# A Method for the Design of Concrete with Combined Steel and Basalt Fiber
## 4 Experimental Results
### Influence of Composition Factors on HFRC Strength

Table 3: Strength of concrete specimens with different SF and PF contents. Mathematical models for strength parameters of fine-grained HFRC are presented in Table 6. Based on the effect on compressive strength, the factors can be placed in the following sequence: \(x_{2}>x_{1}>x_{4}>x_{3}\). The effect on the concrete flexural strength are different as can be observed in Table 6. For the flexural strength, the SF content (\(x_{3}\)) is one of the most important factors. The highest effect of this factor is evident at 28 days (38% of all factors influence). W/C is the second by its influence on flexural tensile strength (33%), and at an early age its influence is higher, compared to SF content (Figure 6). Following Figure 6, adding 4 kg/m\({}^{3}\) of BF (\(x_{4}\)) into concrete mixture increases the flexural tensile strength more than 20% at 1 day, relative to compositions without fiber. This effect is similar to that of SF (\(x_{3}\)). However, as the age increased to 28 days, the influence of \(x_{4}\) decreased while that of \(x_{3}\) increased. The effect of SF content factor (\(x_{3}\)) has a weakly pronounced extreme nature, which is evident from the corresponding graphical dependence (Figure 7) and quadratic coefficient in the corresponding regression equation (Table 6). Increasing the fiber content from 80 (\(x_{3}=-1\)) to 110 (\(x_{3}=0.5\)) kg/m\({}^{3}\) leads to higher flexural tensile strength (increase of 30 to 40%). Further increase of SF has a negligible effect on HFRC flexural tensile strength. If W/C is constant, then the influence of cement content (\(x_{1}\)) on flexural tensile strength is the lowest, as it has the lowest coefficient in the regression equation. The interaction coefficient of factors \(x_{1}\) and \(x_{2}\) is relatively high. Hence their dependence on the change of the other is significant (Figure 7). Figure 4: Compressive strength of HFRC at 28 days vs. cement content (\(x_{1}\)), W/C (\(x_{2}\)), SF (\(x_{3}\)) and BF (\(x_{4}\)). Response surfaces analysis from the viewpoint of the output parameter \(f_{c\prime\prime}\)28 (Figure 8) indicates that for achieving maximum flexural strength values, the value of factor W/C (\(x_{2}\)) should be maintained at the lower variation level, while SF (\(x_{3}\)) and BF (\(x_{4}\)) at the top. It is also evident that \(f_{c\prime\prime}\)28 > 18 MPa can be achieved for a wide range of SF and BF corresponding to 100-120 kg/m\({}^{3}\) of SF and 1-4 kg/m\({}^{3}\) of BF. Analysis of the superplasticizer content effect demonstrates the highest influence of W/C (\(x_{2}\)). Following Table 6, its linear coefficient in the regression equation is significantly higher relative to the two other factors. Higher SF (\(x_{3}\)) and BF (\(x_{4}\)) contents increase the superplasticizer consumption required for obtaining the necessary workability at a potential increase in water demand. If cement content increases from 450 (\(x_{1}=-1\)) to 500 kg/m\({}^{3}\) (\(x_{1}=0\)), the superplasticizer content increases (1.5 to 2) times. For \(x_{1}=0\) and + 1 this dependence significantly decreases. Figure 5: Response surfaces of HFRC compressive strength at 28 days vs. cement content. Figure 6: Factors’ influence degree on flexural tensile strength at different ages. Despite requiring higher water demand which a higher superplasticizer can compensate, the addition of BF improves other properties of the mix and hardened concrete. Results of previous investigations on fine-grained fiber reinforced concrete with compressive strength in the range of 80 to 90 MPa and flexural tensile strength 15 to 18 MPa [23] have demonstrated that optimum SF content is about 100 kg/m\({}^{3}\) (\(\mu\) = 1.3%). Higher fiber content resulted in lower strength due to concrete segregation. Hybrid BF and SF reinforcement allow increasing the SF content to 120 kg/m\({}^{3}\), fulfilling the requirements of concrete mix structure homogeneity and avoiding segregation. The strength of HFRC increases by 10 to 20%, compared to that with a single SF mixture.