## 4 Hybrid Fiber Composition
### Hybrid Combinations of Other Fiber Types
This resulted in a 23.4% increase in flexural strength and is the lowest among hybrid fiber combinations consisting of two of the three fiber types (ST, C and G). The results suggest that the glass fibers had the smallest effect on the improvement in strength, probably because of the lower modulus of elasticity of the G fibers. Figure 9: Results of strength test of UHPC reinforced with steel and glass fibers: (**a**) compressive strength, (**b**) flexural strength. The effect of combining B fibers (12/0.013 mm) and PP fibers (12/0.013 mm) on the flexural strength of concrete was evaluated in [155]. Compared to fiber-free concrete, strength increases of up to 7.5% with a total fiber content of 1% and up to 23.7% with fiber addition of 2% were achieved. The strength of the concrete improved with increasing PP fiber content, regardless of the total amount of fiber. Straight steel fibers with a length/diameter of 15/0.6 mm at 1%, combined with carbon fibers of 20-30/0.018 mm at 1%, influenced the tensile splitting strength of UHPC [85]. This combination of fibers achieved the highest increase in UHPC strength of 32.3% compared to fiber-free concrete. The strength was also 3.8% higher than that of UHPC with 2% ST fibers, demonstrating a positive hybridization effect of C and ST fibers. Increasing the C fiber content to 2% did not result in further improvement in the flexural strength of the UHPC, despite the higher tensile strength of C compared to the ST fibers. This may be due to the smoother surface of the C fibers, which allowed them to be pulled out of the matrix without fully using their tensile capacity. Another type of hybrid combination involved C fibers (20-30/0.018 mm) at 1% and G fibers (6-12/0.012 mm) at 1% [85]. This fiber mixture led to a 15.3% increase in the tensile splitting strength of UHPC compared to fiber-free concrete. However, this strength gain was the lowest among the fiber combinations tested. The result can be explained by the smooth surface of the G fibers and its short length, which made it easier to remove them from the matrix. Carbon fibers, with their higher tensile strength and longer length compared to G fibers, improved their adhesion to the matrix during tensile stress. UHPC with 2% G fibers achieved the smallest strength increase, indicating that G fibers are less effective in increasing the tensile strength of splitting than ST and C fibers. UHPC with the addition of basalt fibers with a length/diameter of 12/0.013 mm and polypropylene fibers of 12/0.013 mm was examined in [155]. Compared to the fiber-free mixture, the tensile splitting strength increased by 19.3% to 43.9% for a total fiber content of 1% and by 36.8% to 52.6% for a fiber content of 2%. Strength increased with increasing B fiber content, both for total fiber content of 1% and 2%. This can be explained by the significantly lower tensile strength of PP compared to B fibers, causing the polypropylene fibers to not be pulled out of the matrix and to break when reaching their maximum strength [211]. Straight steel fibers with a length/diameter of 15/0.6 mm at 1% and carbon fibers of 20-30/0.018 mm at 1% were added to the UHPC mixture [85]. This combination resulted in the highest increase in the elastic modulus of the concrete compared to other hybrid combinations of ST, C, and G fibers. The modulus was 5.7% higher than that of fiber-free UHPC. This can be attributed to the higher stiffness of the ST and G fibers used in the hybrid mixture and their uniform distribution within the matrix. Another type of hybrid combination is the addition of C fibers (20-30/0.018 mm) and G fibers (6-12/0.012 mm) at 1% each to UHPC [85]. The elastic modulus of UHPC increased by 4.6% compared to the plain mixture. This confirms a positive hybridization effect, as the modulus was higher than that of the UHPC with a 2% addition of a single fiber type.