## 2 Acoustic Emission Test on RC Beams
### Factors Affecting Acoustic Emission Tests  
There are many factors affecting AE tests in concrete. One factor is the scaling of specimens, which most researchers use as a basis for their assumptions in creating models. From studies, the mesoscale can be assumed to be closest to the behavior of concrete. Experimentation and the numerical method are employed for notched and un-notched concrete beams during the three-point bending test. The AESL and mesoscale non-linear model is correlated with the damage evolution and characterization of the Fracture Process Zone (FPZ). The initiation of fracture in notched beams is detected earlier than in un-notched beams. An AESL moving along the horizontal axis indicated more ductile behavior [19; 20]. A phenomenological model is made to capture important characteristics that emerge in damage  
Figure 1: Formation of macro- and micro-cracks ([PERSON] et al., 2015 [6]). progression and AE measurements. In the case of masonry specimens, the stored elastic energy is released in the creation of surfaces at the micro/mesoscopic level [21]. Another scale is the multiscale approach. Multiscale methodology should be considered in the AE test of RC structures. Validation using the cracking characterization in a laboratory should be correlated with real structures of different sizes and different ingredients. Specim size, as well as sensor type and sensors separation distance have a strong influence on AE test results [22]. Drawn from previous research, the following are the challenges that need to be addressed by researchers for the improvement of in situ NDTs using AE:  
1. Localization of damage--Determining the localization of cracks inside RC beams is an important concern that researchers need to explore. As an example, time reverse modeling using the AE test is applied in RC specimens and confirmed to be capable of localizing AE activity caused by concrete cracking [23]. In addition, the combination of AE signals and digital imaging can reveal and possibly characterize damage in concrete [24]. 2. Size of specimens--Reinforced beam specimens of three sizes of scales are made to investigate the size effect of concrete beams when the bending test is introduced using digital image correlation (DIC) and AE. In large beams, tensile micro-cracking is observed in the initial and intermediate level of loadings, while shear cracking is evident at the final stages [25]. In another study, zoning is used to explore the behavior of mode of failure near the support and at the midspan of a long beam. Zones near the support tested under the four-point bending test showed that the initial load from 20 to 50% dominantly result in the tensile mode of failure, followed by shear failure over 50 to 100% ultimate load. On the other hand, the middle zone of the beam exhibits a different behavior. It undergoes the dominant tensile mode of failure from 0 to 100% of ultimate load and few shear modes are observed at the final stage of the ultimate load [26]. 3. Mode of failure--Fracture modes are investigated in beams tested in the three-point bending test. Tensile cracks occurred first followed by shear cracks. In addition, elementary finite element models are used to investigate the stress field of tensile stress and shear stress in the beam to redesign the experiment setup to have more dominant shear cracks [27]. Another study considered composite slabs with layers of casted wire mesh, and tested failure modes of punching and flexure. A damage index using AE energy is used when a high AE energy class is found to be associated with activities in the FPZ [28]. In this study, the three challenges mentioned above were addressed and considered as limitations. Particularly, the placement of AE sensors at the midspan of the short RC beam was addressed. The time-difference method was used to localize and determine the AESL hits in the compression zone under the crushing of concrete. The algorithm for the AESL hits is available from the literature [29].