## 4 Hybrid Fiber Composition
### Steel--Glas Hybrid Fiber
#### 4.4.2 Mechanical Properties  
The compressive strength of ultra-high performance concrete with straight steel fibers (length/diameter = 13/0.2 mm) and glass fibers (6, 12, or 18/0.014 mm) was presented in [76]. The best combination effect was achieved with the 12 mm G fibers. However, compared to UHPC with a 2.5% addition of ST fibers, the compressive strength of the hybrid fiber mix decreased, indicating that the compressive strength of UHPC is primarily determined by the more favorable properties of steel fibers. This may also be due to the reduced workability of the mix and the higher probability of particle agglomeration in hybrid fiber reinforced concrete. In the case of short glass fibers, there may also be a slip phenomenon without fiber rupture under full stress, preventing the full use of G fibers in strengthening the mix. In [85], a positive hybridization effect on the compressive strength of UHPC was reported with glass fibers (length/diameter = 6-12/0.025 mm) and steel fibers (15/0.6 mm) at 1%. In addition to the strength increase compared to the control series without fibers, the UHPC also achieved a compressive strength higher than that of the UHCP with 2% ST fibers. Figure 8: Dynamic test results of UHPC reinforced with steel and polymer fibers depending on strain rates: (**a**) compressive strength, (**b**) axial tensile strength, (**c**) impact energy, (**d**) ultimate fracture toughness. Glass fibers limited the spread of microcracks due to their high adhesion to the UHPC matrix and did not interfere with ST fibers in carrying the load after cracking [206; 207]. The combination of G and ST fibers complemented the UHPC matrix due to its different properties, allowing for an increase in the maximum load capacity. A mixture with glass fibers (length/diameter = 6.35/0.025 mm) was supplemented with straight steel fibers (13/0.2 mm) or hooked steel fibers (60/0.75 mm) [82]. Higher compressive strength was achieved with G and straight ST fibers up to 1.9%. Beyond this level, UHPC reinforced with G and hooked ST fibers showed even greater strength. This suggests that the collaboration between ST and G fibers was regulated through synergy. [PERSON] i [PERSON] [84] examined the effect of a hybrid mixture of twisted steel fibers (length/diameter = 25/0.7 mm) and glass fibers (12/0.017 mm) on UHPC strength. They observed a slight decrease in compressive strength of 6.1% for UHPC with 1.5% ST fibers and 1% G fibers compared to the reference mix without fibers. The trend of variation in strength indicates partial fiber agglomeration and increased air content. Glass fibers may cause micropore formation during the mixing process, leading to a reduction in compressive strength, as also noted in [208]. The elastic modulus of UHPC with the addition of straight steel fibers (length/diameter = 12/0.2 mm) at 1% and glass fibers (6, 12, or 18/0.014 mm) increased for every fiber combination tested [76]. As the length of the G fibers increased, the modulus increased by 4% to 37%. Only samples with ST and the longest G fibers achieved a higher modulus than UHPC with 2.5% ST. The shortest G fibers gave the weakest stiffening effect due to the lowest workability of the mixture, causing defects in the UHPC matrix that acted as weak structural areas. The elastic modulus for the hybrid mixture with 1% straight ST fibers (length/diameter = 15/0.2 mm) and 1% G fibers (6-12/0.012 mm) increased by 4.4% compared to the fiber-free mix [85]. However, compared to 2% ST fibers, there was a slight 0.8% decrease in modulus, indicating that despite the significantly higher stiffness of ST compared to G, a remarkably positive hybridization effect of fibers was achieved in UHPC. [PERSON] [82] tested ultra-high performance concrete with G fibers of length/diameter = 6.35/0.025 mm combined with straight ST (13/0.2 mm) or hooked-end ST (60/0.75 mm). The same amount of G/ST fibers = 0.5/0.5% or 1/1% was added to the mixtures. The elastic modulus of UHPC with a total fiber content of 2% was slightly higher than that of UHPC with 1% hybrid fibers.