Materials are used for road construction
Road consist of basically 4 layers:
Sub-grade:
The sub-grade material should be clean and free from
organic matter and should be able to be compacted by roller, to form stable
sub-base. The material should have the following characteristics:
·
Well graded, uniformity coefficient (D60/D10)
should not be less than 3.
·
Fraction passing sieve #200 shall not be
greater than 2/3rd of the fraction passing sieve #40.
·
Should have a L.L not greater than 25%.
·
P.I not greater than 6
·
CBR should not be less than 25.
·
In coarse grain, aggregate retained by #10
sieve, %age of wear shall not be greater than 5%.
·
The maximum dia of any particle shall not
be greater than 2/3rd of the layer thickness of sub-base
Typical particle size distribution for the sub-base
(granular) which will the meet strength requirements are:
2.Sub-base
Course:
·
It is layer of granular material provided
above sub-grade generally natural gravel. It is usually not provided on
sub-grade of good quality.
The materials used may be either unbound granular, or
cement-bound. The quality of sub-base is very important for the useful life of
the road and can outlive the life of the surface, which can be scrapped off and
after checking that the sub-base is still in good condition, a new layer can be
applied. (Unbound granular materials are usually crushed stone, crushed slag or
concrete, or slate.)
3.
Base Course:
·
It is the layer immediately under the
wearing surface (Applied whether the wearing surface is bituminous or cement
concrete and whether its a thick or thin bituminous layer).
·
As base course lies close under the pavement
surface it is subjected to severe loading. The material in a base course must
be of extremely high quality and its construction must be done carefully. The
LA Abrasion test can determine the quality of the aggregate for this purpose.
Types
of Base Course
1.
Granular Base Course:
It is a mixture of soil particles ranging in size from
coarse to fine. Processing involves crushing oversized particles and screening
where it is necessary to secure the desired grading. The requirements of a
satisfactory soil aggregate surface are;
·
Stability
·
Resistance to abrasion
·
Resistance to penetration of water
·
Capillary properties to replace moisture
lost by surface evaporation upon the addition of wearing course requirement
change.
2.
Macadam Base:
Successive layers of crushed rock mechanically locked
by rolling and bonded by stone screening (rock duct, stone chips etc).
3.
In-water bound Macadam:
The crushed stones are laid, shaped and compacted and
then finer materials are added and washed into surface to provide a dense
material.
4.
Treated Bases:
Compose of mineral aggregate and additive to make them
strong or more resistant to moisture. Among the treating agents is bitumen.
4.
Surface Course:
The top layers of pavement which is in direct contact
with the wheel of the vehicle. Usually constructed of material in which bitumen
is used as binder materials.
a.
Bituminous Pavement: (Flexible Pavement)
Consists of combination of mineral aggregate with
bituminous binder ranging from inexpensive surface treatment ¼ in or less thick
to asphaltic concrete. For good service throughout the full life, bituminous
pavement must retain the following qualities:
·
Freedom from cracking or raveling.
·
Resistance to weather including the effect
of surface water heat and cold.
·
Resistance to internal moisture,
particularly to water vapors.
·
Tight impermeable surface or porous
surface (if either is needed for contained stability of underlying base or
subgrade).
·
Smooth riding and non skidding surface.
b.
Concrete Pavement (Rigid pavement):
Rigid pavement is the technical term for any road
surface made of concrete. Concrete roads are called rigid while asphalt-covered
roads are flexible. These terms refer to the amount of deformation created in
the road surface itself when in use and over time.)
There
are four types of concrete pavement:
·
Plain pavements with dowels that use
dowels to provide load transfer and prevent faulting,
·
Plain pavements without dowels, in which
aggregate interlock transfers loads across joints and prevents faulting,
·
Conventionally reinforced pavements that
contain steel reinforcement and use dowels in contraction joints, and
·
Continuously reinforced pavements that
have no contraction joints and are reinforced with continuous longitudinal
steel.
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