Flexible pavement consists of different layers like,

sub grade
sub-base course
base course and
Surface Course

If any one of the above layers becomes unstable or weakened it will result in the failure of the flexible pavement. Therefore it is very important to design and manufacture each layer with utmost care.

The different types of failures encountered in flexible pavements are as follows.

Alligator cracking or map cracking (fatigue)
Rutting of pavement layers
shear failure cracking
longitudinal crack
frost heaving
Lack of binding to the lower course
reflection cracking
Wave formation and corridor
is bleeding
pumping

Table of Contents

1. Alligator or map cracking (fatigue cracking)

This is a common type of failure of flexible pavements. It is also known as fatigue failure.

Following are the primary reasons for this type of failure.

Movement relative to pavement layer material
repeated application of heavy wheel loads
Swelling or shrinking of the subgrade or other layers due to moisture variation

A sidewall with fatigue is visible in Fig-1.

2. Rutting of pavement layers

This type of failure leads to the formation of ruts. A rut is a depression or groove worn in a road by the travel of the wheels.

This type of failure occurs due to the following reasons.

Repeated application of load along the same wheel path results in longitudinal ruts,
as a result of wear of the surface course along the wheel path shallow rut,

Fig-2 shows a sidewalk with routing.

3. shear failure cracking

Shear failure results in the formation of a fracture or crack in the pavement material.

The following are the primary causes of shear failure breakdown.

excessive wheel loading
Low shear resistance of pavement mix

Figure-3 shows the shear failure of the pavement.

4. longitudinal cracking

These types of cracks extend over the entire thickness of the pavement.

The following are the primary causes of longitudinal cracks.

Inter-volume change in subgrade soils
filling material disposal
side slopes

Fig-4 shows a sidewall with longitudinal cracks.

5. frost heaving

Frost heaving causes upheavals in a localized portion of a pavement. The extent of frost heaving depends on the groundwater level and climatic conditions.

Fig-5 shows a sidewalk with frost protection.

6. Lack of bond with the bottom layer (pits and slippery)

When there is a lack of bond between the surface course and the underlying layer, part of the surface course loses the material forming patches and pits. Slippage cracking is one form of this type of failure.

Lack of prime coat or tackle coat between the two layers is the primary reason behind this type of failure.

Fig-6 shows a sidewalk with potholes and Fig-7 shows a sidewalk with slippery cracks.

7. reflection cracking

This type of failure occurs when a bituminous surface course is laid with few cracks on the existing cement concrete pavement. This crack is reflected in the same pattern on the bituminous surface.

Figure 8 shows a sidewalk with a reflecting crack.

8. making waves and corridors

Transverse fluctuations appear at regular intervals due to the unstable surface course due to stop-and-go traffic.

Fig-9 shows a sidewalk along the corridor.

9. is bleeding

Bleeding occurs from excess bituminous binder occurring on the surface of the pavement. The bleeding causes a shiny, glass-like, reflective surface that may be sticky to the touch. Commonly found in wheeled paths.

A sidewalk along the corridor is shown in Fig-10.

10. pumping

Water seeping or pumping out through cracks from under the pavement and is called fine pumping.

Figure 11 shows a sidewalk with pumping.

Reference

Highway Engineering by SK Khanna and CEG Justo