## 3 Results and Discussion
### Flexural Behavior and Toughness  
The load-mid-span deflection curves for all test series are shown in Figure 7.  
According to Figure 7a, plain concrete exhibited deflection softening behavior with a sharp decrease without showing hardness and toughness after cracking. The M2, M3, and M5 samples exhibited a certain deflection hardening by showing an increase in strength after the first crack. In the literature [59], it has been reported that deflection hardening behavior for concretes produced with normal-strength steel fibers occurs when using at least 1% steel fibers. The reason why the M3 and M5 mixtures showed deflection hardening behavior despite the decrease in the steel fiber ratio to 0.60% can be attributed to the positive interaction between steel-basalt and steel-polyppropylene fiber. However, the M4 sample, where the steel fiber ratio was decreased to 0.40%, exhibited a more brittle behavior after cracking and similar deflection hardening behavior was not observed. This situation is an indication that there is a change in deflection pattern when the steel fiber ratio decreases below 0.60%. In addition, this result can be attributed to the inadequacy of bridging macro cracks with the increase in the basalt fiber ratio. With increasing temperature, the initial slopes of the load-deflection curves decreased. These decreases can be attributed to the formation of cracks of different sizes due to thermal stresses and increase in porosity due to heating [18]. At 400 \({}^{\circ}\)C, only deflection hardening was observed in the flexural behavior of the M1 sample. Although the M3 and M4 samples recovered after the first crack, they could not exhibit a second peak load. The M5 series, which showed deflection hardening before heating, also exhibited deflection softening after 400 \({}^{\circ}\)C. This situation can be attributed to the decrease in the bond strength between the fibers and the matrix after heating. Deflection softening behavior was obtained in all series after 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C heating. [PERSON] and [PERSON] [18] detected that the steel  
Figure 7: Load–mid-span deflection curves for all test series. fiber ratio had a minimum limit of 0.9% to improve the post-cracking strength of heated samples with steel fibers. Re-curing the fiber concretes with water after heating changed the behavior of the load-deflection curve after the peak load. Deflection softening behavior was observed in thermally damaged fiber concrete specimens exposed to heating. This situation caused an increase in the flexural strength of the concrete due to the rehydration of cement particles and new phases in the concrete matrix as a result of re-curing, while also increasing the deflection. As a result of the deterioration of the fibers exposed to heating due to oxidation, melting, etc., could not completely accompany the recovery of the concrete and a sharp decrease occurred after the peak load. This sharp decrease can be attributed to the inhomogeneity of the rehydrated compositions and the rapid propagation of micro-macro-size cracks [35]. Toughness is an important indicator of the energy absorption ability of fiber concretes in particular, and can be obtained from the area under the load-deflection curve. Since single and hybrid fiber concretes exhibit significant deflection-hardening behavior, the toughness and load carrying capacity at the modulus of rupture (MOR) was investigated. In addition, since concretes using steel fibers show significant load carrying capacity even at large deflections, the load carrying and toughness parameters were calculated at the L/150 (2 mm), L/100 (3 mm), and L/75 (4 mm) deflection points suggested in previous studies [60; 61]. In Figure 8, the load values and toughness of the fibrous specimens at various deflection points before heating are given. Since plain concrete exhibited brittle fracture, only the load carrying capacity and toughness at the MOR were determined.