## 3 Results and Discussion
### Load Deflection Behavior of UHPFRC Joints  
According the Chinese code for the seismic design of buildings, the shear force (\(V_{j}\)) of a joint can be expressed as [20]:  
\[V_{j}=\frac{\sum M_{b}}{h_{b0}-a_{s}^{\prime}}\Big{(}1-\frac{h_{b0}-a_{s}^{ \prime}}{H_{c}-h_{b}}\Big{)} \tag{2}\]  
in which \(\sum M_{b}\) is the total end bending moment of the beam-adjoining joint core, \(\sum M_{b}=P_{1}L_{1}+P_{2}L_{2}\), particularly for exterior joints, and \(P_{2}L_{2}=0\); \(H_{c}\) is the total column depth; \(h_{b}\) is the total beam depth; \(h_{b0}\) is the effective beam depth; and \(a_{s}^{\prime}\) is the distance from the resultant force center of the compressive reinforcement to the compressive edge. The shear deformation angle (\(\gamma\)) of joints can be calculated according to Equation (1). Figure 5 illustrates the shear force (\(V_{j}\))-deformation angle (\(\gamma\)) hysteretic loops and skeleton curves of the eight specimens, except for specimen EJ-1 with a smaller shear deformation angle. The \(V_{j}\)-\(\gamma\) hysteresis loops of the UHPFRC beam-column joint exhibit the following characteristics:  
1. During the load control stage, little shear deformation occurred at the joint, and the slopes of the loading and unloading curves in the hysteresis loops were basically overlapping prior to joint cracking. 2. During the displacement control stage, in the \(1\Delta_{y}\) displacement stage, the shear deformation angle of the joint was very small, and strength degradation was not observed. During the \(2\Delta_{y}\) stage, the slope of the hysteresis loops began to decline; however, the decline rate was small. The maximum shear carrying capacities of eight specimens were reached in the first \(2\Delta_{y}\) cycle. During the second and third cycles of the \(2\Delta_{y}\) displacement, strength and stiffness degradation of specimens was observed. 3. The shear angles of specimens EJ-1 and EJ-4 were relatively small, which may be explained by the fact that more obvious beam failure was observed for specimens EJ-1 and EJ-4 than other joint specimens. For other specimens, the shear deformation angles of joints became evident in the first \(3\Delta_{y}\) cycle, but the shear strengths decreased only slightly. This indicates that the deformation and toughness of the UHPFRC beam-column joint were effective. As the cycle times increased, the shear deformation angles of the specimens continued to increase, and the strengths began to decrease until failure. Because of the steel fiber, the UHPFRC has a higher toughness than NC; the ultimate shear deformation angles of UHPFRC joint are considerably larger than that at peak shear load. The shear deformation angles ratio between ultimate state and peak shear load range from 1.45 to 8.56, and the average value is 5.79. This indicates that UHPFRC beam-column joints have good deformation capacity. 4. From the comparisons of specimens EJ-2 and EJ-4, and specimens EJ-3 and EJ-5 with same beam reinforcement load bearing capacity, it can be seen that the use of HRB600 grade high-strengthsteel bars with relatively high yield strain and low elastic modulus can improve ductility and strength degradation of UHPFRC joints. For better examination on the effect of test parameters, the envelope curves of hysteretic loops of the same kind of joint are plotted in a single graph, as shown in Figure 6. It can be seen from Figure 6 that an increase in the axial compression load can improve the shear carrying capacity of specimens. This is because the increased column compression extends the compression zone, making the sectional area of the UHPFRC diagonal strut resist the compression increase. Moreover, appropriate compression stresses can improve the initial crack strength of UHPFRC and cause cracks to reclose. Figure 5: Shear force versus shear deformation angle of joint cores: (**a**) specimen EJ-2; (**b**) specimen EJ-3; (**c**) specimen EJ-4; (**d**) specimen EJ-5; (**e**) specimen J-1; (**f**) specimen J-2; (**g**) specimen J-3; and (**h**) specimen J-4.