Abstract:
The pavements
can be classified based on the structural performance into two, flexible pavements
and rigid pavements. In flexible pavements, wheel loads are transferred by grain-to-grain
contact of the aggregate through the granular structure. The flexible pavement,
having less flexural strength, acts like a flexible sheet (e.g. bituminous road).
On the contrary, in rigid pavements, wheel loads are transferred to sub-grade soil
by flexural strength of the pavement and the pavement acts like a rigid plate (e.g.
cement concrete roads). In addition to these, composite pavements are also available.
A thin layer
of flexible pavement over rigid pavement is an ideal pavement with most desirable
characteristics. However, such pavements are rarely used in new construction because
of high cost and complex analysis required.
A highway pavement
is 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. The pavement structure should be able to provide a surface
of acceptable riding quality, adequate skid resistance, favorable light reflecting
characteristics, and low noise pollution. The ultimate aim is to ensure that the
transmitted stresses due to wheel load are sufficiently reduced, so that they will not exceed bearing
capacity of the sub-grade. Two types of pavements are generally recognized as
serving this purpose, namely flexible pavements and rigid pavements. This chapter
gives an overview of pavement types, layers, and their functions, and pavement failures.
Improper design of pavements leads to early failure of pavements affecting the riding
quality.
The pavements
can be classified based on the structural performance into two, flexible pavements
and rigid pavements. In flexible pavements, wheel loads are transferred by grain-to-grain
contact of the aggregate through the granular structure. The flexible pavement,
having less flexural strength, acts like a flexible sheet (e.g. bituminous road).
On the contrary, in rigid pavements, wheel loads are transferred to sub-grade soil
by flexural strength of the pavement and the pavement acts like a rigid plate (e.g.
cement concrete roads). In addition to these, composite pavements are also available.
A thin layer
of flexible pavement over rigid pavement is an ideal pavement with most desirable
characteristics. However, such pavements are rarely used in new construction because
of high cost and complex analysis required.
Types of Flexible
Pavements
The following
types of construction have been used in flexible pavement:
Conventional
layered flexible pavement,
Full
- depth asphalt pavement, and
Contained
rock asphalt mat (CRAM).
Conventional
flexible pavements are layered systems with high quality expensive materials are
placed in the top where stresses are high, and low quality cheap materials are placed
in lower layers.
Full - depth
asphalt pavements are constructed by placing bituminous layers directly on the soil
sub-grade. This is more suitable when there is high traffic and local materials
are not available.
Contained rock
asphalt mats are constructed by placing dense/open graded aggregate layers in between
two asphalt layers. Modified dense graded asphalt concrete is placed above the sub-grade
will significantly reduce the vertical compressive strain on soil sub-grade and
protect from surface water.
Flexible pavements
will transmit wheel load stresses to the lower layers by grain -to- grain transfer
through the points of contact in the granular structure. see Figure
Deflection on
flexible pavement
The wheel load
acting on the pavement will be distributed to a wider area, and the stress decreases
with the depth. Taking advantage of this stress distribution characteristic, flexible
pavements normally has many layers. Hence, the design of flexible pavement uses
the concept of layered system. Based on this, flexible pavement may be constructed
in a number of layers and the top layer has to be of best quality to sustain maximum
compressive stress, in addition to wear and tear. The lower layers will experience
lesser magnitude of stress and low quality material can be used. Flexible
pavements are constructed using bituminous materials.
These can be either in the
form of surface treatments (such as bituminous surface treatments generally found
on low volume roads) or, asphalt concrete surface courses (generally used on high
volume roads such as national highways). Flexible pavement layers reflect the deformation
of the lower layers on to the surface layer (e.g., if there is any undulation in
sub-grade then it will be transferred to the surface layer). In the case of flexible
pavement, the design is based on overall performance of flexible pavement, and the
stresses produced should be kept well below the allowable stresses of each pavement
layer.
Advantages
of flexible pavements include -
» Adaptability
to stage construction
» Availability
of low-cost types that can be easily built
» Ability to
be easily opened and patched
» Easy to repair
frost heave and settlement
» Resistance
to the formation of ice glaze
Disadvantages
include -
» Higher maintenance
costs
» Shorter life
span under heavy use
» Damage by
oils and certain chemicals
» Weak edges
that may require curbs or edge devices
(i) Curbs can
be used of concrete is probably the most common material used for both barrier and
mountable curbs. Concrete curbs are durable and relatively easy to construct.
(ii) Asphalt
is frequently used for curbing, especially where curved parking is lands must be
constructed. They are economical and easy to construct.
(iii) Granite
curbs are not as frequently used as asphalt or concrete unless granite is quarried
in the area. Granite curbs are far more durable than concrete.
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