*2.1. Material Properties*

In this study, three types of steel fibers were used as reinforcement for concrete, which were differentiated in the number of hook-ends, length, diameter, and tensile strength, but with the same aspect ratio (i.e., fiber length-to-diameter ratio). Based on the number of hook-ends, the three types of fibers were named 3D, 4D, and 5D, respectively. Three kinds of steel fibers were produced by Bekaert company in Shanghai, China. The steel fibers having only one hook-end were denoted as 3D fibers, as shown in Figure 1a; those with one and a half hook-ends were named 4D fibers, as shown in Figure 1b; and those with two steel fiber hook-ends were called 5D fibers, as shown in Figure 1c. The physical properties and dimensions of the three types of steel fibers are shown in Table 1. It can be seen that the 4D and 5D fibers both had a length of 60 mm, which was longer than that, i.e., 35 mm, of 3D fibers. In addition, 5D fibers had the highest tensile strength among the three types of fibers.

**Figure 1.** Three types of steel fibers: (**a**) 3D; (**b**) 4D; (**c**) 5D.

**Table 1.** Physical properties and dimensions of steel fibers used in this research.


Note: *lsf* is the length of steel fibers; *dsf* is the diameter of steel fibers; *ft,sf* is the tensile strength of steel fibers; *Esf* is the Young's modulus of steel fibers.

The BFRP bars were produced by Jiangsu lvcaigu new material technology development Co., Ltd. The BFRP bars used as the reinforcement had a diameter of 12 or 14 mm. Their tensile strength and Young's modulus were measured conforming to Chinese standard GB/T 30022-2013 [31], with the average of the results of five samples taken as the relevant representative value as shown in Table 2.

**Table 2.** Tensile properties of the BFRP bars.


In this study, two grades of concrete were prepared with the characteristic strength of 30 and 60 MPa, respectively. Mix proportions of the two grades of concrete were designed according to JG/T 472–2015 [32], as shown in Table 3. All beam specimens are labelled as "BN-CN-VN-SN", where "BN" represents the BFRP reinforcement ratio in percentage, "CN" refers to concrete grade, "VN" refers to steel-fiber volume fraction in percentage and "SN" denotes the type of steel fibers used. For example, "B0.56C60V1.0S3" refers to the beam specimen with a BFRP reinforcement ratio of 0.56% (i.e., 2Φ12), concrete strength of 60 MPa, the steel-fiber volume fraction of 1.0%, and 3D steel fibers.



To ensure the slump of both grades of concrete falling between 50 and 70 mm, a polycarboxylate superplasticizer was added to the concrete mixture. Natural gravels with particle size ranging between 5 and 20 mm was used as coarse aggregates when making concrete. The fine aggregate used was river sand, and its particle size was less than 5 mm. The physical properties of aggregate are shown in Table 4. Grade 42.5R ordinary Portland Cement conforming to Chinese standard GB175-2007 [33] was used as the binder in preparing concrete; its physical properties are shown in Table 5. Six <sup>150</sup> × <sup>150</sup> × 150 mm<sup>3</sup> concrete cubes and six <sup>150</sup> × <sup>150</sup> × 300 mm<sup>3</sup> concrete prisms were also prepared alongside each beam specimen for measuring compression strength and splitting tensile strength of concrete.

**Table 4.** Physical properties of aggregate.



**Table 5.** Physical properties of the used cement.
