## 4 Hybrid Fiber Composition
### Steel--Polymer Hybrid Fiber
#### 4.3.2 Mechanical Properties
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \multicolumn{2}{c}{**Hybrid Fibers**} & & & & & & & **Properties (Effectiveness \%)** & **Ref.** \\ \hline
**Part A** & & & & **Part B** & & & & & \\ \hline
**Type** & **l\({}_{t}\) (mm)** & **d\({}_{t}\) (mm)** & **V\({}_{t}\) (\%)** & **Type** & **l\({}_{t}\) (mm)** & **d\({}_{t}\) (mm)** & **V\({}_{t}\) (\%)** & \\ \hline  & & & 0.25 & & & 0.75 & & compressive strength = 122.3 MPa (\(-\)5.6\%) \\  & & & & & & & & splitting tensile strength = 9.3 MPa (\(+\)36.8\%) \\  & & & & & & & & flexural strength = 8.6 MPa (\(+\)50.9\%) \\  & & & & & & & & elastic modulus = 29.6 GPa (\(-\)8.9\%) \\  & & & & & & & compressive strength = 133.9 MPa (\(+\)3.4\%) \\  & & & & & & & & splitting tensile strength = 10.0 MPa (\(+\)47.1\%) \\  & & & & & & & & flexural strength = 9.2 MPa (\(+\)61.4\%) \\  & & & & & & & & elastic modulus = 25.6 GPa (\(+\)0.0\%) \\  & & & & & & & & compressive strength = 142.7 MPa (\(-\)4.2\%) \\  & & & & & & & & splitting tensile strength = 13.5 MPa (\(+\)51.7\%) \\  & & & & & & & & flexural strength = 9.8 MPa (\(+\)16.7\%) \\  & & & & & & & & elastic modulus = 34.3 GPa (\(-\)10.7\%) \\ \hline  & & & & 0.5 & & & 0.025 & compressive strength = 106.6 MPa (\(-\)6.3\%)\({}^{5}\) \\  & & & & & & & splitting tensile strength = 9.9 MPa (\(+\)98.0\%)\({}^{5}\) \\  & & & & & & & flexural strength = 8.9 MPa (\(+\)8.5\%)\({}^{5}\) \\  & & & & & & & & elastic modulus = 32.4 GPa (\(-\)16.3\%)\({}^{5}\) \\  & & & & & & & & compressive strength = 11.0 MPa (\(-\)2.5\%)\({}^{5}\) \\  & & & & & & & & flexural strength = 11.0 MPa (\(+\)120.0\%)\({}^{5}\) \\  & & & & & & & & flexural strength = 9.4 MPa (\(+\)14.6\%)\({}^{5}\) \\  & & & & & & & & elastic modulus = 39.4 GPa (\(+\)1.8\%)\({}^{5}\) \\  & & & & & & & compressive strength = 110.6 MPa (\(-\)5.5\%)\({}^{6}\) \\  & & & & & & & & flexural strength = 10.5 MPa (\(+\)16.5\%)\({}^{6}\) \\  & & & & & & & & flexural strength = 9.1 MPa (\(+\)16.7\%)\({}^{6}\) \\  & & & & & & & elastic modulus = 106.4 MPa (\(-\)9.1\%)\({}^{6}\) \\  & & & & & & & splitting tensile strength = 10.7 MPa (\(+\)64.6\%)\({}^{6}\) \\  & & & & & & & flexural strength = 10.6 MPa (\(+\)23.1\%)\({}^{6}\) \\  & & & & & & & elastic modulus = 39.2 GPa (\(+\)0.8\%)\({}^{6}\) \\ \hline \end{tabular}  
* comparison with a mix with 1% straight steel fibers; \({}^{2}\) comparison with a mix with 1% straight steel fibers; \({}^{3}\) comparison with a mix with 1.5% polyethylene fibers; \({}^{4}\) comparison with a mix with 1.5% straight steel fibers; \({}^{5}\) water/binding material ratio = 0.3;\({}^{6}\) water/binding material ratio = 0.25.  
\end{table} 
Table 10: _Cont._The mix containing only POM fibers achieved compressive strength of up to 110 MPa as the fiber content increased. Although POM fibers do not significantly affect compressive strength, they do not interfere with the uniform distribution of steel fibers in the matrix, thus helping to form an effective internal fiber network within the concrete. [PERSON] et al. [146] observed slight decreases in compressive strength for a mixture containing 1% straight steel fibers with a length/diameter ratio of 13/0.22 mm and 0.5% or 1% polyethylene fibers of 19/0.023 mm (a reduction of 0.7% and 1.7%, respectively, compared to the mix without fibers). When the steel fiber content was increased to 2%, the addition of 0.5% or 1% polyethylene fibers led to strength increases of 2.8% and 1.4%, respectively. The inclusion of PE fibers trapped more air voids during mixing and could have weakened the fiber-matrix bond. Strength improvement was mainly driven by steel fibers, as they contribute to the delay in crack formation and propagation. Additionally, the high aspect ratio of PE fibers, although potentially helpful for anchoring and twisting, caused fiber grouping and uneven distribution in this case [192].