Maintenance of reinforced concrete (RC) structures is important to ensure long term conservation of concrete structures to serve its intended purpose. The vulnerability of these RC structures to aggressive environment during their service life is a cause of major concern for structural engineers. In general, the structural damage inspection comprises the monitoring and the evaluation of the performance of each component of concrete structure throughout its service life. Any deficiency in performance could be detected and corrected early. The inspection could be routine inspection, in-depth inspection or special inspection. The routine inspection involves a general examination of the structure to look for obvious outward physical evidence of distress that might require repair or maintenance. An in-depth inspection requires a detailed visual examination of all superstructures and substructure elements and this kind of inspection is necessary for old RC structures [1], [2] and [3]. Among the many available technologies, nowadays AE monitoring, one of the non-destructive techniques (NDTs) is used to evaluate the damage in RC structures[4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] and [19]. In general, AE technique is a passive monitoring technique which can be appropriately used for damage assessment of RC structures [4], [20], [21], [22], [23] and [24]. Usually AE monitoring is used to obtain qualitative results by observing the trends of AE parameters recorded during the experiment and the extent of damage is then determined [4], [11], [12], [13], [14], [15] and [17].
Over the past few years, researchers attempted to state the damage in RC beams using parametric based AE techniques [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] and [19]. By defining two ratios namely calm ratio and load ratio based on AE energy and Kaiser effect, researchers assessed the state of damage in RC beams [4], [11] and [17]. Ohtsu et al. made a damage assessment chart on the basis of load ratio and calm ratio and related them with crack mouth opening displacement (CMOD) [11]. Colombo et al. studied AE based b-value which is based on Gutenberg–Richter formula to study the fracture process in concrete beams and concluded that the variation of b-value during fracture process in RC beams showed a significant relationship with micro and macro cracking [12]. Researchers used AE energy parameter to evaluate damage of concrete beams [13], [14] and [15]. By defining a parameter “relaxation ratio” Colombo et al. concluded that there is a significant change in relaxation ratio at 45% of the ultimate failure load [13] and [14]. Ridge and Ziehl used cumulative AE signal strength parameter to evaluate damage in concrete specimens [15]. Nair and Cai used intensity analysis technique to assess damage in concrete bridges [16]. Nowadays most of the researchers are using parametric based AE techniques because of the availability of high speed multi channel AE recording and source location systems. In the present study the limit state of serviceability conditions are used [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [37], [38] and [39].
Research in application of AE technique to RC structures has progressed quite sufficiently. Most of the RC structures built a few decades ago are sufficiently exposed to aggressive environment and therefore both steel and concrete could have undergone damage. It is needed to conduct investigation regarding the state of the structures like existence of invisible cracks, and level of corrosion in steel. It is possible to know the existence of cracks by AE technique. AE technique is a non-invasive one and thus very conversant for structures under use. AE technique can easily quantify the extent of damage [4], [11], [12], [13], [14], [15], [17], [20], [21], [22], [23] and [24].
