## 5 Conclusions  
In this study, the failure modes and ultimate shear capacity of a concave square frustum-shaped wet joint were studied using experimental and numerical methods. Firstly, inspired by the fact that high shear resistance can be achieved by properly choosing the shape of shear keys in dry joints, a concave square frustum-shaped wet joint was proposed to improve the shear capacity. Then, specimens with the concave square frustum-shaped and conventional wet joints (labeled as S-SWJ and S-CWJ) were fabricated and tested for comparison. Finally, a 3D FE model was established to further explore the ultimate shear capacity of S-SWJ. The main conclusions are summarized as follows:  
1. It was found that the ultimate shear capacity of S-SWJ was 73.2% higher than that of S-CWJ, which was caused by changing the wet joint into a concave square frustum shape. This indicates that the shear resistance of wet joints can be significantly influenced by their shape.
2. The ultimate shear capacity of S-SWJ increases almost linearly with increasing concrete strength, while it first increases quickly and then decreases slowly with increasing shear key angle. The recommended shear key angle range is 45\({}^{\circ}\) to 55\({}^{\circ}\) because higher ultimate shear capacity can be achieved within this range.
3. In terms of improving the ultimate shear capacity of S-SWJ, increasing the concrete strength is more effective compared with changing the shear key angle, as the increase of concrete strength is associated with a significantly greater improvement in ultimate shear capacity.  
In sum, this study provides a new possibility to design wet joints with enhanced ultimate shear capacity, and guidelines for obtaining a concave square frustum-shaped wet  
Figure 14: Load–displacement curves of S-SWJ with different shear key angles.  
Figure 15: Ultimate shear strength and the corresponding displacement of S-SWJ with different shear key angles.  
joint with desirable ultimate shear capacity are presented. This study demonstrates that the shape has a significant influence on the ultimate shear capacity of a wet joint, which means higher ultimate shear capacity can be achieved by properly choosing the shape of the wet joint. It should be mentioned that the obtained results may have certain limitations due to the specimen geometry and loading conditions that were adopted in this study. Therefore, in future study, we will perform investigations on wet joints with various geometric shapes. The performance of wet joints under different loading conditions (e.g., earthquake loading, fatigue loading) is also very important and needs to be further explored.  
Conceptualization, F.H. and W.W.; methodology, F.F., W.W. and P.G.; Validation, F.F. and W.W.; Formal analysis, F.F. and Y.T.; Writing--original draft preparation, F.F.; Writing--review and editing, F.F. and Y.T.; Supervision, F.H. and Y.T.; Project administration, F.H. and P.G.; Funding acquisition, W.W. All authors have read and agreed to the published version of the manuscript.  
This research is substantially funded by the National Natural Science Foundation of China (No. 12072375).  
Not applicable.  
The authors declare no conflict of interest.