Damage detection of prestressed concrete beams affected by shear and flexure cracks through vibration monitoring

This paper reports the results of an experimental campaign carried out on prestressed concrete beams (PCB), equipped both with bonded and unbonded strands, with the aim to employ Operation Modal Analysis (OMA) techniques to detect early warning signs (i.e., variation of natural frequencies and/or relevant mode shapes) associated to the onset of structural damages. With respect to similar past experimental studies, the main novelties presented hereafter concern the application of OMA to the following situations: (1) a PCB equipped with bonded strands and affected by a “almost pure” shear-cracks pattern; (2) a PCB equipped with unbonded strands and affected by an almost negligible (i.e., reversible) flexural damage. Among the outcomes of this study, it was found that, especially in the second case, the assessment of variations in the PCB natural fundamental frequencies, is not always an effective index of damage. On the contrary, a qualitative comparison between the undamaged and damaged identified mode shapes can be helpful even for the spatial location of the damage. Eventually, despite the diverging opinions in the reference literature, it is found that OMA technique is not a suitable tool to assess the potential influence of variations of the precompression load on the PCB fundamental frequencies, unless this variation is so high to cause cracking.