Experimental modal analysis shows a useful tool in health monitoring strategies to assess the seismic damage suffered by a structure. This problem is still an open question for existing civil structures where damage due to ageing effects or material degradation superpose to seismic damage. This is particularly true when early warnings are requested and light damage has to be dealt with. The modal parameters (frequencies, mode shapes and damping) are valuable indicators in their own or in conjunction with functional expressions in providing information related to structural changes. Alternatively, when a model of the structure is available, the modal parameters can be used in model updating like strategies to try to capture those model regions and physical parameters affected by damage. In either case safe results can be achieved depending on how much frequencies and mode shapes are actually sensitive to damage.

The paper aims to contribute to the evaluation of modal parameters sensitivity for corrosion damaged beams. To this end the dynamic response relevant to a large set of prestressed concrete beams is investigated. A total of 18 beams (3 groups of 6 joists each) have been corroded and dynamically tested. All the beams share the same geometry and concrete properties so that identical response is expected in the sound conditions. The three groups differ for the amount of prestressing steel wires so that different damage entity is expected for the same corrosion level. Five corrosion levels are analyzed comprising the sound reference state and diffused microcracking to concentrated large cracks.

The modal parameters are identified and compared using time domain, frequency domain and joint time-frequency domain techniques. Mode shapes are accurately identified, but show the less sensitive due to the particular configuration of the corrosion attack considered. Damping shows more sensitive than frequency, even if presents larger scatter in the identification. Both parameters show monotonic and regular trend with damage increase. Changes in modal parameters are detected even in the case of non observable damage.