# Laboratory Validation of Surface-Initiated Transverse Cracking of Asphalt Pavement
## 2 Methodology
### Design of Test Condition  
Table 2 presents a summary of all the tested specimens and test conditions. The conditions through which surface-initiated transverse cracking is observed are also pointed out. All the experiments were carried out by using the _James Cox & Sons_ HWDT equipment. There are in total of 12 specimens tested. The reasons to include these conditions are:  
* _Layer thickness_: Layer thickness has been proved to affect top-down longitudinal cracking significantly [22]. As seen elsewhere [22], pavement thickness could be a critical factor that correlates to surface-initiated transverse cracking. Therefore, it is expected that the initiation of cracks can be observed at different wheel load repetitions by using varied specimen thicknesses. Therefore, HWT samples with different thicknesses are used, consisting of 1 inch, 1.5 inches, and 2.4 inches. * _Aging condition_: Specimens with different degrees of aging (5-day aging, 10-day aging, and 15-day aging) are applied. Five-day aging at 85 \({}^{\circ}\)C is considered to be equivalent to long-term aging in the field pavement in accordance with AASHTO R30 [23]. Ten-day and 15-day agings are used to obtain asphalt mixture with even higher stiffness (more brittleness), which is critical for surface-initiated cracks, as suggested elsewhere [22]. * _With or without existing notch_: Some specimens have joints in the bottom layer, and this is simulated using a notch or saw cut (0.33 inch depth and 0.25 inch width). Both specimens with and without existing notches are used to evaluate the effects of existing notches on the property of surface-initiated transverse cracking. * _Weak base or strong base_: For pavement under traffic load, the modulus of the base can greatly affect the stress magnitude at the pavement surface. Von Mises stress is the maximum octahedral shear stress; it takes into account the multiaxial stress state, including the shear when protentional failure location is near the pavement surface [24]. It was found that the higher the base modulus, the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Design Parameter** & **Mix Design \#1** & **Mix Design \#2** \\ \hline Asphalt PG & 58–28 & 64–22 \\ NMAS, in & 0.38 & 0.38 \\ Asphalt content, \% & 5.9 & 5.8 \\ Gyration number & 75 & 75 \\ \% voids @ N\({}_{\text{max}}\) & 4.0 & 4.0 \\ G\({}_{\text{mb}}\) & 2.449 & 2.462 \\ G\({}_{\text{mm}}\) & 2.551 & 2.527 \\ Aggregate G\({}_{\text{sb}}\) & 2.78 & 2.74 \\ Design ESALs, million & 0.3 to \textless{}3 & 0.3 to \textless{}3 \\ \hline \hline \end{tabular} Note: NMAS indicates nominal maximum aggregate size; ESAL indicates equivalent single axle load.  
\end{table} 
Table 1: Mix design information. smaller the Von Mises stress value at the pavement surface. In contrast, the lower the base modulus, the higher the Von Mises stress value at the pavement surface [8]. The effect of base stiffness on pavement response has also been confirmed by other researchers as well [17; 25]. Specifically, the sensitivity analysis of flexible pavement performance prediction to Mechanistic-Empirical Pavement Design Guide (MEPDG) design inputs shows that the top-down longitudinal cracking is very sensitive to base resilient modulus [25], a greater amount of longitudinal cracking is predicted if the lower base modulus is used. It is also pointed out that pavement with a lower base modulus is more prone to initiate top-down fatigue cracking [17]. Therefore, with a flexible base, it is expected that surface-initiated reflective cracking (SIRC) is easier to be simulated in the laboratory. SIRC refers to such cracks that initiate at the surface of the pavement layer in the transverse direction and propagate downward to match the existing transverse cracking, and there is no cracking in the intralayer. In this paper, rubber with an approximate stiffness range of 400-700 MN/m was placed under test specimen to simulate a weak base, as shown in Figure 2a, and the HWTD steel frame was used as a strong base instead of a rubber pad, as shown in Figure 2b.