## 6 Discussion
### Transition from Laboratory Testing to Practical Application  
The successful transition of the flat-jack test from a controlled laboratory setting to practical engineering applications, particularly for assessing prestress in concrete bridges, necessitates addressing several key practical considerations and operational procedures that go beyond the fundamental feasibility explored in this study, including, e.g., slot depth control, steel bar avoidance, optimal flat-jack size, and adaptation to diverse cross-sectional forms.  
Slot depth control is crucial not only for ensuring the structural integrity of the element but also for the accuracy and repeatability of the test. An excessively shallow cut might not sufficiently relieve the stress in the concrete surrounding the flat jack, leading to an underestimation of the true stress. Conversely, an overly deep cut could damage the structure or increase the risk of encountering internal reinforcement or prestressing tendons. Closely related is the critical aspect of steel bar and tendon avoidance. In a reinforced or prestressed concrete member, the flat-jack slot must be meticulously planned to bypass any embedded steel. Preliminary ground-penetrating radar surveys can aid in locating these elements and define a precise minimum slot distance to avoid damaging steel bars or prestressing tendons.  
Furthermore, the choice of flat-jack size presents a key practical trade-off. While smaller flat jacks offer the advantage of reduced invasiveness and potentially easier application in confined spaces, they may be more susceptible to the localized heterogeneity of concrete and less representative of the average stress across a larger section. Conversely, larger flat jacks tend to average out the effects of local heterogeneity, potentially providing a more representative stress measurement for the overall concrete element, but they inherently require a larger and more invasive cut, which might not always be feasible or desirable on in-service structures. This selection directly impacts both the practical execution of the test and the interpretability of its results in a heterogeneous material.  
Finally, the adaptation of the flat-jack test to different cross-sectional forms, such as T-beams, box girders, or slabs with varying thicknesses, introduces complexities in test point selection and result interpretation. Unlike the axially loaded, uniform concrete slabs examined in this study, real bridge elements exhibit complex stress distributions,often involving combined bending-shear forces, eccentric loads, and non-uniform sections. Selecting test points in such geometries requires careful consideration of stress gradients and the influence of internal structural features. The localized stress measurement obtained by the flat jack must be accurately correlated to the global stress state of the member, which is significantly more intricate in complex cross-sections.  
While these practical considerations are critical for the widespread adoption and reliability of the flat-jack test in the field, their detailed investigation falls outside the primary scope of this preliminary study. The development of comprehensive guidelines concerning slot depth control, steel bar avoidance (including specific minimum distances) and optimal flat-jack sizing strategies, and methodologies for adapting the test to various complex cross-sections, along with their detailed implications on results, represent essential avenues for future research.