## 3 Results and Discussion
### Flexural Behavior and Toughness
P\({}_{\text{MOR}}\) and T\({}_{\text{MOR}}\) values were calculated as 14.08 kN and 4.95 kN mm, respectively. As seen in Figure 8, the M2 specimen with 0.80% steel fiber exhibited the highest load carrying capacity and toughness at all target points. As basalt fiber was added to the mixtures, the load and toughness decreased. For example, while the highest load and toughness values of the M2 specimen at the MOR were 22.37 kN and 21.39 kN mm, these values were 7.82% and 1.26% less in the M3 sample, respectively. The load and toughness values of the specimen with 0.20% PPF (M5) at the MOR were 12.34% and 15.05% less compared to the M2 mixture. The M3 series showed more load and toughness at the MOR compared to the M5 series, while the M5 series started to show higher load and toughness values with the progression in the deflection points (L/150, L/100, and L/75). This finding is consistent with the result stated by [PERSON] et al. [54] that basalt fibers have a restrictive effect on the initiation and propagation of micro-sized cracks, while polypropylene fibers show an inhibiting effect on the expansion of macro-sized cracks. In addition, the basalt fiber samples exhibit poor response by causing damage in the form of fiber rupture after cracking [5]. On the other hand, unlike the crimped ends of steel fibers that provide better anchorage to the cement matrix, basalt fibers are straight and do not have sufficient bond strength, which are possible reasons accounting for this situation. With the increase in basalt fiber content to 0.40%, remarkable decreases in toughness values occurred. Decreases of 30.26%, 32.80%, and 34.75% were reported in the T\({}_{\text{L/150}}\), T\({}_{\text{L/100}}\), and T\({}_{\text{L/75}}\) values of the M4 series, respectively, compared to the M2 mixture. The load values and toughness of the fibrous specimens at various deflection points after exposure to temperatures of 400 \({}^{\circ}\)C, 600 \({}^{\circ}\)C, and 800 \({}^{\circ}\)C and after re-curing are given in Figures 9-11. With the increase in temperature, decreases were obtained in the load carrying and toughness capacities of all the mixture series. Only the P and T values of the reference specimen (M1) at the MOR were calculated. The P\({}_{\text{MOR}}\) and T\({}_{\text{MOR}}\) values for 400 \({}^{\circ}\)C were 8.02 kN and 2.30 kN mm, respectively. The P\({}_{\text{MOR}}\), P\({}_{\text{L/150}}\), P\({}_{\text{L/100}}\), and P\({}_{\text{L/75}}\) values of the M2 mixture exposed to 400 \({}^{\circ}\)C were measured as 14.99, 12.91, 11.14, and 9.29 MPa, respectively. The toughness values of the M2 series at 400 \({}^{\circ}\)C were determined as 15.23, 21.77, 33.79, and 43.93 kN mm at the T\({}_{\text{MOR}}\), T\({}_{\text{L/150}}\), T\({}_{\text{L/100}}\), and T\({}_{\text{L/75}}\) points. Compared to room temperature, 28.80%, 29.54%, 30.52%, and 32.06% decreases in toughness were detected after 400 \({}^{\circ}\)C at the target deflection points for the M2 mixture. With the addition of BF and PPF to the mixtures, decreases in load carrying capacity were observed. The P\({}_{\text{MOR}}\), P\({}_{\text{L/150}}\), P\({}_{\text{L/100}}\), and P\({}_{\text{L/75}}\) values of the M3 and M5 mixtures exposed to 400 \({}^{\circ}\)C were calculated as 14.08, 12.02, 10.94, and 8.67 MPa and 11.86, 9.84, 7.50, and 6.53 MPa, respectively. Similar to the load capacities, the toughness values were also higher in the M3 series than in the M5 series at all deflection points. For example, the T\({}_{\text{L/100}}\) and T\({}_{\text{L/75}}\) values of the M3 series were 13.48% and 18.50% higher than those of the M5 sample. At 400 \({}^{\circ}\)C, basalt fibers had a positive  
Figure 8: Effect of fiber mixture series on flexural properties of concrete before heating: (**a**) load carrying capacity; (**b**) toughness. effect on steel fibers, bridging cracks and increasing toughness, while polypropylene fibers were melted at the target temperature.