The viscous fluids incorporated into seismic dampers may experience in-situ changes in their mechanical properties during the reference structural life for which they are designed. Additionally, leakages in seals, valves and casings of the devices may sometimes occur over time. All these effects can reduce the protective action of viscous devices on the buildings and infrastructures in which they are installed. Therefore, even when not strictly required by relevant Technical Standards, it is advisable to carry out experimental checks on samples of viscous dampers, repeating at considerable time intervals the qualification tests carried out immediately after production, before their installation.

This topic is addressed in the paper, focusing on Pressurized Fluid Viscous (PFV) dampers, which have been the subject of over twenty-five years of research activity by the first two authors. During the first season of this activity, several testing campaigns have been carried out on different types of PFV devices, which allowed to identify their mechanical characteristics, constituting the necessary database for the development of subsequent analytical, numerical modelling, design and application studies.

Some of the experimental investigations conducted in the early 2000s are currently being repeated on the same devices tested at the time. The paper presents a summary of the new laboratory campaign completed on a first PFV spring-damper pair, comparing the results with the ones of the original qualification tests. The identification process developed on both data sets allows to evaluate a complete stability of the mechanical properties and response capacities of the examined devices in this significantly long time interval.