# Experimental Study of a New Precast Prestressed Concrete Joint
## 4 Hysteresis Curve and Skeleton Curve  
The hysteresis curves of the three joints are shown in Figure 16. The hysteresis curves are smooth and full for all joints. At the initial stage of loading, the structures were in the elastic phase, and the forward and reverse loading curves of the hysteresis loops were almost coincident. The hysteresis curves showed nonlinearity when the joints became cracked, and the concrete was damaged. This nonlinearity was manifested by the phenomenon of non-coincidence of the loading and unloading curves and pinching of the hysteresis loops. The hysteresis loops were fusiform as the joints progressed from yielding until peak load. A \"cusp\" appeared in the hysteresis curves from the joint yielding to its complete yielding. These cusps were generally located between 8 and 10 mm. At this cusp, the steel bars began yielding. However, there was a lack of consistency in this yielding due to uncertainties such as the construction of the joint, and the load had to be increased before the joint could be considered fully yielded. After the joints were fully yielded, the pinching phenomenon of the hysteresis loops was seen. The later hysteresis loops gradually develop toward the \"anti-S shape\", but the hysteresis loops remained full. Figure 16: Hysteresis curves of the three joints. The skeleton curves of the three joints are shown in Figure 17. The cracking point is the position on the skeleton curve where the test specimen begins to crack. The yield point is commonly determined through the energy equivalence method and geometric mapping method. The skeleton curve of each joint has a clear inflection point; thus, the inflection point was used to determine the yield point. The corresponding force and displacement values were used as the component of the \"limit point\" when the load on the skeleton curve dropped to 85% of the peak value. The specific values of these parameters determined from the skeleton curves are listed in Table 3.  
It can be seen from Figures 16 and 17, and Table 3 that the cracking displacements of the cast-in-place joint and the precast prestressed concrete joint were different. However, the cracking loads were typically between 31 kN and 42 kN. Generally, the cracking load during forward loading was greater than that during reverse loading. The yield displacement of each joint was relatively close and was about 8-10 mm. The yield point behavior was evident for all joints. The yield, peak, and limit displacements and loads of the two precast prestressed concrete joints were relatively close in both directions, though the load during forward loading was greater than that during reverse loading. The yield, peak, and limit loads of the precast prestressed concrete joint during forward loading were greater than that of the cast-in-place joint, while the loads during reverse loading were smaller than that of the cast-in-place joint. This was because reinforcements of the precast prestressed concrete joints overlap at the beam end, and the effective reinforcement ratio of the beam bottom was improved. Asymmetry of the plastic zone of the beam section was formed since the reinforcement ratio of the beam top was the same as that of the cast-in-place joint. This asymmetry caused greater resistance during forward loading and lower resistance during reverse loading. Figure 17: Comparison of joint skeleton curves: (**a**) left beam; (**b**) right beam.