A series of interconnected cracks caused by fatigue failure of the HMA surface under repeated traffic loading. As the number and magnitude of loads becomes too great, longitudinal cracks begin to form (usually in the wheelpaths). After repeated loading, these longitudinal cracks connect forming many-sided sharp-angled pieces that develop into a pattern resembling the back of an alligator or crocodile.
Alligator cracking near a parking lot drain. The cracking is likely due to water infiltrating pavement cracks and running down to the low point drain underneath the pavement, weakening and removing material from under the already-thin HMA layer. With little remaining support, the HMA will fail even under light loading.
Alligator cracking on a poorly supported road widening. The road was widened by about 2 feet, however the subgrade under the widened portion was not adequately compacted. Therefore, as it settled over time, the HMA layer settled with it and cracked.
Inadequate structural support for the given loading, which can be caused by a myriad of things. A few of the more common ones are:
- Decrease in pavement load supporting characteristics
- Probably the most common reason is a loss of base, subbase or subgrade support from things like poor drainage or spring thaw. Water under a pavement will generally cause the underlying materials to become weak.
- Stripping on the bottom of the HMA layer. The stripped depth contributes little to pavement strength so the effective HMA thickness decreases.
- Increase in loading (i.e., the pavement is being loaded more heavily than anticipated in design)
- Inadequate structural design (i.e., the pavement was designed too thin for the anticipated loads)
- Poor construction (i.e., inadequate compaction)
A fatigue cracked pavement should be investigated to determine the root cause of failure. Any investigation should involve digging a pit or coring the pavement to determine the pavement's structural makeup as well as determining whether or not subsurface moisture is a contributing factor. Once the characteristic alligator pattern is apparent, repair by crack sealing is generally ineffective. Fatigue crack repair generally falls into one of two categories:
- Small, localized fatigue cracking indicative of a loss of subgrade support. Remove the cracked pavement area then dig out and replace the area of poor subgrade and improve the drainage of that area if necessary. Patch over the repaired subgrade.
- Large fatigue cracked areas indicative of general structural failure. Place an HMA overlay over the entire pavement surface. This overlay must be strong enough structurally to carry the anticipated loading because the underlying fatigue cracked pavement most likely contributes little or no strength (Roberts et. al., 1996).