*2.2. Material Properties*

The solid clay bricks were crushed using a brick-crushing machine. The crushed brick aggregates were sieved to obtain coarse brick aggregates with sizes ranging from 5 mm to 20 mm (Figure 1a). The mechanical properties of the bricks were estimated by following the recommendations of ASTM C1314-21 and ASTM C140/C140M-22a [45,46]. The estimated mechanical properties of the bricks are reported in Table 2. However, the standard density of natural aggregates was approximately 2000–2900 kg/m<sup>3</sup> . Actual density tests were not

considered in this study for natural aggregates. The replacement ratio of natural coarse aggregates with brick aggregates was 50% (Figure 1b). The target concrete strengths were 15 MPa and 25 MPa for the low-strength and high-strength concrete, respectively. The mix proportions adopted for the two concrete strengths are shown in Table 3. The ultimate tensile strain and strength of LC-GFRP wraps were 2.04% and 377.64 MPa, respectively, which were determined by following ASTM D3039/D3039M-17 [45]. The thickness of the LC-GFRP sheet was 0.50 mm. sity tests were not considered in this study for natural aggregates. The replacement ratio of natural coarse aggregates with brick aggregates was 50% (Figure 1b). The target concrete strengths were 15 MPa and 25 MPa for the low-strength and high-strength concrete, respectively. The mix proportions adopted for the two concrete strengths are shown in Table 3. The ultimate tensile strain and strength of LC-GFRP wraps were 2.04% and 377.64 MPa, respectively, which were determined by following ASTM D3039/D3039M-17 [45]. The thickness of the LC-GFRP sheet was 0.50 mm.

The solid clay bricks were crushed using a brick-crushing machine. The crushed brick aggregates were sieved to obtain coarse brick aggregates with sizes ranging from 5 mm to 20 mm (Figure 1a). The mechanical properties of the bricks were estimated by following the recommendations of ASTM C1314-21 and ASTM C140/C140M-22a [45,46]. The estimated mechanical properties of the bricks are reported in Table 2. However, the

(**a**) (**b**)

. Actual den-

**Figure 1. Figure 1.** (**a**) Brick aggregates, and ( (**a**) Brick aggregates, and ( **b**) natural aggregates. **b**) natural aggregates.

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**Group Name Strength Corner Radius GFRP Number**

1 SQ-HS-R0-CON High strength R0 - 2

2 SQ-LS-R26-6GFRP Low strength R26 6 2

SQ-LS-R0-CON Low strength R0 - 2 SQ-LS-R0-2GFRP Low strength R0 2 2 SQ-LS-R0-4GFRP Low strength R0 4 2 SQ-LS-R0-6GFRP Low strength R0 6 2

SQ-HS-R0-2GFRP High strength R0 2 2 SQ-HS-R0-4GFRP High strength R0 4 2 SQ-HS-R0-6GFRP High strength R0 6 2 SQ-LS-R26-CON Low strength R26 - 2 SQ-LS-R26-2GFRP Low strength R26 2 2 SQ-LS-R26-4GFRP Low strength R26 4 2

SQ-HS-R26-CON High strength R26 - 2 SQ-HS-R26-2GFRP High strength R26 2 2 SQ-HS-R26-4GFRP High strength R26 4 2 SQ-HS-R26-6GFRP High strength R26 6 2

standard density of natural aggregates was approximately 2000–2900 kg/m<sup>3</sup>

Table 1.

**Table 1.** Summary of tested specimens.

*2.2. Material Properties*

mm radius and strengthened with six layers of GFRP. Further details are provided in

**Table 1.** Summary of tested specimens.


**Table 2.** Mechanical properties of solid clay bricks.


**Table 3.** Mix proportions for concrete.


**Constituent** (/

#### *2.3. Details and Construction of Test Specimens 2.3. Details and Construction of Test Specimens*

High 627 806 358 358 251

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Value 120 3.14 23.27

**Table 2.** Mechanical properties of solid clay bricks.

**Property Density** (/

)

**Table 3.** Mix proportions for concrete.

Each specimen measured 150 mm × 150 mm in cross-section and 300 mm in height to achieve the height-to-width ratio of 2.0. All of the specimens were cast in steel molds of the same dimensions. However, the steel molds were round to a corner radius of 26 mm for the Group 2 specimens, as shown in Figure 2a, whereas the rectilinear-shaped molds were used for the Group 1 specimens. The concrete was poured in three equal layers in each mold, whereas proper compaction (using vibrating poker) was applied to each layer. The specimens were taken out of the molds after one day of casting and cured for 28 days in laboratory environments (i.e., temperature was approximately 30–33 degree centigrade and humidity was 65–75%). The strengthening of specimens was performed after their curing by using a hand layout. Each specimen measured 150 mm × 150 mm in cross-section and 300 mm in height to achieve the height-to-width ratio of 2.0. All of the specimens were cast in steel molds of the same dimensions. However, the steel molds were round to a corner radius of 26 mm for the Group 2 specimens, as shown in Figure 2a, whereas the rectilinear-shaped molds were used for the Group 1 specimens. The concrete was poured in three equal layers in each mold, whereas proper compaction (using vibrating poker) was applied to each layer. The specimens were taken out of the molds after one day of casting and cured for 28 days in laboratory environments (i.e., temperature was approximately 30–33 degree centigrade and humidity was 65–75%). The strengthening of specimens was performed after their curing by using a hand layout.

) **Compressive Strength (MPa) Water Absorption (%)**

**Figure 2.** Steel molds for (**a**) zero corner radius and (**b**) 26 mm corner radius. **Figure 2.** Steel molds for (**a**) zero corner radius and (**b**) 26 mm corner radius.

The surface of each specimen was properly cleaned and smoothed before applying GFRP. Epoxy resin was applied to the concrete surface by using a hand brush to impregnate the surface. This was followed by wrapping GFRP around the specimens. The first layer of GFRP was epoxy impregnated using a brush (see Figure 3a), before the application of the second layer. This process was repeated for the subsequent GFRP layers. Finally, the extended GFRP sheets above and below the top and bottom surfaces were ground, as shown in Figure 3b. The surface of each specimen was properly cleaned and smoothed before applying GFRP. Epoxy resin was applied to the concrete surface by using a hand brush to impregnate the surface. This was followed by wrapping GFRP around the specimens. The first layer of GFRP was epoxy impregnated using a brush (see Figure 3a), before the application of the second layer. This process was repeated for the subsequent GFRP layers. Finally, the extended GFRP sheets above and below the top and bottom surfaces were ground, as shown in Figure 3b. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 6 of 20

**Figure 3.** (**a**) Application of epoxy resin and (**b**) grinding of excessive GFRP portions and smoothening of the top and bottom surfaces. *3.1. Ultimate Failure Modes* **Figure 3.** (**a**) Application of epoxy resin and (**b**) grinding of excessive GFRP portions and smoothening of the top and bottom surfaces.

A monotonic compressive load was applied to each specimen by using a hydraulic

tons. A calibrated load cell was deployed to measure the intensity of the applied compressive load, whereas a logger recorded the measured load. A uniform application of the load was achieved by placing steel plates above and below the specimen, as shown in Figure 4. The axial shortening of the specimens under the applied compressive load was simultaneously measured by using two linear variable differential transducers (LVDTs). The recorded deflection was subsequently converted to the strain, whereas the recorded load was converted to compressive stress by utilizing the geometrical dimensions of

*2.4. Test Setup and Instrumentation*

**Figure 4.** Typical test setup and instrumentation.

**3. Experimental Results**

specimens.

#### *2.4. Test Setup and Instrumentation* A monotonic compressive load was applied to each specimen by using a hydraulic

*2.4. Test Setup and Instrumentation*

specimens.

(**a**) (**b**)

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A monotonic compressive load was applied to each specimen by using a hydraulic Universal Testing Machine. The ultimate capacity of universal testing machine was 200 tons. A calibrated load cell was deployed to measure the intensity of the applied compressive load, whereas a logger recorded the measured load. A uniform application of the load was achieved by placing steel plates above and below the specimen, as shown in Figure 4. The axial shortening of the specimens under the applied compressive load was simultaneously measured by using two linear variable differential transducers (LVDTs). The recorded deflection was subsequently converted to the strain, whereas the recorded load was converted to compressive stress by utilizing the geometrical dimensions of specimens. Universal Testing Machine. The ultimate capacity of universal testing machine was 200 tons. A calibrated load cell was deployed to measure the intensity of the applied compressive load, whereas a logger recorded the measured load. A uniform application of the load was achieved by placing steel plates above and below the specimen, as shown in Figure 4. The axial shortening of the specimens under the applied compressive load was simultaneously measured by using two linear variable differential transducers (LVDTs). The recorded deflection was subsequently converted to the strain, whereas the recorded load was converted to compressive stress by utilizing the geometrical dimensions of

**Figure 4.** Typical test setup and instrumentation. **Figure 4.** Typical test setup and instrumentation.

#### **3. Experimental Results 3. Experimental Results**

#### *3.1. Ultimate Failure Modes 3.1. Ultimate Failure Modes*

The ultimate failure modes of Group 1 specimens are shown in Figure 5. The failure of the two control specimens was due to the sudden concrete crushing and splitting within the top half of their heights. The stress concentrations near the sharp corners of rectilinear concrete specimens are known to exist [43,44]. To avoid premature failure due to these stress concentrations, ACI 440.2R-02 [47] recommends a minimum corner radius of 13 mm. Since no corner radius was provided in the Group 1 specimens, the failure accompanied the rupture of GFRP sheets at the corners, as shown in Figure 5. The resulting failure was brittle, irrespective of the concrete strength and the number of GFRP layers. This suggests that premature failure due to stress concentrations could not be prevented even with the application of six GFRP layers. This could be associated with the relatively lower ultimate strain of GFRP, i.e., 2.04%. Future studies are required to further explore this phenomenon by using FRP composites with higher rupture strains, such as polyester fiber ropes [42].

The failure modes of the Group 2 specimens are shown in Figure 6. In the same way as the control specimens in Group 1, the failure of the Group 2 control specimens was brittle. However, the crushing and splitting propagated all along the full height. The low-strength-strengthened specimens in Group 2 failed due to the rupture of the GFRP sheets in the hoop direction. For the two and four GFRP layers, the tensile rupture of GFRP sheets was found to occur between the corners suggesting that the stress concentrations were mitigated successfully. Specimen SQ-LS-R26-6GFRP failed by the rupture of GFRP sheets at the corners. This can be attributed to the resulting high compressive strength of Specimen SQ-LS-R26-6GFRP due to six GFRP layers that may have eventually resulted in higher stress concentrations near the corners as compared to those in Specimens SQ-LS-R26- 2GFRP and SQ-LS-R26-4GFRP. The failure of high-strength concrete specimens in Group 2 also exhibited rupture of GFRP sheets mainly near the corners, and a similar analogy of high-stress concentrations near the corners can be made due to the high concrete strength. prevented even with the application of six GFRP layers. This could be associated with the relatively lower ultimate strain of GFRP, i.e., 2.04%. Future studies are required to further explore this phenomenon by using FRP composites with higher rupture strains, such as polyester fiber ropes [42].

The ultimate failure modes of Group 1 specimens are shown in Figure 5. The failure of the two control specimens was due to the sudden concrete crushing and splitting within the top half of their heights. The stress concentrations near the sharp corners of rectilinear concrete specimens are known to exist [43,44]. To avoid premature failure due to these stress concentrations, ACI 440.2R-02 [47] recommends a minimum corner radius of 13 mm. Since no corner radius was provided in the Group 1 specimens, the failure accompanied the rupture of GFRP sheets at the corners, as shown in Figure 5. The resulting failure was brittle, irrespective of the concrete strength and the number of GFRP layers. This suggests that premature failure due to stress concentrations could not be

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SQ-HS-R0-CON SQ-HS-R0-2GFRP SQ-HS-R0-4GFRP SQ-HS-R0-6GFRP

**Figure 5.** Ultimate failure modes of Group 1 specimens.

**Figure 5.** Ultimate failure modes of Group 1 specimens.

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SQ-LS-R0-CON SQ-LS-R0-2GFRP SQ-LS-R0-4GFRP SQ-LS-R0-6GFRP

specimens in Group 2 also exhibited rupture of GFRP sheets mainly near the corners, and SQ-LS-R26-CON SQ-LS-R26-2GFRP SQ-LS-R26-4GFRP SQ-LS-R26-6GFRP

SQ-HS-R26-CON SQ-HS-R26-2GFRP SQ-HS-R26-4GFRP SQ-HS-R26-6GFRP

*3.2. Peak Stress and Ultimate Strain*

sharp corners.

SQ-LS-R0-CON was slightly higher than the control specimen SQ-LS-R26-CON. This could be associated with the larger bearing area of the SQ-LS-R0-CON specimen as compared to the control specimen SQ-LS-R26-CON. The low-strength concrete specimens demonstrated an 83%, 103%, and 137% increase in the peak compressive stress due to two, four, and six GFRP wraps, respectively. The corresponding improvement in the ultimate strain was observed at 82%, 194%, and 658%, respectively. Similarly, a considerable improvement in the peak compressive stress of high-strength specimens in Group 1 was also observed as two, four, and six GFRP wraps enhanced the peak compressive stress by 50%, 71%, and 84%, respectively, whereas the enhancement in ultimate strain was 70%, 159%, and 134%, respectively. Group 2 specimens also demonstrated a substantial improvement in the peak compressive stress and strain. The peak compressive stress of the low-strength specimens was improved by 116%, 210%, and 278%, respectively. At the same time, the ultimate strain was enhanced by 222%, 495%, and 752%, respectively. The high-strength specimens exhibited an increase of 84%, 364%, and 563% for the peak compressive stress due to two, four, and six GFRP layers, respectively, whereas the ultimate strain was increased by 84%, 364%, and 563%, respectively. The above discussion concerning Table 4 suggests that the LC-GFRP resulted in a substantial improvement in the peak compressive stress and ultimate strain, which is crucial given the brittle nature of the concrete. Overall, the % increase in peak stresses of the GFRP-confined specimens with 0 mm corner radius was lower than the GFRP-confined specimens with a 26 mm corner radius due to the premature rupture of LC-GFRP at

**Figure 6.** Ultimate failure modes of Group 2 specimens. **Figure 6.** Ultimate failure modes of Group 2 specimens.
