Repeat traffic loading eventually leads to the development of interconnected cracks known as fatigue cracking in pavement. These cracks are caused when fatigue failure impacts the HMA surface/stabilized base. Fatigue cracks originate as a minor issue but if repairs are not made in a timely fashion they can turn into larger and more costly problems.
The Development Of Cracks, Thin Vs. Thick Pavement
Cracks form differently depending on pavement thickness. If pavement is on the thin side, the cracks will start at the bottom and work up to the surface, commonly referred to as “bottom up.” Thicker pavement generally shows signs of cracks starting directly on the surface from weight bearing, tire-pavement interaction and binder aging—commonly referred to as “top down” cracking. After long periods of repeated overloading, you’ll notice longitudinal cracks begin to connect and create multi-faceted and sharp-angled patterns that resemble the patterns on an alligator’s back. That’s why it’s often referred to as alligator cracking.
Are All Fatigue Cracks Alligator Cracks?
Not all fatigue cracks are defined as alligator cracks. Fatigue cracking is the result of “cyclic stresses that are below the ultimate tensile stress.” If fatigue cracks go unfixed, the pavement surrounding the crack will start to deteriorate and turn into alligator cracks.
Three factors must be present in order for fatigue cracks to form:
- The surface is continually put under more weight than it was built to withstand, or is consistently used at peak loading capacity.
- Peak stress levels are high.
- The stress continues over a relatively long period of time.
Other factors that contribute to the development of fatigue cracks include:
- Stress concentration
- Residual stress
- Metallurgical structure
What Causes Fatigue Pavement Cracking?
Fatigue means exhaustion. We become exhausted after we repeatedly do something over and over again, even if it is something we are very proficient at. The same can be said for asphalt. Over time, it begins to break down and experience fatigue, which leads to the development of cracks. This is especially true when it is used at peak capacity on a regular basis.
The most severe fatigue pavement cracks result in structural failure, as the cracks allow water to get inside and cause further damage. Resulting in rough patches of pavement with the potential to form into larger potholes.
The most common causes include:
- A reduction in pavement load supporting characterizes because of:
- Loss of subbase or underlying support system, which can be caused by poor drainage or spring thaw. When moisture gets inside of the pavement, it freezes and thaws in accordance to temperatures. As the water expands and contracts it puts excess pressure on materials and eventually causes cracks to form.
- Stropping beneath the HMA layer.
- An increase in loads, or continually supporting larger loads than initially planned for.
- Improperly constructed
- Not the correct structural design
How To Repair Fatigue Pavement Cracking
Make repairs to fatigue crack pavements as soon as possible for the best results. Prior to repairs, it’s important to determine the cause of the issue. If repairs are made and the cause is not addressed, it will continue to happen, deeming repairs useless. To get to the bottom of the problem, professionals may dig a hole or core the pavement in order to determine its structural makeup and check for the presence of subsurface moisture.
Once fatigue pavement cracking has progressed to the point that it looks like an alligator’s back, it’s too late for crack sealant repairs. More drastic repair measures are necessary.
Generally speaking, there are two different types of fatigue crack repairs:
-Repairs used to fix small-localized fatigue cracking caused by decreasing subgrade support. These repairs include removing sections of cracked pavement, digging out the area and replacing the poor subgrade. Changes to drainage may be necessary to prevent the problem from happening again.
-Repairs used to fix large fatigue cracked areas caused by structural failure. This is more severe and requires placement of an HMA overlay on top of the entire pavement surface. The overlay must be able to support the anticipated loading weight. As the underlying fatigued pavement is not likely to contribute much weight bearing.