## 4 Hybrid Fiber Composition
### Steel--Polymer Hybrid Fiber
#### 4.3.2 Mechanical Properties
Mixtures containing straight steel fibers with a length/diameter of 16.5/0.2 mm in the amount of 0.33% and 19.5/0.2 mm in the amount of 0.67%, combined with polyethylene fibers (18/0.012 mm) or PVA fibers (12/0.012 mm) in the amount of 0.5%, achieved lower compressive strength than the mixture containing 1.5% steel fibers [143]. The results confirm that steel fibers are responsible for increasing the compressive strength of UHPC, provided other parameters are met, such as proper workability, aggregate classification, water-to-binder ratio, and curing process [194]. An analysis of the results for mixtures with a total fiber content of 1% indicates a linear decrease in compressive strength as the proportion of polymer fibers (PVA 6/0.012 mm, PVA 12/0.012 mm, or PE 12/0.02 mm) increased relative to straight steel fibers (19.5/0.2 mm) [140]. No significant differences in compressive strength were observed depending on the type of polymer fiber used, and the strengths were similar for the three types of fibers. This was attributed to the significantly lower stiffness of the polymer fibers, which, although binding to the concrete matrix, were unable to hold it as effectively as the steel fibers. The high aspect ratio of polymer fibers also hindered their uniform dispersion, causing fiber concentration, which led to the formation of a nonhomogeneous structure. The combination of hooked steel fibers with a length/diameter of 30/0.9 mm and polymer fibers with the same shape ratio (30/0.9 mm), with a total content of 2%, resulted in an increase in compressive strength [150]. The highest strength was obtained in the mixture with 1.5% steel fibers, showing a 29% increase in strength compared to the fiber-free mixture. A linear decrease in strength was also observed, reaching the lowest value for concrete with the lowest steel fiber content (0.5%), although it still showed a 9% increase in strength compared to concrete without fibers. Concrete mixtures with a hybrid addition of hooked steel fibers with a length/diameter ratio of 50/1 mm in the amount of 0.5% or 1%, and polypropylene fibers with a length/diameter ratio of 12/0.025 mm in the amount of 0.06%, achieved compressive strength increases of 7.8-11.7% compared to the reference mixture without fibers [151]. The effects of increasing the amount of hooked steel fibers from 1% to 1.5% and polypropylene fibers from 0.05% to 0.1% were described in [153]. The increase in the amount of steel and polypropylene fibers was found to cause a slight decrease in compressive strength as a result of their nonuniform distribution in the mixture. The compressive strength of concrete with the same types of fibers in a total amount of 1% was determined in [152]. With the addition of 0.25% and 0.75% steel fibers, slight decreases in strength of 5.6% and 4.2% were obtained, respectively. A uniform volumetric share of 0.5% ST fibers and 0.5% PP fibers resulted in a slight increase of 3.4% compared to the fiber-free mixture. The effect of the hybrid fibers described above on the compressive strength of UHPC mixtures was confirmed in [154]. Despite the addition of fibers in a total amount of 0.525 to 1.60%, a decrease in compressive strength was obtained compared to the control mixtures. The properties of the fibers were not utilized, which may indicate their non-uniform distribution and the formation of weaker zones without fiber reinforcement through which cracks were initiated. Additionally, the high aspect ratio and the large amount of polypropylene fibers led to their agglomeration and the formation of a more porous matrix [195]. An ultra-high performance concrete mixture with the addition of 1.5% straight steel fibers with a length/diameter ratio of 13/0.2 mm and 0.5% PVA fibers 8/0.038 mm achieved a flexural tensile strength 144% higher than the control UHPC and 10% higher than concrete with a 2% ST addition [139]. After the initial formation of the crack, the load continued to increase until the maximum strength was reached.