An experimental study on the behavior of bonded and unbonded prestressed concrete beams under progressive damage

This work presents an experimental laboratory study on damage detection in prestressed concrete beams, including nine bonded and three unbonded beams tested under service- and ultimate-level conditions with controlled tendon-cut scenarios. In addition to traditional modal testing, the experimental program combined staged static loading with progressive damage scenarios and continuous quasi-static monitoring. A multi-sensor setup featuring MEMS/PCB accelerometers, biaxial inclinometers, and LVDTs for point displacements captured not only frequency and mode-shape changes but also stiffness loss, crack development, and residual deflection growth. On the final bonded prototype, FBG strain gauges offered independent static-strain data in the vicinity of the cut, confirming the multi-sensor observations and broadening the scope of the campaign. Results show that unbonded beams experience larger relative drops in natural frequency and sometimes a mode re-ranking of the first two global modes: after damage, the lateral mode, initially the second mode, can decrease more than the first bending mode, causing the lateral mode to become first and the bending mode to shift to second. On the prototype equipped with FBG sensors, static strain concentrations were observed around the introduced cut region. While frequencies are generally less sensitive to localized damage than static strains, the simultaneous changes observed in both datasets suggest that the damage was severe enough to be detected by multiple indicators. Overall, the findings highlight differences in dynamic response sensitivity between bonded and unbonded beams, suggesting that OMA-based dynamic analysis, combined with FBG static strain measurements, can provide complementary diagnostics for SHM of PC bridge components.