## 3 Mechanical Connectors to Enhance the Interface of Concrete Overlays  
### Design of the Connection between Layers  
To maintain the monolithic structural behavior of concrete overlays after interfacial cracking, we propose to place mechanical connectors in their interfaces. If the connection is designed correctly,pavement layers might remain working together after debonding, thereby preserving the expected service life of the restored pavement. The working principle of connectors is analogous to that of steel rebars and fibers in concrete. Their stress resistance starts when the interface cracks and the load carried by the connectors increases, depending on the crack width [8]. In the design phase, the connection has to provide enough strength to resist the interfacial forces while local failure must be avoided in both the pavement layer and the connectors. Model Code 2010 [43] provides Equation (1) to design mechanical connections subjected to fatigue or dynamic loads when the interfacial adhesive bond cannot be guaranteed. This equation considers that the strength of the interface between old and new concretes (\(\tau_{Rd}\)) depends on (i) the aggregate interlocking, (ii) the shear-friction mechanism, and (iii) the bending resistance of connectors perpendicular to the joint (dowel action). Moreover, the value of \(\tau_{Rd}\) (in MPa) is bounded from above by the product of a coefficient for the strength of the compression strut (\(\beta_{c}\)), a reduction factor for the strength of the diagonal strut (\(\
u=0.55\cdot\left(30/f_{ck}\right)^{1/3}\leq 0.55\)), and the design value of concrete compressive strength of the weakest layer (\(f_{cd}\)). The shear resistance due to interlocking is considered using a coefficient for aggregate interlocking (\(c_{r}\)) and the characteristic compressive strength of concrete (\(f_{ck}\)). The shear-friction strength is defined by a friction coefficient (\(\mu\)), the compressive stress in the joint due to external normal forces (\(\sigma_{n}\)), an interaction coefficient for tensile force activated in the connectors (\(\kappa_{1}\)), the ratio \(\rho\) of the reinforcing steel crossing the interface (\(\rho=A_{s}/A_{c}\); where \(A_{s}\) is the steel cross-section and \(A_{c}\) the concrete area where shear stresses occur), and its design yield strength (\(f_{yd}\)). Finally, dowel-action strength of the connectors is estimated in terms of \(\rho\), \(f_{yd}\), \(f_{cd}\) and an interaction coefficient for flexural resistance of the connector (\(\kappa_{2}\)). \[\tau_{Rd}=0.4\left[c_{r}f_{ck}^{1/3}+\mu\left(\sigma_{n}+\kappa_{1}\cdotp\cdot f _{yd}\right)+\kappa_{2}\cdotp\cdotp\sqrt{f_{yd}\cdotf_{cd}}\right]\leq 0.4\cdotp _{c}\cdotp\cdotf_{cd} \tag{1}\]  
Table 2 shows the values of the coefficients \(c_{r}\), \(\mu\), \(\kappa_{1}\), \(\kappa_{2}\) and \(\beta_{c}\) in terms of the roughness (\(R_{t}\)) of the interface according to Model Code 2010. The value of \(R_{t}\) can be experimentally obtained with the sand path method [44]. Note that smoother surfaces have lower \(c_{r}\), \(\mu\) and \(\kappa_{1}\) values (reduced interlocking and shear-friction strengths) and bigger \(\kappa_{2}\) (high dowel-action strengths in the connectors). On the other hand, Figure 1 depicts the most relevant failure modes of anchors that must be avoided according to Model Code 2010 [43]. These are classified as steel fracture, concrete failure, concrete splitting, bond failure and blow-out. The main origins of these failures modes are insufficient steel strength and anchor transversal area, low concrete strength, and very little distance to borders, corners, and between anchors. Providing the ultimate limit states (ULS) design criteria for all these failure modes would require an extensive explanation which is not the purpose of this work. Nevertheless, accurate information and details on ULS requirements may be found in Reference [45].