Pavement Definition | Types of Pavement in Road

Pavement Definition:

A structure consisting of superimposed layers of processed materials above the natural soil sub-grade, whose primary function is to distribute the applied vehicle loads to the sub-grade.

It is always desirable to construct the pavement in which the subgrade is always dry even during monsoons.

Types of Pavement Structures in Road:

(1) Flexible pavements                 (2) Rigid pavements

(1) Flexible pavements:

The flexible pavement may be constructed in a number of layers and the top layer has to be the strongest as the highest compressive stresses are to be sustained by this layer in addition to the wear and tear due to the traffic. Flexible pavements are commonly designed using empirical design charts or equations taking into account some of the design factors. There are also semi-empirical and theoretical design methods.

TYPICAL LAYERS OF A FLEXIBLE PAVEMENT:

Seal coat is a thin surface treatment used to water-proof the surface and to provide skid resistance.

Tack coat is a very light application of asphalt emulsion diluted with water. And It provides bonding between two layers of binder course.

Prime coat is an application of low viscous cutback Bitumen to an absorbent surface like granular bases om ‘Which binder layer is placed and provides bonding between two layers.

Surface Course is the layer directly in contact with traffic loads and is constructed with dense graded asphalt concrete.

Binder Course purpose is to distribute the load to the base course. binder course requires lesser quality of mix as compared to the course above it.

Base Course provides additional load distribution and contributes to the sub-surface drainage.

Sub-Base Course the primary functions are to provide structural support, improve drainage, and reduce the intrusion of fines from the sub-grade in the pavement structure.

Sub-grade the top soil or sub-grade is a layer of natural soil prepared to receive the stresses from the layers above.

Different Type of Failures of Flexible Pavements:

Different types of failure encountered in flexible pavements are as follow.
1. Alligator cracking or Map cracking (Fatigue)
2. Consolidation of pavement layers (Rutting)
3. Shear failure cracking
4. Longitudinal cracking
5. Frost heaving
6. Lack of binding to the lower course
7. Reflection cracking
8. Formation of waves and corrugation
9. Bleeding
10. Pumping

1. Alligator cracking or Map cracking (Fatigue)

following are the primary cause of this type of failure.

• Relative moment of pavement layer material
• Repeated application of heavy loads
• swelling for shrinkage of subgrade or other layers due to moisture variation

2. Consolidation of pavement layers (Rutting)

• Formation of ruts falls in the type of failure.
• A rut is a depression or groove worn into the road by the travel of wheels

this type of failure is caused due to the following reasons

• repeated application of load along the same wheel path resulting in longitudinal ruts.
• wearing the surface course along the wheel Path result in shallow ruts.

3. Shear failure cracking

shear failure causes upheaval of payment material by forming a fracture or cracking.

Following are the primary causes of shear failure cracking.

• Excessive wheel loading
• low shearing resistance of pavement mixture.

4. Longitude Cracking:

This type of crack extends to the full thickness of the pavement.

Following are the primary causes for longitudinal cracking

• differential volume changes in subgrade soil
• settlement of fill materials
• sliding of side slopes

5. Frost heaving

Frost heaving courses upheaval of a localized portion of the pavement. the extent of Frost heave depends upon the ground water table and climate condition.

6. Lack of binding to the lower course

• when there is a lack of binding between the surface course and the underlying layer, some portion of surface course loses up materials creating patches and potholes.
• slippage cracking is one of the forms of this type of failure.
• lake of primary coat or tack coat in between two-layer is the primary reason behind this type of failure.

7. Reflection cracking

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

8. Formation of waves and corrugation

• transfers undulations appear at regular intervals due to the unstable surface course caused by stop-and-go traffic.

9. Bleeding

• Excess bituminous bonder occurring on the p pavement surface courses bleeding.
• bleeding courses a shiny, glass-like, reflective surface that may be tacky to the touch.
• usually found in the wheel paths.

10. Pumping

• Seeping or ejection of water and fines from beneath the pavement through cracks is called pumping.

 

(2) Rigid pavements:

Rigid pavements are those which possess worthy flexural strength. The rigid pavement transmits the wheel load stresses through a wider area below the slab action. The rigid pavements are made of portland cement concrete. The plain cement concrete slabs are expected to take up about 40 kg/cm2 flexural stress. Joints are also used in the construction of rigid pavement and have high completion costs but low maintenance costs.

A Typical Rigid Pavement Consists of Three layers or Components:

1. Soil subgrade
2. granular sub-base course
3. Base course
4. Cement concrete slab as wearing surface.

Where Rigid Pavement Needed:

Rigid pavements are usually provided under the circumstances.

• very heavy rainfall
• poor soil conditions
• poor drainage
• extreme climatic conditions
• combination of some of these conditions may lead to the development of cracks on the pavement.

Factors Affecting Rigid Pavement:

The factors which affect the design and performance of rigid pavement or cc pavement are listed below

• wheel load
• Temperature variations at the location of the road
• types of joints and their spacing
• sub-grade and other supporting layers

Comparison of Flexible and Rigid Pavements:

• The stresses are not transferred from grain to grain to the lowest layers as in the case of flexible pavement layers. The rigid pavements are made of portland cement concrete – either plain, reinforced, or prestressed concrete.
• The main point of difference in the structural behavior of rigid pavements as compared to the flexible pavement is that the critical condition of stress in the rigid pavement is the maximum flexural stress occurring in the slab due to wheel load and the temperature changes, whereas in the flexible pavement it is the distribution of compressive stresses.
• The rigid pavements are usually designed and the stresses are analyzed using the elastic theory, assuming the pavement as an elastic plate resting over an elastic or a viscous foundation.