Alligator (fatigue) cracking appears as a series of interconnecting cracks caused by fatigue failure of the AC surface. The fatigue failure is most often the result of repeated traffic loading. The cracking initiates at the bottom of the AC surface (or stabilized base) where tensile stress and strain are highest under a wheel load. The cracks propagate to the surface initially as a series of parallel cracks. After repeated traffic loading, the cracks connect, forming many-sided, sharp-angled pieces that develop a pattern resembling the skin of an alligator. The pieces are less than 2 ft (0.6 m) in length on the longest side. Alligator cracking occurs only in areas that are subjected to repeat traffic loadings, such as wheel paths, and is considered a major structural distress. Alligator cracking is measured in area (square feet or square meters).
|Low||Fine, longitudinal hairline cracks running parallel to one another with none or only a few interconnecting cracks. The cracks are not spalled.|
|Medium||Further development of light alligator cracking into a pattern or network of cracks that may be lightly spalled. Medium-severity alligator cracking is defined by a well-defined pattern of interconnecting cracks, where all pieces are securely held in place (good aggregate interlock between pieces).|
|High||Network or pattern cracking has progressed so that the pieces are well defined and spalled at the edges. Some of the pieces rock under traffic and may cause FOD potential.|