## 3 Results and Discussion  
### UPV and Damage Degree (DD)  
The UPV method is considered the preferred nondestructive test in civil engineering to evaluate concrete quality [14; 42]. In addition, the UPV method provides information on the residual strength and the determination of the damaged layer thickness in thermally damaged concrete [43; 44]. In this study, the UPV change of the control and FRC after  
Figure 2: Test set-up of specimen and schematic diagram of a specimen. elevated temperature and re-curing is reported. The results of the UPV values of the specimens used in this study obtained after elevated temperature and re-curing stages are given in Table 4. In the table, re-curing is represented by R.  
UPV values of plain concrete and FRC mixtures at room temperature ranged between 4608 and 4717 m/s. The average UPV value at 23 \({}^{\circ}\)C for all specimens used in the study was approximately 4653 m/s. The addition of SF, hybrid SF and BF, and hybrid SF and PPF to concrete caused a slight decrease of 1.40-2.31% compared to the reference specimen. This may lead to the conclusion that the use of fibers creates more porosity in the concrete content. [PERSON] and [PERSON] [18] reported that there was a decrease in UPV values between 4.60% and 7.90% when single and hybrid SF and PPF were used. [PERSON] et al. [45] stated that 0.20% basalt fiber content in the concrete mixture gave the same UPV result as the control specimen. As the heating temperature increased in the specimens, decreases in UPV values were observed. These decreases in UPV value can be explained by the increase in specimen porosity resulting from water loss and crack growth in the concrete due to heating [46; 47]. At 400 \({}^{\circ}\)C, the UPV values of the mixtures were between 4082 and 4292 m/s. Compared to the M3 (%0.60 SF + %0.20 BF) mixture, a greater decrease in the UPV value of the M5 (%0.60 SF + %0.20 PPF) mixture was detected. This can be attributed to the fact that PPF melts below 400 \({}^{\circ}\)C and forms micro-voids. As the temperature increased above 400 \({}^{\circ}\)C, the cracks in the concrete expanded further with the increase in thermal expansion in the specimens, and, as a result, the velocity of the passed pulse was delayed [48]. After the M1 series was subjected to temperatures of 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C, decreases of 39.80% and 74.14%, respectively, were observed in the UPV results compared to the ambient temperature. This was determined as 40.23% and 70.39% in the M2 series, 40.94% and 70.54% in the M3 series, and 41.62% and 72.50% in the M5 series. As a result, the reference and all fiber series had similar UPV values. Similar findings were reported in the studies of [PERSON] et al. [46]. Significant improvements in the UPV values of the reference and fibrous series were observed after re-curing with water. The DD value of samples exposed to 400 \({}^{\circ}\)C heating varied between 0.08 and 0.11, while after re-curing in water it was approximately between 0.01 and 0.04. Upon exceeding 400 \({}^{\circ}\)C, micro-sized cracks formed in the Interface Transition Zone (ITZ) due to the decomposition in the C-S-H structure and the thermal incompatibility between the aggregate and the cement paste [14]. As the temperature increased from 400 \({}^{\circ}\)C to 600 \({}^{\circ}\)C, the DD increased to a range of approximately 0.39-0.42. A significant improvement was observed in the DD values of the specimens subjected to re-curing after 600 \({}^{\circ}\)C, and the DD decreased to between 0.04 and 0.08. [PERSON] et al.