## 4 Hybrid Fiber Composition
### Steel--Steel Hybrid Fiber
#### 4.1.3. Dynamic Properties  
In all hybrid combinations tested, the increase in strain rate led to notable gains in dynamic compressive strength and fracture energy, unless otherwise noted.  
Under impact, energy is absorbed sequentially through cracking of the matrix, fiber bridging, and eventual fiber pull-out or rupture. This sequence is influenced by fiber geometry, distribution, and anchorage.  
Ultra-high performance concrete (UHPC) with the addition of hybrid steel fibers with a length/diameter ratio of 6 and 13/0.2 mm and a total content of 2% showed a higher dynamic compressive strength compared to the control sample [135]. Shorter fibers form a denser network, increasing the surface of the fiber-matrix interaction and improving the dynamic strength. However, overly short fibers may be ineffective in bridging macrocracks. Under static load, weaknesses such as pores and microcracks expand and a slow increase in load allows their development [179]. However, the behavior under dynamic loading is different. The duration of the load application is too short for these weaknesses to develop fully. This leads to the formation of a large number of microcracks, which requires more energy for their initiation compared to the gradual propagation of macrocracks under static  
Figure 3: Flexural strength of UHPC reinforced with hybrid steel fibers (**a**) straight, and (**b**) with other shapes.  
Figure 2: Compressive strength of UHPC reinforced with hybrid steel fibers (**a**) straight, and (**b**) with different shapes.  
loading. This explains the fracture energy results obtained for the hybrid fiber-reinforced UHPC samples tested. The results were higher than those of mixes containing a single type of fiber, indicating a positive hybridization effect and mutual synergy of fibers within the matrix. The energy required to break concrete into two parts involves both matrix breaking and fiber removal. In the context of short-duration impact loading, slow fiber slippage is excluded, leading to the maximum use of friction at the fiber-matrix interface.  
Impact resistance tests of a UHPC mix with straight steel fibers of length/diameter 6/0.16 mm and straight steel fibers of 13/0.2 mm showed a decrease in fracture energy with an increasing share of short 6 mm fibers [136]. The maximum value was obtained for the mixture with 13 mm fibers at a content of 2%, while at the same fiber content with 6 mm fibers, a decrease of 52% was recorded. Concrete is damaged according to the stress distribution generated during impact. The results indicate that short fibers, because of their insufficient length, do not allow their bridging properties to be fully utilized. As a result, they are pulled from the concrete with a lower force, absorbing less energy compared to long fibers. Table 6 presents and analyzes the results of the dynamic tests of UHPC reinforced with hybrid steel fibers.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Hybrid Fibers/Steel–Steel**} & \multicolumn{3}{c}{**Properties (Effectiveness \%)**} & **Ref.** \\ \hline \multicolumn{1}{c}{**Part A**} & \multicolumn{3}{c}{**Part B**} & & & & & & & \\ \hline Type & I\({}_{\text{f}}\) (mm) & d\({}_{\text{f}}\) (mm) & V\({}_{\text{f}}\) (\  
Figure 4 presents the results of the dynamic compressive strength and fracture energy depending on the impact velocity. For the UHPC mixture with the addition of 6 mm and 13 mm straight steel fibers, an increase in impact velocity resulted in higher dynamic compressive strength and fracture energy. For a total fiber content of 2%, when the proportion of 6 mm ST fibers increased, these parameters also increased. This is due to the higher overall content of shorter fibers and their greater number in the matrix, which creates a more complex internal structure.