Abstract In recent years one of the most important duty of civil engineers is to define methods of rehabilitation both for historic and modern structures. Reinforced concrete is a relatively modern material of construction and it is often damaged both with cracking due to loading or corrosion of reinforcement. In this paper undamaged and damaged reinforced concrete (RC) beams have been experimentally analyzed under bending loading with and without strengthening by near surface mounted (NSM) glass fiber reinforced polymer (GFRP) rod. A nondestructive method of control based on vibration tests has been utilized during the experiments on RC beams: experimental vibration tests have been carried out on RC beams without and with strengthening by GFRP rod assuming free-free ends and hinged ends. In the paper the envelope of Frequency Response functions (FRFs) obtained by the dynamic experimental tests are shown and the changes of natural frequency values are correlated to the damage degree of beam elements. A comparison of experimental and theoretical results is shown and discussed.