PSI - Issue 78

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

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10 cm and 14 cm for the 25 cm and 33 cm flat-jacks, respectively, following relevant guidance (ASTM, 2014; RILEM, 1999). According to them, the gauge length should range between 0.3 and 0.6 times the length of the flat-jack used.

(a) (c) Fig. 2. (a) Concrete sample positioned in the test set-up and layout of cut line and gauge points, dimensions in cm. (b) Flat-jacks involved in the study, dimensions in cm. (c) Pump with manometer connected to the inlet port of the flat-jack. The Flat-Jack Tests were performed under load-controlled conditions at three pressure levels: 2 MPa, 3 MPa, and 4 MPa. At each level, pressure measurements were acquired using the flat-jack. The saw cut was carried out once per test, always at the 2 MPa pressure level. Pressure levels were applied incrementally from 2 MPa to 4 MPa on the intact concrete slab. At each level, the distances between each pair of gauge points were recorded using a mechanical strain gauge, including baseline readings under quasi-unloaded conditions (approximately 0 MPa). A slot approximately 25 cm or 33 cm in length, depending on the test, was made perpendicular to the applied load direction, and the corresponding flat-jack, connected to the hydraulic pump, was inserted (Fig. 2c). For each pressure level, three cycles of flat-jack pressure measurements were performed, consisting of: (i) acquisition of post-cut displacements (flat-jack unpressurized), (ii) gradual pressurization of the flat-jack until pre-cut distances were restored, with the corresponding pressure recorded, and (iii) complete depressurization. The final pressure was defined as the average value from the three cycles. This procedure was repeated for each pressure level. Additionally, the effective geometry of the cut, including slot length, opening width, and depth, was measured. 3. Results The results of the Flat-Jack Tests performed according to Section 2 are presented hereinafter. Fig. 3 shows examples of the baseline measurements taken on a concrete slab, with the two different gauge lengths of 10 cm (left panel) and 14 cm (right panel). The diagrams show the relative displacement (Δ) between each pair of gauge points as a function of the stress applied to the slab ( σ c ), with respect to the measurement at 0 MPa. Results for the 10 cm gauge length are represented by triangles (Fig. 3a), while those for the 14 cm gauge length are shown as dots (Fig. 3b). At each stress level, the relative displacements for the 10 cm gauge length are more consistent among each other compared to those observed for the 14 cm gauge length. This trend was confirmed in all tests (diagrams not reported for sake of simplicity). (b)