Experimental estimation of structural damage of RC bridge decks through the change of dynamic properties

Monitoring techniques to control the in-service performance of civil structures have been increasingly used for the last years. Many of them rely on the evaluation of dynamic properties. However, significant research is still needed to establish a close correlation between the change of dynamic properties and structural damage of, for example, concrete structures. This paper presents a contribution to understand the change of dynamic properties of reinforced concrete bridges against both static and fatigue transverse loads. An experimental campaign on specimens reproducing the cross-section of a box girder bridge has been performed. At relevant stages of the experiments, impact excitation tests were carried out and free vibration accelerations were measured. The high sampling rate (2400 Hz) made it possible to estimate accurately the change of natural frequencies of high order modes (up to 800 Hz), whose contribution in the recorded acceleration is larger than the first natural frequencies. The change of dynamic properties was compared with the evolution of damage, with emphasis on the fatigue process: development of cracks, cyclic decrease of tension stiffening and fatigue fracture of the rebars. The study indicates that the decrease of natural frequencies can be used to establish levels of damage against fatigue loads when the results of the static test are used as a reference.