Ultra-high-performance concrete (UHPC) is one of the latest advances in concrete technology and is a new concrete series [1]. Compared to conventional concrete (CC), UHPC shows superior mechanical properties, durability, and ductility after cracking ^[2][3][4][5][2,3,4,5]. At present, there is no widely accepted definition of UHPC, but it is generally considered that its compressive strength is ≥150 MPa and its tensile strength after cracking is ≥5 MPa ^[6][7][6,7]. Under the same bearing capacity, the weight of a structure made of UHPC is only 1/3~1/2 that of CC [8]. Therefore, UHPC is often used in long-span structures and high-rise buildings. In addition, UHPC is, furthermore, energy efficient and environmentally friendly when considering the entire life cycle of the building [6].

The tensile strength is an important factor that affects the overall performance of concrete structures, and it is, therefore, crucial to have the correct material input in the design calculations [9]. In some projects, structural damage is often caused by an insufficient tensile strength and compression zones that have not reached their bearing limits. Steel fibers are crucial for improving the tensile properties of UHPC, so it is important to investigate the effect of steel fibers on the tensile strength of UHPC. Usually, the common methods to improve the tensile properties of UHPC by steel fiber are as follows ^{[10][11][12][13]}[10,11,12,13]: (a) increase the fiber content; (b) use different types of fibers; (c) increase the fiber length; (d) hybrid of different types of fibers. However, due to the complexity of the fiber geometry and the uncertainty of the fiber distribution direction, the tensile properties of UHPC materials vary greatly ^{[14][15][16][17][18]}[14,15,16,17,18]. At the same time, the physical properties of the steel fibers and the bonding strength between the fibers and the UHPC matrix also affect the mechanical properties of the UHPC ^[17][19][20][17,19,20]. This is also a core factor to be considered in structural design. In addition, the steel fibers are also the main reason for the high cost and carbon footprint of UHPC [21]. Therefore, combing and summarizing the knowledge of steel fibers on the tensile properties of UHPC is beneficial for improving the structural design theory of UHPC and reducing the carbon footprint.

With the application of UHPC to engineering practice, the issue of insufficient tensile properties has received increasing attention [22]. By reviewing the latest reviews of UHPC, it was found that the reviews of the effect of steel fiber on the tensile strength of UHPC lacks its influence on the splitting tensile strength ^{[20][21][23][24][25][26][27][28]}[20,21,23,24,25,26,27,28], which must be supplemented. It is very important to detect the cracks in concrete, which is usually carried out by optical means or an extensometer [29]. Digital image correlation (DIC) technology is a non-contact, modern optical measurement experimental technology, which can directly measure the mechanical behavior of materials and structural surfaces. Due to its high measurement accuracy and operation, it has gradually attracted more and more attention in the research on UHPC, especially for the tensile behavior of steel fibers-reinforced UHPC.

2. Advantages and Significance of Steel Fibers in Concrete

Concrete is a quasi-brittle material according to its macroscopic mechanical behavior. As its strength increases, its brittleness increases correspondingly, with the disadvantages of low tensile strength and minor ultimate elongation. To overcome these shortcomings, many researchers have incorporated steel fibers into concrete and have proven its feasibility ^[30][31][32][30,31,32]. Steel fibers can improve the ductility of the cement matrix by bridging cracks, thus, improving the brittle fracture of concrete ^[33][34][35][33,34,35]. Usually, adding steel fibers into concrete can play two main roles: (a) improving the tensile strength and (b) preventing the occurrence and propagation of cracks. Figure 1 shows that compared with normal concrete, the tensile strength and toughness of UHPC when steel fibers are added are significantly improved. Steel fiber is one of the most commonly used fibers in UHPC. Those properties related to the tensile strength of a UHPC depend largely on the content and geometric size of the steel fiber ^[21][37][21,37]. The geometric differences among (in Figure 2) and fiber content (vol.%) of steel fibers have been discussed in many studies ^[38][39][38,39]. The motivation for this kind of research is that steel fiber makes an important contribution to the tensile properties, and steel fiber is one of the most expensive materials in UHPC [21]. Therefore, discussing the influence of steel fiber on UHPC is beneficial for improving the tensile strength of UHPC, reducing the cost of UHPC, and making it more suitable for engineering practice. According to previous experience, steel fibers can be divided into micro steel fibers (𝑙_𝑓/𝑑_𝑓 ≤ 13/0.2) and macro steel fibers (𝑙_𝑓/𝑑_𝑓 > 13/0.2). Based on their length, they can also be divided into short steel fibers (6 mm < 𝑙_𝑓 < 13 mm), medium–long steel fibers (13 mm ≤ 𝑙_𝑓 ≤ 20 mm), long steel fibers (20 mm < 𝑙_𝑓 < 30 mm), and ultra-long steel fibers (30 mm ≤ 𝑙_𝑓). It is generally believed that a reasonable hybrid of two or more kinds of steel fibers can be added to a UHPC, which can not only play up the advantages of the different fibers, but also reflects their synergistic effect, which can significantly improve some properties of the UHPC, and reduce costs. Figure 3 shows the synergy between macro and micro steel fibers.