*3.1. Impermeability Test*

#### 3.1.1. Experimental Design

The ability of concrete to resist seepage under the action of water pressure is called impermeability, which is an important index that reflects the durability of concrete. Based on the GBJ82-2009 [31] specification, a water penetration test was carried out using the penetration height method. A digital display automatic adjusting concrete impervious meter was selected as the measuring device. Additionally, Φ 175 × 185 × 150 cylindrical specimens were selected, with six pieces in each group. The test was carried out after 28 days of standard maintenance of the specimens, and the loading device is shown in Figure 6.

**Figure 6.** Impermeable loading device.

Considering that shaft lining structures are subjected to high underground water pressure in engineering practice, 3.6 MPa was selected to stabilize water pressure, and the following measures were taken during the impermeability test.


**Figure 7.** Process chart of impermeability test. (**a**) Sample grinding; (**b**) Sample sealing wax; (**c**) Sample installation; (**d**) impermeability test; (**e**) Take out the sample; (**f**) Split specimen.

#### 3.1.2. Test Results and Analysis

After measuring the seepage height of the concrete, the relative permeability coefficient of the concrete was calculated using Formula (1).

$$K\_r = \frac{aD\_m^2}{2TH} \tag{1}$$

where *Kr* is the relative permeability coefficient of concrete, *α* is the water absorption of concrete (0.03), *Dm* is the average seepage height of concrete (mm), *T* is the constant pressure time (h), and *H* is the water column height corresponding to the water pressure (1 MPa corresponds to 100 m water column height).

Table 8 shows the average height of water seepage and the calculated relative permeability coefficient of the two groups of specimens after 24 h of pressure stabilization at 3.6 MPa water pressure. It can be seen from Table 8 and Figure 8 that the water seepage height of the two groups of specimens was not large, which indicates that impermeability of both specimen groups is good and meets the impermeability requirements of highperformance concrete. This is mainly because the water–binder ratio is relatively low; at the same time, in the admixture, the slag and silicon powder generated hydration products to improve the compactness of concrete and reduce the number of pores. In addition, the average water permeability height and relative permeability coefficient of the concrete in the hybrid fiber group were significantly lower than those in the reference group, and the average water permeability height was 31.7% lower than that in the reference group. Compared with the reference group, the relative permeability coefficient decreased by 53.3%. This shows that the incorporation of hybrid fiber can further improve the impermeability of concrete.



**Figure 8.** Concrete seepage height chart: (**a**) Reference group; (**b**) hybrid fiber group.

### *3.2. Freeze–Thaw Resistance Cycle Test*

#### 3.2.1. Experimental Design

Freezing resistance capacity is another important indicator reflecting the durability of concrete. Considering the particularity of the environment in which the structure of a frozen shaft is located, the concrete of the shaft lining should meet certain requirements of freezing resistance [32,33]. The TEST-1000 high and low temperature TEST chamber was selected, and the temperature control range can reach −60 ◦C~+150 ◦C. As shown in Figure 9, small cube specimens with a side length of 100 mm were selected for the test. The slow freezing method was adopted to first maintain the specimens in the curing box and then in the water tank. At this time, the water surface temperature no lower than that of the surface of the specimens, and then, they were placed in the test box for freeze–thaw cycle testing. The specimens were frozen at −15 ◦C for 4 h and then melted in a 20 ◦C water tank for 6 h. After freezing and thawing, it was regarded as a freeze–thaw cycle.

**Figure 9.** Freeze–thaw cycle test device.

In this test, the freezing resistance of the C60 benchmark group specimens and those in the hybrid fiber group was studied. The number of freeze–thaw cycles were set as 25, 50, 75, and 100, with three specimens in each freeze–thaw cycle.

#### 3.2.2. Test Results and Analysis

The mass loss rate of concrete specimens was calculated using the following formulas.

$$
\Delta m\_{\rm ni} = \frac{m\_{\rm 0i} - m\_{\rm ni}}{m\_{\rm 0i}} \times 100\% \tag{2}
$$

$$
\Delta m\_{\rm nl} = \frac{\sum\_{i}^{3} \Delta m\_{\rm mi}}{3} \times 100\% \tag{3}
$$

where Δ*mni* stands for the mass loss rate of the *i*th specimen after *n*(*n* = 0, 25, 50, 75, and 100) freeze–thaw cycles, *m*0*<sup>i</sup>* stands for the mass of the *i*th specimen before the freeze–thaw cycle *mni* stands for the mass of the *i*th specimen after *n* freeze–thaw cycles, and Δ*mn* stands for the average mass loss rate of each group of specimens after *n* freeze–thaw cycles (the mass decreases when Δ*mn* > 0; and increases when Δ*mn* < 0).

The results of mass loss are shown in Table 9.


**Table 9.** Concrete mass loss results after freeze–thaw cycles (%).

Meanwhile, the relationship of concrete compressive strength and mass loss rate with the number of freeze–thaw cycles were drawn, as shown in Figure 10.

**Figure 10.** Relation chart of compressive strength and mass loss factors with the number of freeze– thaw cycles.

It can be seen from Figure 10 that the mass loss rate of concrete showed a negative value in the early stage and changed slowly, that is, the mass increased slowly. After 75 freeze–thaw cycles, the concrete surface began to shed mud, thus reducing the mass, and the change speed of the specimen mass accelerated. The mass loss in the hybrid fiber group was not significant during the whole freeze–thaw test period, and the mass loss rate after 100 freeze–thaw cycles was 78.3% lower than that of the reference group. With the increase in the number of freeze–thaw cycles, the compressive strength of the two groups of specimens continued to decrease slowly, and the specimens remained in a high–strength state after 100 freeze–thaw cycles. The strength loss rates of the reference group after 25, 50, 75, and 100 cycles were 1.56%, 3.13%, 4.98%, and 8.11%, respectively, and those of the hybrid fiber group were 0.83%, 1.80%, 3.46%, and 5.54%, respectively. The strength loss rate of the hybrid fiber group was 0.73%, 1.33%, 1.52%, and 2.57% lower than that of the reference group, respectively. This can be explained by the fact that the mixture of FST fiber and PVA fiber has a positive hybrid effect, which can effectively reduce the number of micropores in the matrix and improve the antispalling ability of the concrete. Therefore, it can be seen that the incorporation of hybrid fiber can effectively reduce the strength loss of concrete after freeze–thaw cycles and improve the frost resistance of concrete.

A morphological comparison of each group of specimens after 100 freeze–thaw cycles is shown in Figure 11. Many gullies appeared on the surface of the reference concrete group, especially in the corner zone, which is more serious. This is because the concrete freeze–thaw damage generally starts with the spalling of the surface cement mortar, when the deterioration of the reference concrete has already begun. On the contrary, the surface of the hybrid fiber concrete was relatively intact, which indicates that the PVA-FST fiber hybrid can effectively reduce the spalling of concrete subjected to freezing and thawing.

**Figure 11.** Contrast of morphology of concrete after 100 freeze–thaw cycles: (**a**) Reference group; (**b**) hybrid fiber group.

#### *3.3. Sulfate Corrosion Resistance Test*

#### 3.3.1. Experimental Design

In addition to bearing complex loads, the shaft lining structure in a long-term deep formation environment is also affected by various harmful corrosive substances gathered in soil and water, among which sulfate is a typical one [34,35]. A long-term immersion method was adopted in this sulfate corrosion resistance test. In order to accelerate corrosion, a 10% NaSO4 solution with mass concentration was selected as the immersion solution, which was prepared from anhydrous sodium sulfate and tap water. And the specimens were soaked in NaSO4 solution after curing, and the soaking time was set as 30d, 60d, 90d, and 120d. In order to ensure that the concentration of the solution was not reduced by crystal precipitation and water evaporation, the solution was replaced regularly (every 30 days in this test). After reaching the expected soaking time, the specimens were washed and wiped dry, and then, the mass loss and strength loss were measured.
