## 3 Results and Discussion
### Flexural Strength
As the temperature to which the specimens were exposed increased, the relative residual flexural strength values of the fiber and non-fiber specimens began to approach each other. This result is an indication that the bonds between the matrix and fibers were disrupted due to the increase in the oxidation rate on the surface of the steel fibers along with the deterioration in the concrete [18; 21]. At room temperature, the strength of the M2 mixture was approximately 7.82% and 24.81% higher than the M3 and M4 mixtures, respectively. These rates decreased to 5.13% and 19.92% and 2.38% and 11.11% after 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C. This is an indication that the use of BF has positive effects on strength at 600 \({}^{\circ}\)C and above. Basal fiber exhibits approximately 10% strength loss at 600 \({}^{\circ}\)C [22], which strengthens it  
Figure 6: Relative residual flexural strength results. against high temperatures by restricting crack expansion against thermal damage. This result shows that the use of fibers with high tensile strength and very good thermal stability, such as SF and BF, positively affects the strength. Re-curing resulted in increased strength of concrete in all batches. This resulted in the rehydration of thermally damaged cement particles and newly formed crystals that bridged and narrowed the cracks [35]. The average flexural strength of mixture M1 increased from 3.61 and 1.91 MPa to 4.07 and 3.70 MPa at 400 \({}^{\circ}\)C and 600 \({}^{\circ}\)C, respectively, which reached 64.29% and 58.32% of its value at room temperature. The relative strengths recovered in the M2 series were calculated as around 80.69% and 71.86% at 400 \({}^{\circ}\)C and 600 \({}^{\circ}\)C, respectively. This increase in the M2 mixture compared to the control specimen is due to the improvement in the fibers and matrix ITZ bond strength by the rehydration products formed as a result of re-curing [35]. A similar trend was observed in the M3 mixture containing %0.20 basalt fibers as in the M2 mixture. The average flexural strength of mixture M3 increased from 6.34 and 3.92 MPa to 7.39 and 6.53 MPa at 400 \({}^{\circ}\)C and 600 \({}^{\circ}\)C, respectively, which reached 79.63% and 70.38% of its value at ambient temperature. The relative strength recovered by increasing the basalt fiber ratio from 0.20% to 0.40% at 400 \({}^{\circ}\)C and 600 \({}^{\circ}\)C was calculated as 76.07% and 66.16%, respectively. Compared to the mixtures containing steel and basalt fibers, the relative strength recovery was lower in the M5 mixture using 0.20% PPF, at 70.48% and 63.57%. This result can be attributed to the decrease in the total volume of fibers to bridge the cracks due to the melting of the PPFs. As the temperature increased to 800 \({}^{\circ}\)C, the relative strength recovery rate of the M1 and M2 series approached each other and became 43.17% and 43.63%, respectively. This situation shows that the SF lost its favorable effect in terms of improving the strength. This may be due to the significant deterioration at the matrix-and-fiber interface resulting from remarkable oxidation of the steel fibers. The residual relative strength ratio of the M3 and M4 series containing basalt fiber was higher than that of the M2 sample. This is evidence that basalt fiber is more effective at temperatures above 600 \({}^{\circ}\)C, similar to the behavior after high temperature. As a result, higher values were obtained in all series with water re-curing than the flexural strength obtained at the previous temperature. For example, the flexural strength of the M2 series at 400 \({}^{\circ}\)C was 6.75 MPa, while this value was determined as 7.23 MPa after re-curing at 600 \({}^{\circ}\)C.