PSI - Issue 84

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

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installation of the displacement reference points across the intended cut line. Baseline measurements are then collected to establish the initial spacing between the gage points prior to any intervention. Subsequently, a slot is carefully cut in the concrete element, inducing a local release of the internal stress state, and the resulting displacement is measured. Once the slot has been completed, the flat-jack is inserted into the opening and properly positioned to ensure uniform contact with the slot surfaces. An initial pre-seating phase is performed to stabilize the device and eliminate any potential slack. The flat-jack is then pressurized incrementally, while the displacement between the reference points is continuously monitored. The loading process is continued until the measured spacing returns to the initial pre-cut value within the prescribed tolerance. At this stage, the corresponding internal pressure of the flat-jack is recorded and used for the subsequent estimation of the stress acting in the concrete. The tests were executed for three pressure level: 2 MPa, 3 MPa and 4 MPa. With reference to Equation (1), the pressure p is a known quantity; however, the conversion factors K m and K a need to be determined. The conversion factor K m was determined through laboratory calibration of flat-jack devices in accordance with ASTM (2014). The calibration setup is composed of the compression machine used for flat-jack calibration, a manual hydraulic pomp equipped with a manometer, and a control panel for recording the load cell readings. The calibration of the flat-jacks was carried out using a compression testing machine equipped with two rigid steel plates. Proper alignment with the loading axis was ensured by positioning each flat-jack so that the eccentricity of its centroid did not exceed 6 mm. Different load cells were employed depending on the flat-jack size, with capacities of 75 kN and 500 kN for the 25 cm and 33 cm devices, respectively. Steel shims with thicknesses of 4 mm and 5 mm were introduced between the plates to reproduce realistic slot openings. Each flat-jack was connected to a manual hydraulic pump and subjected to incremental pressurization up to 90 Bar, with pressure steps held constant for approximately 30 seconds while the corresponding force was recorded. The procedure was repeated for both flat-jacks and gap widths, performing three pressurization cycles for each configuration. The results from the first cycle were discarded due to initial seating effects, and the conversion factor K m was calculated using data from the stabilized third cycle. The correction factor K a is defined as the ratio between the effective area of the flat-jack and the mean area of the slot after testing. The slot geometry can be determined using conventional depth gauges, as adopted in the present experimental campaign. 4. Results This section first presents the results of the calibration performed on the two flat-jack devices. Subsequently, the outcomes of the experimental tests carried out on the concrete slabs are reported. The purpose of the flat-jack calibration was to determine the conversion factor K m , which was obtained according to Equation 2. = ℎ / − (2) where: • P machine [MPa] is the contact pressure that the flat-jack applies to the plates of the machine. It is calculated dividing the load recorded from the load cell by the nominal flat-jack area; • P flat-jack [MPa] is pressure inside the flat-jack read by the manometer. Figure 2 shows the results of the calibration procedure, presenting the conversion factor K m as a function of the flat-jack pressure recorded on the manometer for both devices. The plots indicate a nonlinear behavior common to both flat-jacks, with lower K m values at low pressures. As the flat-jack pressure increases, K m also rises, becoming proportional to P flat-jack above 50 Bar. The slot width noticeably affects the conversion factor K m for the 33 cm flat jack, likely due to variations in the jack’s shape factor, whereas its influence is practically negligible for the 25 cm flat-jack.