## 4 Experimental Program  
### Materials and Methods  
The study involved three concrete samples, each consisting of a slab made of C20/25 concrete with dimensions of 34 \(\times\) 150 \(\times\) 50 cm (length \(\times\) height \(\times\) width). The dimensions were chosen to minimize edge effects. The maximum aggregate size of concrete was 20 mm. The experimental set-up is sketched in Figure 4 and consisted of two reaction plates anchored to a strong floor and a servo-hydraulic jack. The concrete slab was positioned horizontally and carefully aligned with the axis of the jack to prevent load eccentricity. The load was applied to the slab using rigid metal plates at both ends, ensuring a uniformly distributed pressure on the surface. The servo-hydraulic jack was operated in load-controlled mode, and the force applied to the sample was read by a calibrated load cell. The procedure followed the one described in Section 3. After identifying the test area on the surface of the slab, the cut line and the position of the gage points were drawn (Figure 5). The cut line was located 18 cm from the end of the slab. Four rows of gage points were created: two rows at 14 cm gage length and two rows at 20 cm, according to  
Figure 4: Experimental set-up: (**a**) Schematic representation. (**b**) Concrete slab positioned in the test set-up. the layout sketched in Figure 5a. Each row comprised 5 gage points, with 5 cm spacing between two consecutive points. The gage points consisted of small metal plates with a conical seat at the center, fitting the tips of the mechanical extensometer. The plates were bonded to the concrete surface using a suitable adhesive. 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 (Figure 5c). It must be noted that in applications in PT reinforced concrete elements, the height of the flat jack was constrained by the maximum allowable depth of the cut, which must not damage rebar or tendons. The tests were labelled by the acronym _MXY_, where \"\(X\)\" indicates the slab tag and \"\(Y\)\" refers to the flat-jack: \"A\" stands for the smaller device, and \"B\" stands for the larger one (Table 1). Only two tests with the 33 cm flat jack were performed because at the end of test M1B (test M4B was already performed), the jack became stuck in the slot. During the attempt to remove it, the device was damaged (specifically, the weld was compromised). Therefore, fearing that its reliability had been affected, further tests with this flat jack were discontinued. Figure 5: Implementation of the testing procedure: (**a**) Layout of cut line and gage points (dimensions in cm). (**b**) Cut line and gage points positions drawn on slab 1. (**c**) Flat jacks involved in the study. (**d**) Cutting of the slot with a circular saw, where the red arrow indicates the longitudinal axis of the concrete slab. The tests involved the assessment of the flat jacks at three levels of stress in the concrete slab: 2, 3, and 4 MPa. First, the pressure levels were applied incrementally (from 2 MPa to 4 MPa) on the intact concrete slab, and baseline measurements of the distance between each pair of gage points were recorded at each level. Measurements were taken using two extensometers, one with a gage length of 14 cm and the other with 20 cm gage length. At each stress level, readings were taken approximately 5 min after the load was applied to allow for stabilization. Once the baseline readings were obtained at all three stress levels, the flat-jack tests were performed following the protocol described below:  
* The load corresponding to the highest pressure level (4 MPa) was applied to the slab. * A cut of approximately 25 cm in length was made perpendicularly to the direction of application of the external load using a circular saw with a blade with diameter of 350 mm and thickness of 3 mm (Figure 5d). The compressive force on the slab was controlled and held constant during the operation.