## 4 Hybrid Fiber Composition
### Steel--Steel Hybrid Fiber
#### 4.1.2 Mechanical Properties
In combination with straight fibers of 15/0.2 mm at a dose of 1.5%, the addition of fibers of 6 mm at 1% led to a higher compressive strength. The results are consistent with those of [PERSON] and [PERSON] [136], who evaluated the behavior of hybrid mixtures reinforced with steel fibers of 13 and 6 mm lengths. In [138] a hybrid mixture was examined with the addition of straight steel fibers with lengths/diameter of 13 and 19.5/0.2 mm and 30/0.3 mm, with a total content of 2%. For the mixture containing 19.5 and 30 mm fibers, an increase in compressive strength was observed with a higher proportion of long fibers. The highest result was achieved with a fiber content of 1.5%. The increase in strength was possible because of the uniform distribution of fibers, which allowed the use of their properties. The longer the fibers used, the greater the friction forces between them and the matrix. Therefore, to overcome these forces, a greater external force was required. In the case of a mixture with 13 mm and 30 mm fibers, the highest compressive strength was observed with the largest proportion of short fibers, which contrasts with the results of the previous analysis. According to research in [162], macro-steel fibers can have a positive effect by inhibiting cracks and increasing maximum strength, but also a negative effect by increasing air content. A UHPC mixture with 2% steel fibers achieved the highest compressive strength with a combination of 1% straight fibers with a length/diameter of 13/0.2 mm and 1% hooked-end fibers of 30/0.5 mm--an increase of 18.6% compared to the mix without fibers [139]. The effect of increasing the amount of straight steel fibers to 5% was also investigated, and it did not lead to a further increase in strength. The results obtained were lower than those for the hybrid mixture with 2% fibers. This indicates the existence of a certain ultimate volumetric content of fibers, which depends not only on their type but also on the fiber content and the selection of other UHPC ingredients. In each of the above mixes, a water-reducing additive was added, the amount of which increased with fiber content, leading to increased matrix porosity and reduced compressive strength [163]. The largest amount of reducer was added to the mixture containing 2% hooked-end fibers. As the fiber content increased, fiber agglomeration and the need for more energy was observed to evenly distribute them in the mixture. [PERSON] et al. [141] reported compressive strength results of UHPC with a total addition of 2% steel fibers, in which straight steel fibers with a length/diameter of 19.5/0.2 mm and hooked-end steel fibers of 30/0.38 mm or twisted fibers of 30/0.3 mm were used. The highest compressive strength was achieved by samples with a 1.5% addition of straight 19.5 mm fibers and 0.5% twisted fibers. Each mixture contained a 2% fiber addition. However, the number of 19.5 mm fibers was approximately 8.3 and 5.2 times higher than the number of hooked end and twisted fibers, respectively. As the percentage of hooked-end or twisted fibers increased, the compressive strength decreased. Based on the described results, straight fibers with a length of 19.5 mm can be concluded to be the most effective in improving compressive strength, as replacing hooked end and twisted fibers with straight fibers resulted in an increase in strength. The results were confirmed in [164], where the effect of two types of steel fibers with a total amount of 2% was investigated in the ultra-high performance concrete mix. The replacement of 1% of hooked-end fibersof 30/0.375 mm with straight fibers of 13, 16.3, or 19.5/0.2 mm resulted in an increase in compressive strength in the range of 1.5-6.1%. Furthermore, the hooked-end and twisted fibers were difficult to evenly distribute in the mix, leading to the formation of weak zones [165]. [PERSON] et al.