**5. Acoustic emission results**

### **5.1 Average frequency (AF) and rise angle (RA)**

A parametric analysis was performed between AF and RA values by moving averages based on over 100 AE hits [20, 25]**.** In the present case, the proportion of AF and RA values are likewise set to 1:200. The plot of Average frequency (AF) and rise angle (RA) values for all three damage levels, for S-series and G-series RC beams, is shown in **Figures 17**–**19** respectively. AF-RA value plot gives a fair indication of the cracking modes in steel-RC and GFRP-RC beams and is used for crack classification. The diagonal line represents the transition line between tensile and shear cracks and is used as a reference line for crack classification.

In damage level I, the plot of AF-RA for the S-0.33-1, S-0.52-1, and S-1.11-1 RC beam **(Figures 17(a), 18(a)**, and **19(a)**) shows the development of cracks initially

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

#### **Figure 18.**

*Variation in AF vs. RA values at different levels of damages. (a) Damage level I. (b) Damage level II. (c) Damage level III.*

due to tensile cracking whereas for the G-0.33-1, G-0.52-1, and G-1.11-1 RC beam, it is in shear cracking mode (**Figures 17(a)**, **18(a)**, and **19(a)**)**.** A high average AF value of 32.2, 53.73, and 50.64 kHz with a lower RA value of 579.9, 398.78, and 205.0 μs values were noticed in the S-0.33-1, S-0.52-1, and S-1.11-1 RC beam as against lower average AF value of 15.0, 16.20, and 26.73 kHz and a higher RA value of 6432.5, 7324.80, and 10752.52 μs for the G-0.33-1, G-0.52-1, and G-1.11-1 RC beam (**Tables 5-7**), respectively. These values indicate that the steel-RC beams can resist and bridge the cracks better owing to the perfect bond between concrete and steel and the high modulus of elasticity of steel bars as compared to GFRP reinforced concrete beams. In this damage level, the hairline cracks were visible in both beams.

With the increasing load in damage level II, invisible cracking is observed in the steel-RC beam at higher AF and smaller RA value indicating tensile cracking mode whereas, in the case of the GFRP-RC beam, a reversed trend is observed with a slight increase in average AF value and drop in RA value pointing towards a shift from shear to tensile cracking in the GFRP-RC beam (**Figures 17(b), 18(b)**, and **19(b)**). Further, in damage level III, a slight decrease in the average AF value of 37.3, 40.99, and 47.41 kHz with a minute increase in the RA value of 1327.0, 1225.25, and 454.92 μs value was noticed in the S-0.33-1, S-0.52-1, and S-1.11-1 RC beam. A

#### **Figure 19.**

*Variation in AF vs. RA values at different levels of damages. (a) Damage level I. (b) Damage level II. (c) Damage level III.*


#### **Table 5.**

*Variation in RA and AF values in S-0.33-1 and G-0.33-1 RC beams.*


#### **Table 6.**

*Variation in RA and AF values in S-0.52-1 and G-0.52-1 RC beams.*


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

### **Table 7.**

*Variation in RA and AF values in S-0.1.11-1 and G-1.11-1 RC beams.*

plausible explanation for this can be attributed to the reduction in the cross-sectional area due to the yielding of steel bars in the steel-RC beam.

On the other hand, a continuous increase in AF value of 42.2, 43.66, and 39.40 kHz with a significantly lower RA value of 3317.6, 5130.68, and 5255 μs was noticed in the G-0.33-1, G-0.52-1, and G-1.11-1 RC beam (**Figures 17(c), 18(c)** and **19(c)).** AFRA plots at the failure point suggest that the steel-RC beam experiences flexural cracks localisation as shown in **Figure 10a, c**, and **e**, whereas the GFRP-RC beam experiences shear cracking as also observed visually as shown in **Figure 10b, d**, and **f.**

Hence, AF-RA plots can be exploited to predict the initiation and progression of invisible and visible crack formation in concrete as indicated by the density of dots in the plot of the AFRA value. Hence, the AE plot of AF and RA value can effectively demonstrate the variation in initiation and progression of damage, classification of cracking, and failure modes in steel as well as GFRP-reinforced beams.
