# Laboratory Validation of Surface-Initiated Transverse Cracking of Asphalt Pavement
## 7 Conclusions  
Surface-initiated transverse cracking has been continuously reported but has not been validated by laboratory tests. By using Hamburg equipment and gyratory compacted specimens, this paper evaluates the key factors that are correlated to surface-initiated transverse cracking. The conclusions of this paper are:  
1. The specimen thickness, base stiffness, material stiffness (aging effect), existing notch, and binder source are critical factors for surface-initiated transverse cracking. Specifically, laboratory tests indicate that such a crack type is seen easier for the softer base, stiffer asphalt mixture, with the existing transverse notch.
2. In terms of pavement thickness, although some studies in the literature indicate that thicker pavement may crack easier due to shear failure, laboratory tests indicate that specimens with thinner thickness (1 inch) takes less wheel load passes before the crack was observed. Two reasons could be used to explain this difference: (a) a thin specimen cannot be directly equal to field thin HMA layers, the scale-down factor needs to be considered in laboratory tests, and (b) sufficient thickness is important to shift the critical distress position to the surface of the pavement and lead to surface-initiated cracking. However, there may exist a threshold value, and if the value above the threshold, the pavement with thinner thickness is more prone to develop SIRC and cracks may appear with fewer traffic repetitions that need a further demonstration.
3. Surface-initiated transverse cracking can be induced under traffic load. Although the thermal load gradient (cooling cycles) could accelerate the development of the surface-initiated transverse cracking, the laboratory evaluation proved that such a crack type can be caused by repeated traffic load at intermediate temperature.
4. The possible mechanism that induces the surface-initiated transverse cracking is that with the existence of a transverse joint at the pavement bottom, it is possible that when the traffic load tracks back and forth in the middle of the road, a bending is induced in an upward direction near the load area. Therefore, tensile stress is caused on the pavement surface and tears the pavement apart under repeated wheel loads. Compression is caused on the pavement bottom surface and  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Binder PG**} & \multirow{2}{*}{**Sample \#**} & \multirow{2}{*}{**Observed Crack**} & \multicolumn{2}{c}{**Cycle at Crack**} & \multicolumn{2}{c}{**Rut Depth**} & \multirow{2}{*}{**Total Test**} & \multirow{2}{*}{**Final Rut**} \\  & & & **Crack** & & & **Initiation**, & & **Cycle** \\ \hline
64-22 & 1 & Yes & 40,000 & 5.0 & 70,000 & 7.0 \\
64-22 & 2 & Yes & 40,000 & 5.5 & 80,000 & 7.0 \\
64-22 & 3 & Yes & 180,000 & 2.0 & 240,000 & 3.0 \\
64-22 & 4 & No & N/A & N  
leads to the asphalt mixture squeezing together, which has been validated by the laboratory tests. Such bending may only occur when the base structure is relatively weak. If a strong base is applied, no bending is expected, and surface-initiated transverse cracking is not seen anymore. When the tensile stress is higher than the strength of the material, the crack initiates. The asphalt material with higher stiffness or brittleness may crack easier than soft material, since soft asphalt mixture can relax stress faster.