*3.4. Flexural Strength*

Figures below illustrate the flexural strength of geopolymer concrete and NFRGC with fiber addition after 28 and 90 days of curing, respectively. The flexural strength of GPC was observed to be enhanced with fiber added compared to plain GPC, and this improvement increased as the volume percent of fiber in the GPC increased.

From Figure 8, it was found that the inclusion of geopolymers led to an increment of the flexural strength of a concrete to a maximum of 0.5%, or 4.43 MPa. Meanwhile, normal geopolymer concrete without fiber addition exhibits a flexural strength of 4.39 MPa. Furthermore, when the nylon66 content exceeded 0.5%, the flexural strength began to decline. This was due to the samples' poor workability when nylon66 fibers were added in large quantities. It is believed that this poor workability has an impact on the distribution of nylon66 fibers. As a result, when loading was applied, the absorption capacity inside the sample was unbalanced, thus causing crack formation.

**Figure 8.** Flexural strength for 28 days.

Due to the limited availability of fiber, none of the three varieties have the same forms, surface smoothness, or aspect ratio, making direct comparisons using normalized or deleted measurements of the same level difficult. The tensile strength of the fiber has the greatest influence on post-crack behavior, and these factors only have an impact on the first fracture load. Furthermore, for each type of fiber used, various fiber volume fractions are generated, taking into account cost, density, and fiber dispersion in the concrete mix.

As shown in Figure 9, the nylon66 fiber played a role in enhancing flexural strength by inhibiting crack propagation during flexural testing by bridging at the crack region. With the addition of Nylon66 fibers, the sample was able to sustain a larger flexural force prior to complete failure. Photographic observation of the crack and final fracture in NFRGC and geopolymer concrete with various fiber additions is shown in Figure 9.

**Figure 9.** Shows the sample of bending test (**a**) geopolymer concrete without fiber addition and (**b**) with fiber addition.

The fiber failure mode demonstrates which type of feature dominates the flexural performance of geopolymer concrete. The majority of fibers do not draw out. All fibers are not extracted, particularly in the NFRGC. In this case, the binding property between the fiber and the concrete significantly influences how effectively the structure bends. The majority of the fibers rip apart at the fracture surface, indicating that fiber tensile behavior has a major influence on reinforced concrete flexural performance.

With an increase in the percentage of fiber volume, the number of fibers spread over the fracture surface increases, and the post-cracking performance is also enhanced. Fracture toughness, also known as post-crack performance, is expressed by the energy absorbed by a sample during deformation and failure.

Figure 10 shows that until the addition of 0.5%, the flexural strength of the geopolymer concrete increases to 4.99 MPa. Meanwhile, the plain geopolymer concrete without fiber addition has a flexural strength of 4.87 MPa. Flexural performance decreases when nylon66 fiber addition exceeds 0.5%. This is due to the samples' poor workability when substantial amounts of Nylon66 fibers are introduced. It is believed that the low workability affects the distribution of nylon66 fibers. As a result, when loading is applied, the absorption capacity within the sample is unbalanced. The sample frequently cracks due to the fewer fibers available. The results of the comparison between 28 and 90 days show that the curing period is the most important factor in the development of the flexural strength.

**Figure 10.** Flexural strength for 90 days.

Results for 90 days are better than those for 30 days since geopolymers are still hydrating slowly at that point. The pattern matches the compressive strength result exactly. In comparison to 30 days, the geopolymerization at 90 days enhanced the geopolymers' characteristics, leading to better microstructure properties. According to Figure 10, it was found that Nylon66 fiber improved the flexural strength of geopolymer concrete. Regardless of the fiber type, the improvement in first-crack strength was expected due to the increase in fiber volume fraction.

Moreover, the nylon66 fiber reinforced geopolymer concrete was noted to be primarily responsible for the fiber bridging effect. Therefore, the characteristics of strain hardening and the flexural strength may be adversely affected by an increase in fiber content due to the uneven geometry of fiber from the recycling process.

When the specimen was subjected to the bending load, the area between the two loading pins, where the flexural stress was at its highest, began to deform and split. Once the matrix's bending strength was exceeded, the first crack in composite materials began to form. After that, the crack continued to spread until it reached a nylon66 fiber with a low rigidity. The fracture attempted to penetrate through the fiber at this point due to the flexural tension being applied, which caused it to elongate, rupture, or pull out.

According to Wang et al. [27], the fiber addition was found to significantly improve the flexural strength of PPRGPC. The percentage of addition was varied at 0.1%, 0.15%, and 2.0%, respectively. This study used the PP fiber straight type. According to the result obtained, plain GPC obtained a flexural strength of 4 MPa and the strength was increased to 4.3 MPa with 0.1% fiber addition. Meanwhile, the flexural strength started to decrease for fiber addition at 0.15% (4.1 MPa) and 2.0% (4 MPa).

The geopolymerization product that had adhered to the nylon66 fibers' surface suggested that the binding strength might be high enough to activate this mechanism. The interfacial bond strength, in contrast, is weaker than the applied stress. By redistributing

the localized stress, fiber bridging caused the specimens to develop many microcracks. Increases in ductility and post-cracking toughness were brought on by the ongoing process of microcrack development.

Flexural strength at 28 and 90 days yielded results with several decimals since there was little significant variation in strength. The result is 5 MPa to 3 MPa between 28 and 90 days. Flexural strength displays the improvement and gap strength for each fiber added for 0.5%, 1%, 1.5%, and 2.0% with numerous decimals. The fiber distribution inside the geopolymers cannot be controlled, which is a problem.
