PSI - Issue 84

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

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monitoring the displacement between the reference points. Pressurization is continued until the measured distance between the gauge points returns to its original pre-cut value. At this stage, the pressure registered by the manometer is associated with the stress acting normal to the cut section. The estimated stress is then refined by applying a dimensionless calibration coefficient, , and a geometric correction factor, , which account for the mechanical characteristics of the flat-jack and the test configuration. A detailed description of these correction factors is provided in the following sections. According to the ASTM (2014), the average compressive stress in the element, f m , is calculated as: = ∙ ∙ (1) Where p [psi or MPa] is the flat-jack pressure required to restore the initial pre-cut distance between the gauge points. The conversion factor accounts for the inherent stiffness of the flat-jack, which resists to expansion. As a result, the internal fluid pressure within the device is higher than the stress applied to the tested element. thus links the internal oil pressure of the flat-jack to the stress applied to the tested element. The correction factor , on the other hand, accounts for the difference between the area of the flat-jack and the area of the cut slot. Since the slot area is usually larger than the surface of the inserted device, the effective loaded area differs from the surface area of the device. 3. Experimental Program The experimental campaign was conducted on four concrete specimens, each consisting of a slab cast with C20/25 concrete and dimensions of 34 × 150 × 50 cm (length × height × width). The test configuration, schematically illustrated in Fig. 1a, comprised two reaction frames rigidly anchored to a strong floor and a servo-hydraulic actuator used to apply axial compression. The concrete slabs were positioned horizontally and carefully aligned with the actuator axis in order to prevent eccentric loading. The compressive force was transferred to the specimens through rigid steel bearing plates placed at both ends, ensuring a nearly uniform stress distribution across the cross-section. The actuator was operated under load-controlled conditions, and the applied force was continuously measured using a calibrated load cell. Two semicircular flat-jacks were investigated. The smaller one measured 25 cm in length and 5 cm in height, while the larger one had a length of 33 cm and a height of 12 cm (Fig. 1b). A total of five tests were conducted using the smaller flat-jack, while three tests were performed with the larger device.

a) b) Fig. 1. a) Test setup and configuration; b) Flat-jacks, dimensions in cm (adapted from Rossi et al., 2025)

The testing protocol is organized into a sequence of operational steps, as introduced by Rossi et al. (2025). The procedure begins with the identification and marking of the test area on the surface of the specimen, followed by the