**2. AE monitoring technique**

AE has been recently recognised as one of the most reliable passive tools for insitu health monitoring of civil engineering RC structures [19]. It employs surfacemounted AE-sensors to capture the energy bursts in the form of the transient elastic-stress waves. These elastic waves are generated due to the rapid release of energy during deformation or crack propagation in RC structures during any type of loading [20, 25]. These AE sensors convert transient elastic waves into electrical signals. In AET, various AE parameters are extracted and used to correlate to damage initiation and progression in various kinds of infrastructures as well as localise and quantify it. Some of the key AE parameters reported for damage analysis are cumulative AE hits and AE counts, AE energy (MARSE), and AE signal strength are shown in **Figure 1**. Acoustic emission hits in AE bursts are described as the number of times an AE transient signal crosses the threshold value of the anticipated signals in a structure. As the cumulative AE hits and counts increase, it points towards damage progression in the structure and gives information about the intensity of the AE event. AE energy is the transient elastic energy released during an AE event and is measured as the area under the AE signal. A significant jump in the AE signal energy in the form of a 'Knee" indicates severe damage in the form of macro-cracking in the structure [21, 22].

Recent studies have also demonstrated the potential of AE techniques to detect the onset and propagation of damage/cracking in RC structures under flexural loading. Characterisation of cracks in plain and reinforced concrete beams subjected to flexural loading up to failure has also been reported [25]. It has been reported that as the level of damage in the RC beam increases, an increase in AE parameters of AE hits, counts, AE energy, rise time, and duration has been observed. AE parameters of average frequency (AF) and rise angle (RA) have been correlated with the cracking pattern and its type-tensile or shear cracks [27]. It has been observed that AE-based Ib-value along with RA and AF has been successfully used for the

**Figure 1.** *Waveform of AE signal and its various parameter [26].*

evaluation of flexural deformation of RC beams under cyclic loading [27]. AET has also been recently used for monitoring the fracture behaviour of different types of composite concrete beams [28]. AE not only determines accurately the onset of cracking and monitors the development of fracture but also indicates various kinds of damage and fracture modes in the form of de-bonding and concrete cracking in these beams. Hence, it can be concluded that AET has been established as a potential NDT tool for monitoring the performance of RC structures under loading when subjected to various types of damages. In this work, the efficacy of AET to understand and compare the failure pattern of steel and GFRP reinforced concrete is explored to establish its effectiveness as a potential NDT tool for concrete structures.

In this study, steel-RC and GFRP-RC beams with varying tension reinforcement were prepared and tested under the four-point bending test associated with AE equipment. The main objectives of this chapter are to examine the behaviour of cracks at each stage of the mechanical behaviour of the RC beams from loading to failure using the AE parameter analysis-based method. Moreover, this chapter also attempts to examine the effect of the changes in the varying percentage tension reinforcement ratio of steel-RC and GFRP-RC beam and level of damage on the parameters (cumulative AE hits, amplitude, rise angle, and A-FRQ) of the AE parameter analysis-based method. Furthermore, this chapter also aims to classify the crack types and classification of damage level occurs in two differently reinforced concrete beams along with varying percentage tension reinforcement ratio.
