PSI - Issue 78

Dalila Rossi et al. / Procedia Structural Integrity 78 (2026) 98–104

103

4. Discussion and Conclusions The effectiveness of the Flat-Jack Test to estimate the stress in compressed concrete elements was preliminary investigated to explore its use for assessing the residual prestress in PT concrete members. Although the study was limited to a small number of samples, the results provide valuable insights and offer recommendations for the practical application of this methodology. Larger flat-jacks may be more accurate, likely due to the greater cutting depth they allow and a reduction in scale effects. The variability observed with the 25 cm flat-jack may be attributed to the limited cutting depth required (5 cm), which may not be sufficient to adequately represent the stress distribution within the slab. The 33 cm flat-jack, on the other hand, allows for a greater cutting depth (12 cm), thus reducing scale effects due to the larger size of the device. However, the size of the flat-jack is necessarily constrained by the dimensions of the element under investigation, particularly the maximum allowable cutting depth, which must not damage internal reinforcement or post-tensioned cables. Additionally, the flat-jack calibration should reflect actual testing conditions. In particular, the slot cut opening should be carefully measured on-site and replicated in the calibration set-up. The preliminary findings suggest that the Flat-Jack Test can achieve accuracy and repeatability comparable to, or even better than, other existing stress measurement methodologies. These promising results highlight the need for further research to refine and expand the application of this technique. Further research should focus on enhancing the performance of the 25 cm flat-jack, which may be preferable for post-tensioned elements where cutting depth must be limited. The stress distribution as a function of cutting depth could be investigated through finite element modelling, similar to studies performed for the saw-cut method (Kralovanec et al., 2024). This would help confirm and further understand the performance differences between flat jacks of different sizes, and support improvements to the testing protocol. Additional testing on reinforced concrete members is recommended to evaluate the influence of internal reinforcement on the accuracy and reliability of the

Flat-Jack Test results. Acknowledgements

This study was supported by FABRE – “Research consortium for the evaluation and monitoring of bridges, viaducts and other structures” (www.consorziofabre.it/en) through the project ”SAFOTEB - a reviewed safety format for structural reliability assessment of post-tensioned concrete bridges”. The Authors are grateful to Mr. Daniele Spinelli and Dr. Giacomo Vazzana of the Materials Testing Laboratory (LPM) at Politecnico di Milano, Italy for their assistance during the execution of the tests and the calibration of the

equipment. References

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