## 2 Debonding Mechanisms and Reduction of Debonding Risk  
The debonding of the whitetopping and overlays occurs mainly due to excessive interfacial stresses caused by direct and indirect loads. Direct loads are (1) vertical and (2) horizontal traffic loads while indirect ones are generated by (3) the autogenous shrinkage of the new concrete layer and (4) thermal and (5) moisture (drying shrinkage) exchanges with the environment [7]. All of them may simultaneously act in a real situation, thereby increasing the risk of interfacial debonding. Table 1 shows the mechanisms of debonding classified by the type of loads and the typology of interfacial stresses. Vertical traffic loads (vehicle weight) generate normal interfacial stresses (\(\sigma_{\text{tr,v}}\)) that emerge near joints, cracks, or edges [8]. Owing to slab discontinuities, top and bottom layers are unable to deform jointly, and consequently \(\sigma_{\text{tr,v}}\) emerges in the interface. These stresses increase when load transmission between adjacent slabs is poor and are also the leading cause of premature failure in whitetoppings [10; 12]. Horizontal traffic loads (braking, acceleration, and centrifugal forces of vehicles) are applied by the wheel on the surface of the pavement. They make the slab tend to slip, so shear stresses (\(\tau_{\text{tr,h}}\)) appear in the opposite direction to avoid slippage. The magnitude of these stresses might be a major cause of interfacial failure [5; 9; 11; 13]. On the other hand, thermal exchange, drying shrinkage, and autogenous shrinkage generate length changes that are translated into both normal and shear stresses [18; 19; 20; 21; 22; 23]. In general, length changes are non-linear, so their effects may be decomposed into uniform and non-uniform components. The uniform component may be understood as a rough simplification that helps to understand the debonding mechanisms. It generates shear (\(\tau_{\text{lc,u}}\)) stresses at the edge of overlay discontinuities to make deformations compatible. Concomitantly, normal tensile stresses (\(\sigma_{\text{lc,u}}\)) also arise close to the discontinuities due to the peeling moment generated by \(\tau_{\text{lc,u}}\) and its eccentricity to the interface [7]. Finally, non-uniform length changes are related to curling and warping of overlay slabs. Upward and downward curvatures of concrete slabs induce normal tensile stresses (\(\sigma_{\text{lc,nu}}\)) to the interface due to the restraint produced by its adhesion to the original pavement. Sometimes, the magnitude of \(\sigma_{\text{lc,nu}}\) is not negligible, as it can cause premature cracking of the interface [6]. It is possible to optimize the overlay design to reduce the stresses in the interface, enhance the adherence between layers, and to reduce the risk of interfacial debonding. The attributes to be optimized are the five cornerstones of design: (1) thickness of the concrete layer, (2) slabs size, (3) joint properties, (4) interfacial treatment, and (5) concrete mixture [15]. \begin{table}
\begin{tabular}{c c c} \hline
**Load** & **Steaves** & **Mechanisms** \\ \hline Traffic (Vertical) & Normal & \\ \hline Traffic (Brokenhall) & Shear & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanism of interfacial debonding due to direct and indirect loads. Doubling the thickness of the overlay might reduce normal tensile stresses in the interface due to vertical loads about 40 to 60% [12] and also decrease shear stresses due to horizontal traffic loads by about 24% [13]. Nevertheless, increasing the thickness increases the normal peeling stresses due to uniform length changes. This is due to the higher eccentricity of the interfacial shear stresses to the half the thickness of the overlay [7]. However, thicker overlays suffer shorter interfacial cracking lengths when subjected to drying shrinkage and temperature gradients due to the beneficial effect of self-weight forces [24]. The size of slabs may govern the failure mode of bonded whitetoppings on asphalt pavements [18; 25; 26].