# Shear Behavior of Superposed Perfobond Connectors Considering Lateral Constraints
## 3 Experimental Results  
### Failure Modes  
The final cracking phenomenon of a concrete slab is shown in Figure 7. During the loading process, all specimens were first cracked vertically along the direction of the steel plate. When the load increased, the cracks gradually extended to the edge of the slab until a main fracture was formed. Meanwhile, transverse cracks centered on the perfobond rib began to appear and develop. For certain specimens, a small number of oblique cracks were observed on the concrete block.  
Based on the test results and the previous studies [10; 11; 25; 26], lateral constraints have a significant impact on the failure mode of connectors. As shown in Figure 7a-c,j-l, the increase of the diameter of the perforating rebar can not only provide the vertical force but also enhance the lateral constraints of the concrete dowel, thus limiting the crack width of the concrete slab. Therefore, with the increase of the diameter of the perforating rebar, the number of cracks in the concrete slab increases and the width of the cracks becomes smaller. Compared with Figure 7a,h,i, the existence of transverse reinforcement improved the lateral constraints on the concrete dowel so that the specimens can maintain a high bearing capacity without immediate damage after the peak load, thus allowingmore time for concrete slab cracks to fully unfold. Specimen NP-1 with a plain concrete slab had longitudinal splitting failure soon after reaching the ultimate bearing capacity, and the vertical cracks gradually developed and widened until the whole concrete slab was penetrated. Specim SP-1 with segmental stirrups had the largest number of cracks, most of which were small in width, and several oblique cracks appeared in the concrete blocks. In the case of the same size of specimens, the concrete wrapping force is mainly affected by the strength of concrete. However, by comparing Figure 7a-cj-l, the failure modes of specimens with different strengths of concrete had little difference, and the crack pattern was similar. It can be seen from Figure 7a,e,g that when the lateral constraints were basically the same, the cracking phenomenon of the concrete slab of P-1 with a small hole area was more obvious than that of Specimens P-5, LP-1 and HP-1 with a large hole area.  
To investigate the shear failure mechanisms of perfobond connectors, the specimens were broken to observe the inner failure modes. Figure 8a-c shows the internal failure modes of perfobond connectors with circular, long, and high holes, respectively. The three failure modes differed slightly, though their shear mechanisms remained essentially identical. Connectors featuring a perforating rebar eventually formed a relatively complete shear surface. On the surface, coarse aggregate can be found to be cut off, the top of the concrete dowel was almost squeezed out, the bending and shear deformation occurred through the perforating rebar, and the deformation of perforating rebar in connectors with large hole areas was smaller. As illustrated in Figure 8d, the failure modes of circular hole connectors without transverse reinforcement were similar to those with transverse reinforcement. Meanwhile, the shear failure surface was not as smooth as that of the transversally reinforced connectors, less of the concrete dowel was extruded, and very little deformation occurred in the perforating rebar. As shown in Figure 8e, the single-hole connector formed a fairly smooth shear plane, and the perforating rebar was ultimately cut off. It can be seen from Figure 8f that the concrete dowel without perforating rebar underwent failed shear, its fracture surface was relatively flat, and a white powder was generated by friction after the coarse aggregate was cut off. It can be concluded from Figure 8 that the stronger the lateral constraint, the closer the concrete dowel is to the forced shear failure. Whether the perforating rebar is set does not affect the failure mode of the concrete dowel when in a state of good lateral constraint.  
Figure 8: _Cont._