Transportation Engineering II
The Pavement
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What is a Pavement?
• A multi layer system that distributes
the vehicular loads over a larger area
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What is a Pavement?
OR
• Highway pavement is a structure
consisting of superimposed layers of
selected and processed materials whose
primary function is to distribute the applied
vehicle load to the sub grade.
OR
• It can also be defined as “structure which
separates the tires of vehicles from the
under lying foundation.”
3
What is a Pavement?
• Pavement is the upper part of
roadway, airport or parking area
structure
• It includes all layers resting on the
original ground
• – It consists of all structural elements
or layers, including shoulders
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Functions of the Pavement
• Reduce and distribute the traffic loading so as
not to damage the subgrade
• – Provide vehicle access between two points
under all-weather conditions
• – Provide safe, smooth and comfortable ride
to road users without undue delays and
excessive wear & tear
• – Meet environmental and aesthetics
requirement
• – Limited noise and air pollution
• – Reasonable economy
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Requirements of pavement structure
• Sufficient thickness to spread loading to a
pressure intensity tolerable by subgade
• Sufficiently strong to carry imposed stress due to
traffic load
• Sufficient thickness to prevent the effect of frost
susceptible subgade
• Pavement material should be impervious to
penetration of surface water which could weaken
subgade and subsequently pavement
• Pavement mat. shd be non-frost susceptible
• Pavement surface shd. be skid resistant
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Classification of Pavements
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Types of Pavement
PAVEMENTS
Flexible Pavements
Rigid Pavements
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Flexible Pavements
Asphalt Concrete
Aggregate Base Course
Natural Soil (Subgrade)
Aggregate Subbase Course
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Flexible Pavements
Given Wheel Load
150 psi
3 psi
Wearing C.
Base
Sub-base
Sub-grade
Load Distribution in Flexible Pavements
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Rigid Pavements
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Rigid Pavements
Load Distribution in Rigid Pavements
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Pavements Comparison
Flexible pavements:
• Deep foundations / multi layer construction
• Energy consumption due to transportation of materials
• Increasing cost of asphalt due to high oil prices
Rigid pavements
• Single layer
• Generally last longer
• May require asphalt topping due to noise / comfort
issues
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Pavements Comparison
• Heavy vehicles consume less fuel on rigid
pavements
• Rigid pavements more economic when
considering environmental / life-cycle costing
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Flexible Pavement
• “A flexible pavement is a structure that
maintains intimate contact with and distributes
load to the sub grade and depends on
aggregate interlock, particle friction and
cohesion for stability”
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Flexible Pavement
OR
• It is a structure which distributes the traffic
loading stresses to the soil (sub grade) at a
magnitude that will not shear or distort the
soil i.e., from 150 psi to 3 psi
OR
• Pavement which reflects deformation of sub
grade & the subsequent layers on to the
surface” i.e.; load is transmitted from grain to
grain through contact points of granular
material, i.e. in a compressive way.
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Flexible Pavement
Flexible pavements
• Elastic
• Three main layers
– Surfacing
• Wearing course
• Base course
– Road base
– Sub base
• Supported by Sub-grade
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Flexible Pavement
Introduction
• Flexible pavements are so named
because the total pavement structure
deflects, or flexes, under loading.
A
flexible pavement structure is typically
composed of several layers of different
materials.
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Flexible Pavement
• Each layer receives the loads from
the above layer, spreads them out,
then passes on these loads to the
next layer below. Thus, the further
down in the pavement structure a
particular layer is, the less load (in
terms of force per unit area) it must
carry.
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Load Distribution In Flexible Pavements
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Structure of Flexible Pavement
• In order to take maximum advantage of
this property, material layers are usually
arranged in order of descending load
bearing capacity with the highest load
bearing capacity material (and most
expensive) on the top and the lowest load
bearing capacity material (and least
expensive) at the bottom.
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Structure of Flexible Pavement
– Surface Course: This is the top layer and the layer
that comes in contact with traffic.
– Base Course: This is the layer directly below the
surface course and generally consists of
aggregates (either stabilized or un-stabilized).
– Sub-base Course: This is the layer (or layers)
under the base layer. A sub-base is not always
needed.
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Structure of Flexible Pavement
– Sub-grade Course: The "sub-grade" is the
material upon which the pavement structure is
placed. Although there is a tendency to look at
pavement performance in terms of pavement
structure and mix design alone. The sub-grade
can often be the overriding factor in pavement
performance.
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Surface Course
• The surface course is the layer in contact
with traffic loads and normally contains the
highest quality materials. It provides
characteristics such as friction,
smoothness, noise control, rut and shoving
resistance and drainage.
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Surface Course
•
In addition, it serves to prevent the entrance
of excessive quantities of surface water into
the underlying base, sub-base and subgrade. This top structural layer of material is
sometimes subdivided into two layers.
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Wearing Course
• This is the layer in direct contact with traffic
loads. It is meant to take the brunt of
traffic wear and can be removed and
replaced as it becomes worn. A properly
designed (and funded) preservation
program should be able to identify
pavement surface distress while it is still
confined to the wearing course. This way,
the wearing course can be rehabilitated
before distress propagates into the
underlying intermediate/blinder course
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Intermediate/Binder Course
– Intermediate/Binder Course: This layer
provides the bulk of the HMA
structure. It's main purpose is to
distribute load.
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Base Course
• The base course is immediately beneath
the surface course. It provides additional
load distribution and contributes to
drainage and frost resistance.
Base
courses are usually constructed out of:
– Aggregates: Base courses are most typically
constructed from durable aggregates that will
not be damaged by moisture or frost
action. Aggregates can be either stabilized or
un-stabilized.
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– HMA: In certain situations where high
base stiffness is desired, base courses
can be constructed using a variety of
HMA mixes.
In relation to surface
course HMA mixes, base course mixes
usually
contain
larger
maximum
aggregate sizes, are more open graded
and are subject to more lenient
specifications.
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Lime rock Base Course Undergoing Final Grading
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Sub-base Course
• The sub-base course is between the base
course and the sub-grade. It functions
primarily as structural support but it can
also:
– Minimize the intrusion of fines from the subgrade into the pavement structure.
– Improves drainage.
– Minimize frost action damage.
– Provides a working platform for construction.
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Sub-base Course
• The sub-base generally consists of lower
quality materials than the base course but
better than the sub-grade soils.
• A sub-base course is not always needed
or used.
• For example, a pavement constructed over
a high quality, stiff sub-grade may not
need the additional features offered by a
sub-base course so it may be omitted from
design.
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• However, a pavement constructed over a
low quality soil such as a swelling clay
may require the additional load distribution
characteristic that a sub-base course can
offer. In this scenario the sub-base course
may consist of high quality fill used to
replace poor quality sub-grade.
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Sub-grade
• Although a pavement's wearing course
is most prominent, the success or failure
of a pavement is more often than not
dependent upon the underlying subgrade , the material upon which the
pavement structure is built.
34
Sub-grade
• Sub-grades be composed of a wide
range of materials although some are
much better than others. This
subsection discusses a few of the
aspects of sub-grade materials that
make them either desirable or
undesirable and the typical tests used to
characterize sub-grades.
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Sub-grade Preparation
Sub-grade Failure Crack
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Types of Flexible Pavement
Dense-graded
Open-graded
Gap-graded
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Flexible Pavement – Construction
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Types of Pavements
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Types of Pavements
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Types of Pavements
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Types of Pavements
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Types of Pavements
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Types of Pavements
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Types of Flexible Pavements
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Types of Flexible Pavements
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Types of Flexible Pavements
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Typical Load Distribution in Flexible Pavement
Wheel Load
Bituminous Layer
Sub-grade
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Typical Stress Distribution in Flexible Pavement.
Vertical stress
Foundation stress
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Load Transfer Mechanism
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Load Transfer Mechanism
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Rigid Pavement
• Rigid pavements are those, which contain
sufficient beam strength to be able to bridge
over the localized sub-grade failures and
areas of in adequate support.
OR
• Load is transmitted through beam action of
slab in rigid pavements.
OR
• Rigid pavements are those, which reduces the
stress concentration and distributes the
reduced stresses uniformly to the area under
the slab.
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Rigid Pavement
• Rigidity – does not deform under stress
• Concrete – air entrained increases resistance
to frost damage and de-icing salt corrosion
• Reinforcement – may be bars or mesh.
Continuous rigid pavements have heavy
reinforcement
• Joints – used in non-continuous pavements to
allow for thermal movement. Includes a ‘filler’
and surface sealant
• Rigid pavements – laid as single layer by
‘concrete paver’
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Rigid Pavements
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Basic Components of Concrete Pavement
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Concrete paver
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Rigid Pavements
Load Distribution in Rigid Pavements
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Rigid Pavements
Introduction
• Rigid pavements are so named because
the pavement structure deflects very little
under loading due to the high modulus of
elasticity of their surface course. A rigid
pavement structure is typically composed
of a PCC surface course built on top of
either
– the sub-grade or
– an underlying base course.
58
Rigid Pavements
• Because of its relative rigidity, the
pavement structure distributes loads over
a wide area with only one, or at most two,
structural layers.
• There are other types of surfaces also i.e.;
reinforced, continuously reinforced etc.
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Structure of Rigid Pavement
– Surface course. This is the top layer, which
consists of the PCC slab, reinforced or
continuously reinforced slabs .
– Base course. This is the layer directly below
the PCC layer and generally consists of
aggregate or stabilized sub-grade.
– Sub-base course. This is the layer (or layers)
under the base layer. A sub-base is not
always needed and therefore may often be
omitted.
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Surface Course
• The surface course is the layer in contact with
traffic loads and is made of PCC or RCC. It
provides characteristics such as friction,
smoothness, noise control and drainage. In
addition, it serves as a waterproofing layer to the
underlying base, sub-base and sub-grade.
• The surface course can vary in thickness but is
usually between 150 mm (6 inches for light
loading) and 300 mm (12 inches for heavy loads
and high traffic). Figure shows a 300 mm (12 inch)
surface course.
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PCC Surface
Rigid Pavement Slab
(Surface Course) Thickness
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Base Course
• The base course is immediately beneath the
surface course. It provides
•
•
•
•
Additional load distribution,
Contributes to drainage and frost resistance,
Uniform support to the pavement and
A stable platform for construction equipment. Bases also
help and prevent sub grade soil movement due to slab
pumping. Base courses are usually constructed out of:
– Aggregates base.
A simple base course of
crushed aggregates has been a common option
since the early 1900s and is still appropriate in
many situations.
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Base Course
– Stabilized aggregate or soil. Stabilizing agents
are used to bind otherwise loose particles to
one another, providing strength and
cohesion. Cement treated bases (CTB s) can
be built to as much as 20 - 25 percent of the
surface course strength.
– Dense-graded HMA. In situations where high
base stiffness is desired base courses can be
constructed using a dense-graded HMA layer.
– Permeable HMA. In certain situations where
high base stiffness and excellent drainage is
desired, base courses can be constructed using
an open graded HMA.
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Base Course
– Lean concrete. Contains less Portland cement
paste than a typical PCC and is stronger than
a stabilized aggregates. Lean concrete bases
(LCB s) can be built to as much as 25 - 50
percent of the surface course strength. A lean
concrete base, functions much like a regular
PCC surface course and therefore, it requires
construction joints and normally cracks over
time. These joints and cracks can potentially
cause reflection cracking in the surface course.
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Completed CTB with Curing Seal
Lean Concrete Base Material
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Sub-base Course
• The sub-base course is the portion of the pavement
structure between the base course and the subgrade. It functions primarily as structural support but
it can also:
– Minimize the intrusion of fines from the sub-grade into the
pavement structure.
– Improves drainage.
– Minimizes frost action damage.
– Provides a working platform for construction.
• The sub-base generally consists of lower quality
materials than the base course but better than the
sub-grade soils. Appropriate materials are
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aggregates and high quality structural fill.
Sub-grade
• Sub grade provides support to the
overlying concrete slab. If it is of good
quality then slab can be laid over it without
providing sub-base otherwise if it is
extremely poor then a sub-base layer
should be incorporated .
• For design purpose the only thing to know
about sub-grade is its classification and
the unit pressure coming from slab to subgrade should be calculated for its
selection. However, it must be resistant to
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moisture damages.
Types of Rigid Pavement
• Jointed Plain Concrete Pavement
(JPCP)
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Types of Rigid Pavement
• Continuously Reinforced Concrete
Pavement (CRCP)
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Photo from the Concrete Reinforcing Steel Institute
Rigid Pavement –
Construction
Slipform
Fixed form
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Pavements Comparison
Flexible pavements:
• Deep foundations / multi layer construction
• Energy consumption due to transportation of materials
• Increasing cost of asphalt due to high oil prices
Rigid pavements
• Single layer
• Generally last longer
• May require asphalt topping due to noise / comfort
issues
72
Pavements Comparison
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Types of Pavement Failure
•
•
•
•
•
•
•
•
•
Failure criteria
Flexible Pavements
Fatigue Cracking,
Rutting,
Thermal Cracking,
Rigid Pavements
Fatigue Cracking,
Pumping or Erosion
Others: Faulting, spalling, joint deterioration
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Flexible vrs. Rigid Pavements
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Flexible vrs. Rigid Pavements
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Airport-Highway Pavements
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