PSI - Issue 84

Andrea Nino Consiglio et al. / Procedia Structural Integrity 84 (2026) 914–921

915

1. Introduction The management of existing stocks of bridges and, more generally, civil infrastructures has become a particularly complex challenge, shaped by technical, social, economic and political factors (Zanini et al. 2022). Prestressed Concrete (PC) members play a central role, as they are widely used in modern infrastructures and bridges to meet the increasing structural demands associated with higher traffic loads. The use of prestressing in concrete, introduced in the first half of the 20 th century, has gradually become a well-established technique for a wide range of applications from railway sleepers to large bridges, as well as lightweight precast flooring and roofing systems. The safety and serviceability of PC structures heavily rely on the effective level of prestressing force, which helps limit deflections and partially counterbalance the effects of permanent and variable loads (Bonopera et al. 2022). Consequently, important prestressing losses may lead to significative deformations, cracking phenomena, and an overall reduction in performance, particularly in long-span PC girder-bridges (Gandelli et al. 2024). As a result, an accurate evaluation of the in situ prestressing force is essential for monitoring tension losses in PC bridges. To ensure a proper control of the structural operational state, non-destructive testing methods are required to support condition assessments throughout the service life of the structure (Ciolko and Tabatabai 1999; Kralovanec et al. 2024; Mateu-Sánchez et al. 2025). While direct instrumentation of ducts or tendons—such as through load cells—is feasible during construction (Williams et al. 2023), it is generally impractical for existing PC girder-bridges, making the development and application of reliable non-destructive approaches as indispensable ways for indirectly estimating prestressing forces. A comparison between a destructive and a non-destructive method to estimate residual prestressing in concrete beams was here illustrated. The destructive approach was related to the “cracking re-opening” method according to the flexural capacity of the PC beam under investigation. Contrariwise, the non-destructive method was based on the Timoshenko theory, where the procedure identifies the prestressing by at least using one small-deflection measured along the beam axis. Moreover, such a non-destructive method requires information on the PC beam’s flexural rigidity, and relies on static response parameters only. Particularly, the static deflected shape of a simply supported pre– tensioned concrete beam, and subjected to an additional vertical loading, was measured as part of an experimental campaign. Thus, the identified pre–tensioning forces proved the feasibility of both methods, even in presence of moderate measurement errors. Yet, both approaches are applicable to PC girder-bridges within their decompression serviceability limit state, even if the “cracking” method can exclusively be applied to decommissioned PC bridges (Labia et al. 1997; Osborn et al. 2012; Guo et al. 2018; Consiglio et al. 2025). Concluding remarks considering further research directions were reported at the end of the article. 2. Methods for evaluating residual prestressing in concrete beams The authors focused on two experimental approaches presented in the literature, i.e., the “cracking” method (or “decompression” method) and the “static deflected shape” method (or “magnification factor” method) revised by the Timoshenko theory. Both the destructive and non-destructive approach, adopted in the experimental campaign, were illustrated as follows. In addition, the corresponding procedures and evaluations of the effective pre–tensioning in concrete girder-bridges were described.

Fig. 1. Laboratory setup.

2.1. Destructive method The “cracking” method consists of three phases. During the first one, a vertical load is applied to create and locate a series of flexural cracks to be instrumented by strain gauges or displacement transducers. The second phase is instead executed to determine the decompression load in the PC beam, according to the strain or displacement measurements of the crack openings regarding the cracks identified during the first phase. Conversely, the third phase involves overloading to the failure for determining the PC beam’s maximum capacity. In this experimental campaign, a four-