PSI - Issue 84

D. Rossi et al. / Procedia Structural Integrity 84 (2026) 337–343

342

the larger M33 flat-jack show markedly lower errors and variability. The mean error for the M33 device remains below 10% for all stress levels, decreasing to about 6% at 4 MPa, while the corresponding CoV values are consistently limited to the range of 6–7%. These results indicate that the larger flat-jack provides more accurate and repeatable stress estimations, whereas the smaller device is more sensitive to local effects and exhibits greater measurement scatter.

Fig. 3. Mean Error ( E m ); Coefficient of Variation (CoV) of test campaign.

5. Discussion and Conclusions The experimental investigations, performed under controlled laboratory conditions, provided preliminary evidence of the applicability of the FJT to concrete, a material stiffer than those traditionally investigated in masonry. The results indicate that the method can yield reasonably consistent stress estimates, with accuracy and repeatability strongly influenced by parameters such as flat-jack size and gauge length. Although greater measurement scatter was observed compared to masonry applications (where flat-jack testing typically achieves accuracies within ±10–20% and coefficients of variation on the order of 15–30%), the accuracy achieved using optimized configurations, particularly the 33 cm flat-jack, is considered sufficient for estimating the prestressing level. Despite the simplified experimental setup, this pilot study highlights the potential of the FJT as a semi-destructive technique for prestress assessment. Its ability to directly measure stress, combined with limited invasiveness and operational simplicity, makes it a promising alternative to more intrusive or complex methods currently used in bridge inspections. Nevertheless, further research is required to refine the methodology, including the development of stress transfer models, extended experimental campaigns covering a wider range of conditions, and the definition of practical guidelines for in-situ applications. Acknowledgement This research was carried out with the support of the Materials Testing Laboratory (LPM) of Politecnico di Milano, Italy. The Authors gratefully acknowledge Mr. Daniele Spinelli, Dr. Giacomo Vazzana, and Mr. Marco Antico for their valuable assistance during the experimental tests and the calibration of the equipment. The Authors also thank GRConsulting s.r.l. for their technical and practical support, as well as for providing the testing equipment. Special thanks are extended to Mr. Paolo Rossi for his professional contributions and continuous support throughout the experimental activities. References American Society for Testing and Materials (ASTM), 2014. C1196-14 Test Method for In Situ Compressive Stress Within Solid Unit Masonry Estimated Using Flatjack Measurements. ASTM International, West Conshohocken, PA, USA. https://doi.org/10.1520/C1196-14 Fedele, R., Maier, G., 2007. Flat-jack tests and inverse analysis for the identification of stress states and elastic properties in concrete dams. Meccanica 42, 387–402. https://doi.org/10.1007/s11012-007-9061-y