A typical flexible pavement consist of following 4 layers
- Soil subgrade
- Sub-base course
- Base course
- Surface or wearing course
If due to any reason, any of the above mentioned layers loses its stability, it will lead to failure of the whole pavement. There are many types of failures of flexible pavement, such as formation of pot holes, ruts, cracks, localized depression etc. when any one form of failure found on the surface, then it is an indication of failure of pavement. Therefore it is essential that each layer of the pavement should be carefully designed and constructed to maintain its stability.
What Causes Failure of Flexible Pavement?
The 3 common causes of failure of flexible pavement are as follow
- Failure of subgrade
- Failure of sub-base or base course
- Failure of surface or wearing course
1. Failure of Subgrade
This is the main cause of flexible pavement failure. When there is excessive deformation in subgrade soil, it will result in failure of whole pavement. The failure of subgrade soil can be detected by the following forms of defects causing unevenness of pavement surface.
- Excessive undulation & corrugation on surface
- Depression followed by heaving at surface
- Lateral shoving of pavement near the edge along the wheel path
The two primary reasons of failure of subgrade soil are
- Inadequate stability
- Excessive stress application
- Inadequate stability: Stability is the resistance to deformation under stress. When soil used for construction of subgrade is of inferior quality, it will not be able to resist the load coming from wheel, and ultimately it will fail.
Another reason causing loss of stability of subgrade soil is improper compaction of soil during construction. Presence of excessive moisture at subgrade level without proper drainage control also affects the stability of subgrade.
- Excessive stress application: Thickness of the pavement should be so designed, that it can distribute the wheel load properly. If pavement thickness becomes less than that of the required value, then it will result in failure of subgrade. Also if the wheel load applied on pavement is in excess of design value, it will result in failure of subgrade.
2. Failure of Sub-base or Base Course
There are 5 primary reasons behind failure of sub-base or base course as described below.
- Inadequate stability or strength: Role of a sub-base or base course is to transform the wheel load from surface course or wearing course to the subgrade. Therefore the strength of the sub-base or base course is always higher than that of subgrade. Strength of the sub-base or base course can be achieved by taking following measures.
- Using good quality of aggregate
- Proper mix design
- Providing sufficient thickness
- Proper quality control
If there is any deviation occurs in any of the above mentioned factors, it will lead to failure of pavement.
- Loss of binding action: When wheel load is repeatedly applied on road surface, it causes internal movement of particles in the sub-base or base course. This results in relative movement between surface course and sub-base or base course. In other word, instead of acting as whole, different layer acts separately. This is the cause of alligator or map cracking on bituminous surface.
Therefore a layer of tack coat or prime coat is applied on top of the base course before placing surface course. This creates a better bonding of these two layers.
- Loss of base course materials: When there is no wearing course or surface course on the base course, or if the wearing course has completely worn out, that will result in loss of base course material. This happens because of suction caused by the tyre and exposed base course materials. Also loss of stone aggregate creates pot holes on surface course.
- Inadequate wearing course: If the thickness of wearing course is less, then water will find its way to the base course causing damage to it.
Therefore it is essential to consider type, intensity and volume of traffic before deciding thickness of wearing course.
- Use of inferior material: The materials to be used for construction of base course should be so chosen in a manner so that it can resist the wheel load and weathering actions. Inferior quality of material should not be used.
3. Failure of Wearing Course
Wearing course or surface course is the layer having more strength than all the other pavement layers. This is because the wheel load is directly applied on this layer. Along with the vertical load, it has also to resist the abrading effect of wheel and weathering effect of climate.
Therefore design and construction of wearing course should be done properly. A pervious layer of wearing course can damage all the underlying layers. The following measure should be employed during design and construction of wearing courses
- Proper mix design
- Sufficient thickness
- Good quality of binder
- Proper amount of binder
- Good quality aggregate
High degree of quality control should be employed during construction of wearing course.
Oxidation or aging of binder, also make the bituminous surface brittle and creates cracks on pavement surface. This results in ingress of moisture to underlying layers and weakening of the layers.