Flexible pavement consist of different layers such as,
Base course and
If any one of the above mentioned layers becomes unstable or weak then it will result in failure of flexible pavement. Therefore it is very important to design and construct each layer with utmost care.
Different types of failure encountered in flexible pavements are as follow.
Alligator cracking or Map cracking (Fatigue)
Consolidation of pavement layers (Rutting)
Shear failure cracking
Lack of binding to the lower course
Formation of waves and corrugation
1. Alligator or Map Cracking (Fatigue Cracking)
This is a common type of failure of flexible pavements. This is also known as fatigue failure.
Followings are the primary causes of this type of failure.
Relative movement of pavement layer material
Repeated application of heavy wheel loads
Swelling or shrinkage of subgrade or other layers due to moisture variation
Fig-1 shows a pavement with fatigue cracking.
2. Consolidation of Pavement Layers (Rutting)
Formation of ruts falls in this type of failure. A rut is a depression or groove worn into a road by the travel of wheels.
This type of failure is caused due to following reasons.
Repeated application of load along the same wheel path resulting longitudinal ruts.
Wearing of the surface course along the wheel path resulting shallow ruts.
Fig-2 shows a pavement with Rutting.
3. Shear Failure Cracking
Shear failure causes upheaval of pavement material by forming a fracture or cracking.
Followings are the primary causes of shear failure cracking.
Excessive wheel loading
Low shearing resistance of pavement mixture
Fig-3 shows shear failure cracking of pavement.
4. Longitudinal Cracking
This types of cracks extents to the full thickness of pavement.
The following are the primary causes of longitudinal cracking.
Differential volume changes in subgrade soil
Settlement of fill materials
Sliding of side slopes
Fig-4 shows a pavement with longitudinal cracking.
5. Frost Heaving
Frost heaving causes upheaval of localized portion of a pavement. The extent of frost heaving depends upon the ground water table and climatic condition.
Fig-5 shows a pavement with frost heaving.
6. Lack of Binding with Lower Layer (Potholes & Slippage)
When there is lack of binding between surface course and underlying layer, some portion of surface course looses up materials creating patches and potholes. Slippage cracking is one form of this type of failure.
Lack of prime coat or tack coat in between two layers is the primary reason behind this type of failure.
Fig-6 shows a pavement with potholes & Fig-7 shows a pavement with slippage cracking.
7. Reflection Cracking
This type of failure occurs, when bituminous surface course is laid over the existing cement concrete pavement with some cracks. This crack is reflected in the same pattern on bituminous surface.
Fig-8 shows a pavement with reflection cracking.
8. Formation of Waves & Corrugation
Transverse undulations appear at regular intervals due to the unstable surface course caused by stop-and-go traffic.
Fig-9 shows a pavement with corrugation.
Excess bituminous binder occurring on the pavement surface causes bleeding. Bleeding causes a shiny, glass-like, reflective surface that may be tacky to the touch. Usually found in the wheel paths.
Fig-10 shows a pavement with corrugation.
Seeping or ejection of water and fines from beneath the pavement through cracks is called pumping.