Functions and Testing of Various pavement components/layers(Overview)

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Functions of various components of Pavement:

There are four pavement layers:

1.     Sub-grade

2.     Sub-base

3.     Base

4.     Surface course

Functions of each layer are described as below:

·         Sub-Grade :

·         Functions: The load is transferred by the sub-grade effectively to the earth mass. However the locally available earth is used to construct the sub-grade but it becomes necessary that the sub-grade should be of required strength.

·         Testing: This makes it necessary to check the strength characteristics of the sub-grade by using the various methods. Here is the list of the tests that are employed to check the strength of the sub-grade:

a.  California bearing ratio test.

b.  California resistance value test.

c.  Tri-axial shear test.

d.  Plate load test.

California bearing ratio test is a kind of penetration test, in which we get the value of the California bearing ratio of the given soil sample. The test can also be performed in the field to take the in-situ measurements. 

The value is used to design the thickness of the flexible pavements by using some empirical charts.This test is also used to calculate CBR value for the other layers too. 

California resistance value is got by using the Hveem stabilometer. This test is used in empirical methods of the flexible design methods, based on the soil strength.

Tri-axial shear test: This test gives the value of the shear strength of the soil, but it is not done, because the values obtained from this test are used only in a very few theoritical methods of flexible pavement design.

Plate Load Test:    In this test we make use of the plates of some larger diameter. In this test we get the values of the elastic modulus of sub-grade. These values are used in the methods of flexible pavement designs like that in Mcleaod method and in method of layer system approach for analysis by Burmister. 

The modulus of sub-grade reaction is also calculated for the rigid pavement analysis by the Westergaard's approach.

·         Sub-base and base course: 

Base course and sub-base course is used in the flexible pavement to disperse the upcoming loads to large area through a finite thickness, so as to increase the load bearing capacity of the pavement. The material used is the broken stones, or gravels, or bound or unbound aggregates. 

Sometimes the bricks can also be used as the sub-base and base materials. Generally the sub-base layers have the inferior quality of aggregates, than that used in the base course.  

Generally smaller aggregates are used because the larger gravels or aggregates have the tendency to sink to the lose sub-grade soil under the heavy loading.

The function of the sub-base or the base layer in case of the rigid pavement is to -

1.  Check the pumping action.

2.   Protecting the sub-surface against the frost action.

The basic purpose of the sub-base and base layer is to transfer the load through a dispersed/larger area to the sub-grade. So it must of sufficient strength to do its job. These layers are evaluated using a suitable strength or stability tests like the plate load test, California bearing ratio test, and the Hveem stabilometer test, where each test has its own significance and importance. 

Wearing course:   Top most layer serves as the smooth riding surface for the traffic, and it wears all the abrading forces. The top most layer is constructed with the superior quality of aggregates because it has to wear the maximum intensity of loads. 

·         It has to be water proof to stop the water penetration to the lower layers, so in case of flexible pavements the bituminous concrete is used and it is well compacted, and in rigid pavements the cement concrete is used and is well compacted so that no water penetration takes place. 

·         There is no direct test to check the stability of the surface course but the bituminous concrete used in the flexible pavement is checked for its suitability. Marshall stability test is used to determine the optimum content of the bitumen to be used in the aggregate mix, based on the stability density, VMA and VFB of the given grading of the aggregate mixture.

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