Unit:1 Road Pavement Note (Highway Engineering -II)

1.1) Definition, types, difference between flexible & rigid pavement.

► Definition:- A stable layer constructed over the natural soil can be defined as the road pavement. The main function of pavement is to support & distribute the wheel load of vehicle over a wide area of the underlying sub-grade soil & permitting the deformation within the permissible range.

# Pavement Layers

There are different types of pavement layers according to different practice which are shown below.

- Surface Wearing
Surface Binder Base
Base Base Road Base
Sub-Base Sub-Base Sub Base
Sub-Grade Sub-Grade Sub-Grade

Fig: British Practice Fig: American Practice Fig: Indian Practice

► Types of Pavements

i) Flexible Pavements ii) Rigid Pavements iii)Semi-Rigid pavements iv) Composite pavements

i) Flexible Pavements:-

The pavement which have negligible flexural strength in their structural action under the load is known as flexible pavement. The lower layer of flexible pavements reflects the deformations upto the surface of the layer. So if there is undulation in subgrade soil, flexible pavements also get undulated.

ii) Rigid Pavements:-

The pavement which have worthly flexural strength are known as rigid pavement. The rigid pavement are made of plain reinforced or pre-stressed concrete. Rigid pavement structure consists of a cement concrete slab, below which granular subbase or base course over the sub-grade soil.

iii) Semi-rigid pavement:-

When bounded materials like lean cement concrete or soil cement are used in the base course or sub-base course layer of the pavements they are called semi-rigid pavements.

iv.) Composite Pavements:-

It comprises of multiple, structurally significants layers of different composition

E.g:- Brick, Sand, Stone dust, aggregates etc.

# Typical cross-section of flexible & rigid pavements.

# Differences between Flexible & Rigid pavement structure

S.N Criteria Flexible Pavement Rigid Pavement

1 Design Accuracy Relatively less because these pavements are mainly designed by emperical method. More precise & accurate Structural analysis techniques can be used.

2 Life Well designed pavement has about10-20 years. About 40 years.

3 Repair & Maintenance Frequent repair & maintenance is needed. Practically little repair & maintenance is needed.

4 Initial Cost Low High

5 Stage Construction Possible in flexible pavement in which minimum initial thickness can be provided & later on additional overlays can be provided. Not possible

6 Surface Characteristics Relatively poor riding quality. Good riding Quality because free from rutting,potholes,&corrugations.

7 Penetration of water Easily penetrate through the layers due to their less imperviousness. Difficult due to high imperviousness.

8 Glare & night visibility Surface is black in color so no light reflects hence more street light are needed for night visibility. Grey in color so good glare & needed less street lights for night visibility.

9 Traffic dislocation during Construction Road can be opened quickly for traffic operation. Traffic can be only opened after 28 days.

10 Availability of materials Bitumen is a scarce material it has limited desposited in few countries. Cement can be manufactured easily by limestone in many countries.

11 Environmental Consideration It degrades none by burning bitumen. Less degrades.

12 Overall Economics Costly according as life cycle. Cheaper

13 Design philosophy Elastic layered theory. Plate theory.

S.N	Criteria	Flexible Pavement	Rigid Pavement
1	Design Accuracy	Relatively less because these pavements are mainly designed by emperical method.	More precise & accurate Structural analysis techniques can be used.
2	Life	Well designed pavement has about10-20 years.	About 40 years.
3	Repair & Maintenance	Frequent repair & maintenance is needed.	Practically little repair & maintenance is needed.
4	Initial Cost	Low	High
5	Stage Construction	Possible in flexible pavement in which minimum initial thickness can be provided & later on additional overlays can be provided.	Not possible
6	Surface Characteristics	Relatively poor riding quality.	Good riding Quality because free from rutting,potholes,&corrugations.
7	Penetration of water	Easily penetrate through the layers due to their less imperviousness.	Difficult due to high imperviousness.
8	Glare & night visibility	Surface is black in color so no light reflects hence more street light are needed for night visibility.	Grey in color so good glare & needed less street lights for night visibility.
9	Traffic dislocation during Construction	Road can be opened quickly for traffic operation.	Traffic can be only opened after 28 days.
10	Availability of materials	Bitumen is a scarce material it has limited desposited in few countries.	Cement can be manufactured easily by limestone in many countries.
11	Environmental Consideration	It degrades none by burning bitumen.	Less degrades.
12	Overall Economics	Costly according as life cycle.	Cheaper
13	Design philosophy	Elastic layered theory.	Plate theory.

1.2) Different layers in pavement structure & their function:

i) Sub-grade:-

# Characteristics:

*Strength *Drainage *Ease of compaction # Function:-

*It is the compacted natural earth & top of it is called formation level.

*It is layer of natural soil prepare to receive the other layer of the pavement.

*Loads on the pavements are ultimately support by the soil sub-grade & dispersed to the earth mass below.

ii) sub-base course:-

# Characteristics:-

* Weak * moisture sensitive insitu soil.

# Function:-

* Provide additional help is distributing the loads.

* Layer of selected granular soil, stabilized soils or gravels, boulders, broken stones, bricks are used to constructed these layers.

* Usually provided on fine grained soil.

* To improve drainage.

* To eliminate frost heave.

* To bear all load, stress transfer from surface course, Base course & sub-base course.

iii) Base course

# characteristics:-

* Strong * Free draining * less expensive. # Function:-

The base course which lies immediately next below is the medium, through which the stresses imposed are distributed uniformly to the underline layers.