*3.3. Compressive Stress-Strain Curves*

The recorded stress–strain behavior of Group 1 specimens is shown in Figure 7. Figure 7a shows the stress–strain curves for the low concrete strength specimens in Group 1. It is evident that a substantial improvement in the peak compressive stress was observed. The important parameter to be observed is the range of strain for which the peak sustained stress was maintained. This suggests that LC-GFRP imparted a considerable ductility to the concrete, which is crucial for strengthening against dynamic loads. The post-peak behavior of Specimens SQ-LS-R0-2GFRP and SQ-LS-R0-4GFRP was descending, whereas a stable second branch was observed for Specimen SQ-LS-R0-6GFRP. On the contrary, the second branch of the compressive stress–strain curves of the high-strength specimens in Group 1 was descending irrespective of the number of LC-GFRP layers. The second difference in

*Sustainability* **2022**, *14*, x FOR PEER REVIEW 9 of 20

**Table 4.** Summary of the peak stress and the corresponding strain.

SQ-LS-R0-CON 8.66 0.707 - 0.0053 - SQ-LS-R0-2GFRP 15.87 4.243 83 0.0096 82 SQ-LS-R0-4GFRP 17.59 0.000 103 0.0155 194 SQ-LS-R0-6GFRP 20.51 1.414 137 0.0400 658 SQ-HS-R0-CON 14.41 4.950 - 0.0051 - SQ-HS-R0-2GFRP 21.62 4.243 50 0.0086 70 SQ-HS-R0-4GFRP 24.67 0.707 71 0.0131 159 SQ-HS-R0-6GFRP 26.48 2.121 84 0.0119 134 SQ-LS-R26-CON 7.74 0.707 - 0.0084 - SQ-LS-R26-2GFRP 16.71 1.414 116 0.0270 222 SQ-LS-R26-4GFRP 23.99 1.414 210 0.0500 495 SQ-LS-R26-6GFRP 29.29 8.485 278 0.0717 752 SQ-HS-R26-CON 13.39 2.121 - 0.0062 - SQ-HS-R26-2GFRP 23.16 1.414 73 0.0115 84 SQ-HS-R26-4GFRP 32.07 1.414 140 0.0290 364 SQ-HS-R26-6GFRP 40.73 6.364 204 0.0414 563

**Increase in Peak Stress (%)**

**Ultimate Strain**  

**Increase in (%)**

**tion**

*3.3. Compressive Stress-Strain Curves*

**Specimen ID Peak Stress (MPa) Standard Devia-**

Figure 7a,b is the value of the ultimate strain. The high-strength specimens were able to sustain compressive stress to lower strain values as compared to low-strength specimens. were able to sustain compressive stress to lower strain values as compared to low-strength specimens.

The recorded stress–strain behavior of Group 1 specimens is shown in Figure 7. Figure 7a shows the stress–strain curves for the low concrete strength specimens in Group 1. It is evident that a substantial improvement in the peak compressive stress was observed. The important parameter to be observed is the range of strain for which the peak sustained stress was maintained. This suggests that LC-GFRP imparted a considerable ductility to the concrete, which is crucial for strengthening against dynamic loads. The post-peak behavior of Specimens SQ-LS-R0-2GFRP and SQ-LS-R0-4GFRP was descending, whereas a stable second branch was observed for Specimen SQ-LS-R0-6GFRP. On the contrary, the second branch of the compressive stress–strain curves of the high-strength specimens in Group 1 was descending irrespective of the number of LC-GFRP layers. The second difference in Figure 7a,b is the value of the ultimate strain. The high-strength specimens

**Figure 7.** Compressive stress vs. strain response for Group 1 specimens with (**a**) low concrete strength and (**b**) high concrete strength. Figure 8a,b shows stress–strain curves for Group 2 specimens with low and high concrete strength, respectively. A significant difference between Figures 7 and 8 is ob-

Figure 8a,b shows stress–strain curves for Group 2 specimens with low and high concrete strength, respectively. A significant difference between Figures 7 and 8 is observed in the second branch. It is recalled that the sharp corners in the Group 2 specimens were rounded to a 26 mm corner radius. The corresponding result is depicted in Figure 8a, as the stiff initial branch was followed by an ascending branch highlighting the importance of the corner radius. The peak compressive stress and the ultimate strain were found to increase with the number of LC-GFRP sheets. However, the improvement in the axial ductility was limited by the concrete strength. It is evident in Figure 8 that the failure of the high concrete strength specimens for the same number of LC-GFRP layers occurred at lower strains than those of the low concrete strength specimens. This could be related with the lower lateral dilation of the high-strength concrete as compared to the low-strength concrete. served in the second branch. It is recalled that the sharp corners in the Group 2 specimens were rounded to a 26 mm corner radius. The corresponding result is depicted in Figure 8a, as the stiff initial branch was followed by an ascending branch highlighting the importance of the corner radius. The peak compressive stress and the ultimate strain were found to increase with the number of LC-GFRP sheets. However, the improvement in the axial ductility was limited by the concrete strength. It is evident in Figure 8 that the failure of the high concrete strength specimens for the same number of LC-GFRP layers occurred at lower strains than those of the low concrete strength specimens. This could be related with the lower lateral dilation of the high-strength concrete as compared to the low-strength concrete.

**Figure 8.** Compressive stress vs. strain response for Group 2 specimens with (**a**) low concrete strength and (**b**) high concrete strength. **Figure 8.** Compressive stress vs. strain response for Group 2 specimens with (**a**) low concrete strength and (**b**) high concrete strength.

The effect of the concrete strength on the gain in peak compressive stress due to LC-GFRP confinement is shown in Figure 9. It is evident that the gain in peak compres-

number of LC-GFRP layers, the specimens with low concrete strength demonstrated greater improvement in the peak compressive stress than high strength specimens. Another observation that can be made from Figure 9 is the effect of corner radius on the improvement in peak compressive stress. For the case of no corner radius (see Figure 9a), the maximum increase in the peak compressive stress observed for Specimen SQ-LS-R0-6GFRP was 137%, whereas the corresponding value for a 26 mm corner radius

was 278%, observed for Specimen SQ-LS-R26-6GFRP.

*3.4. Effect of Concrete Strength and Corner Radius*
