# Laboratory Validation of Surface-Initiated Transverse Cracking of Asphalt Pavement
## 4 Potential Mechanism  
It is interesting to note that the specimen bottom is squeezed from outside toward the specimen middle in Figure 5d. By seeing Figure 5c,d together, it is reasonable to assume that when the wheel load moves to one side of the existing transverse cracking (the near side), the specimen on this side tends to bend downward under the wheel load, especially when the support is relatively soft. If without constraint, the other side of the specimen (the far side) should consequently be tilted upward. However, the bonding with the base layer and the self-weight of the specimen restrict the upward movement of the far-side specimen. Therefore, tensile stress at the surface of the specimen directly on top of the bottom crack or notch is created. At the same time, the bottom crack has the potential of being squeezed and pushed together. A schematic to explain the hypothesized causes of such a crack is shown in Figure 6.  
Figure 5: Specim #1 (1 inch thickness, 10 days aging, with a notch) with surface-initiated reflective cracking (SIRC), (**a**) specimen with propagated transverse crack; (**b**) transverse crack that initiates at the other side; (**c**) SIRC matches well with existing notch, and (**d**) specimen bottom.  
A repetition specimen (#2 in Table 1) uses the same conditions (specimen thickness of 1.0 inch, 10-day aging, with an existing joint and with a weak base) is tested and the specimen after 80,000 passes is shown in Figure 7. Apparently, the transverse crack initiates from the surface and the specimen bottom tends to squeeze to the middle. Similar to the first specimen shown in Figure 5, the repetition specimen has the same crack direction (perpendicular to wheel load) and location (at the specimen surface) and matches well with the existing notch. The repetition test proves that the appearance of surface-initiated transverse cracking does not happen by accident in the laboratory test; it should appear as long as the specific conditions are satisfied (weak base, long-term aged material, proper layer thickness, with existing joint). Again, recordable rut depth is observed for this specimen.  
Specimen #3 listed in Table 1 also showed surface-initiated reflective cracking. Specifically, the specimen is 1.5 inches in thickness, 10-day aged, with the existing notch at the specimen bottom and with a weak base placed below the sample. The testing result shows that it takes in total 180,000 passes before the surface transverse crack was observed. As seen in Figure 8, there is a transverse crack in the middle of the specimen under wheel load, and the specimen bottom severely squeezes to the middle. The test was continued for an additional 60,000 passes, but the crack was not further developed. A recordable rut depth is seen as well for this specimen.  
Figure 6: Mechanism sketch for SIRC.  
Figure 7: Repetition specimen #2 (1-inch thickness, 10 days aging, with a notch) with surface-initiated transverse cracking, (**a**) specimen top and (**b**) specimen bottom.  
Considering that the only difference between specimen #3 and specimens #1 and #2 is layer thickness, it can be concluded that the layer thickness is a critical factor for SIRC. It is possible that with the increase of layer thickness, the traffic load-induced bend on the specimen end decreases as well. This has been confirmed by the finite element method (FEM) analysis in another study [8] that showed that the magnitude of the Von Mises stress at the pavement surface decreases with the increase of HMA layer thickness. More traffic repetitions are required to induce the same stress level for thicker pavement compared with the thinner pavement. In addition, unlike the specimens that are shown in Figures 5 and 7, the crack in Figure 8 is within the wheel load, while no crack is found on the two sides; the reason for such a crack location needs to be further evaluated.