*3.4. Effect of Concrete Strength and Corner Radius*

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 compressive stress is dependent on the concrete strength. For both of the groups and for the same 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. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 11 of 20 *Sustainability* **2022**, *14*, x FOR PEER REVIEW 11 of 20

**Figure 9.** Increase in peak compressive stress in (**a**) Group 1 and (**b**) Group 2. **Figure 9.** Increase in peak compressive stress in (**a**) Group 1 and (**b**) Group 2. **Figure 9.** Increase in peak compressive stress in (**a**) Group 1 and (**b**) Group 2.

The effect of concrete strength on the improvement in ultimate strain is exhibited in Figure 10. A similar trend as that for the peak compressive stress is observed. This is evident as the low-strength specimens demonstrated a greater improvement in their ultimate strains than the high-strength specimens. The second branch of stress–strain curves of the Group 1 specimens was either descending or stable, whereas an ascending second branch was observed for the Group 2 specimens. As a result, the Group 2 specimens demonstrated a higher increase in the peak compressive stress. The effect of concrete strength on the improvement in ultimate strain is exhibited in Figure 10. A similar trend as that for the peak compressive stress is observed. This is evident as the low-strength specimens demonstrated a greater improvement in their ultimate strains than the high-strength specimens. The second branch of stress–strain curves of the Group 1 specimens was either descending or stable, whereas an ascending second branch was observed for the Group 2 specimens. As a result, the Group 2 specimens demonstrated a higher increase in the peak compressive stress. The effect of concrete strength on the improvement in ultimate strain is exhibited in Figure 10. A similar trend as that for the peak compressive stress is observed. This is evident as the low-strength specimens demonstrated a greater improvement in their ultimate strains than the high-strength specimens. The second branch of stress–strain curves of the Group 1 specimens was either descending or stable, whereas an ascending second branch was observed for the Group 2 specimens. As a result, the Group 2 specimens demonstrated a higher increase in the peak compressive stress.

The accurate prediction of the peak compressive strength and the ultimate strain of strengthened concrete is important from both the design and analysis considerations. In existing studies, the confined concrete peak strength is often related to the lateral con-

The accurate prediction of the peak compressive strength and the ultimate strain of strengthened concrete is important from both the design and analysis considerations. In existing studies, the confined concrete peak strength is often related to the lateral con-

= 1 +<sup>1</sup> (

= 1 +<sup>1</sup> (

 ′

 ′

) (1)

) (1)

**Figure 10.** Increase in ultimate strain in (**a**) Group 1 and (**b**) Group 2. **Figure 10.** Increase in ultimate strain in (**a**) Group 1 and (**b**) Group 2. **Figure 10.** Increase in ultimate strain in (**a**) Group 1 and (**b**) Group 2.

finement pressure that is generated by the external confinement as:

finement pressure that is generated by the external confinement as:

 

 

**4. Analytical Investigations**

**4. Analytical Investigations**

where

as:

′
