## 4 Conclusions  
This study investigated the toughness and mechanical properties of specimens without fiber, with steel fiber, and with steel-basalt hybrid fiber after ambient temperature and exposure to elevated temperatures. It was conducted to understand the effect of water re-curing on the specimens after heating. In addition, a mixture was prepared with steel-polyppropylene hybrid fiber content to make a comparison with steel-basalt fiber. The following findings were obtained as a result of the experimental study:  
* At ambient temperature, fiber content slightly decreased the UPV values of the specimens compared to the reference mixture. As the specimens were exposed to temperature, remarkable decreases in the UPV results were observed, especially at 600 \({}^{\circ}\)C and above.
* An increase in compressive strength was obtained in all series except for the M4 mixture with 0.40% basalt fiber content at room temperature. This situation shows the negative effect of increasing basalt fiber content on compressive strength. The use of fibers caused significant improvements, especially in terms of flexural strength. Decreases in the strength of all series were observed with increasing temperatures. However, especially at temperatures above 600 \({}^{\circ}\)C, basalt fiber contributed to reduc  
Figure 12: Section features of specimens.  
ing the residual flexural strength loss. In mixtures where the same proportion of basalt and polypropylene fiber was used, basalt fiber gave better results in residual flexural strength.
* In contrast to flexural strength, polypropylene fiber gave better results than basalt fiber in terms of load carrying and toughness after the peak of the load-deflection curve at room temperature. This situation can be attributed to the weak response of basalt fiber specimens after peak load and fiber breakage.
* Water re-curing had a significant effect in recovering the performance of thermally damaged fibrous and non-fibrous concretes after heating. This recovery was especially evident at 600 \({}^{\circ}\)C and above.
* In terms of flexural strength, after exposure to 600 \({}^{\circ}\)C, the reference specimen reached 58.32% of its initial strength, while the M2 mixture reached 71.86%. These values were 30.11% and 41.08%, respectively, before re-curing. Re-curing after 800 \({}^{\circ}\)C brought the recovery rates of the mixtures closer to each other.
* Significant increases were obtained in the toughness properties of the specimens as a result of re-curing. However, although re-curing at temperatures of 600 \({}^{\circ}\)C and above caused a significant increase in the load carrying capacity at the MOR, there were sudden decreases in the load values at the L/100 and L/75 deflection points and even lower than the before-re-curing temperature at some points. This situation can be attributed to the fact that despite the improvement in the matrix, the fibers could not accommodate the sudden stress increase in the concrete due to the deterioration of the fibers at high temperatures.  
Conceptualization, Z.C.; methodology, Z.C. and Y.U.; validation, Z.C. and Y.U; investigation, Z.C. and Y.U.; data curation, Y.U.; writing--original draft preparation, Z.C. and Y.U.; writing--review and editing, Z.C.; visualization, Z.C. and Y.U.; supervision, Z.C.; project administration, Z.C.; funding acquisition, Z.C. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
This research was supported by the research project \"High- Temperature Resistance of Steel--Basalt Hybrid Fiber Concretes\" from the Ataturk University, Scientific Research Projects (BAP) Program in Erzurum, Turkey (Grant number FYL-2024-13225).  
The authors declare no conflict of interest.