Damage Detection of Concrete Beams using Nonlinear Features of Forced Vibration

A new indicator is proposed to facilitate the detection of newly induced damage in reinforced concrete (RC) beams, based on the transient characteristics of nonlinear vibration. Two full-scale RC beams, one reinforced with externally bonded fiber-reinforced-polymer sheets, are tested to develop the proposed damage indicator. Both beams are statically and dynamically loaded to correlate the dynamic characteristics of the beams to the damage level. A phenomenological model is developed to simulate the general behavior of cracked concrete members with a softening Duffing oscillator. Numerical results and test data show that the indicator rapidly increases with the severity of damage and is very sensitive to cracking even under service loads. The indicator is directly related to the transient features along crack surfaces and requires no baseline in practical applications. Experimental test results also show that the fundamental natural frequency of the strengthened beam suddenly decreases at the initiation of cracking and then remains nearly constant while natural frequency of the unstrengthened beam decreases continuously as the beam experiences concrete cracking and reinforcement yielding.