## 1 Introduction  
Fire represents one of the most severe durability problems, negatively and irreversibly affecting the physical, chemical, and mechanical properties of a concrete structure [1, 2, 3]. In recent decades, the use of FRC has increased significantly because of its excellent propertiescompared to plain concrete [4] in terms of increased initial crack strength [5], remarkable improvement in flexural behavior [6], and high toughness and energy absorption capacity [7; 8]. However, if exposed to high-temperature effects, significant decreases may occur in the mechanical properties of cementitious materials [9; 10]. For this reason, it is important to know the properties of FRC in terms of determining its post-fire resistance [4]. The post-heating residual characteristics of FRC vary remarkably by the type and proportions of fiber used. Most of the previous studies aimed to investigate the after-elevated-temperature resistance of SF- and PPF-reinforced cementitious composites [11; 12; 13]. Substituting steel fiber into concrete prevents the spread of cracks and improves heat transfer properties [14]. It has been determined that the presence of SF in cementitious composites subjected to elevated temperatures increases the residual compressive [15], tensile [16; 17], flexural [18], and impact strength [19], and fracture energy [20]. [PERSON] and [PERSON] [15] found that 1% steel fiber in normal concrete increases the residual compressive strength by approximately 5-15% compared to concrete without fiber addition, regardless of the heating temperature. [PERSON] and [PERSON] [18] reported in their research that 0.6% steel fiber caused a remarkable improvement in the flexural strength. Basalt fiber, which is produced from basalt rock and is effective in strengthening cementitious composites with its high modulus of elasticity and tensile strength, attracts attention with its high melting temperature [21; 22]. In addition to the mechanical properties of basalt fiber, its potential for use compared to other fiber types needs to be investigated in terms of its behavior against important environmental effects such as high temperature. In this context, in recent decades, the fire resistance of BF has become a subject of research for both single [22; 23; 24] and hybrid [21; 25; 26] use. The presence of basalt fiber in cement-based composites significantly strengthens elevated temperature resistance and has been reported to increase strength [22]. [PERSON] et al. [21] determined the effects of elevated temperature on mortar specimens containing basalt and steel fiber. The study indicated that the addition of basalt and steel fiber can inhibit crack formation and propagation. They also reported that although the hybrid use of steel and basalt fiber reduced the strength, it still had a significant effect in preventing cracks even at 900 \({}^{\circ}\)C. [PERSON] et al. [25] proved that steel and basalt fiber can still seal cracks in mortar specimens exposed to high temperatures. They also reported that basalt fiber maintains its performance even with temperature increase due to its high melting temperature, but an oxide layer forms on the surface of SF at approximately 500 \({}^{\circ}\)C.  
Studies on the improvement of engineering structures exposed to high temperatures have become an important research topic in recent years. Existing studies in the literature have shown that the strength and durability characteristics of cement-based composites exposed to high temperatures will improve after re-curing with water or being kept in a humid environment [27; 28; 29; 30]. This recovery has been attributed to the formation of C-S-H and carbonate phases in the concrete by the hydration of initially unhydrated cement grains in the concrete exposed to high temperatures [31; 32]. The majority of studies on post-fire re-curing have focused on the effect of different cementitious materials [31; 33; 34].