## 6 Discussion
### Reproducibility of the Test  
It is worth comparing the large measurement spreads obtained experimentally with the typical variability of the flat-jack test conducted on masonry structures, where this method is widely adopted and standardized.  
In masonry the flat-jack test is generally considered a reliable method for assessing in situ compressive stress and deformability. This reliability stems from extensive research and practical experience, with various national and international standards [30; 31] providing guidelines for its application. Accuracy can often be within \(\pm\)10-20% of the actual stress, particularly for average stress determination over large areas [5; 8]. In general, the variability ranges from 15 to 30%, depending on the heterogeneity of the masonry, the skill of the operator, and the consistency of the cutting and jacking procedures. The standard ASTM C1196 [30] acknowledges that the \(CoV\) of this method can be as great as 20% and recommends performing at least three tests in the same general area to verify the test results. Despite this variability, the method is commonly accepted in in situ structural testing as the test effectively averages out the macro-heterogeneity of units and mortar over the tested area.  
The use of flat-jack tests in concrete is less prevalent and not as well documented as in masonry. However, the variability is expected to be influenced by factors, reinforcement layout, and concrete curing conditions. When applying the flat-jack test to concrete, especially for residual prestress in post-tensioned elements, the material's properties such as aggregate size distribution and the presence of embedded steel introduce new complexities that can influence measurement accuracy and spread. To the knowledge of the authors of this paper, specific data on the \(CoV\) for flat-jack tests in concrete are missing, [PERSON] et al. [34] discuss the application of flat-jack tests in concrete dams, highlighting the method's potential for assessing stresses and Young's modulus in deteriorated concrete structures. However, while they emphasize the need for careful calibration and interpretation, specific \(CoV\) values are not provided. [PERSON] et al. (2017) and [PERSON] et al. (2019) report the results of prestressing load estimation in post-tensioned concrete beams but do not provide specific information on the spread of measurements.  
In the current study, \(CoV\) values from tests performed on unreinforced concrete slabs ranged from 25% to 43% for the 25 cm flat jack and 10% to 40% for the 33 cm flat jack, which were significantly higher than the typical \(CoV\) values reported for masonry. Several factors which can contribute to this divergence, including heterogeneity of concrete properties among the tested slabs and interaction between the flat jack and the randomly distributed micro-heterogeneities of concrete (aggregates, cement paste, and voids), have been already discussed. However, the small dataset means that the high spreads are indicative of trends rather than definitive statistical conclusions. A larger experimental campaign would be necessary to confirm these observed divergences and better understand their statistical distribution.