Evaluation of low-cycle fatigue damage in RC exterior beam-column subassemblages by acoustic emission

Since Acoustic Emission (AE) is effective in monitoring the initiation of cracks in materials and structures, it has been widely used as a real-time evaluation technique for damage in reinforced concrete (RC) elements such as beams or columns in moment-resisting frames. This paper investigates its applicability for assessing the low-cycle fatigue damage in RC exterior beam-column subassemblages, which are especially vulnerable parts of the RC frames when subjected to earthquakes. Two 3/5 scale specimens representing connections from the lower and upper part of a prototype building located in a moderate-seismicity area are tested under cyclic loading until collapse. Important differences between the two specimens are observed owing to the different size and amount of reinforcing steel. First, a significant increase in AE activity is observed when the steel reinforcement begins to undergo plastic deformations. Second, by means of the AE improved b-value method, it is clearly shown that the macroscopic fracture processes occur mainly during the loadings paths. Thus, on the basis of AE source location with four sensors situated around the beam-column joint, the AE is successfully correlated with the observed cracks. A strong correlation is also found between the accumulated plastic strain energy dissipated by the concrete and the accumulated AE energy. Based on this fact, a formula is proposed to predict the level of damage and the closeness to failure of an exterior beam-column subassemblage from the AE as recorded by sensors located near the joint.