PSI - Issue 78

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

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2. Methods and Materials The Flat-Jack Test is used to assess the normal stress acting on a specific cross-section of a structural member. The procedure involves making a cut perpendicular to the outer surface of the element. If the element is under compression, the internal force causes the edges of the resultant slot to close. A displacement transducer (e.g., a mechanical strain gauge) measures the change in distance between each pair of gauge points placed at either side of the cut line. The flat-jack device is then inserted into the cut slot and connected to a hydraulic pump. The oil pressure inside the flat jack is gradually increased until the initial pre-cut distance between the gauge points is restored. Finally, the stress acting perpendicularly to the cut section is determined based on the internal pressure of the flat-jack measured using a manometer. Subsequently, this value is corrected by applying the dimensionless conversion factor K m and the correction factor K a . Further clarification of these constants is provided below. 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 K m 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. K m thus links the internal oil pressure of the flat-jack to the stress applied to the tested element. The correction factor K a , 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. The conversion factor K m was determined through laboratory calibration of flat-jack devices in accordance with ASTM (2014). For brevity, the specific details of the calibration procedure are not included in this paper. The resulting calibration curves represent the conversion factor K m as a function of the pressure inside the flat-jack read on the manometer. These curves were developed for two slot gap openings, 4 mm and 5 mm respectively. Therefore, the actual slot gap must be measured to select the relevant calibration curve. The correction factor K a is the ratio of the measured area of the flat-jack to the average measured area of the cut slot after the test. The latter measurement can be done using a depth gauge (as was done in this experimental program), or a laser scanner as proposed by Lulić et a l. (2023). Fig. 1 presents a flowchart that schematically illustrates the described process.

Fig. 1. Flowchart representing the calculation process of the concrete stress from the flat-jack pressure measurements.

The study was conducted on three C20/25 concrete slabs (dimensions: 34×150×50 cm) tested using a servo hydraulic jack and two reaction plates anchored to a strong floor (Fig. 2a). The load was applied axially through rigid plates to ensure uniform pressure distribution. 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. 2b). Each slab underwent two tests (one per flat-jack), for a total of six tests. The tests were labelled as n°MX , where “ n° ” is the sample number and “ X ” identifies the device used. The cut line was marked approximately 54 cm from the slab end, and three rows of gauge points (A, B, C) were placed at 5 cm intervals across it (Fig. 2a). Each gauge base consisted of two gauge points with conical seats, fitting the tips of the mechanical extensometer. Gage lengths were