Experimental study on the friction behaviour of polytetrafluoroethylene bridge bearings under in-service ageing conditions

The long-term performance of polytetrafluoroethylene (PTFE) sliding bearings in bridges is still poorly documented, particularly for devices aged under real service conditions. This study investigates four PTFE bearings removed after 50 years of service from an Italian viaduct in a marine environment. The bearings, originally designed for a maximum vertical load of 2000 kN, were tested up to 2500 kN to evaluate reserve capacity and failure mechanisms. Semi-static and dynamic displacement-controlled tests were performed, with sliding velocities up to 50 mm/s. The results show that, despite visible PTFE degradation and permanent deformation of the steel base plate, the steel–PTFE frictional behaviour remains consistent: the friction coefficient decreases non-linearly with increasing vertical load and increases with sliding velocity, with the two effects appearing independent. Maximum edge pressures exceeded allowable limits at the highest load level in both loading regimes; however, PTFE extrusion was observed only under dynamic conditions. This indicates that, beyond stress exceedance, speed-related effects, including frictional dissipation and transient reduction of the effective contact area, play a major role in triggering localised extrusion. These findings provide experimental evidence on the behaviour of aged PTFE bearings under realistic conditions and support the assessment and design of existing bridge bearing systems.