**5.2 AE XY plots monitoring**

To further understand the sequences of AE events, their locations within the steel-RC and GFRP-RC beam have been plotted at different times in the form of event maps. The locations of the AE events during the entire process of damage are presented in X-Y plots and these AE XY-event plot profiles were obtained by using AE-win software (**Figures 20**–**28**). It may be recalled that the cracks can be located only in the zones covered by the AE sensors. Every crack is labelled as an event recorded by three or more sensors. The red spawn (dots) in AE plots represents the location of each AE event recorded to indicate the frontal surface condition of the steel-RC and GFRP-RC beams at different stages of damage under flexural loading. AET is supposed to increase the efficiency of structural inspection by indicating the initiation and progression of cracking and the surface strains in the two types of beams respectively.

It is apparent from **Figures 20(a), 23(a)**, and **26(b)** that, during the damage level I i.e., at cracking loads of 5.58, 6.99, and 9.54 kN for S-0.33-1, S-0.52-1, and S-1.11-1 RC beam and 7.89, 8.01, and 10.31 kN for G-0.33-1, G-0.52-1, and G-1.11-1 RC beam respectively. It is indicated by the appearance of AE events in the XY-plot at the same instant which suggests the formation of invisible cracks in steel-RC and GFRP-RC at the same location as shown in **Figures 20(b), 23(b)**, and **26(b).** Hence, it can be concluded that invisible cracking which is not visible to the naked eye can be reliably displayed by AE XY- event plots.

Further, with an increase in loading, it is visually observed that at a yield load (Py) of 28.93, 45.85, and 81.44 kN in S-0.33-1, S-0.52-1, and S-1.11-1 RC beam the earlier invisible crack starts to become visible and eventually coalesce together to form visible cracks propagating vertically upwards. This indicates the progression of damage to level II as shown in **Figures 21(a), 24(a)**, and **27(a)**. These cracks do

**Figure 20.** *(a) Beam sample (b) AE XY plots at damage level I.*

**Figure 21.**

*(a) Beam sample (b) AE XY plots at damage level II.*

not result in sudden failure of the beam as their propagation is arrested by the presence of shear stirrups. On the contrary, in the case of the GFRP-RC beams, owing to the elastic behaviour of GFRP bars, the beam continues to carry load linearly and invisible cracking is observed at a load of 33.6, 51.80, and 57.46 kN. On the other hand, the AE event plot shows the congregation of red dots pointing towards the coalescence of invisible cracks into visible cracks at approximately (1.2, 1.2, and 1.1) m and (1.3, 1.2, and 1.1) m distance from the left support for S-0.33-1,

*Crack Classification in Steel-RC and GFRP-RC Beams with Varying Reinforcement Ratio Using… DOI: http://dx.doi.org/10.5772/intechopen.101305*

#### **Figure 22.**

*(a) Beam sample (b) AE location- XY plots at damage level III.*

#### **Figure 23.**

*(a) Beam sample (b) AE XY plots at damage level I. (c) Beam span (m).*

**Figure 24.**

*(a) Beam sample (b) AE XY plots at damage level II.*

**Figure 25.** *(a) Beam sample (b) AE location- XY plots at damage level III.*

**Figure 26.** *(a) Beam sample (c) AE XY plots at damage level I.*

S-0.52-1, and S-1.11-1 RC and G-0.33-1, G-0.52-1, and G-1.11-1 RC beams. This is in a close match with the actually cracked beam and the same can also be observed in the actual beam sample and compared with the AE X-Y plots (**Figures 21(b), 24(b)**, and **27(b)**).

With further loading i.e. damage level III, the steel bars yield leading to flexural failure followed by concrete crushing (**Figures 22(a)**, **25(a)**, and **28(a))**. The same can also be confirmed with the actual beam sample. Moreover, the G-0.33-1, G-0.52-1, and G-1.11-1beams fail typically in shear followed by concrete crushing at damage level III. The corresponding ultimate load of 48.94, 56.74, and 76.98 kN in G-0.33-1, G-0.52-1, and G-1.11-1beams and is also depicted by extremely dense AE event plots at the same locations at 1.33, 1.33, and 1.33 m from the left support in

*Crack Classification in Steel-RC and GFRP-RC Beams with Varying Reinforcement Ratio Using… DOI: http://dx.doi.org/10.5772/intechopen.101305*

**Figure 27.** *(a) Beam sample (c) AE XY plots at damage level II.*

**Figure 28.**

*(a) Beam sample (b) AE location- XY plots at damage level III.*

**Figures 22(b)**, **25(b)**, and **28(b)**. A clear trajectory of transverse vertical cracks is also observed from the AE event plot. This is also validated by the image of the actually cracked beams. Thus, AE events maps provide a reliable and real-time indication of the initiation and progression of cracking inside concrete in steel as well as GFRP-RC beams undergoing flexural loading.

Hence, it can be concluded that AE has the potential to serve as an online nondestructive monitor that can map the progression of cracking in RC structures. Various AE parameters like cumulative AE hits and their amplitudes, the plot of AF, and RA can effectively capture the initiation and progression of cracking in the steel and GFRP-RC beams, much before these are visible to the naked eye. Moreover, AE XY-plot has ample potential to serve as an effective tool to monitor cracking in the terms of AE XY-plot, and much before the actual cracking is visible to the naked

eye. Hence, the advanced AE tools can be effectively used in conjunction with NDE of various types of RC structures incorporating steel as well as GFRP bars.
