## 1 Introduction
Therefore, if preliminary bridge inspections reveal widespread defects, a detailed assessment is necessary to accurately determine the residual tension level. Tension loss can affect the behavior of the bridge at both the serviceability limit states (SLS) and the ultimate limit states (ULS), directly compromising the structure's ability to carry loads safely and maintain long-term durability. At the SLS, the prestressing force contributes to shifting the position of the neutral axis, thereby controlling deflections and delaying the onset of cracking. A significant loss of prestressing will lead to increased deflections, earlier cracking, and enhanced penetration of aggressive agents, which accelerate degradation. At the ULS, the prestressing force provides a direct contribution to both flexural and shear resistance. Its reduction can therefore compromise the overall load-bearing capacity of the structural element. Different types of brittle failure can occur depending on the structural scheme: simply supported beams become more susceptible to shear failure; segmental structures may experience monolithic collapse due to joint opening,and in continuous beams, maximum moments typically occur at the supports, making it more challenging to detect critically stressed areas. The evaluation of the structural safety and performance of post-tensioned concrete structures therefore requires a precise assessment of the residual prestress in concrete. Unforeseen or excessive prestress loss can lead to undesirable cracking; increased deflections; reduced fatigue life; and ultimately, a compromised structural integrity, potentially jeopardizing both serviceability and ultimate limit states and, in severe cases, leading to structural failure. The issues discussed above highlight the need for reliable methods to assess the prestressing of post-tensioned concrete structures. Accurately determining the residual prestress is crucial for evaluating the actual condition and identifying severe deterioration of the PT system, otherwise difficult to detect; predicting the remaining service life; and making informed decisions regarding inspection, maintenance, and potential strengthening interventions to ensure the continued safety of these structures. However, as it will be discussed later, existing technologies for this purpose often present significant limitations, particularly in terms of invasiveness and operational complexity, which hinder their widespread and practical application. This justifies the ongoing investigation into alternative, more practical methods. In this context, the flat-jack test [5; 6; 7] appears promising for two key reasons: it is relatively simple and practical to perform, and it is minimally invasive as it does not require direct exposure of the steel strands. Flat jacks are widely used in structural and geotechnical investigations to measure in situ stresses, assess masonry and rock stability, monitor deformations, and evaluate the effects of reinforcement or restoration interventions. However, their application to prestressed reinforced concrete structures remains largely unexplored. This study aims to explore the fundamental feasibility of the flat-jack test for estimating the prestress in concrete members and identify the initial challenges and opportunities associated with this method. Section 2 of the paper presents a review of existing available non-destructive methods proposed for the assessment of the residual prestress in reinforced concrete (RC) elements, while Section 3 illustrates the general application of the flat-jack test methodology to concrete elements. In Section 4 a preliminary experimental study, consisting of a series of laboratory tests on concrete slabs, is presented. The slabs were subjected to the application of a controlled compression force, and flat-jack tests were performed using different instrumentations and setups. The experimental findings are shown and analyzed in Section 5, where the implications of the dimensions of the flat jack and the gage length are discussed.