# Analytical Method and Analysis of Cold-Joint Interface
## 2 Shear Interface Behaviour  
Interface strength and stiffness depend on the following mechanisms: cohesive bonds, friction (due to an external compressive force or connectors subjected to tension) and  
Figure 1: Behaviour of a layered concrete member, where M\({}_{1}\) and M\({}_{2}\) are the bending moments of each layer and T is the shear force acting at the interface between the layers. connector dowel action [5; 21; 30]. Shear mechanisms are depicted in Figure 2. It is important to emphasize that the influence of each shear mechanism on shear transfer changes during different interface behaviour stages. For instance, when shear forces are mainly resisted by interface cohesive bonds, dowel action does not have a significant impact on interface behaviour, and on the contrary, when shear forces are resisted by dowel action, chemical bonds between the layers are lost and cohesive contribution disappears [16; 21]. Researchers who have studied concrete interfaces subjected to shear describe the process of failure of interfaces crossed by connectors and the performance of individual shear mechanisms [5; 16; 21]. When the interface has not yet shown any signs of cracking or damage, cohesion is the main shear transfer mechanism. For the dependence of shear-layer displacement at this stage, rigid behaviour is usually observed. When cracks start to appear and when the displacement reaches a value that should not be ignored, shear force transfer is gradually taken over by other shear mechanisms. The second mechanism, interface friction, is caused by compressive forces that occur as a result of an external load perpendicular to the interface and the connectors crossing the interface. As cracks develop throughout the interface and concrete layers slip, connectors join the shear transfer and resist the slippage. This causes clamping (compression) in the interface. This phenomenon significantly increases shear stress at the interface in the case of considerable surface roughness (e.g., when concrete aggregates protrude through the surface plane). As an uneven concrete surface slips, cracks in the interface open in a direction perpendicular to the interface and connectors are subjected to even greater tension. In the literature, this phenomenon is called dilation [20]. As a result, even higher compression is achieved. The last shear mechanism is dowel action. During this mechanism, bars which cross the interface are subjected to shear and local bending [31]. [PERSON] and [PERSON] (1981) [32] stated that dowel action does not significantly influence the interface behaviour as long as the interface is not noticeably cracked. This confirms that, in the earlier stages of interface behaviour, shear force is mostly resisted by cohesive bonds. Observing the dependence of shear stress-layer displacement, it can be noticed that the influence of dowel action increases with increasing displacement of layers. An Interface which is not crossed by any type of transverse bar behaves slightly differently to an interface with connectors. It can be assumed that an interface starts cracking and fails at the same time. This means that the interface behaves rigidly up to  
Figure 2: Shear mechanisms: (**a**) friction due to surface roughness and external compression; (**b**) friction due to connector clamping force; (**c**) dowel action. N (section (**a**))—external compressive force acting perpendicular to the interface. T\({}_{\text{f}}\) –shear force due to friction. \(\sigma_{\text{s}}\)—tensile stress in the connectors. \(\sigma_{\text{cs}}\)—resulting clamping stress. T\({}_{\text{cs}}\)—shear stress due to clamping force. M—friction coefficient. P—connector ratio. N (section (**c**))—tensile force acting on the connector. the highest shear-bearing capacity. When the interface fails, the only shear mechanism that governs its behaviour is the surface friction. At this stage, shear stress decreases to a value that is close to 0.