## 1 Introduction  
Steel fiber reinforced concrete (SFRC) is a concrete admixing short and random distributed steel fibers [1, 2, 3]. The main features of SFRC concentrate on the promotion of the tensile performance of conventional concrete by strengthening the tensile properties and related performances with the addition of a certain amount of steel fibers [4, 5, 6, 7]. The structural application of SFRC can improve the mechanical properties and durability of reinforced and prestressed concrete structures [8, 9]. To realize the above purposes, the bond performance of steel fiber in concrete matrix is a key point that should ensure its synergistic workings [2, 7, 10, 11]. After decades of research, the mechanism of steel fiber reinforcing concrete becomes clearly understood. Similar to the deformed steel bars in concrete, the bond between deformed steel fiber and a cementitious matrix comes from the chemical adhesion, the physical friction, and the mechanical anchorage [6, 7, 10]. The chemical adhesion is mainly determined by the properties of the interfacial transition zone between steel fiber and matrix, which plays a role in the initial pull-out process of deformed fiber in matrix. The physical friction is mainly dependent on the surface characteristic of steel fiber. The mechanical anchorage is mainly provided by the geometric deformation of steel fiber and the matrix with rational tensile property, which acts accompanied with the physical friction and plays a major role if the slip of steel fiber takes place [11; 12]. Therefore, the deformed geometry of steel fiber has a decisive effect on the bond behavior. This provides plenty of space for the design of steel fiber geometry. Accompanied with different manufacturing techniques, many kinds of steel fibers are supplied in the market. Theoretically, rational bond performance comes from the properties of steel fiber and concrete themselves, and it matches through their interface. As complex geometry and the rough surface of steel fiber provide higher bond strength, a high tensile strength of steel fiber is needed to avoid breakage during the pulling out of concrete. However, high-strength steel fiber tends to pull out more from a low-strength matrix and presents a smaller anchoring effect compared to low-strength steel fiber [13; 14]. These results show insufficient work of high-strength steel fiber. In high-strength concrete, low-strength steel fiber is more prone to break compared to high-strength steel fiber at the same length, which contributes a to the pull-out work and bond toughness [15]. Thus, to determine the reinforcement effects on concrete strength and toughness, the bond behaviors of steel fiber is essential to the rational application of deformed steel fibers. Meanwhile, this is also important to upgrade the mix proportion design of SFRC with high-performance coming from the matching up of steel fibers with concrete [16; 17; 18; 1; 4; 16; 1]. The bond properties of some deformed steel fibers in a cement-based matrix have been experimentally studied [19; 20; 21]. The bond of straight steel fiber relies on the material characteristics and has no relation to fiber diameter [22; 23]. Hooked-end steel fiber presents more flexible characteristics and higher fracture toughness in the pull-out process, and it fails due to the end hook being straightened without fiber breakage [24; 25; 26]. The peak pull-out load increases with the strength and diameter of hooked-end steel fiber in a concrete matrix with a constant strength [13; 24]. The peak pull-out load of hooked-end steel fiber with a tensile strength of 2600 MPa embedded in concretes with a cubic compressive strength of 84 MPa and 44 MPa are nearly two times those of hooked-end steel fiber with a tensile strength of 1250 MPa, respectively [24]. In addition, excellent bond properties and pull-out behaviors were observed in 4D-type and 5D-type hooked-end steel fibers [27; 28]. Crimped steel fiber always has a large peak pull-out load, and it tends to increase the tensile strength of concrete [29].