## 2 State of the Art of NDTs for Concrete Prestress Evaluation
In contrast to the previous methods, stress release methods such as the drilled hole method and the saw-cut method have the advantage of being performed on concrete without requiring exposure of the strands. The drilled hole method has been widely used in mechanical and aeronautical fields since 1934 and has a standardized test procedure for steel [16]. Later, several authors [17; 18] explored improvements to make it suitable for concrete structural members, where small holes, suitable for steel, are impractical due to concrete's heterogeneity. To address this issue, the International Federation for Structural Concrete (FIB) [19] recommends holes ranging from 77 to 79 mm in diameter and between 100 and 175 mm in depth. However, in post-tensioned concrete beams with parabolic cable layouts, the ducts may be too close to the concrete surface for this method to be effectively applied. To overcome the limitations of the drilled hole method, Kukay in 2008 introduced the saw-cut method [20], which involves isolating a small section of concrete through a transverse saw cut and measuring the strain relief by means of strain gages bonded to the concrete surface. While this method seems promising for in situ testing, it has yet to be thoroughly validated, and the literature presents conflicting results [21; 22]. In particular, no direct evaluation of the error has been reported to date. The only available studies compare the results of the method with stress values obtained through analytical calculations of residual stress or finite element models [22; 23]. A significant source of uncertainty in the results arises from the difficulty of fully isolating the concrete section as the cut depth must be limited in practice to avoid damaging the tendons. One of the primary limitations of stress release methods lies indeed in their invasiveness. They require physically cutting into or drilling holes in the concrete element, which inherently causes localized damage to the structure. This damage must then be repaired, adding to the cost, time, and complexity of the assessment. For in-service structures, such invasive procedures can disrupt operations, compromise structural integrity (even if localized), and raise concerns about the long-term performance of the repaired sections. Furthermore, the operational complexity of these methods can be high, requiring specialized equipment, precise execution to ensure accurate stress relief, and often skilled personnel to perform the tests and interpret the results correctly. The process can be time consuming, especially when multiple test locations are required across a large structure. The flat-jack test is also performed directly on concrete, minimizing its impact on the structure. Compared with the drilled hole and saw-cut methods, it is operationally simpler and less invasive as it requires significantly less material removal, making it a promising alternative for evaluating concrete prestress. The flat jack was invented and patented by [PERSON] in 1939 [24], and after about a decade of improvements to its design, it began to be used in rock engineering. Over the next two decades, the flat-jack test became widely employed for the determination of deformability and in situ stress conditions in existing structures, such as masonry buildings [5; 25] and dams [6]. The minimally invasive nature of this test is further confirmed by its application to buildings of historical and cultural significance, as documented by various studies [26; 27; 28]. However, despite its extensive use in structural assessment, there is a notable lack of research on its application to reinforced and prestressed concrete structures. In particular, the few references in this respect mainly regard its use on post-tensioned beams, as illustrated in Figure 1[7; 29], but these references are limited and not well established.