PAVEMENT
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The Pavement is the structure which
separates the tires of vehicles from
the underlying foundation material.
The underlying foundation material is
generally the soil, but it may be
structural concrete or a steel bridge
deck.
Types of Pavements
There are two types of the Pavement:
1. Flexible Pavement
A layered system pavement which has low flexural
strength.
2. Rigid Pavement
It has a slab action and possess High flexural strength.
3. Composite Pavement
A pavement composed of Both HMA and PCC. The use
of PCC as a bottom layer and HMA as a top layer.
If the pavement itself is very strong, but it is constructed on
loose and poor subgrade, it can fail.
FUNCTIONS OF PAVEMENT
 The most important function of the pavement is to withstand
the load applied from a vehicle such as a truck or an aircraft,
without deforming excessively.
 The layered structure of the pavement is meant for ensuring
that the load is spread out below the tire, such that the
resultant stress at the bottom layer of the pavement, the
subgrade, is low enough not to cause damage.
 When the existing soil is not stiff enough to support the
relatively small stress, then there is a need to improve the
soil.
FUNCTIONS OF PAVEMENT
 There is also a need to improve the soil if it is susceptible to
moisture. Such a problem can be solved by treating the soil
with an additive, such as lime and a Portland cement.
 Since pavements are exposed to the environment, a very
important factor in the design of pavements is the
consideration of water, which could be coming from rain/snow
(surface water) and/or from the ground (ground/subsurface
water).
 Since water can be detrimental to a pavement, a basic
necessity of designing a proper pavement is to provide
adequate drainage for both surface and subsurface water.
FUNCTIONS OF PAVEMENT
 Standing water on a pavement can cause
hydroplaning, skidding, and accidents. There is a
need to make sure that water from precipitation is
drained away quickly and effectively.
 Water present in frost-susceptible soils in the
subgrade can freeze, causing heaving and failure of
the pavement. Therefore, frost-susceptible
materials should be avoided.
 Generally the layers in a pavement improve in
quality as one goes up from the bottom to the
surface layer.
 The surface layer, which can be asphalt or concrete, is the
most expensive and stiff/durable layer in the entire pavement
structure. Components of this layer are mostly naturally
occurring materials.
 Generally several layers are present in an asphalt pavement.
From the bottom up, the layers are known as the subgrade,
subbase, base, and binder and/or surface.
 While designing, adequate thickness to each layer is assigned,
so as to obtain the desirable properties in the most costeffective way.
 Concrete pavements may not have as many layers, and in many
cases the concrete slab rests on a stabilized subgrade, which
consists of soils modified with some additives.
IMPORTANT ISSUES
Traffic keeps on increasing, while the costs of materials and
methods keep on climbing, as budgets dwindle everywhere. The
only way to keep up and still have good roads is through learning
and applying good principles, implementing proven new concepts
and technologies, and continually researching for better
materials and methods.
Issues can be summarized as follows:
1.
Drainage is needed to drain water away from the pavement.
2. The materials must be evaluated and selected properly so that
they can withstand the effects of the traffic and the
environment.
IMPORTANT ISSUES
3. The mix must be designed properly such that it can withstand
traffic and environmental factors.
4. The structure should be designed properly such that it has
adequate thickness to resist excessive deformation under
traffic and under different environmental conditions.
5. The pavement must be constructed properly such that it has
desirable qualities.
6. The pavement must be maintained/managed properly through
periodic work, regular testing, and timely rehabilitation.
7. Generation of knowledge through research is critical for
ensuring good pavements in the future.
Conventional Flexible Pavements
Following figure shows the cross section of a conventional flexible
pavement . Starting from the top, the pavement consists of seal
coat, surface course, tack coat, binder course, prime coat, base
course, subbase course, compacted subgrade, and natural sub grade.
The use of the various courses is based on either necessity or
economy, and some of the courses may be omitted .
Conventional Flexible Pavements
Seal coat is a thin asphalt surface treatment used to waterproof the
surface or to provide skid resistance where the aggregates in the surface
course could be polished by traffic and become slippery .
The surface course is the top course of an asphalt pavement, sometimes
called the wearing course. It is usually constructed of dense graded HMA .
It must be tough to resist distortion under traffic and provide a smooth
and skid-resistant riding surface .
The binder course, sometimes called the asphalt base course, is the
asphalt layer below the surface course.
There are two reasons that a binder course:
First, the HMA is too thick to be compacted in one layer,
Second, the binder course generally consists
of larger aggregates and less asphalt, so replacing a part of the surface
course by the binder course results in a more economical design.
Conventional Flexible Pavements
A tack coat is a very light application of asphalt, usually asphalt
emulsion diluted with water, used to ensure a bond between the
surface being paved and the overlying course.
Tack coats are also used to bond the asphalt layer to a PCC base or
an old asphalt pavement .
A prime coat is an application of low-viscosity cutback asphalt to an
absorbent surface, such as an untreated granular base on which an
asphalt layer will be placed .
The difference between a tack coat and a prime coat is that a tack
coat does not require the penetration of asphalt into the underlying
layer, whereas a prime coat penetrates into the underlying layer,
Conventional Flexible Pavements
The base course is the layer of material immediately beneath the
surface or binder course . It can be composed of crushed stone,
crushed slag, or other untreated or stabilized materials .
The subbase course is the layer of material beneath the base
course . The reason that two different granular materials are used
is for economy. Instead of using the more expensive base course
material for the entire layer, local and cheaper materials can be
used as a subbase course on top of the subgrade .
Subgrade The top 6 in . (152 mm) of subgrade should be scarified
and compacted to the desirable density near the optimum moisture
content . This compacted subgrade may be the in-situ soil or a
layer of selected material .
Conventional Flexible Pavements
Full-Depth Asphalt Pavements are constructed by placing one or
more layers of HMA directly on the subgrade or improved
subgrade.
The asphalt base course in the full-depth construction is the
same as the binder course in conventional pavement.
As with conventional pavement, a tack coat must be applied
between two asphalt layers to bind them together .
Characteristics of Pavement
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It should be sound enough to support the
wheel loads imposed on it from traffic
moving over it.
It should be thick enough to distribute the
external loads and stresses on the earthen
subgrade, so that it can safely bear it.
It should provide comfort to the road user
at high speeds.
Characteristics of Pavement (Contd.)
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The surface of the pavement should have
adequate roughness to prevent skidding of
vehicles.
The surface should be impervious.
The Pavement should have long life and the cost of
maintaining it annually should be low.
It should provide a hard wearing surface, so that
abrading action of wheels does not damage it.
Pavement Thickness Design
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It is the determination of required
thickness of various pavement layers to
protect given soil condition for a given
wheel load.
Given wheel load
150 Psi
Asphalt concrete thickness ?
Base Course Thickness ?
Sub base Course Thickness ?
Given In Situ Soil Condition
3 Psi
Design Process
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Estimating the amount of traffic and the number
of equivalent standard axles that will use the road
over the selected design life.
Assessing the strength of subgrade soil over
which the road is to be built.
Selecting the most economical combination of
pavement materials and layers thickness that will
provide service over the design life of the
pavement.
Factors Affecting Pavement Design
A.
Design Life
The period of time for which the initially
designed pavement structure will last
before any rehabilitation is needed.
The design life can be dependent on the
type and level of maintenance provided
over the design life.
For Flexible Pavement it is normally ranges
(10 -40) years
Factors Affecting Pavement Design (Contd.)
Functional Performance Curve
Perfect
Ride Quality
Rehabilitation
Unacceptable Limit
Traffic / Age
Rehabilitation
Capacity
Structural
Structural Performance Curve
Perfect
Traffic / Age
Structural Failure
Factors Affecting Pavement Design (Contd.)
B.
Reliability
It is the level of assurance that the designed
pavement will serve satisfactorily during the
entire design life. As per AASHTO the
recommended levels are:
National Highways
State Highways
Major District Road
Other District Roads
Urban
Rural
85 – 99.5
80-99
80-95
50-80
80-99.9
75-95
75-95
50-80
Factors Affecting Pavement Design (Contd.)
C.
Traffic Factors
 Wheel Load, Causes stresses and strain in the pavement
layers and subgrade, the tyre pressure
determines the area of application
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Repetition of Wheel Loads,
The cumulative load application during the
design life causes elastic and plastic deformations
Considering the growth rate and design life, the
traffic in terms of number of commercial vehicles per
day, can be estimated as follows:
A = P (1+r)n+L
Where, A = number of commercial vehicles per day for
design
P = Number of vehicles per day at last count
r = Annual growth rate
n = Number of years b/w the last count and the
year
of completion of construction
L = Design life (in years)
Factors Affecting Pavement Design (Contd.)
D.
Climatic Factors
 Temperature magnitude and its variation affects
the pavement materials.
 At high temp. bitumen looses its stability and at
low temp. it becomes very hard and stiff.
 The subgrade can show loss in strength in
frequent occurrence of freeze – thaw cycles.
 Subgrade soils and other paving materials weaken
appreciably when saturated. Even certain soils may
show some volume change.
Factors Affecting Pavement Design
Contd.)
E.
Subgrade Strength and Drainage
 Subgrade soil type and compacted density
significantly affect pavement design
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Surface and subsurface drainage of pavements and
from adjoining land also affects subgrade strength.
AASHTO pavement design method uses drainage
coefficients, based on the quality of drainage.
Factors Affecting Pavement Design
(Contd.)
F.
Material Properties
Proper design of pavement requires a thorough study of
properties of materials used in subgrade and other layers of
pavement.
Generally the strength of subgrade soil and of sub-base / base
courses is determined in terms of CBR.
AASHTO uses Resilient Modulus (MR).
If Soaked CBR <10, MR (in psi) = 1500 * CBR
For Rigid pavement; the various material properties considered
are:
The modulus of subgrade reaction, concrete Elastic Modulus;
Concrete Modulus of Rupture etc