## 1 Introduction
In the course of ongoing research, steps have also been taken to investigate the effects of other types of fiber combinations, such as carbon and steel fibers [84] or glass and carbon fibers [85]. Furthermore, to improve the properties of UHPC mixes, combinations of basalt and polypropylene fibers can be used [86; 87; 88; 89]. The addition of such types of fiber can improve impact resistance, reduce shrinkage, increase crack resistance, and improve flexural and tensile strength, with a limited increase in compressive strength. The increase in UHPC dynamic strength has been recognized to have potential for further research, offering prospects for the use of hybrid fibers in structures exposed to impact and explosion loads [90; 91; 92; 93; 94]. Hybrid fiber combinations have been found to produce better results in achieving higher dynamic performance parameters compared to single-type fiber [42]. The mix can be designed so that some fibers are pulled out of the concrete, while others absorb energy during cracking. When ultra-high performance concrete reinforced with hybrid fibers is used in protective structures, this material may be subjected to destructive dynamic loads, such as high-speed impacts [95; 96]. Another problem is the appearance of explosive spalling, which occurs in ultra-high performance concrete during exposure to fires due to its low permeability and dense structure [97; 98]. This phenomenon is caused by high steam pressure and thermal stresses that develop under fire conditions, which can be reduced by adding fibers [99]. However, the addition of steel fibers alone is not effective in preventing explosive spalling of concrete [100; 101; 102]. An effective solution to avoid explosive spalling is the hybridization of steel fibers with polymer or natural fibers [103; 104; 105; 106; 107]. Polymer fibers melt at approximately 200 \({}^{\circ}\)C, helping to release trapped steam through a network of newly formed microchannels [108]. After the addition of basalt fibers, it was possible to determine residual strength after exposure to temperatures ranging from 500 to 1000 \({}^{\circ}\)C, while the mix containing only steel fibers experienced spalling and did not maintain structural integrity, preventing the assessment of residual strength. Basalt fiber, with its favorable mechanical properties, chemical stability, environmental friendliness, and relatively low production cost, can be used as a single type of fiber additive or in hybrid fiber mixes [109; 110; 111; 112; 113]. The skillful application of fibers enables the creation of more slender structural elements while maintaining the same load-bearing capacity, preserving the properties of the mix and reducing the overall amount of reinforcement. This, in turn, results in a reduction in weight, lower overall construction costs, shorter construction times, and an increase in usable space in buildings with the same building area. The current state of knowledge is primarily based on studies evaluating the effect of single-type fibers on the properties of normal-strength concrete under static loads. On the contrary, research focusing on hybrid fiber systems in high- and ultra-high performance concrete, especially under dynamic conditions, is still limited. Although there are some review articles on UHPC or on concrete with selected hybrid fiber types, a consolidated and wide-ranging review that examines various fiber combinations and their influence on UHPC performance parameters including workability, static strength, and dynamic resistance is still lacking. This article aims to address this gap by comprehensively presenting the effects of hybrid fiber use on UHPC. It incorporates current knowledge, highlights technological challenges, and outlines future research directions. The study offers a comparative, data-driven synthesis that can support the design of more durable and resilient concrete systems for demanding civil and military infrastructure applications.