# A Method for the Design of Concrete with Combined Steel and Basalt Fiber
## 4 Experimental Results
### Influence of Composition Factors on HFRC Strength  
The obtained experimental results demonstrate the possibility of improving the structure and the properties of HFRC using BF. More detailed research was carried out to study the influence of water-cement ratio, cement content, SF and BF content and volume ratio on fine-grained HFRC strength. With this aim, a three-level four-factor B4--type experiment plan was used [20]. Tables 4 and 5, respectively, present the conditions of experiment planning and the obtained results. Figure 3: Dependence of flexural tensile strength at 7 days vs. BF length, SF and BF contents. The experimental data were processed and analyzed statistically. As a result, corresponding mathematical models of HFRC compressive and flexural tensile strength were obtained (Table 6). Following this table, the obtained models have a form of polynomial regression equations. Those models were used for obtaining graphs and response surfaces of two investigated parameters vs. the influence factors (Figures 4 and 5). In order to obtain these graphs, two factors that are not shown in each of the graphs were fixed at zero level (Table 4). Following the experimental-statistical models, water-cement ratio, (\(x_{2}\)) is the dominant factor for compressive strength. Decreasing it from +1 to -1 (from W/C = 0.4 to W/C = 0.3) yields a corresponding strength increase of 40%. Moreover, the increase is observed at different cement contents and at different concrete ages. Following Figure 4, this factor has a linear effect in the selected variation range, which is about 70% of all factors that were considered. Higher cement content (\(x_{1}\)) within the variation limits yields correspondingly higher compressive strength (by 8-20%). If the water-cement ratio is constant, varying the SF content (\(x_{3}\)) has a negligible effect on concrete strength. However, following Figure 5, BF (\(x_{4}\)) can yield higher concrete strength at 28 days. This is also indicated by the values of linear coefficients in the regression equations (\(-\)0.1 for \(x_{3}\) and 1.0 for \(x_{4}\)). In the authors' opinion, the different effect of hybrid SF and BF on concrete strength is due to higher specific surface of the hybrid fiber and correspondingly higher adhesion with the concrete matrix. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multicolumn{2}{c}{**Factors**} & \multicolumn{2}{c}{**Variation Levels**} & \multicolumn{1}{c}{**Variation**} \\ \cline{2-6}  & **Code** & **Natural Value** & \(\mathbf{-1}\) & **0** & **+1** & **Interval** \\ \hline
1 & \(X_{1}\) & Cement content, kg/m\({}^{3}\) (C) & 450 & 500 & 550 & 50 \\
2 & \(X_{2}\) & W/C & 0.3 & 0.35 & 0.4 & 0.05 \\
3 & \(X_{3}\) & SF, kg/m\({}^{3}\) & 80 & 100 & 120 & 20 \\
4 & \(X_{4}\) & BF, kg/m\({}^{3}\) & 0 & 2 & 4 & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Experiment planning conditions for obtaining HFRC compositions. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**SF**, kg/m\({}^{3}\)**} & **Total Fibers** & **PF Part in the** & **Compressive** & **Flexural Tensile** \\  & **PF, kg/m\({}^{3}\)** & **Content by Volume,** & **Fiber Volume,** & **Strength at 7 Days** & **Strength at 7 Days** \\  & & \(\mu\)**, \%** & \(n\) & \(f_{cm}\)**7,** & \(f_{c,tf}\)**7,** & \(f_{c,tf}\)**7,** \\  & & & **MPa** & **MPa** \\ \hline \multicolumn{5}{c}{PF (\(l\) = 12 mm)} \\ \hline \multirow{3}{*}{80} & 0 & 1.03 & 0.00 & 64.5 & 10.3 \\  & 1 & 1.14 & 0.10 & 64.2 & 10.1 \\  & 2 & 1.25 & 0.18 & 64.6 & 9.4 \\ \hline \multirow{3}{*}{120} & 0 & 1.54 & 0.00 & 66.8 & 13.3 \\  & 1 & 1.65 & 0.07 & 67.2 & 12.7 \\  & 2 & 1.76 & 0.13 & 67.3 & 12.1 \\ \hline \multicolumn{5}{c}{PF (\(l\) = 18 mm)} \\ \hline \multirow{3}{*}{80} & 0 & 1.03 & 0.00 & 64.5 & 10.3 \\  & 1 & 1.14 & 0.03 & 63.2 & 9.8 \\  & 2 & 1.25 & 0.06 & 62.9 & 8.6 \\ \hline \multirow{3}{*}{120} & 0 & 1.54 & 0.00 & 66.8 & 13.3 \\  & 1 & 1.65 & 0.07 & 65.3 & 10.6 \\ \cline{1-1}  & 2 & 1.76 & 0.13 & 65.9 & 7.8 \\ \hline \hline \end{tabular}
\end{table}