## 4 Hybrid Fiber Composition
### Steel--Polymer Hybrid Fiber
#### 4.3.3. Dynamic Properties  
A UHPC mix with a total fiber content of 2%, consisting of hooked-end steel fibers (length/diameter = 30/0.9 mm) and polymer fibers (30/0.9 mm), achieved an increase in dynamic compressive strength ranging from 0.2% to 17.2%, depending on the mix  
Figure 6: Compressive strength of hybrid UHPC reinforced with steel and polymer fibers: (**a**) ST + PE, (**b**) ST + PP, (**c**) ST + PVA, and (**d**) ST + other types of polymer fibers. Figure 7: Flexural strength of UHPC reinforced with steel and polymer fibers. and strain rate, compared to the control UHPC [150]. As the strain rate increased, the compressive strength also increased, reaching the highest gains for concrete with the lowest number of synthetic fibers. The results indicate that the hooked-end steel fibers were responsible for the greater reinforcing effect. This phenomenon can be explained by the homogeneous dispersion of fibers within the matrix, which effectively compensates for internal defects. Synthetic fibers, which have a lower elastic modulus than steel fibers, were less effective at inhibiting crack propagation under dynamic load, as their greater flexibility results in lower stiffness and reduced capacity to transfer tensile stresses across developing cracks. However, the mixture containing 1.5% synthetic fibers achieved slightly lower strength, suggesting that these fibers help disperse stress concentrations and slow the formation and growth of microcracks. At lower strain rates (45-55 s\({}^{-1}\)), the fiber-free mixture fractured into large fragments, while the samples with fiber addition maintained their integrity. This is due to the fibers preventing the coalescence of emerging cracks, acting as effective bridges between cracked surfaces, and distributing tensile stresses more evenly across the damaged areas. Increasing the strain rate (93-105 s\({}^{-1}\)) caused greater damage, with some fibers detached from the matrix near the edges of the specimen. Additionally, hybrid fiber mixtures exhibited less severe cracking under impact loading compared to samples reinforced with a single type of fiber. Strain rates greater than 160 s\({}^{-1}\) caused all samples to lose integrity and show severe damage. As a result of the expansion of stresses around steel fibers, matrix spalling was observed. Polymer fibers, characterized by high deformability, effectively controlled crack propagation within the matrix through stretching and eventual rupture upon reaching their tensile limits. Consequently, cracks in the hybrid mixtures were less abrupt than those in the mixtures containing only steel fibers. UHPC with straight steel fibers (length/diameter = 13/0.2 mm) and POM fibers (12/0.2 mm), with a total fiber content of 3%, underwent dynamic tests [42]. The increase in strain rate resulted in a higher strength for each mix due to the stronger impact on the structure of the material [199,200]. At lower strain rates (40-80 s\({}^{-1}\)), only a few cracks were observed, bridged by steel fibers, along with minor edge spalling. Increasing the strain rate (80-100 s\({}^{-1}\)) led to more cracks, fiber pull-out, and fiber breakage. The further increase in the strain rate caused fragmentation to become progressively smaller pieces. Dynamic values of compressive strength, elastic modulus, and fracture resistance were higher for the mixture with 2% ST. This is due to the lower strength and elastic modulus of POM fibers compared to steel fibers. ST fibers were pulled out, while POM fibers were pulled out or broken under dynamic loads, due to their lower tensile strength and weaker interfacial bond with the concrete matrix, which results in lower resistance during extraction. Importantly, short steel fibers do not create localized energy concentration points, unlike long hooked-end fibers, which exhibit particularly abrupt cracking, because of their geometry that generates stress concentrations at their ends, leading to localized failure in the surrounding matrix.