Unit 3 RIGID AND FLEXIBLE PAVEMENTS

8/12/2019 Unit 3 RIGID AND FLEXIBLE PAVEMENTS

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Basically, all hard surfaced pavement types can be categorized

into two groups, flexible and rigid. Flexible pavements arethose which are surfaced with bituminous (or asphalt)

materials. Rigid pavements are composed of a PCC surface

course. Such pavements are substantially "stiffer" than flexible

pavements due to the high modulus of elasticity of the PCC

material. Further, these pavements can have reinforcing steel,which is generally used to reduce or eliminate joints.

INTRODUCTION


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Flexible Pavement Basics

Flexible pavements are so named

because the total pavement structure

deflects, or flexes, under loading. A

flexible pavement structure istypically composed of several layers

of material. Each layer receives the

loads from the above layer, spreads

them out, then passes on these loads

to the next layer below.


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The typical flexible pavement structure consist of: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 HMA

layer and generally consists of aggregate (either stabilizedor 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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Typical layers of a conventional flexible pavement includes seal

coat, surface course, tack coat, binder course, prime coat, base

course, sub-base course, compacted sub-grade, and natural sub-grade .

Seal Coat: Seal coat is a thin surface treatment used to water-proof

the surface and to provide skid resistance.

Tack Coat: Tack coat is a very light application of asphalt, usuallyasphalt emulsion diluted with water. It provides proper bonding

between two layer of binder course and must be thin, uniformly

cover the entire surface, and set very fast.

Prime Coat: Prime coat is an application of low viscous cutback

bitumen to an absorbent surface like granular bases on which binder

layer is placed. It provides bonding between two layers. Unlike tack

coat, prime coat penetrates into the layer below, plugs the voids, and

forms a water tight surface.

Typical layers of a flexible pavement


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Surface course

Surface course is the layer directly in contact with traffic

loads and generally contains superior quality materials. Theyare usually constructed with dense graded asphalt concrete

(AC). The functions and requirements of this layer are:

It provides characteristics such as friction, smoothness,

drainage, etc. Also it will prevent the entrance of excessivequantities of surface water into the underlying base, sub-base

and sub-grade,

It must be tough to resist the distortion under traffic and

provide a smooth and skid- resistant riding surface,

It must be water proof to protect the entire base and sub-

grade from the weakening effect of water.


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Binder course

This layer provides the bulk of the asphalt concrete structure. It's chief

purpose is to distribute load to the base course The binder course

generally consists of aggregates having less asphalt and doesn't requirequality as high as the surface course, so replacing a part of the surface

course by the binder course results in more economical design.

Base course

The base course is the layer of material immediately beneath the

surface of binder course and it provides additional load distribution and

contributes to the sub-surface drainage It may be composed of crushed

stone, crushed slag, and other untreated or stabilized materials.


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Sub-Base course

The sub-base course is the layer of material beneath the base course

and the primary functions are to provide structural support, improvedrainage, and reduce the intrusion of fines from the sub-grade in the

pavement structure If the base course is open graded, then the sub-

base course with more fines can serve as a filler between sub-grade

and the base course 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. In such situations, sub-base course may not be provided.

Sub-grade

The top soil or sub-grade is a layer of natural soil prepared toreceive the stresses from the layers above. It is essential that at no

time soil sub-grade is overstressed. It should be compacted to the

desirable density, near the optimum moisture content.


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There are many factors that affect pavement design which can be

classified into four categories as traffic and loading, structuralmodels, material characterization, environment.

Traffic and loading

Traffic is the most important factor in the pavement design. The

key factors include contact pressure, wheel load, axle

configuration, moving loads, load, and load repetitions.

Contact pressure:

The tyre pressure is an important factor, as it determine the contact

area and the contact pressure between the wheel and the pavement

surface. Even though the shape of the contact area is elliptical, forsake of simplicity in analysis, a circular area is often considered..

FACTORS AFFECTING PAVEMENT DESIGN


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Wheel load:

The next important factor is the wheel load which determines the

depth of the pavement required to ensure that the sub grade soil isnot failed. Wheel configuration affect the stress distribution and

deflection within a pavement. Many commercial vehicles have

dual rear wheels which ensure that the contact pressure is within

the limits. The normal practice is to convert dual wheel into anequivalent single wheel load so that the analysis is made simpler.

Axle configuration:

The load carrying capacity of the commercial vehicle is further

enhanced by the introduction of multiple axles.

Moving loads:The damage to the pavement is much higher if the vehicle is

moving at creep speed. Many studies show that when the speed is

increased from 2 km/hr to 24 km/hr, the stresses and deflection

reduced by 40 per cent.


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Repetition of Loads:

The influence of traffic on pavement not only depend on themagnitude of the wheel load, but also on the frequency of the load

applications. Each load application causes some deformation and

the total deformation is the summation of all these. Although the

pavement deformation due to single axle load is very small, thecumulative effect of number of load repetition is significant.

Therefore, modern design is based on total number of standard

axle load (usually 80 kN single axle).


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Structural models

The structural models are various analysis approaches to determine

the pavement responses (stresses, strains, and deflections) at various

locations in a pavement due to the application of wheel load. Themost common structural models are layered elastic model and visco-

elastic models.

Layered elastic model:

A layered elastic model can compute stresses, strains, and

deflections at any point in a pavement structure resulting from the

application of a surface load. Layered elastic models assume that

each pavement structural layer is homogeneous, isotropic, and

linearly elastic. In other words, the material properties are same at

every point in a given layer and the layer will rebound to its originalform once the load is removed. The layered elastic approach works

with relatively simple mathematical models that relates stress,

strain, and deformation with wheel loading and material properties

like modulus of elasticity and poissons ratio.


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Material characterization

The following material properties are important for bothflexible and rigid pavements.

When pavements are considered as linear elastic, the elastic

moduli and poisson ratio of subgrade and each component

layer must be specified.If the elastic modulus of a material varies with the time of

loading, then the resilient modulus, which is elastic modulus

under repeated loads, must be selected in accordance with a

load duration corresponding to the vehicle speed.

When a material is considered non-linear elastic, theconstitutive equation relating the resilient modulus to the state

of the stress must be provided.


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Basic Structural Elements

A typical flexible pavement structure consists of the surface

course and the underlying base and sub base courses. Each of

these layers contributes to structural support anddrainage. The surface course is the stiffest and contributes

the most to pavement strength. The underlying layers are less

stiff but are still important to pavement strength as well as

drainage and frost protection. A typical structural design

results in a series of layers that gradually decrease in material

quality with depth.


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The surface course is the layer in contact with traffic loadsand normally contains the highest quality materials. It

provides characteristics such as friction, smoothness, noise

control, rut and shoving resistance and drainage. In

addition, it serves to prevent the entrance of excessivequantities of surface water into the underlying base, sub

base and sub grade.

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.

Intermediate/Binder Course. It's chief purpose is to

distribute load.

Surface Course


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The base course is immediately beneath the surface course. Itprovides additional load distribution and contributes to drainage

and frost resistance. Base courses are usually constructed out of:

Aggregate. 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.

Base Course


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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 sub grade into the

pavement structure.

Improve drainage.

Minimize frost action damage.Provide 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. 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. 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.

Sub base Course


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Design criteria

The flexible pavements has been modeled as a three layer

structure and stresses and strains at critical locations have

been computed using the linear elastic model. To giveproper consideration to the aspects of performance, the

following three types of pavement distress resulting from

repeated (cyclic) application of traffic loads are

considered:

vertical compressive strain at the top of the sub-grade

which can cause sub-grade deformation resulting in

permanent deformation at the pavement surface.

horizontal tensile strain or stress at the bottom of the

bituminous layer which can cause fracture of thebituminous layer. pavement deformation within the

bituminous layer.


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Design procedure

Based on the performance of existing designs and using analytical

approach, simple design charts and a catalogue of pavement

designs are added in the code. The pavement designs are given for

subgrade CBR values ranging from 2% to 10% and design traffic

ranging from 1 msa to 150 msa for an average annual pavement

temperature of 35 C. The later thicknesses obtained from the

analysis have been slightly modified to adapt the designs to stageconstruction. Using the following simple input parameters,

appropriate designs could be chosen for the given traffic and soil

strength:

Design traffic in terms of cumulative number of standard axles;and

CBR value of sub grade


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To carry maximum load with in the specified limit and to carry greater load, dual

wheel, or dual tandem assembly is often used. Equivalent single wheel load (ESWL)

is the single wheel load having the same contact pressure, which produces same value

of maximum stress, deflection, tensile stress or contact pressure at the desired depth.

The procedure of finding the ESWL for equal stress criteria is provided below. This is

a semi-rational method, known as Boyd and Foster method, based on the following

assumptions:

equalancy concept is based on equal stress; contact area is circular;influence angle is 45o; and

soil medium is elastic, homogeneous, and isotropic half space.

The ESWL is given by:

where P is the wheel load, S is the center to center distance between the two

wheels, d is the clear distance between two wheels, and z is the desired depth.

ESWL


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Rigid Pavement Basics

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 istypically composed of a PCC surface course built on top of either (1)

the sub grade or (2) an underlying base course. Because of its

relative rigidity, the pavement structure distributes loads over a wide

area with only one, or at most two, structural layers

The typical rigid pavement structure consist of:

Surface course. This is the top layer, which consists of the PCC

slab.

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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Basic Structural Elements

A typical rigid pavement structure consists of the surface

course and the underlying base and sub base courses (ifused). The surface course (made of PCC) is the stiffest and

provides the majority of strength. The underlying layers are

orders of magnitude less stiff but still make important

contributions to pavement strength as well as drainage andfrost protection.


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Surface Course

The surface course is the layer in contact with traffic loads and is

made of PCC. 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 (for light loading) and 300 mm (for heavy loads

and high traffic).

PCC Surface Rigid Pavement Slab

(Surface Course) Thickness

B C


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Base Course

The base course is immediately beneath the surface course. It provides (1)

additional load distribution, (2) contributes to drainage and frost resistance,

(3) uniform support to the pavement and (4) a stable platform forconstruction equipment. Bases also help prevent sub grade soil movement

due to slab pumping. Base courses are usually constructed out of:

Aggregate base. A simple base course of crushed aggregate has been a

common option since the early 1900s and is still appropriate in many

situations today.Stabilized aggregate or soil. Stabilizing agents are used to bind Cement

treated bases (CTBs) can be built to as much as 20 - 25 percent of the

surface course strength.

Lean concrete. Contains less Portland cement paste than a typical PCC

and is stronger than a stabilized aggregate. A lean concrete base functionsmuch like a regular PCC surface course and therefore, it requires

construction joints and will crack over time. These joints and cracks can

potentially cause reflection cracking in the surface course if they are not

carefully matched.


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Sub base Course

The sub base course is the portion of the pavement structure

between the base course and the sub grade. It functionsprimarily as structural support but it can also:

Minimize the intrusion of fines from the sub grade into the

pavement structure.

Improve drainage.

Minimize frost action damage.

Provide 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 aggregate and high qualitystructural fill. A sub base course is not always needed or

used.


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Joints

Joints are purposefully placed discontinuities in a rigid pavement

surface course. The most common types of pavement joints,defined by their function, are: contraction, expansion and

construction.

Contraction Joints

A contraction joint is a sawed, formed, or tooled groove in a

concrete slab that creates a weakened vertical plane. It regulates

the location of the cracking caused by dimensional changes in the

slab. Unregulated cracks can grow and result in an unacceptablyrough surface as well as water infiltration into the base, sub base

and sub grade, which can enable other types of pavement

distress. Contraction joints are the most common type of joint in

concrete pavements.


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Contraction joints are chiefly defined by their spacing and their

method of load transfer. They are generally between 1/4 - 1/3

the depth of the slab and typically spaced every 3.1 - 15 m withthinner slabs having shorter spacing. These patterns typically

use a repeating sequence of joint spacing (for example: 2.7 m

then 3.0 m then 4.3 m then 4.0 m . Transverse contraction joints

can be cut at right angles to the direction of traffic flow.

Rigid Pavement Showing

Contraction Joints


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Expansion Joints

An expansion joint is placed at a specific location to

allow the pavement to expand without damaging

adjacent structures or the pavement itself. However,

expansion joint are not typically used today because

their progressive closure tends to cause contractionjoints to progressively open. Progressive or even

large seasonal contraction joint openings cause a loss

of load transferparticularly so for joints without

dowel bars.


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Construction Joints

A construction joint is a joint between slabs that results whenconcrete is placed at different times. This type of joint can be

further broken down into transverse and longitudinal

construction joints . Longitudinal construction joints also allow

slab warping without appreciable separation or cracking of the

slabs.

Longitudinal and Transverse Construction Joints


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Dowel Bars

Dowel bars are short steel bars that provide a mechanical

connection between slabs without restricting horizontal jointmovement. They increase load transfer efficiency by allowing

the leave slab to assume some of the load before the load is

actually over it. Dowel bars are typically 32 to 38 mm in

diameter, 460 mm long and spaced 305 mm apart. In order toprevent corrosion, dowel bars are either coated with stainless

steel or epoxy. Dowel bars are usually inserted at mid-slab

depth and coated with a bond-breaking substance to prevent

bonding to the PCC. Thus, the dowels help transfer load but

allow adjacent slabs to expand and contract independent ofone another.


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Dowel Bars in Place at a

Construction Joint- the GreenColor is from the Epoxy

Coating


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Unit 3 RIGID AND FLEXIBLE PAVEMENTS

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