Flexible Pavements...

Flexible Pavements are constructed from bituminous or unbound material and the stress is transmitted to the sub-grade through the lateral distribution of the applied load with depth.

It transmits load to the subgrade through a combination of layers. Flexible pavement distributes load over a relatively smaller area of the subgrade beneath. The initial installation cost of a flexible pavement is quite low which is why this type of pavement is more commonly seen universally.Stress Distribution in Flexible Pavement

A valuable advantage of flexible pavement is that it can be opened for traffic within 24 hrs after completion. Also the repair and maintenance of flexible pavement is easy and cost effective.

Today 96% of all paved roads and streets in Pakistan are surfaced with asphalt. Almost all paving asphalt used today is obtained by processing crude oils. Man-made asphalt consists of compounds of hydrogen and carbon with minor proportions of nitrogen, sulfur and oxygen.

The different types of failures in flexible pavements are described below:

1)Subgrade failure.

2)Base failure or base course failure.

3)Surface failure or wearing course failure.

1)Subgrade Failure

Excessive deformation in subgrade soil of a flexible failure is called subgrade failure. This is one of the main causes of failure of flexible pavements. This type of failure causes undulations (ups and downs) and corrugations in the pavement surface.

Causes Of Subgrade Failure

=Inadequate stability.

=Inadequate road drainage.

=Excessive stress application

2) Base Course Failure

Excessive deformation in the base or foundation course of flexible pavement is known as base course failure. This type of failure causes potholes, waves, and corrugations on the pavement surface.

Causes Of Base Course Failure:

=Insufficient strength.

=Loss of binding action.

=Crushing of base course material.

=Lack of lateral confinement of the granular base course.

=Insufficient wearing course.

=Inadequate quality control.

=Inadequate road drainage

3. Surface Failure

Excessive deformation in the wearing course of flexible pavement is called surface failure or wearing course failure. This type of failure causes ruts, potholes, cracks, etc in the pavement structure.

Causes Of Surface Failure:

=lack of proper mix design

=Use of the inferior type of binder.

=Inadequate quality control.

=Volatization and oxidation of bind

Results Of Flexible Failure

1. Ruts

The longitudinal depressions or cuts in the flexible pavement are known as ruts. These are usually formed ion earth or W.B.M roads of one lane width due to repetitive traffic wheel loads on the same location, particularly under wheeled traffic.

2. Pot Holes

The isolated depressions, more or less circular in shape formed in flexible pavements are called potholes or patches.

These are usually formed in all types of flexible pavements due to the disintegration of road metal or lack of binding surface course with the underlying base.

3. Frost Heaving

A localized heaving up of pavement portion is known as frost heaving. The water which may find access to pavement structures freezes in cold climates.

The expansion accompanying this frost action causes upheaval which in turn may crack the pavement surface.

4. Shear Failure Cracking

The formation of a fracture or cracking due to upheaval of pavement portion followed by a depression called shear failure cracking. This type of failure occurs due to localized weakness in the pavement.

5. Longitudinal Cracking

The formation of cracks in the longitudinal direction of road pavement is called longitudinal cracking.

This failure is caused due to frost action, different volume changes in subgrade, settlement of filling material, or due to sliding of side slopes.

Shear Failure Cracking

The formation of a fracture or cracking due to upheaval of pavement portion followed by a depression called shear failure cracking. This type of failure occurs due to localized weakness in the pavement.

6. Map Cracking

The development of irregular cracks, usually formed on bituminous surfacing is called map cracking.

This type of flexible road failure is due to excessive wear of the road surface or localized weakness in the underlying base course.

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