# A Method for the Design of Concrete with Combined Steel and Basalt Fiber
## 4 Experimental Results
### Influence of Steel and Basalt Fiber Contents on Concrete Strength  
The values of HFRC strength at 7 days for different combinations of SF and BF are presented in Table 2. As it follows from this table, adding BF has a mainly positive effect in terms of flexural tensile strength up to a certain fiber dosage. For instance, the flexural strength for control concrete specimens with SF content of 80 kg/m\({}^{3}\) is 10.3 MPa, whereasfor specimens with hybrid SF and BF reinforcement is 12.4 MPa (for 4 kg/m\({}^{3}\) of 12 mm BF). The average increase in flexural tensile strength is from 7% to 20%.  
The flexural strength for control concrete specimens with SF content of 120 kg/m\({}^{3}\) is 13.3 MPa, and its maximum value for hybrid SF and BF is 14.9 MPa (for 4 kg/m\({}^{3}\) of 12 mm BF). Thus, the maximum increase in strength is 12%. The lower effect, in this case, could be due to the attainment of the maximum possible fiber content by volume.  
Using BF with a length of 24 mm has demonstrated a relatively lower performance. The flexural strength increase was just by 7% at BF content of 2 kg/m\({}^{3}\) at SF content of 80 kg/m\({}^{3}\). Further increase in the BF content yielded a maximum decrease in flexural strength up to 40%. [PERSON] et al. (2020) observed that flexural strength increases with the increase of fiber length to a maximum aggregate size ratio of up to about 3 and gradually decreases after that. For the 24 mm BF in the current study this ratio is close to 5. The fiber length initially has a positive effect due to the bridging effect of the fibers. However, when the length exceeds the optimum value, the fibers tend to bend during mixing reducing the effective length of the fiber and in turn reducing its reinforcing effect. Additionally, longer fibers tend to have less distribution in the fiber reinforced concrete that has a further effect on concrete structure.  
As shown in Figure 3, the optimum fiber content for the 12 mm BF is 2 to 4 kg/m\({}^{3}\) at both dosages of SF. For the 24 mm BF the optimum BF content is 2 kg/m\({}^{3}\) at both SF dosages. Higher BF content yields an increase of the dispersed reinforcement specific area, increasing the water-cement ratio, required to achieve the necessary workability. As expected, the compressive strength of HFRC is almost independent of the BF content (see Table 2).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**SF**, **kg/m\({}^{3}\)**} & **\multirow{2}{*}{**BF**, **kg/m\({}^{3}\)**} & **Total Fibers** & **BF Part in the** & **Compressive** & **Flexural Tensile** \\  & & **Content by Volume,** & **Fiber Volume,** & **Strength at 7 Days** & **Strength at 7 Days** \\  & & & \(\mu\)**, \%** & \(n\) & \(f_{cm}\)**7,** & \(f_{c,t\ell}\)**7,** \\  & & & & **MPa** & **MPa** \\ \hline \hline \multirow{4}{*}{80} & 0 & 1.03 & 0.00 & 64.5 & 10.3 \\  & 2 & 1.10 & 0.07 & 65.2 & 12.1 \\  & 4 & 1.18 & 0.13 & 67.1 & 12.4 \\  & 6 & 1.25 & 0.18 & 66.2 & 10.2 \\ \hline \multirow{4}{*}{120} & 0 & 1.54 & 0.00 & 66.8 & 13.3 \\  & 2 & 1.61 & 0.05 & 66.5 & 14.8 \\  & 4 & 1.69 & 0.09 & 69.2 & 14.9 \\  & 6 & 1.76 & 0.13 & 65.4 & 11.5 \\ \hline \hline \multicolumn{5}{c}{BF (\(l\) = 24 mm)} \\ \hline \multirow{4}{*}{80} & 0 & 1.03 & 0.00 & 64.5 & 10.3 \\  & 2 & 1.10 & 0.07 & 67.2 & 11.0 \\  & 4 & 1.18 & 0.13 & 63.1 & 9.6 \\  & 6 & 1.25 & 0.18 & 62.8 & 6.5 \\ \hline \multirow{4}{*}{120} & 0 & 1.54 & 0.00 & 66.8 & 13.3 \\  & 2 & 1.61 & 0.05 & 68.6 & 13.5 \\ \cline{1-1}  & 4 & 1.69 & 0.09 & 63.1 & 11.9 \\ \cline{1-1}  & 6 & 1.76 & 0.13 & 61.7 & 7.9 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Strength of concrete specimens with different SF and BF contents.