Thursday, May 8, 2014

Failures of Flexible Pavements

Hi there,

Failures of flexible pavements can occur due to the failure of one of the four layers. Failure of Sub-grade will result in the failure of the pavement, because it will get reflected in the top surface. 

Similarly failure of the sub-base or base layer will also result in the failure of the pavement. There are various causes of the failure of the flexible pavement, I will post another article for this. 

Here we will talk about the general failures that occur in the flexible pavements.

  1. Alligator(Map) cracking
  2. Shear Failure
  3. Frost Heaving
  4. Longitudinal cracking
  5. Consolidation failure
  6. Wearing of the Surface
  7. Reflection cracks

  • Alligator Cracking: 
Alligator or map cracking occurs on the surface of the flexible pavement due to the relative movement of the material or failure of the materials of the pavement layers. This may be caused by the repeated application of the heavy wheel loads resulting in fatigue failure due to the moisture variations resulting in swelling and shrinkage of sub-grade and other pavement materials. A localized weakness of the under-lying base course would also cause a cracking of the surface course in this pattern.
  • Shear Failure: 
Shear failure of the flexible pavement occurs due to the weakness of the pavement mixtures, the shearing resistance being low due to in-adequate stability of excessive heavy loading. The shear failure causes upheaval of pavement material by forming a fracture or cracking. 
  • Frost Heaving: 
Areas having cold climates, are prone to frost heaving. When the water present in the pores of the layers turns into ice, it causes swelling of the ice, and therefore results in the upheaval of the area affected by the frost. 

It must be remembered that the difference between the frost heaving and the shear failure is that in shear failure the depression is followed by the upheaval of the adjacent area but in case of the frost heaving, there is no depression.
  • Longitudinal Cracking: 
Longitudinal cracking may occur due the differential settlement of the pavement due to the differential volume change. The area of the pavement near to the pavement edge is more prone to moisture and therefore it may swell more as compare to the interior region of the pavement sub-grade. 
This will cause a differential volume change of the pavement and therefore may lead to the longitudinal cracking of the pavement. Generally, these longitudinal cracks traverse through the full pavement thickness.
  • Consolidation Failure: 
Consolidation of the subgrade due to the continuous action of the wheel load along the wheel path results in the formation of the ruts along the wheel path. 
  • Wearing of the Surface: 
Generally wearing of the surface may be caused due to the use of inferior material or due to the lack of the inter-locking of the surface layer with the bottom layers. 
Lack of the interlocking may be a result of the non-use of the prime and tack coat. Specifically in case of the overlays over the existing cement concrete pavements or the soil cement roads have poor inter-locking.
  • Reflection cracks: 
Reflection cracks are formed in the overlays laid over the existing cement concrete pavements. In such overlays, if any cracks are there in the existing cement concrete pavements, will get reflected in the surface layer also. These cracks are known as the reflection cracks.

Thanks for your kind visit!

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