Fatigue Cracking in Asphalt Concrete

What does it look like?

Fatigue cracking can be recognized by its distinctive pattern of closely spaced and inter-connected cracks, especially in its more advanced form where the pattern of cracks had a chance to grow and develop into a clearly defined area:

Fatigue cracking in the right wheel path. Transversal cracking is also present, perpendicular to the direction of traffic. Sealed longitudinal cracking can be observed along the lane divider. Photo by Dragos Andrei, 2017.

In the photograph above, several types of cracking are present. Unlike longitudinal and transverse cracking, fatigue cracking is primarily caused by traffic loading. This is why we expect to find this type of distress in the wheel paths where the asphalt concrete is subject to a large number of load repetitions. In contrast, shoulders, the area between lanes or in the middle of the lanes are likely to see much less loading than the wheel paths.

In its incipient stage, fatigue cracking may appear as longitudinal cracking in the wheel paths, as shown in this photograph:

Fatigue cracking, low severity. Photo by Dragos Andrei, 2017.

With time, more fine cracks develop parallel to the direction of traffic and grow into a pattern that resembles the back of an alligator - hence the name "alligator cracking." You be the judge:

Alligator skin. Photo by Chirag Saini on Unsplash

High severity alligator cracking. Photo by Dragos Andrei, 2016.

In this last photograph alligator cracking extends to the entire road surface. This indicates an aged, oxidized surface, that has become too brittle with time.

Fatigue cracking normally starts in the wheel paths but may also appear in localized areas of soft subgrade, such as around utilities and patches where the soil was not compacted sufficiently or where water accumulates in the base and subgrade.

To learn more about identifying and measuring pavement distress refer to the Federal Highway Administration Pavement Distress Identification Manual.

Why is this a problem?

Alligator cracking is a serious issue and the most common repair method is to saw cut the entire area of cracking, remove the existing asphalt concrete, repair the base and subgrade if necessary, and patch with new asphalt concrete. Patching is a temporary solution and it will only address localized areas of distress. If alligator cracking occurs along the entire length of a pavement segment, either the pavement is past its design life or it was under-designed. In such cases, reconstruction may be the only option. This is also the most expensive option.

If damage were limited to the surface we could simply remove and replace the asphalt concrete on the surface, but alligator cracking usually causes further damage deeper into the pavement structure. Why? Watch this video to learn more:

After watching this video, I hope you recognize that alligator cracking is a serious issue.

How can we measure resistance to fatigue cracking?

We use a laboratory test, AASHTO T 321: Determining the Fatigue Life of Compacted Hot Mix Asphalt (HMA) Subjected to Repeated Flexural Bending.

To see a more detailed description of the test go to this page from pavementinteractive.org

To see a video of the equipment, go to this video from Controls Group