**1. Introduction**

In coal mining, the vertical shaft is an important channel for moving personnel up and down, ventilation, drainage, and coal lifting [1–3], and its supporting structure is called the shaft lining structure [4,5]. When a newly established shaft passes through deep alluvium, it needs to be constructed by a special drilling method. Freezing shaft sinking has the advantages of strong construction adaptability, flexible supporting structure, and fast completion speed; therefore, it has become the main method of construction in China [6–8]. In order to resist strong external loads, high-strength concrete is usually selected as the building material for walls in the structural design of frozen shaft lining in deep alluvium, and the outer shaft wall is required to have the characteristics of early strength [9,10]. With the increase in the depth of the well construction, the thickness of the inner and outer sidewalls of the deep alluvium shaft is greatly increased, the hydration heat generated during the pouring of the shaft is high, and the environmental temperature varies greatly during construction, so the high-strength concrete of the shaft lining easily produces temperature cracks [11]. Especially in the method of shaft sinking by freezing in deep alluvium, the freezing pressure on the outer wall of the shaft is obviously unevenly distributed, due to the uneven distribution of the saline flow at the freezing hole deviation, which is extremely adverse to the force on the shaft lining and can easily cause the tensile failure of the high-strength concrete [12,13]. Figure 1 shows the cracks of high-strength concrete in shaft lining.

Frozen shaft lining concrete in deep alluvium has been used under a harsh engineering environment for a long time [14–16]. In addition to bearing loads, such as high ground

**Citation:** Yao, Z.; Fang, Y.; Zhang, P.; Huang, X. Experimental Study on Durability of Hybrid Fiber-Reinforced Concrete in Deep Alluvium Frozen Shaft Lining. *Crystals* **2021**, *11*, 725. https://doi.org/10.3390/cryst11070725

Academic Editors: Cesare Signorini, Antonella Sola, Sumit Chakraborty, Valentina Volpini and Ing. José L. García

Received: 11 May 2021 Accepted: 21 June 2021 Published: 23 June 2021

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pressure and freezing pressure, the built shaft was also faced with problems, such as water seepage, freezing, and thawing, and the erosion of harmful ions in groundwater [17–20]. Traditional high strength concrete has obvious deficiencies in impermeability, frost resistance, and corrosion resistance, which is difficult to be applied to such formation conditions as deep alluvium. Therefore, scholars have carried out a series of studies on the durability of shaft lining concrete. In order to solve the problems of many cracks and serious water seepage caused by thermal stress, Yang, L. et al. [21] prepared excellent concrete as a building material for walls by adding hybrid fiber and an expansive agent, and impermeability, crack resistance, and microscopic tests were carried out. The results show that it can solve the problems of shaft lining cracking and water seepage. Zhao, X. M. et al. [22] carried out the study on the frost resistance of fiber reinforced concrete and found that the incorporation of hybrid fiber could improve the compressive strength of concrete after freeze–thaw cycles and reduce the mass loss rate. Yang, D. et al. [23] carried out dry–wet cycle corrosion tests on concrete cube specimens at different corrosion periods. The results show that the splitting tensile strength of concrete decreases continuously with the increase in corrosion depth. Hou, Y. F. et al. [24] analyzed the evolution of concrete corrosion depth by means of ultrasonic testing and microstructure observation, and discussed the factors causing shaft lining concrete corrosion. Liu, J. H. et al. [25] studied the mechanical properties of polypropylene steel fiber-reinforced shaft lining concrete under the coupling action of early stress and negative temperature through a self-made test device, and concluded that concrete's load-bearing capacity and Cl−<sup>1</sup> corrosion resistance were improved.

**Figure 1.** The cracks of high-strength concrete in shaft lining: (**a**) Temperature cracks caused by temperature stress; (**b**) Rupture cracks caused by uneven freezing pressure.

Based on the above analysis, in this research, experimental studies on the static properties and durability of hybrid fiber-reinforced concrete were carried out to improve the bearing capacity and durability of a frozen shaft lining structure in deep alluvium, based on the frozen wall, Polyvinyl alcohol fiber and polypropylene steel fiber were added to commonly used C60 high-strength concrete to create a high-performance hybrid fiber concrete as a building material for the wall. The research results can provide a scientific basis for engineering applications.
