## 3 Results and Discussion
### Flexural Strength  
Table 6 shows the flexural strength results for the non-fibrous and fibrous mixture series. As seen in Table 6, the addition of different fibers to concrete mixtures has different improvement effects on the flexural strength. The bridging effect due to the fiber addition increased the crack resistance and, consequently, the flexural strength was increased. The highest flexural strength value before heating was obtained from the M2 series, with 10.07 MPa. This result was approximately 58% higher than the reference specimen. Decreases in flexural strength were observed with the addition of 0.20% BF (M3) and 0.20% PPF (M5) substituted for steel fiber. However, a remarkable increase of 46% and 39%, respectively, was detected in the M3 and M5 mixture compared to the control specimen. BF has a more pronounced restriction on the initiation and propagation of microcracks, while PPF has a more pronounced restriction on the expansion of macro cracks [54]. As a result, the hybrid use of BF with SF has a synergistic positive effect in terms of bridging micro- and macro cracks (Figure 5). With the increase in basalt fiber content from 0.20% to 0.40%, the flexural strength decreased from 9.28 MPa to 7.65 MPa. This value was 20.66% higher compared to the non-fiber mixture. This result shows the importance of SF in improving the flexural strength at room temperature. \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & \multicolumn{4}{c}{**Flexural Strength (MPa)**} \\ \hline  & \multicolumn{4}{c}{**Temperatures**} \\
**Mixtures** & **23 \({}^{\circ}\)C** & **400 \({}^{\circ}\)C** & **600 \({}^{\circ}\)C** & **800 \({}^{\circ}\)C** \\ \hline M1 & 6.34 & 3.61 & 1.91 & 1.18 \\ M1-R & & 4.07 & 3.70 & 2.74 \\ M2 & 10.07 & 6.75 & 4.14 & 2.02 \\ M2-R & & 8.12 & 7.23 & 4.39 \\ M3 & 9.28 & 6.34 & 3.92 & 1.97 \\ M3-R & & 7.39 & 6.53 & 4.37 \\ M4 & 7.65 & 4.99 & 3.31 & 1.79 \\ M4-R & & 5.82 & 5.06 & 3.89 \\ M5 & 8.82 & 4.97 & 3.38 & 1.36 \\ M5-R & & 6.22 & 5.61 & 3.72 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Flexural strength results of mixture specimens. Figure 5: Bridging effect of fibers after flexural test of M4 sample. The residual relative flexural strength (%) results of the mixtures used in this study are given in Figure 6. The flexural strength of specimen M1 at 400 \({}^{\circ}\)C decreased by 43.03%, while this rate was determined as 32.99% in specimen M2. This result can be attributed to the fact that the addition of steel fiber restricts the volume change in concrete during the heating increase and its higher thermal conductivity than the cement matrix and aggregate [55]. After the M1 mixture specimen, the highest decrease in flexural strength at 400 \({}^{\circ}\)C was obtained from the M5 specimen containing 0.20% PPF. The decrease in strength of the M5 (%0.60 SF + %0.20 PPF) was 39.46%. In concretes to which PPF is added, the pores formed by the melting of PPF after high temperatures [56; 57] have a negative effect on the performance under tension, and the residual flexural strength decreases significantly at temperatures around 400 \({}^{\circ}\)C. Although the highest flexural strength at 400 \({}^{\circ}\)C was obtained from the M2 series, the least decrease in strength was observed in the M3 mixture, with 14.66%. The decrease in strength of the M4 mixture was determined as 31.72%. This result is evidence that basalt fiber is effective in bridging microcracks formed at 400 \({}^{\circ}\)C.  
Significant decreases in flexural strength were measured for all series above 400 \({}^{\circ}\)C. This can be attributed to the decomposition of Ca(OH)\({}_{2}\) and CaCO\({}_{3}\) above 400 \({}^{\circ}\)C and the decomposition of C-S-H above 600 \({}^{\circ}\)C [18; 58]. Series M1 exhibited 69.89% and 81.32% reductions in flexural strength at 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C, respectively. These decrease rates were calculated as 58.92% and 79.97% at 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C in the M2 sample, respectively.