Damage detection in elastic properties of masonry bridges using coda wave interferometry

Structures may be subjected to damage and deterioration over different time scales and monitoring their health status may allow to perform maintenance actions before the functionality limit is reached. Masonry arch bridges, in particular, are sensitive to the bearings loss produced by scour of the streambed soil at the pier foundations. In this study we measured the changes in the elastic properties of a 1:2 scaled model of a masonry arch bridge built in laboratory to study the evolution of the damage mechanism related to the application of foundation movements. Specifically, the bridge is realized to model the effect of erosion of the ground underneath the central pier. We analysed the accelerometric records acquired along the structure generated by a sledge hammer hitting the bridge walls. We used the method of coda wave interferometry to detect the changes in the elastic properties of the medium. After selecting the specific frequency band exciting coda waves, we progressively measured the time lag between signals acquired in the intact and two damaged stages of the bridge for each source-receiver couple and we fit the data to get the relative wave velocity changes. We found that the average relative velocity changes for the two different damaged steps are v/v = -5.08 ± 0.08% and v/v = -8.2 ± 0.6%, consistently measured at all the analysed source-receiver couples. These values correspond to an average estimation of the velocity changes occurred within the structure, because the associated wavelengths are comparable with the bridge size and the damage is spread over a large portion of the structure.