**2. Preparation of Hybrid Fiber-Reinforced Concrete for Shaft Lining** *2.1. Raw Materials*

This study used P.O52.5R cement, which was produced by the Fengyang Conch Cement Plant. The main parameters are shown in Table 1. The coarse aggregate was continuously graded basalt gravel with a 5~20 millimeter particle size; the crushability index was 6.7%, and the mud content was detected before the test to meet the requirements of the specification. The fine aggregate was natural river sand with a mud content of less than 1.6%, which belongs to medium and coarse sand. The fineness modulus is shown in Table 2. Slag powder and silica fume were selected as the mineral admixtures; The slag powder (Anhui Qingya Building Material Co., Ltd., Huainan, China) had a specific surface area greater than 350 m2/kg; The silica fume (Shanxi Dongyi Ferroalloy Factory, Shanxi, China) had a specific surface area greater than 18,000 m2/kg; Table 3 summarizes the mineral composition of the admixture. Moreover, a high-efficiency NF water-reducing agent was used, and its main properties are presented in Table 4. The fiber materials used were polyvinyl alcohol fiber (PVA) and polypropylene steel fiber (FST). Their basic performance parameters are shown in Table 5, and the physical appearance is shown in Figure 2.


**Table 1.** Main performance parameters of P.O52.5R cement.

**Table 2.** Fineness modulus of sand.


**Table 3.** Mineral composition of admixture.


**Table 4.** Properties of water reducer.


**Table 5.** Basic performance parameters of fibers.


**Figure 2.** Appearance of fibers: (**a**) FSTF; (**b**) PVAF.

#### *2.2. Test Mix Ratio*

In order to better resist the effect of uneven freezing pressure, based on C60 highstrength concrete commonly used in freezing walls, a high-performance hybrid fiber concrete was prepared with the addition of PVA fiber and FST fiber. The PVA content and FST content were selected as the two influencing factors of the orthogonal test, and three levels were set for each factor. The PVA fiber content levels were set to 0.728 kg/m3, 1.092 kg/m3, and 1.456 kg/m3; FST fiber content levels were set to 4.0 kg/m3, 5.0 kg/m3, and 6.0 kg/m3. At the same time, another group of reference concrete was set as the control group, resulting in 10 groups of tests in total. The reference mix ratio of the C60 high-strength concrete is shown in Table 6.

**Table 6.** The reference mix ratio of the C60 high-strength concrete.


Annotation: Admixtures = Slag powder (102.12 kg·m−3), Silicon powder (20.5 kg·m−3), NF (7.38 kg·m−3).

#### *2.3. Preparation and Curing of Specimens*

Firstly, after weighing, the gravel and sand were poured into a mixer for dry mixing for 2 min; then, cement and admixture were added, and the dry mixing was continued for 2 min; Secondly, the fiber was evenly added in batches, and then stirred for 2 min; Finally, water was added to the mixture quickly followed by slow wet mixing for 2 min.

The slump of the mixture was measured during mixing to evaluate the influence of the hybrid fiber on the fluidity of the concrete. Then, the concrete mixture was poured into a test mold and transported to a shaking table for vibration. The mixture was added or removed according to the situation during the vibration process. It was placed on indoor flat ground for natural curing for 24 h, and the mold was then removed accordingly. After numbering, it was transported to a standard curing box (temperature 20 ± 2 ◦C; relative humidity 97%) to the age.

#### *2.4. Test Results and Analysis*

The specimens were taken out after standard curing for 28 days, and cube compression, splitting tensile, and flexural strength tests were carried out. A CSS-YAW3000 electrohydraulic servo press was adopted to determine the loading method and rate according to the relevant provisions of CECS13-2009 [26]. Cube specimens of 100 mm × 100 mm × 100 mm were selected for the compression and splitting tensile strength tests, and the results were multiplied by size conversion coefficients of 0.95 and 0.85, respectively. The orthogonal test results are shown in Table 7.


**Table 7.** Orthogonal test results.

Annotation: CS = Compressive strength, TS = Splitting tensile strength, FS = Flexural strength.

As shown in Table 7, the fluidity of the hybrid fiber concrete is significantly lower than that of the reference group, and the greater the fiber content is, the greater the decrease is. At the maximum content level, the slump is only 160 mm, which is 35 mm lower than that of the reference group; therefore, the amount of hybrid fiber should not be too high, as otherwise, it is difficult to meet the working performance requirements of concrete. The test results of each group show that the compressive strength of the hybrid fiber group is the same as that of the reference group, with an increase of –5.5~3.0%, while the splitting tensile strength and flexural strength are significantly increased compared with those of the reference group, with a maximum increase of 32.4% and 25.6%, respectively. The failure patterns of each group of specimens are shown in Figures 3–5.

**Figure 3.** Comparison of compression failure modes of specimens: (**a**) Reference group; (**b**) hybrid fiber group.

The comparison of the compressive failure patterns of the specimens in Figure 3, shows that the base concrete was completely crushed, and the fragments fell off, showing the characteristics of brittle failure, while the hybrid fiber-reinforced concrete specimen remained intact and did not break down; only the outer surface was slightly raised and several vertical cracks appeared, similar to hoop failure in the circumferential direction. As can be seen from the comparison of the splitting and tensile failure modes of the specimens in Figure 4, the reference concrete was directly split into two along the center line, while the hybrid fiber-reinforced concrete had vertical cracks along the center line, and the bridging fiber could be seen at the crack section. As shown in Figure 5, flexural strength test, the reference group concrete was directly broken, while the hybrid fiber group showed failure cracks. Thus, hybrid fiber can improve the brittleness of concrete during failure, make it show ductility; and effectively improve the crack resistance of concrete. The analysis shows that PVA fiber has good crack resistance in the early stage, and FST fiber can inhibit crack

expansion in the late stage. When the two fibers are mixed, they show a positive hybrid effect, improving toughness, crack resistance, and deformation constraint. Meanwhile, based on the range analysis of the test results in Table 7, obtained using the SPSS software, it was found that the optimal contents of the two kinds of fibers in this experiment were 1.092 kg/m3 PVA fiber and 5 kg/m<sup>3</sup> FST fiber, which was determined to be the fiber content of the shaft lining hybrid fiber concrete to be prepared.

**Figure 4.** Comparison of splitting failure modes of specimens: (**a**) Reference group; (**b**) hybrid fiber group.

**Figure 5.** Comparison of flexural failure modes of specimens: (**a**) Reference group; (**b**) hybrid fiber group.
