## 6 Conclusions  
Dynamic mechanical properties testing of water-filled frozen cement mortar specimens with the precast joints of different transfixion degrees (0%, 20%, 40%, 60%, 80%, 100%) by using a split Hopkinson pressure bar (SHPB) system assembled with axial and perimeter compression devices. The stress-strain curve, peak stress, peak strain, energy composition and absorption, destruction characteristics, and other dynamic mechanical properties of cement mortar were analyzed. Based on energy constitution theory, the energy density per unit time index (\(E_{\rm VT}\)) and porosity change rate were connected and revealed the action relationship between microscopic damage and energy dissipation of cement mortar specimens with different transfixion degrees of joint. The conclusions of the study were as follows.  
(1) The dynamic compressive strength of cement mortar specimens with different transfixion degree joints was controlled by the effect of penetration and saturation freezing. Average dynamic compressive strength was reduced sequentially to transfixion degree of 20%, 40%, 0%, 60%, 80%, and 100%. With the increase of the joint transfixion degree, the  
Figure 9: MRI and failure morphology of different transfixion joint specimens: (**a**) 0%, (**b**) 20%, (**c**) 40%, (**d**) 60%, (**e**) 80%, (**f**) 100%. Specimen destruction pattern diagram: the green lines are prefabricated, the red lines are tensile cracks or shear cracks formed by the impact test, and inside the white ring are the specimens shedding parts or large pores.  
peak stress of the specimens showed a significant linear downward trend. The average attenuation rate of peak strength of 20-100% axial transfixion joint specimens is 5.53 MPa.  
(2) The cement mortar structure subjected to dynamic load should have better dynamic performance after the water-saturated freeze pre-conditioning process. The 80% transfixion degree joint specimen has the weakest ability to carry the impact load, and the 40% transfixion degree joint specimen has the strongest ability to carry the impact load. With the increase of joint transfixion degree, there was a similar trend relationship of first decreasing and then rising in both the average change of porosity and the change of \(E_{\text{VT}}\).  
(3) Joint specimens with 20-80% transfixion degree gradually expanded along the precast joint tip and eventually formed a penetration crack. The final failure mode of the 0-40% transfixion degree joint specimens were pull-shear composite destruction. The final failure mode of 60-100% transfixion degree joint specimens were split and tensile failure.  
Conceptualization: S.B. and Z.X.; methodology: S.B., H.M. and D.W.; formal Analysis: S.B., L.G. and J.Z.; writing--original draft preparation: S.B.; writing--review and editing: S.B. All authors have read and agreed to the published version of the manuscript.  
This work was carried out in the School of Mining Engineering, University of Science and Technology Liaoning, and supported by the Foundation of Liaoning Educational Committee of China (Grant No. IJKZ0282).  
Not applicable.  
The datasets generated and analyzed during the current study are available from the corresponding author upon reasonable request.  
The authors would like to acknowledge [PERSON] and [PERSON] for the help in the sample testing.  
The authors declare no conflict of interest.