## 6 Discussion
### Technical Positioning of the Flat-Jack Test  
Existing methods for assessing the prestress in concrete, or alternatively the tensioning force in the steel tendons, are described in Section 2 of this paper and include X-ray diffractometry and stress release methods such as the drilled-hole method, the saw-cut method, and the proposed flat-jack test. Each method has distinct advantages and disadvantages. Understanding these distinctions clarifies the technical positioning of the flat-jack test as a promising alternative. Invasiveness is a key differentiating factor. XRD is highly invasive, often requiring complete removal of concrete cover and grout to expose steel for direct measurement, making it impractical for routine bridge inspections. Drilled-hole and saw-cut methods are semi-destructive, creating localized damage (namely, a small hole or a slot, respectively) that requires repair. The flat-jack test is also semi-destructive, involving a slot cut, but its localized damage is manageable and repairable, often considered comparable to or slightly less impactful than a full saw cut for stress relief. Each method presents its own nuances regarding detection accuracy. XRD measures the stress in exposed steel wires and requires a detailed mechanical model to relate such measurement to the overall tensile force in the tendon. Its accuracy is limited by its surface-sensitive nature and the inherent difficulty of accessing internal components without altering the stress state, making it less reliable for representing the overall prestress within the concrete matrix or at greater depths. Drilled-hole and saw-cut methods provide moderate to high local accuracy for localized stress measurements immediately around the cut. Their accuracy, however, heavily depends on meticulous execution, accurate strain measurement, and the correct application of theoretical or empirical correlations, with factors like material heterogeneity and precise cut geometry influencing the results. The flat-jack test provides moderate accuracy, sensitive to precise jack fit in the slot and accuracy of its calibration coefficients, and requires advanced models to translate local readings to global stress states. Setup conditions like gage length and jack size can also influence its accuracy and must be included in the model. In terms of operational efficiency, XRD is very low for field applications due to bulky equipment, specific power and environmental requirements, and the need for highly trained personnel. Drilled-hole and saw-cut methods have medium efficiency, requiring specialized cutting or drilling tools along with strain gages and data acquisition systems. The process can be time-consuming per test point, encompassing cutting, strain gage installation, measurement, and subsequent repair, often demanding complex site logistics. The flat-jack test also has medium operational efficiency; its equipment is generally less complex and more portable than XRD, and its procedure can be comparatively quicker than some drilled-hole setups, making it moderately efficient for field deployment, although it still necessitates post-test repair. From a cost perspective, XRD is typically the most expensive due to high equipment and specialized labor cost. At present stage it is still reserved for fundamental research. Drilled-hole and saw-cut methods fall into the moderate to high cost range. Significant investment in specialized equipment may be required, and the labor involved in preparation, testing, and post-test repair substantially contributes to the overall expense, with costs escalating quickly for multiple test points. The flat-jack test for masonry structures typically lies in the medium cost range, and the same is expected when transferred to concrete. While the initial investment for specialized equipment (jacks, pump, gages, and cutting tools) can be notable, the per-test cost might be competitive once the equipment is acquired.