## 4 Conclusions  
In order to solve the high brittleness problem of vertical shaft lining under dynamic loading, the authors of this paper propose the addition of steel and polypropylene fibers to prepare high performance concrete. The static and dynamic mechanical properties and crack failure characteristics of steel-polyppropylene-hybrid fiber-reinforced concrete were tested and studied, and the main conclusions were as follows:  
1. According to the results of the static test, the compressive strength of the single addition steel fiber and hybrid fibers group slightly increased compared to that of the reference group, but the splitting tensile strength significantly increased. The splitting tensile strength of the optimal group increased by 40.9% compared to that of the single addition steel fiber group.
2. The SHPB test showed that the peak stress and strain of the optimal group (SPB2) were significantly higher than those of the other groups. The peak stress and strain of the optimal group (SPB2) were 24.2% and 65.28% higher than those of the single steel fiber group, respectively, and the toughness index was 81.9% higher. These results indicate that the hybrid fiber group had higher dynamic splitting tensile strength and dynamic toughness.
3. The DIC technique was used to analyze the strain distribution before visible cracks appeared in the specimens, and the results showed that compared to the reference group, greater strain dispersion appeared in the single addition steel fiber and hybrid fiber groups. Additionally, the time required for the first crack to appear in the optimal group (namely SPB2) was the longest, and the time to arrive at the first visible crack increased by 57.1% compared to the single steel fiber addition group.  
Figure 9: Crack failure modes of each group of specimens at different moments: **(a)** The crack failure form corresponding to the time before cracks appeared in the specimen (i.e., T0); **(b)** the crack failure form corresponding to the time when cracks first appeared in the specimen (i.e., T3); **(c)** the crack failure form corresponding to the moment when the main crack first appeared in the specimen (i.e., T4).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name of Specimens** & T0/**\(\upmu\)s** & T3/**\(\upmu\)s** & T4/**\(\upmu\)s** \\ \hline S0P0 & 0 & 175 & 350 \\ S2P0 & 0 & 245 & 525 \\ SPB1 & 0 & 315 & 595 \\ SPB2 & 0 & 385 & 665 \\ SPB3 & 0 & 280 & 565 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Corresponding schedule of crack failure form in each group.  
(4) The analysis of the crack failure form of the specimens showed that, compared to the reference group, the steel fiber and hybrid fiber groups showed the characteristics of continuous and slow ductile failure. Additionally, the main crack took the longest time to appear in the optimal group (SPB2), and the time before the main crack appearance was increased by 26.7% compared to that of the steel fiber group. These results indicate that the hybrid fiber was able to effectively improve the brittleness of concrete. Therefore, the steel-polypropylene-hybrid fiber-reinforced concrete studied in this paper can effectively resist impact loading and allow shaft lining to better cope with the effects caused by deep coal mining.  
Investigation, Q.Z. and W.Z.; methodology, Q.Z., Y.F. and X.H.; software, W.Z. and X.H.; data curation, Y.F. and Y.X.; formal analysis, Q.Z., Y.F. and X.H.; writing--original draft, Q.Z. and Y.F.; visualization, W.Z. and X.H.; writing--review and editing, Y.F., Y.X. and X.H.; conceptualization, Q.Z., Y.F. and X.H.; resources, W.Z.; supervision, X.H. and Y.F.; project administration, W.Z., Y.F. and Y.X.; funding acquisition, W.Z. and X.H. All authors have read and agreed to the published version of the manuscript.  
Not applicable.  
The authors declare no conflict of interest.