## 4 Hybrid Fiber Composition
### Steel--Steel Hybrid Fiber
#### 4.1.2 Mechanical Properties
[48] noted that the highest flexural tensile strength was achieved in the case of a UHPC mix with the addition of 0.5% straight steel fibers with a length/diameter of 10/0.12 mm and 1% straight steel fibers of 13/0.12 mm, which was twice as much as the control sample. Longer fibers (10 and 13 mm) had a greater impact in improving maximum tensile strength compared to shorter fibers (6 mm). It was also observed that 13 mm fibers, despite their lower number, had a greater effect than 10 mm fibers. As the width of the crack in the matrix increased, the stress between the cracks was redistributed, and the load was transferred by the fibers due to anchoring and friction between the matrix and the fibers [166]. The effect of 6 and 13/0.2 mm straight steel fibers on flexural tensile strength for hybrid UHPC mixes with a total fiber content of 2% was confirmed in [135]. A mixture with the addition of 1.5% 13 mm fibers and 0.5% 6 mm fibers achieved a 78% increase in strength compared to the control sample. In the case of the reverse fiber content, the mixture achieved a 25% increase in strength, which was a three-fold lower improvement in flexural strength with the same volume content of fibers. When the UHPC maximum strength was reached, the fibers took over the load bearing, which was noted by distinctly audible fiber tearing sounds. The test results of UHPC mixes with a total hybrid steel fiber content of 2.5% indicate that in combinations with steel fibers of length/diameter 15/0.2 mm, higher flexural tensile strength with the addition of 6/0.2 mm fibers compared to 10/0.2 mm fibers was obtained [137]. This was because the short fibers allowed the sample to carry the load up to the point of crack formation. After cracking, long fibers played a key role, allowing a further increase in load until the strength was exceeded and the cross section failed [167; 168; 169]. The analysis of the results of the flexural tensile strength for a mixture with the addition of 2% straight fibers with a length/diameter of 13 or 19.5/0.2 mm, and 30/0.3 mm was presented in [138]. The highest strength was achieved for a combination of 1.5% 19.5 mm fibers and 0.5% 30 mm fibers. Based on the results obtained, it was concluded that longer fibers allow for an increase in flexural strength as a result of a larger contact surface between the matrix and the fiber. The results were confirmed in [167], where the effect of steel fibers 13, 16.3, and 19.5/0.2 mm on the flexural strength of UHPC was determined, showing an improvement with increasing fiber length. Longer fibers offered better slip resistance, increased energy dissipation, and crack bridging, allowing increased flexural strength [170]. The impact of the hybrid combination of straight steel fibers with a length/diameter of 13/0.2 mm and hooked fibers of 30/0.5 mm as an additive to the UHPC mix was presented in [139]. The highest flexural strength for a total fiber content of 2% was achieved by the mixture with 1% of each type of fiber. No agglomeration was observed, allowing the fibers to operate effectively in areas where cracks occurred under increasing load. Hooked-end fibers had better anchorage during pull-out than straight fibers, but increasing their content to 5% by volume did not result in improved strength, indicating the occurrence of agglomeration and interlocking of fibers through their curved ends. The high density and poor workability prevented an even distribution of the ingredients. [PERSON] et al. [141] presented the results of the flexural strength test for a hybrid UHPC mix with straight fibers of length/diameter 19.5/0.2 mm and hooked-end fibers 30/0.38 mm or twisted fibers 30/0.3 mm. For a total fiber content of 2%, a smaller effect was observed for hooked-end fibers as their content increased from 0.5% to 1.5%. This was due to the fact that a high concentration of hooked-end fibers caused cracking in the matrix, reducing the ability of neighboring fibers to pull out.