Most existing Italian bridges, built since the 1960s with prestressed reinforced concrete, have reached 50 years of service life and exhibit a state of degradation that has led to significant consequences on the operability of the road network, including total or partial collapses. Consequently, the Italian Guidelines for the assessment of existing bridges foresee the execution of inspections to assess the degradation states at specified time intervals. The large amount of data collected during the inspections can be useful not only to determine the risk associated with the bridge (Class of Attention, CoA), but even to evaluate proactive and immediate maintenance actions. Therefore, the paper aims to analyse the degradation of post-tensioned prestressed reinforced concrete beams belonging to a set of bridges in the Basilicata region (southern Italy). A degradation index is proposed and computed for every beam allowing to compare the deterioration of spans, edge beams and internal ones, or bridges with each other. Subsequently, further analysis of these data is conducted to determine which defects are most frequent and to verify the existence of possible correlations among them, in order to outline the presence of cause-and-effect relationships. As a result, it emerges that the presence of deep defects, such as reinforcement corrosion, are well correlated with superficial defects due to poor water drainage systems. Furthermore, some defects are also related to the condition of the deck slabs. In fact, when these are not waterproof, the degradation, usually more pronounced on the edge beams, also affects the internal ones. These observations allow for the optimal selection of beams to be subjected to special inspections identifying the most degraded elements, as well as suggesting the execution of expeditious maintenance action to restore the deck slab waterproof layer.