Studies on damage and FRP strengthening of reinforced concrete beams by vibration monitoring

In this paper, modal characteristics of reinforced concrete beam specimens are identified using experimental test data. Nine concrete beam specimens were gradually damaged and the changes of dynamic parameters were monitored from initial to the failure state. Six of the specimens were strengthened with externally bonded CFRP sheets when the load reached approximately half of the predicted failure load. The identification methods based on measured modal parameters were utilized and the efficiencies of these methods were investigated. These methods include frequency changes, Modal Assurance Criterion (MAC), Coordinate Modal Assurance Criterion (COMAC) and modal curvatures. The comparison of the methods shows that the frequency changes are not fully capable of detecting damage and predicting the potency of strengthening. The MAC values can reveal the changes of overall stiffness of the beams during the load steps. The change of stiffness at each degree of freedom of beams evaluated by COMAC and modal curvatures show that the damage identification of the beam specimens is best described by modal curvature method.