PSI - Issue 78

Carlo Pettorruso et al. / Procedia Structural Integrity 78 (2026) 1190–1196

1191

1. Introduction The assessment of internal post-tensioning (PT) systems in reinforced concrete bridges has become an increasingly pressing concern for infrastructure managers and researchers, particularly due to the challenges of detecting hidden defects and the sheer number of aging structures. PT systems, widely used in European bridge construction after World War II, consist of tensioned steel strands housed in ducts, typically filled with grout to ensure bonding and corrosion protection. However, poor grouting practices, material aging, and exposure to aggressive agents like chlorides can lead to tendon corrosion, reduced pre-stress, and potential structural failure. The long-term durability of many PT bridges was not adequately considered during their original design and construction, resulting in widespread degradation. High-profile collapses have raised awareness of these vulnerabilities and triggered stronger emphasis on inspection, maintenance, and risk assessment. In response, the Italian Ministry of Infrastructure has issued national guidelines (LLGG2020) promoting a tiered inspection strategy, including Special Inspections (SI) for more vulnerable structures, based on internationally recognized guidelines such as FHWA and CS465, whose effectiveness and reliability have been widely validated. However, current SI procedures lack clarity, and traditional testing methods often fall short in detecting internal defects, highlighting the need for advanced Non-Destructive Testing (NDT) techniques. Given limited resources and the vast scale of bridge networks, prioritizing interventions and selecting appropriate NDT methods is crucial. This paper addresses that need by applying a decision-making framework to rank NDT techniques based on factors such as accuracy, usability, cost, and traffic impact. The framework supports infrastructure managers in developing efficient, targeted inspection strategies across both individual assets and entire networks. 2. Summary of NDT methods for internal PT systems assessment The assessment of the condition of internal post-tensioning (PT) systems and the evaluation of residual pre-stress can be performed through a variety of Non-Destructive Testing (NDT) techniques. Many of these methods require only minimal intervention and are considered non-destructive due to their localized and reversible nature. A preliminary step typically involves Visual Testing (VT) which focuses on direct visual inspection of accessible areas, such as anchorage zones. While VT can reveal evident defects or signs of deterioration (Trejo et al. 2009), it is limited by accessibility and often fails to detect early-stage damage. To access internal, non-visible zones, borescope inspection can be employed. This involves inserting a small camera through pre-drilled holes into the duct to obtain localized images. While useful for targeted inspections, it remains a point-based method with limited spatial coverage. Ground-Penetrating Radar (GPR) enables the detection of metallic ducts by analyzing the reflection of electromagnetic waves. Although effective under certain conditions, its accuracy decreases in the presence of HDPE ducts or heavily congested reinforcement, and the interpretation of results requires experienced operators (Giannopoulos et al. 2002). One of the most promising techniques for detecting corrosion is Magnetic Flux Leakage (MFL) (Scheel et al. 1997). This method exploits distortions in magnetic field lines caused by discontinuities in prestressing wires. MFL is effective up to 20 cm of depth and is particularly suitable in lightly reinforced structures or in the presence of isolated strands. To identify voids or defects in duct grouting, the Impact Echo (IE) method uses low-frequency stress waves. It is recommended by various technical guidelines; however, its precision in defect sizing is limited, particularly when water is present in the duct (Terzioglu et al. 2018). Ultrasonic techniques, such as Ultrasonic Pulse Echo (USE) and Ultrasonic Tomography (UST), detect internal discontinuities by propagating acoustic waves through the concrete. While USE is more suited for localized assessments, UST provides a more de-tailed evaluation, especially effective in identifying defects near the surface in metallic ducts. Nevertheless, their effectiveness is influenced by the concrete quality and the depth of the target (Mazzatura et al. 2023).