## 1 Introduction
Previous studies [27; 28] have demonstrated that the addition of steel fibers (SF) can improve many properties of concrete, including tensile strength, impact resistance, energy absorption capacity, and crack propagation resistance. Moreover, in the case of fire, a more uniform temperature distribution can be achieved inside the concrete with the inclusion of randomly distributed SF. This mainly originates from the high thermal conductivity of SF (28-50 W/mK) compared with that of the concrete (1.1-1.3 W/mK) [29; 30]. Accordingly, stress gradients originating from abrupt temperature variations are reduced, thereby preventing the spalling phenomenon in the concrete structure [31]. It should be indicated that with the incorporation of SF in the concrete, both the spalling resistance and residual mechanical properties such as residual compressive strength, residual tensile strength, and residual elastic modulus improve. This is because SF can bridge and retard the crack propagation in the concrete subjected to fire heating [32; 33; 34; 35]. Studies approve this analysis and show that SF's inclusion in the concrete protects the inner structure from direct damage in the case of fire. However, this protective effect appears only when the tensile forces over a crack can be sufficiently transferred through the heated fibers. On the other hand, fibers are heated severely so that their tensile strength decreases significantly and the protection cannot be fulfilled effectively. It should be indicated that a single type of fiber or a combination of different types of fibers can be used to reinforce the SCC. More specifically, the cocktail fiber in concrete has a positive hybrid effect on the concrete performance so that it has attracted many scholars. It is worth noting that each fiber operates as a complement to cover the weaknesses of other fibers within the concrete matrix [36; 37; 38; 39]. Reviewing the literature indicates that there is not in-depth knowledge about the performance of hybrid fibers combined with SF and PF [40; 41]. The performed investigations in this issue reveal that the presence of fibers increases the tensile strength and the porosity, thereby reducing the fire-induced spalling in SCC. Moreover, hybrid fibers improve the residual mechanical attributes of SCC and increase the concrete resistance to explosive spalling originating from high temperatures [42; 43; 44; 45; 46]. [PERSON] et al. [41] carried out experiments on four reinforced reactive powder concrete (RPC) beams and, accordingly, studied the flexural behavior when three sides were exposed to fire. The obtained results showed that adding 2% SF and 0.2% PF by volume is an effective way to increase the resistance of reinforced RPC beams to fire-induced spalling. Although studies show that the incorporation of PF in concrete can significantly prevent fire-induced spalling, the influence of PF on the residual mechanical characteristics of the concrete has not been determined yet. In this regard, [PERSON] et al. [46] showed that the incorporation of PF in HPC does not have a considerable impact on the residual mechanical properties of the concrete after heating to 600 \({}^{\circ}\)C. On the other hand, they found that the inclusion of 1 kg/m\({}^{3}\) PF in HPC significantly improved the residual splitting strength and residual compressive strength when the concrete was exposed to temperatures up to 300 \({}^{\circ}\)C. [PERSON] et al. [29] studied the influence of various fibers on the residual compressive strength, flexural tensile strength and flexural toughness of SCC after heating to different target temperatures (300 \({}^{\circ}\)C, 600 \({}^{\circ}\)C, and 900 \({}^{\circ}\)C). The obtained results showed that although the inclusion of PF significantly prevents fire-induced spalling in SCC, there is no clear rule about the residual mechanical properties of SCC.