*3.2. Flexural Strength*

The flexural strength biochar concrete specimens are displayed in Figure 7 at 7, 14, and 28 days of testing. The results indicate that, unlike compressive strength, adding biochar, even at minimal dosages (0.25–0.5 wt%), was favourable to enhance the flexural strength of the concrete beam. The flexural strength of the control mix was recorded as 4.75, 4.95, and 5.06 MPa at 7, 14, and 28 days, respectively. The flexural strength of all biochar-concrete mixes showed higher flexural strength than the control specimen. Adding biochar from 0.25 to 0.75 wt% in concrete showed a linear increase in flexural strength. However, with high biochar loading above 0.75 wt%, the flexural strength was not significantly enhanced. For instance, at a 7-day age, 0.25%, 0.50% and 0.75% biochar indicated a 9%, 13% and 16% improvement in flexural strength as compared to the control mix. Consequently, an increase in biochar loading, i.e., 1.00% and 1.5%, caused only 14% and 13% improvement in flexural strength, which was almost similar to 0.75% biochar-concrete but still better than the control mix. Likewise, at a 14-day and 28-day age, similar trends were noticed, 0.25%, 0.50% and 0.75% biochar resulted in around 7%, 10%, and 12% increase,

respectively, at 14 days and 8%, 10%, and 11% increase, respectively, at 28 days while 1.00% and 1.50% biochar showed 10% and 9% increment at 14 days and 9% and 8% increment at 28 days in flexural strength than that of control mix. The substantial improvement in flexural strength due to the addition of biochar could be due to the flexibility provided by biochar in concrete, which functions as a link between biochar particles and hydrated cement, preventing premature fracture. Maljaee et al. [32] concluded that biochar-based mortar's flexural strength improved due to the addition of biochar. The concrete becomes dense and tough due to the addition of porous biochar, contributing micro-reinforcement effect. This ultimately resists the crack propagation, deflects the crack path, and increases flexural strength [2]. However, a negative impact was noticed after adding biochar by greater than 0.75%. A large amount of biochar in the cement matrix may cause the aggregation of biochar particles leading to an increase in inhomogeneity in the tensile plane of the cement-biochar matrix. Similar behaviour was also reported by Ahmed et al. when using biochar in a cement composite [33]. The results indicate that the optimum biochar dosage was 0.75%, and the flexural strength produced by 0.75% biochar was improved up to 16%, 12%, and 11% at 7, 14, and 28 days compared to the control specimen.

**Figure 7.** Flexural strength of concrete beam specimens.
