# Shear Behavior of Superposed Perfobond Connectors Considering Lateral Constraints
## 3 Experimental Results
### Shear Failure Mechanism of Connectors  
The strain analysis results for the perfobond rib (Figure 9) show that the steel plates underwent no notable deformation; this suggests that the failure modes of the concrete dowels and perfobond rib should be the primary focus when studying the shear mechanism. Using the inner failure states of the connectors in Figure 8 and the data measured in the test, the force-transfer mechanism of the connector was analyzed using a single hole as an example. As shown in Figure 11, the vertical load acting upon the steel was transmitted to the concrete dowel via the local bearing surface between the perfobond rib and concrete, and subsequently transferred to the surrounding concrete through the shear concrete surface and perforating rebar, in the form of shear load. Owing to its dilatancy, concrete not only provides a shear bearing capacity but also produces an additional horizontal  
Figure 10: Load–strain curve of perforating rebar: (**a**) P-1; (**b**) P-4.  
Figure 9: Load–strain curve of perfobond rib: (**a**) P-1; (**b**) P-4.  
dilatancy effect, which is constrained by the surrounding concrete, the rebar in the hole, and the transverse steel bar [11; 12], namely \"lateral constraints\" in this paper. Previous studies have found that lateral constraints significantly determine the bearing capacities and failure modes of connectors [11; 12; 25; 26; 30]. Therefore, the shear mechanisms of perfobond connectors can be classified into two categories: those with and without a perforating rebar.  
The connectors featuring the three different hole shapes exhibited the same shear failure mode; therefore, the shear failure mechanism can be investigated by using the circular hole as a typical example, as shown in Figure 12a. The load was first transferred to the concrete dowel through the partial bearing surface, and this was associated with compression deformation of the concrete between the perfobond rib and perforating rebar. When an increasing load was transmitted to the rebar, bending and shearing deformation occurred throughout the reinforcement, in response to which the vertical and axial forces of the rebar provided shear resistance and constrained the concrete dowel, respectively. Meanwhile, the load was transferred to the surrounding concrete in the form of a shear load through the shear plane. This indicates that the concrete in the hole undergoes a tri-axial compression through the perforating rebar, the perfobond rib, and the surrounding concrete [20; 22]. During cracking, the concrete wrapping and transverse reinforcement produced lateral constraints on the concrete dowel, alongside the axial force through the reinforcement.  
Figure 11: Force-transfer mechanism of the connector.  
Figure 12: Shear-failure mechanism of the perfobond connector: (**a**) Shear-failure mechanism of perfobond connector with perforating rebar; (**b**) Shear-failure mechanism of perfobond connector without perforating rebar.  
It can be seen from Figure 12b that in the connectors without a perforating rebar, the load was initially transferred to the concrete dowel via the steel plate before being shifted to the surrounding concrete through the shear surface. When the shear plane exhibited clear shear-slip deformation, the concrete cracks propagated along the direction of the perfobond rib. The lack of a rebar in the concrete dowel meant that the shear plane of the concrete dowel was confined only by the concrete wrapping and transverse reinforcement.  
The findings of [PERSON] [31] and [PERSON] et al. [32], combined with the shear failure mechanism described in this paper, indicate that double-sided shear failure occurs in the connectors when the thickness of the perfobond rib exceeds 9 mm.