## 1 Introduction  
Concrete is widely used in engineering structures because of its low cost, simplicity of preparation, and excellent strength [1]. However, concrete also has disadvantages such as a low tensile strength, poor toughness, and high brittleness, which adversely affect the safety, applicability, and durability of the concrete structure [2]. Many studies have shown that the mechanical properties and durability of concrete can be effectively improved by incorporating fibers into it, thus obtaining fiber-reinforced concrete for high strength, toughness, and durability [3,4]. Fibers can be incorporated into concrete in two ways: incorporation of a single type of fiber, and incorporation of fibers of different types or sizes. Incorporation of a single type of fiber provides limited improvement in concrete performance. When hybrid fibers obtained by mixing fibers of different types or sizes are incorporated into concrete, the hybrid fibers can induce their respective reinforcing effects in different layers and stress stages of concrete, and consequently, the improvement in the concrete performance is more remarkable [5]. Steel-polyppropylene fiber-reinforced concrete is currently the most widely used hybrid fiber-reinforced concrete. The elastic modulus and strength of steel fiber (SF) are high, because of which its incorporation into concrete can effectively reduce the brittleness of concrete, and improve its mechanical properties and load-bearing capacity. Polypropylene fiber (PF) has a low elastic modulus and good ductility; as a result, its incorporation into concrete can effectively improve the splitting tensile strength and flexural strength of concrete [6; 7]. [PERSON] et al. reported that SF and PF could significantly improve the mechanical properties of pumice concrete and that the maximum increments in the compressive strength and splitting tensile strength were 76% and 110%, respectively [8]. [PERSON] et al. studied the flexural properties of steel-polypropylene fiber-reinforced concrete and found that the hybrid fibers could effectively improve the mechanical properties and toughness of concrete [9]. [PERSON] et al. demonstrated that the compressive strength and elasticity modulus of steel-polypropylene fiber-reinforced concrete were higher than those of SF-reinforced concrete and PF-reinforced concrete [10]. However, the chemical composition of SF is the same as that of reinforcing bars; because of this, SF rusts easily in a corrosive environment, which limits the application scope of steel-polypropylene fiber-reinforced concrete [11]. Basalt fiber (BF) is made from basalt as raw material via melting and drawing at high temperatures. BF has advantages such as a high tensile strength, high-temperature resistance, corrosion resistance, and a high elastic modulus. Moreover, it is a type of a green, environmentally-friendly fiber and a good substitute for SF [12; 13]. Therefore, basalt-polypropylene fiber-reinforced concrete (BPFRC)--prepared by mixing BF instead of SF with PF--has a wider application range. The mechanical properties of BPFRC have been preliminarily studied by some researchers. [PERSON] et al. reported that compressive strength improved significantly when it was incorporated with BPFRC composed of 0.1% BF and 0.1% PF [4]. [PERSON] et al. demonstrated that when the BF and PF contents were 0.15% and 0.033%, respectively, the compressive strength and splitting tensile strength of concrete increased by 14.1% and 48.6%, respectively [14]. [PERSON] et al. investigated the mechanical properties of basalt-polypropylene hybrid fiber-reinforced concrete under high-temperature conditions [15]. They showed that the compressive strength and splitting tensile strength of concrete reinforced with hybrid fibers were higher than those of plain concrete. However, several other studies showed that incorporation of hybrid fibers composed of BF and PF improved the splitting tensile strength of concrete, but lowered its compressive strength [16; 17].