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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.
Thanks for visit!
