Metrological evaluation of non-destructive testing methods for post-tensioned reinforced concrete bridges

Non-destructive testing (NDT) techniques are widely used to assess the condition of prestressed concrete bridges, although their real effectiveness is still subject to uncertainty and requires experimental validation. This study evaluates three NDT techniques commonly applied to post-tensioning systems: digital radiography, ground penetrating radar (GPR), and reflection impulse measurement test (RIMT). The investigation is carried out on the Alveo Vecchio viaduct, a decommissioned post-tensioned reinforced concrete bridge representative of construction practices from the 1960s. All inspections were performed by trained operators from specialised NDT service providers, who also interpreted the results according to their best professional practice, focusing on grout injection defects and prestressing steel corrosion. Following the NDT campaign, the bridge is subjected to controlled dissection, allowing direct observation of the actual defect conditions. The predictions obtained from each technique were compared with the observed defects, and their correlation is quantified using statistical analysis tools. The results indicate that DR is the only technique that shows appreciable effectiveness in detecting grout injection voids. In contrast, RIMT exhibits a poor correlation with grout defects. Furthermore, both GPR and RIMT demonstrate limited capability in identifying corrosion of prestressing steel, highlighting significant limitations in current NDT approaches for reliable condition assessment.