## 4 Hybrid Fiber Composition
### Steel--Glas Hybrid Fiber
#### 4.4.2 Mechanical Properties
Compared to fiber-free UHPC, there was a 12% increase for the mix with straight ST fibers and a 13% increase for the mix with hooked-end ST fibers. The elastic modulus of UHPC with 1.5% crimped ST fibers (25/0.7 mm) and 1% G fibers (12/0.017 mm) was 2% higher compared to the fiber-free mixture, but 19% lower than UHPC with 1.5% ST [84]. The results confirmed the observed trend of decreasing the modulus of UHPC with combinations of G and ST fibers as the fraction of the lower-stiffness glass fibers increased. The flexural strength of UHPC with 1% straight ST (15/0.2 mm) and 1% G fibers (6-12/0.012 mm) increased by 39% compared to fiber-free UHPC [85]. The strength also exceeded that of UHPC with 2% ST fibers, indicating a positive hybridization effect when replacing 1% ST with 1% G fibers. A hybrid mix 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) was tested for flexural strength [82]. The combination of hooked-end ST and G fibers achieved the highest flexural strength at a total fiber content of 2%, increasing the strength of UHPC by 163% compared to fiber-free concrete. For the mixture with straight ST fibers, the strength increased by 54.3%, indicating that long hook-end ST is more effective in improving flexural strength. Long ST better bridged cracks and delayed crack propagation. Furthermore, the hooked ends ensured good anchorage in the matrix, preventing fiber slippage, which was not observed for the straight and short ST fibers. However, no brittle fractures were observed in any of the fiber combinations tested. The tensile strength of the mixture with 1.5% crimped ST fibers (25/0.7 mm) and 1% G fibers (12/0.017 mm) increased by 6% compared to the fiber-free mixture, as reported in [84]. However, compared to UHPC with 1.5% ST, the tensile strength was 46% lower, suggesting a lower impact of fiber hybridization. Before cracking occurred, the interaction between the fibers and the UHPC matrix did not affect the flexural behavior. The appearance of cracks depends on the strength of the concrete matrix, while the type of fiber influences the subsequent development of strength [209]. Furthermore, the fiber bridging ability is dependent on chemical adhesion, static friction, and its mechanical anchorage [117]. Axial tensile strength tests showed a positive hybridization effect for each UHPC mixture containing 2% straight ST (12/0.2 mm) and 0.5% G fibers with dimensions of 6, 12, or 18/0.014 mm [76]. UHPC with hybrid fibers achieved higher tensile strengths than the mixture with 2.5% ST, and the strength increased as the length of the G fibers increased. These results can be related to the crack-bridging effect, which is worse for short fibers, and to the reduced workability of UHPC with a higher fiber content. The tensile splitting strength of UHPC with 1% straight ST of length/diameter 15/0.2 mm and 1% G fibers 6-12/0.012 mm increased by 24% compared to the concrete without fibers [85]. Compared to the mixture with 2% steel fibers, there was a slight 2% decrease. UHPC with G fibers of length/diameter 6.35/0.025 mm and straight ST 13/0.2 mm or hooked ST 60/0.75 mm was subjected to tensile splitting tests [82]. The strength of UHPC increased for the straight ST and G fiber mixture by 10.7% and 15.5%, respectively, with a total fiber content of 1% and 2%. For the hooked-end fibers, the strength increases were 9.5% and 14.3%. The static properties of UHPC reinforced with steel and glass fiber mixtures are presented in Table 12.  
Figure 9a illustrates the results of the compressive strength of UHPC with the addition of steel and glass fibers. The highest strengths were achieved with a hybrid fiber content of 2%. These concretes contained straight steel fibers of 13 mm in length or hooked steel fibers of 60 mm in length, as well as glass fibers of 6.35 mm or 6-12 mm. Increasing the fiber content to 2.5% led to a decrease in strength. The results of the tensile strength tests are presented in Figure 9b.