## 4 Experimental Validation
### Mechanical Behavior of Concrete Overlays with Screw Anchors under Heavy Traffic Loads  
The pull-off tests demonstrated that the placement of screw anchors in the interface of the whitetoppings might be a suitable solution to enhance the tensile interfacial strength without a substantial increase in construction time. A 229.4 m long, 3.10 m width and 10 cm thick overlay was built (see Figure 8a) to assess that this technical solution was valid for the construction of real pavements. The pavement was located in a cement plant and closely placed to a cement clinker kiln, so the area was subjected to temperatures higher than its surroundings. The objective of this test was to determine if concrete overlays with screw anchors were capable of resisting a high flux of heavy-weight vehicles without structural damage. The original pavement was cold-milled to leave a box with the width and thickness of the reinforcement (Figure 8b). Four rows of screws were arranged in the milled surface following the symmetrical scheme in Figure 9. Since normal and shear stresses due to direct and indirect loads mainly concentrate next to discontinuities, the double density of the screws were installed in the borders of the overlay. As presented in Figure 10a, the 4 cm of the extreme of the screw core was inserted into the concrete to subsequently embed the remaining 4 cm and the head into the overlay. Then, 24 h after that, the concrete was poured and extended in a single day (without construction joints) using an asphalt paver (see Figure 10b) and then compacted with rollers. Joints were cut at a distance of 3.5 m. The total duration of building the test setup was two days. Table 6 presents the mix of the concrete overlay. The amount of water was adapted to improve the extension and compaction processes. For the mechanical characterization, six cylindrical specimens (\(\phi\)150 mm \(\times\) 150 mm) and six beams (150 mm \(\times\) 150 mm \(\times\) 600 mm) were cast. After 28 days,  
Figure 8: Original concrete pavement (**a**). The appearance of the cold-milled box (**b**). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Slab** & **Code** & **Screw Anchors** & **Pmax,PO (kN)** & **\(\sigma_{\text{max,PO}}\) (MPa)** \\ \hline \multirow{3}{*}{\(1.\)a} & SA-1 & & 4.0 & 0.51 \\  & SA-2 & YES & 4.0 & 0.51 \\  & SA-3 & & 4.5 & 0.57 \\ \hline \multirow{3}{*}{\(2\)} & NA-1 & & 1.0 & 0.13 \\  & NA-2 & NO & 2.5 & 0.32 \\  & NA-3 & & 3.0 & 0.38 \\ \hline \multirow{3}{*}{\(3\)} & NA-4 & NO & 4.5 & 0.57 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Pull-off test results. compression (f\({}_{\text{c}}\)) and flexural (f\({}_{\text{ct,fl}}\)) strength tests were carried out. Results of f\({}_{\text{c}}\) ranged from 55.45 MPa to 60.36 MPa while the values of f\({}_{\text{ct,fl}}\) were between 5.62 MPa and 6.41 MPa. Two days after construction, a visual survey was made to analyze the presence of pavement damage. Since the opening to heavy traffic was made after 7 days, potential damage would be associated with the very early age behavior of concrete. As depicted in Figure 11a-c, transversal and longitudinal cracks appeared in most of the slabs, which could be explained by excessive shrinkage. It must be stated that during concrete hardening, many of the leading causes for shrinkage could have come together, i.e., a very high rate of concrete evaporation due to both high ambient temperature (above 25 \({}^{\circ}\)C) and direct solar radiation, the use of rapid-hardening cement in the concrete mix, an insufficient curing and/or an excessive distance between joints. Nevertheless, after three years of construction, no significant damage was noticed, so it could be affirmed that the overlay with screws performed adequately in terms of structural behavior under heavy traffic.