This paper deals with vibration based damage detection methods applied to AAR-affected structures within the framework of tests carried out on concrete beams made of reactive aggregates. The resonant frequency variations of the tested beams had been measured as the chemical reactions were progressing. Experimental results show evidence that the stiffness of the reactive beams first decreased significantly, subsequent increase up to the initial value was then observed. The relationships between the variations of the resonant frequencies and the physical properties are discussed taking into account water content effects, boundary conditions and the nature of the damage.