, such as mechanical gauges, positioned across the cut line. After the stress release has been measured, a flat hydraulic jack is placed within the slot and connected to a hydraulic pressurization system. The internal pressure of the flat-jack is progressively increased while continuously