## 2 Materials and Methods
### Jointed Specim Preparation  
Experimental investigation using natural rock materials have the following limitations. Firstly, as rock contains various minerals and internal defects, there is a certain dispersion of physical and mechanical parameters, which may lead to low consistency of repeated test results and cause errors. Secondly, it is difficult to process joints with various roughness on natural rock surface, and the natural rock may be damaged during the preparation process, resulting in test results not reflecting the effect of the joint roughness on the dynamic  
Figure 1: Schematic diagrams of serrated joints.  
Figure 2: Schematic diagram of SHPB apparatus and specimen placement. (**a**) Variable cross-section SHPB apparatus, (**b**) Jointed specimen sandwiched between incident bar and transmission bar.  
mechanical properties. In contrast, cement mortar has physical and mechanical properties similar to rock materials, and it can be poured into any shapes, which can effectively solve the above problems.  
Based on relevant literature [3; 23; 27] and orthogonal tests, five sets of mix proportion were designed, and a static uniaxial compression test has been conducted using an electro-hydraulic servo universal testing machine. In order to eliminate the discreteness of test results, three parallel tests have been carried out for each mix proportion of cement mortar. Ordinary Portland cement with 42.5 grade was adopted. Fine sand with maximum particle size of 0.6 mm was selected. According to the static test results, the mix proportion of the cement mortar was selected as cement:water:fine sand = 1:0.4:0.7. The corresponding physical and static mechanical parameters of cement mortar is shown in Table 1. In Table 1,\(f\) is static uniaxial compressive strength, \(\rho\) is density, \(E\) is elastic modulus, and \(\mu\) is Poisson's ratio.  
In order to prefabricate serrated joints in cement mortar specimens, 3D printing technology has been used to manufacture serrated joint surface molds with various JRCs and joint inclination angles.  
The production process of jointed cement mortar specimen can be divided into five steps. Firstly, put the serrated joint surface mold into a standard cylindrical mold with lubricant oil on the inner wall, then brush the serrated joint surface with lubricant oil and compact moderately to prevent slippage during the vibration process. Secondly, weigh cement, water, and fine sand using an electronic scale according to the mix proportion, then put the cement and fine sand into a mixer and stir for 90 s, subsequently pour the water into the mixer and stir for 240 s. Thirdly, pour the cement mixture into the cylindrical mold with the serrated joint surface mold, then vibrate for 90 s. Fourthly, remove the mold after curing for 24 h. Half jointed cement mortar specimens with various JRCs are shown in Figure 3a. Finally, bond the two-half jointed cement mortar specimens by gypsum after curing for 28 d. The jointed cement mortar specimens after bonding are shown in Figure 3b. Before impact tests, both ends of jointed specimens were polished by a grinding machine to ensure that the non-parallelism at both ends was less than 0.02 mm.