## 3 Results and Discussion
### Compressive Strength  
As a result of the experimental study, the compressive strength results of the mixture series subjected to different processes are given in Table 5.  
The compressive strength of the control and fiber-containing mixtures varied between 51.44 and 59.52 MPa. The highest compressive strength value was determined in the M2 series containing single steel fiber, which was 11.74% more compared to the reference specimen. A decrease in the compressive strength of the M3 and M5 mixture series, in which 0.20% BF and PPF were substituted instead of steel fibers, was observed compared to the M2 series. This decrease was determined as 2.90% and 7.86% in the M3 and M5 mixtures, respectively. This can be attributed to the higher strength and elastic modulus  
Figure 3: Crack distribution on the surface after high temperature and re-curing. values of steel fibers, which are more efficient in healing macro-cracks, compared to basalt and polypropylene fibers [49]. In addition, the compressive strengths of the M3 and M5 series were approximately 9.10% and 4.21% higher than the reference sample, respectively. This can be related to the ability of fiber content in appropriate proportions to delay the spread of cracks in concrete, change their direction, and delay the rate of expansion [49,50]. However, a 2.08% decrease in compressive strength was reported for the M4 series, where the BF volume fraction increased from 0.20 to 0.40 compared to the reference sample. This result can be attributed to the fact that adding more than certain amounts of basalt fiber volume fraction to the concrete will cause defects such as more cracks, voids, and weak interfaces in the concrete due to the decrease in workability, as seen in Table 3, which will reduce the strength. This is consistent with previous studies [51, 52, 53], which indicated that the increase in basalt fiber content cannot improve or significantly reduce the strength. The relative compressive strength results of the reference and fibrous mixture series after elevated temperature and re-curing are given in Figure 4. \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multicolumn{5}{c}{**Compressive Strength (MPa)**} \\ \hline  & \multicolumn{4}{c}{**Temperatures**} \\
**Mixtures** & **23 \({}^{\circ}\)C** & **400 \({}^{\circ}\)C** & **600 \({}^{\circ}\)C** & **800 \({}^{\circ}\)C** \\ \hline M1 & 52.53 & 42.36 & 21.54 & 10.34 \\ M1-R & & 45.10 & 31.77 & 18.34 \\ M2 & 59.52 & 49.17 & 26.27 & 10.72 \\ M2-R & & 51.64 & 37.85 & 19.56 \\ M3 & 57.79 & 48.36 & 26.08 & 10.20 \\ M3-R & & 50.37 & 35.96 & 17.74 \\ M4 & 51.44 & 43.79 & 24.30 & 9.60 \\ M4-R & & 46.54 & 33.98 & 16.67 \\ M5 & 54.84 & 38.40 & 22.52 & 9.33 \\ M5-R & & 47.95 & 32.55 & 17.09 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Compressive results of mixture specimens. Figure 4: Relative residual compressive strength results after heating and re-curing. The relative residual compressive strengths of the mixture series at 400 \({}^{\circ}\)C were between 70.02% and 85.13%. At 400 \({}^{\circ}\)C, the reference specimen strength decreased by 19.36% to 42.36 MPa. The M2 mixture series using single steel fiber had the highest compressive strength at 400 \({}^{\circ}\)C with 49.17 MPa, and its relative strength was determined as 82.61%. The highest relative strength results were obtained from the M3 and M4 series using basalt fiber, with 83.68% and 85.13%, respectively. This may be an indication that steel fiber prevents the expansion of macrocracks in the cement matrix and basalt fiber prevents the expansion of microcracks and that the use of hybrid fibers has a significant synergistic effect. The most drastic decrease in compressive strength at 400 \({}^{\circ}\)C was obtained from the M5 mixture with 0.20% PPF added. The residual relative compressive strength of the M5 mixture was determined as 70.02%.