## 3 Results and Discussion
### Compressive Strength
This result indicates that the restriction effect on the spread of microcracks decreased due to the melting and vaporization of PPFs during heating, and the decrease in strength because of the formed porous structure reached a significant level [14,18]. As the concrete samples were exposed to temperatures of 600 \({}^{\circ}\)C and higher, the decomposition of aggregates Ca(OH)\({}_{2}\) and CaCO\({}_{3}\) into lime and water vapor increased the degree of damage remarkably [52]. After temperatures of 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C, the residual relative compressive strength of plain concrete was obtained as 42.49% and 19.68%, respectively. The highest residual relative compressive strength at 600 \({}^{\circ}\)C was determined in the M4 series as47.24%. This result shows that the use of steel and basalt in appropriate proportions has a positive effect on the residual compressive strength. The minimum decrease in strength when exposed to 800 \({}^{\circ}\)C was determined in the control specimen. The decreases in compressive strength of the control, M1, M2, M3, M4, and M5 mix series at 800 \({}^{\circ}\)C were determined as 80.32%, 81.99%, 82.35%, 81.34%, and 82.99%, respectively. In contrast to the residual relative compressive strengths at 400 \({}^{\circ}\)C and 600 \({}^{\circ}\)C, the best result at 800 \({}^{\circ}\)C was obtained from the reference concrete. However, despite the decrease in residual strength, the highest compressive strength is still obtained from the M2 series, with 10.70. This means that the serious rustiness of steel fiber beyond 600 \({}^{\circ}\)C can only reduce the reinforcement of concrete, not eliminate it [21]. Twenty-eight days of water re-curing showed a significant effect on the compressive strength recovery of thermally damaged non-fibrous and fibrous concrete samples. Among the samples exposed to 400 \({}^{\circ}\)C, the strength improvement was partially achieved in the control and steel-basalt fiber samples. The residual relative compressive strength of the M2 mixture containing only steel fiber increased from 82.61% to 86.76%. This increased from 83.68% to 87.16% and from 85.13% to 90.47% in the M3 and M4 series with 0.20% and 0.40% basalt fiber, respectively. This result can be attributed to the fact that the damage between the single steel fiber and hybrid steel-basalt fibers and the cement matrix and ITZ did not reach a serious level [14]. The lowest improvement in strength occurred in the plain concrete sample, with 1.41%. In contrast to these mixture series, the compressive strength of the M5 mixture containing 0.20% PPF increased from 38.40 MPa to 47.95 MPa. This result was recorded as a striking improvement of 17.42% in the relative residual strength. This phenomenon is due to the filling of the voids formed by the melting and evaporating PPF fibers after heating with the rehydrated products. This result is consistent with the studies in the literature [14,32]. The strength recovery rate of the concrete mixture series after re-curing with water continued to increase as the temperature increased. At 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C, the residual relative compressive strength of the control sample increased from 42.49 to 57.43% and from 19.68% to 34.91%, and the amount of improvement was calculated as 14.94% and 15.23%, respectively. The fiber-added mixtures had a compressive strength of 600 \({}^{\circ}\)C. The strength improvements at 600 \({}^{\circ}\)C were better compared to the control sample. The residual relative compressive strength of the M2 mixture series improved by 19.45% and increased up to 63.59%. The strength recovery rates of the M3 and M4 series at 600 \({}^{\circ}\)C were 17.10% and 18.83%, respectively. This remarkable recovery can be attributed to the formation of rehydration products such as AFt and C-S-H and the carbonation of Ca(OH)\({}_{2}\) and CaO. The recovery rates of hybrid fiber-reinforced mixtures at 800 \({}^{\circ}\)C approached each other and ranged between 13.05% and 14.85%.