## 2 Background and Design Provisions of Interface Shear
### Eurocode 2 (En 1992-1-1:2004) [20]  
_Eurocode 2_[20] (Article 6.2.5) suggests an equation for predicting interface shear resistance between concretes cast at different times. As indicated in Equation (7), _Eurocode \begin{table}
\begin{tabular}{l c c} \hline \hline
**Contact Surface Condition** & \(c\) **(MPa)** & \(\mu\) \\ \hline Concrete placed monolithically & 1 & 1.4 \\ \hline Concrete placed against a clean concrete surface, with surface intentionally roughened to an amplitude of 5 mm & 0.5 & 1 \\ \hline Concrete placed against a clean concrete surface, but not intentionally roughened & 0.25 & 0.6 \\ \hline Concrete anchored to as-rolled structural steel by headed studs or by reinforcing bars & 0 & 0.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Coefficients for different interface types. \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Interface Type** & \(c\) **(MPa)** & \(\mu\) & \(K_{1}\) & \(K_{2}\) **(MPa)** \\ \hline Concrete placed monolithically & & & & \\ _For normal-weight concrete_ & 2.8 & 1.4 & 0.25 & 10.3 \\ _For lightweight concrete_ & 1.7 & 1 & 0.25 & 6.9 \\ \hline Cast-in-place concrete slab on clean concrete girder surfaces, with surface roughened to an amplitude of 6 mm & & & \\ _For normal-weight concrete_ & 1.9 & 1 & 0.3 & 12.4 \\ _For lightweight concrete_ & 1.9 & 1 & 0.3 & 9 \\ \hline Concrete placed against a clean concrete surface, with surface intentionally roughened to an amplitude of 6 mm & & & \\ _For normal-weight concrete_ & 1.7 & 1 & 0.25 & 10.3 \\ _For lightweight concrete_ & 1.7 & 1 & 0.25 & 6.9 \\ \hline Concrete placed against a clean concrete surface, but not intentionally roughened & 0.52 & 0.6 & 0.2 & 5.5 \\ \hline Concrete anchored to as-rolled structural steel by headed studs or or by reinforcing bars where all steel in contact with concrete is clean and free of paint & 0.17 & 0.7 & 0.2 & 5.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Coefficients for different interface types. 2 considers the cohesion mechanism in relation to the lower design tensile strength of concrete and the friction mechanism related to clamping stress and externally applied stresses perpendicular to the interface. The dowel action effect is neglected. The factors of cohesion and friction are proposed for four different interface types. \[v_{Rdi}=cf_{ctd}+\mu\sigma_{n}+\rho f_{yd}(\mu\sin\alpha+\cos\alpha)\leq 0.5 \psi f_{cd} \tag{7}\]  
where \(c\) and \(\mu\) are the factors which depend on the surface roughness (see Table 5), \(f_{cd}\) is the characteristic compressive cylinder strength of concrete, \(f_{ctd}\) is the design tensile strength, \(f_{yd}\) is the design yield strength of reinforcement, \(f_{cd}\) is the design value of concrete compressive strength, \(v\) is the strength reduction factor for concrete, \(\rho\) is the interface shear reinforcement ratio = \(A_{qf}/A_{cv}\), \(A_{qf}\) is the area of the shear reinforcement crossing the interface, \(A_{cv}\) is the area of the concrete section resisting shear transfer, \(\sigma_{n}\) is the stress per unit area (positive for compression, such that \(\sigma_{n}\) <0.6\(f_{cd}\), and negative for tension; when \(\sigma_{n}\) is tensile, \(cf_{ctd}\) should be taken as 0), \(\alpha\) is the angle of the interface shear reinforcement measured from the horizontal interface shear plane, and \(v_{Rdi}\) is the design shear strength at the interface. _Fib model code 2010_[21]  
The _Fib model code 2010_ (_Fib MC 2010_) [21] (Article 7.3.3.6) considers all three load transfer mechanisms, as indicated in Equation 8: The cohesion mechanism related to the lower characteristic compressive strength of concrete, the friction mechanism as a function of externally applied stresses and clamping stress, and the dowel action mechanism due to flexural deformation. It should be noted that this code is the first to consider the dowel action.