Acoustic emission (AE) is an important nondestructive evaluation (NDE) technique used in the field of structural engineering for both case local and global monitoring. In this study AE technique with a new approach was employed to investigate the process of fracture formation in reinforced concrete structure. A number of reinforced concrete (RC) one story frames were tested under loading cycle and were simultaneously monitored using AE. The AE test data was analyzed using the relaxation ratio and calm and load ratio method. Also, the relaxation ratio was dominated with approaching load to 58% of the ultimate load. In addition three levels of damage using calm and load ratio were distinguished. The trend of relaxation ratio and calm and load ratio method during loading and unloading showed that these methods are strongly sensitive with cracks growth in RC frame specimens and were able to indicate the levels of damage. Also, results showed that AE can be considered as a viable method to predict the remaining service life of reinforced concrete. In addition, with respect to the results obtained from relaxation ratio and, load and calm ratio indicated, a new chart is proposed. 1. Introduction AE testing is a useful nondestructive technique for real-time examination of the behaviour of materials deforming under stress [1]. The main goal of the monitoring of AE phenomena is to provide a series of useful information by the correlation of AE signals with growing fracture process [2]. The wide range for application of acoustic emission is between seismic event as the largest scale and the movement of small numbers of dislocation in material as the small scale [3]. The primarily sources of AE are microcracking, macrocracking, compression failure, yielding, fracture, debonding between materials, sliding, and friction between interfaces [4]. AE data can be evaluated by means of several methods. Relaxation ratio and calm and load ratio which are derived from events during unloading and loading are reasonable methods for evaluation of structure under cyclic load. The relaxation ratio and calm and load ratio have been used by several researchers to evaluate structural damage. A few works found that relaxation ratio have been used for evaluation of concrete structure such that [5–7] have been investigating several RC beams under cyclic load using relaxation ratio. They found that relaxation ratio is more than one when approximately 45% of the ultimate bending load was reached. In addition, several works found that calm and load ratio has been used for evaluation of
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