15 March 2001
DISTRESS 41, ALLIGATOR OR FATIGUE CRACKING
3.1. Description. Alligator or fatigue cracking is a series of interconnecting cracks caused by
fatigue failure of the asphalt surface under repeated traffic loading. The cracking initiates at the
bottom of the asphalt surface (or stabilized base) where tensile stress and strain is 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 and form many-sided, sharp-angled pieces that
develop a pattern resembling chicken wire or the skin of an alligator. The pieces are less than
2 feet (0.6 meters) on the longest side. Deduct curves for alligator cracking are shown in
Alligator cracking occurs only in areas that are subjected to repeated traffic loadings, such as
wheel paths. Therefore, it would not occur over an entire area unless the entire area was
subjected to traffic loading. (Pattern-type cracking, which occurs over an entire area that is not
subject to loading, is rated as block cracking, which is not a load-associated distress.)
Alligator cracking is considered a major structural distress.
3.2. Severity Levels.
3.2.1. Low (L). Fine, longitudinal hairline cracks running parallel to each other with no or only a
few interconnecting cracks. The cracks are not spalled (Figures 3.2., 3.3., and 3.4.).
3.2.2. Medium (M). Further development of light alligator cracking into a pattern or network of
cracks that may be lightly spalled (Figures 3.5. and 3.6.).
3.2.3. High (H). Network or pattern cracking progressed so that pieces are well-defined and
spalled at the edges; some of the pieces rock under traffic (Figure 3.7.).