## 1 Introduction
However, the use of a single type of fiber does not always guarantee a sufficient improvement in compressive strength, as observed in the case of UHPC containing 18 mm basalt fibers, 10 mm polypropylene fibers, or 12 mm glass fibers [40]. [PERSON] et al. [41] also reported a decrease in the compressive strength of ultra-high performance concrete with the addition of up to 2.5% basalt, glass, or polypropylene fibers. With the advancement of production technology, two or more types of fiber of different sizes, types, and shapes began to be used, which complemented each other due to their diverse properties and allowed a reduction in final costs [42; 43; 44; 45; 46; 47]. In the case of hybrid combinations, it was also possible to achieve higher tensile strength, abrasion, and fatigue resistance, as well as energy absorption capacity [48]. In such complex reinforcement systems, short fibers inhibit microcrack formation, acting during the lower load phase. On the other hand, the addition of long fibers improves strength under higher loads by slowing macrocrack propagation and improving ductility through the crack bridge [49; 50; 51]. Due to the larger contact surface between the long fibers and the concrete matrix, and sometimes their special shape, which enhances anchoring, their extraction is more difficult [52]. This enables a transformation of the behavior of UHPC from brittle to ductile, improving energy absorption capacity, increasing safety, and reducing the risk of structural failure [53]. A properly selected amount of fibers creates a spatial skeletal structure that strengthens and hardens the UHPC, bridges, and reduces cracks [8; 22]. The first type of fiber used was steel fibers of various lengths (from 6 to 50 mm) and shapes (straight, twisted, wavy, or hooked ends) [54; 55]. They are particularly effective in improving fracture resistance and increasing compressive and tensile strength [56; 57; 58; 59]. However, they are highly dense, are relatively expensive to produce and exhibit poor corrosion resistance. Additionally, the carbon dioxide emissions generated during their production have a negative impact on the natural environment. Another type of fiber that has attracted researchers' attention is basalt fiber [60; 61; 62], which, due to its thermal stability, corrosion resistance, lower cost, and lower energy consumption during production, has become an alternative to steel fiber [63; 64; 65; 66; 67]. Basalt fibers are available in various dimensions, that is, lengths ranging from 12 to 50 mm and diameters ranging from 0.012 to 0.45 mm. Furthermore, they demonstrate high resistance to corrosion in salt solutions, especially aqueous solutions, and high resistance to frost after freeze-thaw cycles [68; 69]. The addition of basalt fibers improves the strength properties and impact resistance of concrete, increases crack resistance, and extends its useful life. A hybrid mix containing steel and basalt fibers allows optimal use of the properties of both types of fibers [70; 71]. The compressive and flexural strength of UHPC can increase by more than 50%, while the splitting tensile strength can reach twice the value. Furthermore, the hybrid fiber-reinforced UHPC demonstrates high resistance to dynamic impacts, such as explosions or projectile strikes [72]. Another addition to concrete mixes in the form of hybrid fibers includes micro- and macro-sized polymer fibers with various chemical compositions [73; 74; 75; 76]. These include polyvinyl alcohol (PVA), polyvinylene (POM), polyethylene (PE), polypropylene (PP), and polyester (PET) fibers, with lengths ranging from 13 to 40 mm and diameters from 0.012 to 0.9 mm, depending on the performance requirements for the concrete mix [77; 78; 79; 80]. Glass fibers are also used, with lengths of 6-18 mm and diameters of 0.012-0.025 mm. They can be added to mixes as a single type of fiber or combined with steel fibers, resulting in high mechanical performance values [81; 82; 83].