## 4 Experimental Validation  
### Pull-Off Behavior of Whitetopping Interfaces with Screw Anchors  
A real-scale whitetopping was constructed and pull-off tests [47; 48] were performed to study the installation of the screws and their influence on the mechanical behavior of an overlay interface. This test was selected because it is one of the most widely used methods to evaluate the bond between layers [37]. As Figure 4 shows, the whitetopping consisted of a 32.4 m\({}^{2}\) and 10 cm thick overlay on an asphalt concrete pavement in good condition and without slope. The whitetopping was formed by 4 slabs (slabs 1.a, 1.b, 2 and 3) of 1.80 m width, length between 2.60 and 6.00 m, and constructed without joints. A total of 110 screw anchors were installed on the interface of slabs 1.a and 1.b at a distance of 15 and 45 cm from the borders and spacings of 30 and 25 cm, respectively. As seen in Figure 5a, the installation of the screws was performed by drilling a hole 5 cm deep and then placing and tightening the screw with a manual impact screwdriver. The 4 cm of the extreme of the screw core was inserted into the asphalt pavement while the rest of the core and the head were embedded into the concrete overlay (Figure 5b). The installation time of each screw was less than 30 s. The process of drilling, placing, and tightening was done by the same operators who later constructed the overlay. About 15 min after placing the screw anchors in the interfaces of slabs 1.a and 1.b, the concrete layer of the whitetopping was constructed. For this, fixed formworks were placed, and subsequently, the surface of the asphalt was cleaned with water. After that, the concrete was poured onto the wet asphalt surface visually ensuring a similar moisture content in the areas where the pull-off test would be performed, i.e., the borders of slabs 1.a and 1.b and the entire surface of slabs 2 and 3 (Figure 6a). Figure 3: Mechanical behavior of a screw anchor under tensile (**a**) and shear (**b**) stresses. Weather conditions during the placement of the overlay were cold (temperature at about 10 \({}^{\circ}\)C) and wet ambient without rainfall episodes. The compaction and levelling of the concrete were done using a vibrating screed (Figure 6b). The presence of screws did not interfere with these tasks. Table 4 shows the mixes of the two concretes used to construct the whitetopping and their compressive strength at 24 h of age (measured according to [49]). A plain concrete without fibers was used for slabs 1.a and 3. For slabs 1.b and 2, the mix contained 3.0 kg/m\({}^{3}\) of 30 mm long polyolefin  
Figure 4: Appearance of the whitetopping. Figure 5: Screw anchor: installation (**a**) and appearance after installation (**b**). Figure 6: Construction of the whitetopping: concrete pouring (**a**) and compaction (**b**). fibers. Both mixes had a high content (500 kg/m\({}^{3}\)) of CEM I 52.5 R cement with rapid development of mechanical properties, that allowed reaching strengths of about 50 MPa in 24 h.  
The pull-off tests to measure the tensile strength of the interfaces were performed on 7 points after 135 days. As observed in Figure 7a, the tests were carried out on 100 mm diameter cores drilled to the bottom layer of asphalt. Threaded rods were attached to drilled holes in the top side of the cores using an epoxy resin. After that, the rods were pulled using an anchor tester which indicated the maximum tensile force performed during the test with a nominal precision of \(\pm\)2.5% (Figure 7b). Table 5 represents the results of the maximum forces (P\({}_{\text{max,PO}}\)) and tensile stresses (\(\sigma_{\text{max,PO}}\)) reached by performing pull-off tests after 7 days of overlay hardening. All the results of tests were accepted according to the statistical [PERSON]'s test for outliers [50]. The four cores without screws at the interface (slabs 2 and 3) failed through the interface between the concrete and the asphalt following the same failure mechanism.