## 6 Discussion
### Application Differences Between Concrete and Masonry  
The flat-jack test, traditionally a well-established method for assessing in situ stress and deformability in masonry structures, presents distinct application differences when transferred to concrete, particularly prestressed concrete. These differences primarily stem from the varying forms of material heterogeneity and the presence of embedded steel elements. In masonry structures, heterogeneity is intrinsic to their composite nature, comprising discrete units (bricks and stones) bound by mortar joints. The mechanical properties of units and mortar can vary significantly, and the presence of bed and head joints creates planes of weakness and anisotropy. The slot cut for the flat jack typically intersects both units and mortar, leading to a complex stress redistribution, which is further enhanced by geometrical heterogeneities like voids created by the cut. Steel shims are commonly used to achieve a tight fit and bridge over any interior voids. The interpretation of the measured stress relief and subsequent re-compression must account for this inherent heterogeneity as the test aims to capture an average stress state over the tested area, influenced by the varying stiffnesses and potential localized cracking within the composite material. On the other hand, concrete structures, although seemingly more homogeneous than masonry, possess their own forms of heterogeneity due to the presence of aggregates, cement paste, and micro-voids. The size and distribution of coarse aggregates can influence the cutting process and the localized stress field around the flat-jack slot, potentially affecting readings. Nevertheless, when the dimension of the flat jack is sufficiently large with respect to the size of the aggregate, the effect of such heterogeneities can be assumed to be negligible. A more significant and unique challenge in concrete, especially post-tensioned members, is the distribution of steel reinforcement and prestressing tendons. Unlike masonry, where the test primarily interacts with the load-bearing masonry fabric, in concrete, the flat-jack slot must be carefully positioned to avoid cutting through reinforcing bars or prestressing tendons (which can be detected, e.g., by performing a preliminary ground-penetrating radar survey). The presence of these embedded steel elements fundamentally alters the stress flow within the concrete. The flat-jack test in this context aims to measure the residual compressive stress in the concrete cross-section, which is a direct consequence of the prestressing force exerted by the tendons. Therefore, understanding the influence mechanisms of concrete's inherent heterogeneity and, more importantly, the precise location and stress contribution of the steel reinforcement and prestressing system, becomes paramount for accurate test execution and reliable interpretation of results. This distinction highlights why the application of the flat-jack test to prestressed concrete constitutes a novel and more complex investigation, requiring a thorough understanding of these material and structural interactions. For its practical application to concrete, it is necessary to develop a comprehensive stress transfer model that accounts for the complex material and structural interactions between the concrete and embedded steel elements, effectively correlating the localized stress measurement obtained by the flat jack to the actual, overall stress distribution within the reinforced concrete cross-section and to the tensile force in the tendons. However, given the preliminary nature of this study, such a detailed model has not yet been developed. The study was intentionally kept simple, and a controlled experimental setup using C20/25 concrete under axial compression was designed to establish a baseline understanding and gather initial, reliable data. This approach was crucial for establishing a baseline understanding before embarking on a more extensive and complex research program.