PSI - Issue 84

Silvia Manarin et al. / Procedia Structural Integrity 84 (2026) 231–239

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1. Introduction The evaluation of residual prestressing force in post-tensioned concrete structures is a critical aspect of structural health monitoring and safety assessment. The effective tension in the prestressing strands is fundamental for maintaining the long-term structural integrity and overall performance of prestressed concrete elements, which are elements often used in critical infrastructure such as bridges, see Chen and Wissawapaisal (2005). Due to the hidden and embedded nature of prestressing tendons, they are not directly accessible for traditional direct condition assessment methods. Consequently, several non-destructive testing (NDT) techniques, the most common of which are illustrated in Manarin et al (2025), have been developed and calibrated to provide reliable stress measurements in steel strands and wires. Among these techniques, X-ray diffraction (XRD) has emerged as a promising tool for direct stress evaluation, despite the practical challenges associated with in-situ applications. This method is based on Bragg’s law, which is explained, for instance, by Fitzpatrick et al (2005), and relates the diffraction angle of X-rays to the interplanar spacing of atomic lattice planes in a crystal. Worth mentioning, this method suffers from the problems of providing a very superficial measure, as the X-ray penetrates few μ m inside the matter, and as a consequence, it may be influenced by residual stresses induced by thermal treatments. This work summarizes the preliminary results obtained through a large experimental campaign aimed at evaluating the accuracy of the method under laboratory conditions. 2. Experimental Setup and Instrumentation The experimental campaign is carried out using a GNR SpiderEDGE X-ray diffraction (XRD) system mounted on a photographic tripod and equipped with a metallic arm to ensure alignment of the X-ray beam normal to the wire axis. SpiderEDGE is a portable X-ray diffractometer specifically developed for the non-destructive characterization of metallic materials. Its compact architecture and battery-powered operation render it particularly suitable for on-site measurements. The system is fully compliant with relevant international standards, including UNI EN 15305 and ASTM E915, thereby ensuring the reliability, reproducibility, and traceability of the obtained results. The instrument consists of four principal components: the diffractometer head, a main control unit , an X‑ray warning indicator, and a rechargeable battery. For each measurement, it is essential to define the following parameters: • Collimator diameter: three collimators are available, with diameters of 2 mm, 1 mm, and 0.5 mm. The choice of collimator diameter is governed by the accessible surface area, with smaller diameters being preferable for more spatially constrained regions. The acquisition time must be adjusted accordingly: a reduction in collimator diameter necessitates a longer acquisition time to achieve an adequate signal‑to‑noise ratio. • Acquisition time: as the collimator diameter decreases, the acquisition time required to obtain reliable data increases. According to the instrument manufacturer, the recommended minimum acquisition times are 30 s for the 2 mm collimator, 60 s for the 1 mm collimator, and 120 s for the 0.5 mm collimator. • X‑ray incidence angle: this angle corresponds to the goniometer tilt angle of the instrument, generally on the order of ±40°. Prior to defining the scan range, it is necessary to verify that no mechanical interference or collision occurs between the instrument and adjacent surfaces during operation. In situ constraints may require a reduction of this angle due to limited accessibility. Nonetheless, it is advisable to employ the largest feasible angle to enhance measurement accuracy. For the instrument used in this study, the maximum permissible incidence angle for α‑iron is ±45°. • Number of points: this parameter specifies the number of discrete intervals into which the incidence angle range is subdivided and at which measurements are acquired. In most applications, a minimum of nine points is employed. The parameters listed above define the essential input required by the instrument’s proprietary software, EDGE. Once the measurement is initiated, with total duration given by the product of the acquisition time and the number of points, the instrument returns the stress value. These stresses can subsequently be evaluated using one of the three built-in algorithms implemented in the software: GNR PVII, PARABOLA, or CENTROID. These algorithms, which are well