# A Method for the Design of Concrete with Combined Steel and Basalt Fiber
## 1 Introduction
It was reported that the optimum content of BF and steel fiber (SF) for concrete class C30/37 is 0.75% and 0.5% by volume, respectively. The increase in concrete compressive strength with optimum BF was 9.3% and for SF 11.9%. The increase in splitting tensile strength of concrete was 43% and 48%, correspondingly. BF increased flexural concrete strength by 9.5% and SF by 12.5%. It was concluded that concrete with SF is better overall compared to that with BF. However, the authors believe that adding 0.75% of BF could be problematic from the viewpoint of uniform SF distribution in concrete. [PERSON] et al. investigated mechanical properties and technological and application aspects of BF and SF in C40 grade concrete [13]. Beams, cubic and cylindrical specimens with 0%, 0.25%, 0.5%, 0.75%, 1% and 1.25% of fiber were tested. The relationship between compressive, flexural and tensile strength at 7 and 28 days was obtained. The results show that compressive strengths at 1% and 1.25% of BF and SF are lower compared to normal cement concrete. Splitting tensile and flexural strengths become lower at 1.25% of BF and SF. The concrete with BF performed better than the SF counterpart in terms of compressive, flexural and splitting tensile strengths. [PERSON] and [PERSON] [14] studied the effect of elevated temperature on SF and BF reinforced concretes. Both types of fibers reduced spalling and explosive failures. [PERSON], [PERSON] [15] performed a study on the toughness, flexural strength and load-deflection behavior of concrete beams with hybrid BF and SF reinforcement. Different fiber content and aspect ratio were selected. Reinforced concrete beams with SF and BF were cast for comparison. The ultimate load in beams with hybrid fiber content of 2% is 25% higher, compared to those without fibers. From the viewpoint of ultimate load, hybrid fiber reinforcement was more effective compared to single BF or SF by 20% and 9%, respectively. Beams with hybrid fiber exhibited higher deflections relative to specimens with single SF or BF. Specimens with single BF reinforcement demonstrated lower energy absorption, compared to hybrid fiber reinforced beams. These positive features were achieved because hybrid fiber has higher crack bridging effect compared to single SF or BF. [PERSON] et al. [16] studied the effect of hybrid fiber content on the shear strength, toughness, flexural strength and residual load capacity. The best shear strength and toughness were obtained at 180 kg/m\({}^{3}\) of SF and 4.5 kg/m\({}^{3}\) BF. It is evident that concrete with hybrid fiber has better performance. SF makes significant improvements on the mechanical behavior of concrete by controlling the cracking and improving the flexural strength and toughness. BF which has high resistance to radiation and oxidation and improved fracture energy and abrasion resistance leads to increased flexural strength [17]. Synthetic fiber types such as polypropylene fiber (PF) are resistant to chemicals and are capable of arresting micro crack growths. They are effective in reducing plastic shrinkage cracks and can be used in different harsh environments. More research on the mechanical properties of high-performance concrete with hybrid fiber reinforcement is required, as experimental data reported in the literature is limited. In most cases, the available data are focused on concrete classes with compressive strength up to 50 MPa. The present research is focused on investigating the effect of hybrid SF and BF and concrete mix composition on high-performance concrete strength. Such concrete has application prospects in areas requiring high strength concretes with improved post-cracking behavior.