3.3.2. Test Results and Analysis

In order to analyze the mass change of concrete after sulfate corrosion, the mass change factor S was defined, and the expression is as follows:

$$S = \frac{m\_t - m\_0}{m\_0} \times 100\% \tag{4}$$

where *S* is the mass variation factor (the mass increases when *S* > 0, and decreases when *S* < 0), *mt* is the mass of the specimen after tage corrosion, and *m*<sup>0</sup> is the mass of the uncorroded specimen.

The results of concrete mass change are shown in Table 10.

Table 10 shows that with the increase in corrosion exposure time, the change factor of concrete mass increases at first and then decreases as the concrete reacts with NaSO4 solution and gradually generates ettringite, gypsum, and other corrosive substances in the initial stage of corrosion [36]. At the same time, some salt crystals invade the specimen and fill the micropores inside the concrete, thus improving the density and mass of the specimen. As the corrosion continues, the filling material inside the specimen continues to accumulate and expand, which destroys the pore structure, produces microcracks and gradually expands; and is accompanied by exfoliation of the epidermis, which finally leads to an increase in the number of cracks and easier invasion of harmful erosion materials; the corrosion is also more serious.


**Table 10.** Mass change factors of specimens at different corrosion exposure (%).

Additionally, compressive strength tests were carried out on the specimens soaked for various periods of time, and the test results at different exposure times are shown in Table 11.

**Table 11.** Results of compressive strength after sulfate corrosion (MPa).


As shown in Table 11, when the corrosion exposure time were 30d, 60d, 90d, and 120d, the strong growth rates of the specimens in the reference group and the hybrid fiber group were 2.8%, 8.3%, 4.4%, and –2.6%, and 2.6%, 6.9%, 7.5%, and 2.1%, respectively. After 120d of corrosion, the strength of the reference group specimens decreased by 2.6%, while the strength of the hybrid fiber group specimens was still in the growth stage, and the strength was 1.076 times that of the reference group, which indicated that the hybrid fiber could effectively slow down the strength loss of concrete after sulfate corrosion. In order to analyze the relationship between the change factors of the compressive strength and mass of concrete specimens and the change in corrosion exposure time more intuitively, the results in Tables 10 and 11 were drawn into a double-Y-axis, columnar-broken line diagram, as shown in Figure 12.

As can be seen from Figure 12, the mass factor of concrete increased rapidly after 30 days of corrosion exposure time, which involved the process of the filling and compaction of erosive materials in the first stage, but this process obviously slowed down after 60 days. This is mainly due to the increase in expansion stress in the specimen, resulting in the generation and expansion of microcracks, and the decline in mass along with the shedding of the outer skin and cement mortar, and the corrosion of concrete intensified, due to the increase in cracks. Meanwhile, the compaction process of erosive material filling continued; therefore, the mass change factor of concrete still shows an increasing trend, but the rate decreases obviously. At 90d of corrosion, the mass variation factor of concrete in each group reached the maximum value, and at this time, the mass variation factor of concrete in the hybrid fiber group was 44.6% lower than that in the reference group. However, after 90d of corrosion, the deterioration cracking and shedding process intensified, and the mass variation factor of concrete decreased sharply. When the corrosion exposure time was 120 days, the mass variation factors of the two groups of concrete were still positive. However, if the trend is developed, the mass of the two groups of concrete will decrease sharply when the corrosion exposure time is further increased. The compressive strength of the specimens in each group increased first and then decreased with the increase in corrosion exposure time. The initial decrease in compressive strength in the reference group occurred after corrosion for 60 days, while that in the hybrid fiber group occurred after 90 days. This is because the erosive material invades and fills the concrete, the number of pores in the matrix is reduced, the compactness is increased, and the strength is also improved to a

certain extent. With continuous corrosion, the filling material in the matrix continues to accumulate and expand, and the expansion stress increases continuously, which eventually exceeds the tensile strength of the concrete, resulting in internal deterioration and cracking, and the pressure-bearing capacity is also reduced. The hybrid fiber can improve the tensile capacity of the matrix, hinder the expansion of cracks and increase the compactness of the concrete; therefore, it slows down the rate of corrosion deterioration to a certain extent.

**Figure 12.** Relation chart of compressive strength and mass change factor with corrosion.

Overall, after 120 days of sulfate corrosion, there was no obvious mass loss in the reference group and hybrid fiber group, and the compressive strength of the hybrid fiberreinforced concrete did not decrease but rather even slightly improved. Additionally, the apparent integrity of specimen morphology was relatively high, while the concrete of reference group has obvious surface spalling, as shown in Figure 13. Therefore, the hybrid fiber concrete has good corrosion resistance.

**Figure 13.** Appearance of specimens after 120 d immersion: (**a**) Reference group; (**b**) hybrid fiber group.

#### **4. Discussion**

In this research, according to the static mechanical test, we can see that the flexural strength and splitting tensile strength of the hybrid fiber concrete were significantly higher than those of the reference group, because the bridging effect of the hybrid fiber improves the bond strength between the matrix and the fiber, and hinders the expansion of microcracks and macro cracks in the concrete. This is consistent with the discussion of

the strength mechanism of steel fiber reinforced concrete by Liu, J. H. et al. [25]. In the durability test, various properties of the hybrid fiber concrete group were better than those of the reference group. 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 improves the compactness of concrete so as to resist water pressure penetration, improve the freeze–thaw resistance of concrete, and slow down the speed of sulfate corrosion cracking [21,22]. Therefore, the hybrid fiber concrete prepared is an excellent wall building material for the shaft, which can be applied in engineering practice. Unfortunately, we also acknowledge that there are some shortcomings in this paper. For example, we failed to consider the performance of hybrid fiber concrete under the combined action of freeze–thaw cycles and sulfate. Additionally, we can increase the number of freeze–thaw cycles and extend the exposure time of corrosion tests in the subsequent studies to achieve better test results.

#### **5. Conclusions**

In order to improve the bearing capacity and durability of frozen shaft lining structures in deep alluvium, experimental studies on the static properties and durability of hybrid fiber-reinforced concrete were carried out. The static mechanical properties, impermeability, frost resistance, and corrosion resistance of hybrid fiber concrete were studied, and the conclusions are as follows:


**Author Contributions:** Investigation, Y.F. and P.Z.; methodology, Z.Y., Y.F. and X.H.; software, P.Z. and X.H.; data curation, Y.F. and P.Z.; formal analysis, Z.Y, Y.F. and X.H.; writing—originaldraft, Z.Y. and Y.F.; visualization, P.Z. and X.H.; writing—review and editing, Y.F., P.Z. and X.H.; conceptualization, Z.Y.,Y.F. and X.H.; resources, Z.Y.; supervision, Z.Y. and P.Z.; project administration, Z.Y., Y.F. and P.Z.; funding acquisition, Z.Y. and X.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Anhui University Discipline Professional Talented Person (No.gxbjZD09), Anhui Provincial Natural Science Foundation Youth Project (1908085QE185), Anhui Provincial College of Natural Science Research Key Project (KJ2018A0098), Project Funded by China Postdoctoral Science Foundation (2018M642502), and the Science Research Foundation for Young Teachers in Anhui University of Science and Technology (QN2017211).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
