# Experimental Study of a New Precast Prestressed Concrete Joint
## 3 Test Results
### Cast-In-Place Joint  
The force-displacement curve of the cast-in-place joint was linear prior to cracking of the specimen; the reciprocating curves almost coincide with each other, and the elastic behavior is obvious. The first vertical crack appeared at the edge of the top of the left beam when the displacement reached 2 mm, and the crack width was 0.04 mm. When the loading was reversed, a vertical crack also appeared at the edge of the bottom of the right beam, and the crack width was 0.06 mm. At this time, the slope of the force-displacement curve began decreasing, and the positive and negative curves no longer coincided. These observations indicate that the cracking decreased the concrete's tensile strength. Thus, the load was transferred from the concrete to the steel bar and increased the strain in the steel bar. At a displacement of 4 mm, the left and right beams of the joint were observed to have a second vertical crack that was approximately 100 mm long and was about 100 mm from the edge of the column. The distribution of the initial cracks for the cast-in-place beam is shown in Figure 7.  
When the displacement was 6 mm, cracks at the bottom of the beam began developing obliquely. These cracks were typical bending shear cracks. Increasing the displacement to 8 mm resulted in the longitudinal steel bar strain near the edge of the right beam of the joint to be greater than the yield strain of 0.0022. Thus, the steel yielded during this displacement cycle. When the loading was reversed, the longitudinal reinforcements of the upper portion of the left beam near the edge of the column also began yielding. Thus, \(\Delta\) = 8 mm is taken as the yield displacement \(\Delta\)y. At this time, the number of yielded steel bars was small, and the plasticity had not fully developed. Therefore, the force-displacement curve of the joint did not yet have a significant inflection point. Further displacement increased the amount of the steel bars entering the plasticity stage at the beam end and the strain. At a displacement of 10 mm, the steel bar strain at the end of the beam reached 0.0032, and the longitudinal reinforcement of the beam 100 mm from the edge of the column yielded. The force-displacement curve had a fairly obvious inflection point at this time. The \"cusp point\" indicated that the reinforced concrete at the beam end was fully yielded. After the joint yielded, the number of bending cracks on the beam no longer increased, and about six bending cracks were seen respectively in the left and right beams. These positive and negative bending cracks mutually interpenetrated one another, and some were staggered with the crack spacing between 100 and 180 mm. The crack farthest from the edge of the column was 682 mm away and had a crack width of 0.38 mm. The main crack was found at the beam end and had a crack width of 1.08 mm. The development of  
Figure 7: The initial cracking of the cast-in-place beam. this main crack and the yielding of the steel bar caused the bending cracks away from the edge of the column to develop slowly. This crack development indicates that the energy was mainly absorbed by the reinforced concrete in the main crack zone. Thus, the development of cracking and plasticity of reinforced concrete in other areas of the beam was delayed. The distribution of bending cracks in the beam when the joint was fully yielded is shown in Figure 8.  
The strengthening phase occurred after the joint was yielded and displacement continued. When \(\Delta=2\Delta\)y, the width of the main crack on the beam at the edge of the column reached 2.86 mm. At this time, the strain gauge of the beam reached its limit and lost measuring capability. When \(\Delta=2.5\Delta\)y, small oblique cracks appeared on the periphery of the core area of the joint, and the concrete at the bottom and top of the beam were lifted due to the pressure and slightly enlarged. When \(\Delta=4\Delta\)y, a main cross-diagonal crack appeared in the core area of the joint and had a width of 0.12 mm.