PAVEMENT STRUCTURAL ANALYSIS

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The University of Mustansiriyah Subject: Pavement Structural Analysis Faculty of Engineering Dr. Rana Amir Yousif Highway and Transportation Engineering Department 3

rd Year Stage

The University of Mustansiriyah Pavement Structural Analysis Faculty of Engineering 2021-2022 Highway and Transportation Engineering Department 1

st Lecture

PAVEMENT STRUCTURAL ANALYSIS

Assost Prof Dr Abeer K. Jameel & Dr Rana Amir Yousif

This course covers the structural analysis of pavements

Syllabus:

1. Introduction: Pavement types

2. Flexible Pavement analysis: Stresses and strains in flexible pavements

- Homogeneous Mass: solutions by charts, solution by Axis symmetry, Nonlinear Mass

- Layered systems: single-layer elastic solution, two layers elastic solution, multilayer linear

elastic solution, multilayer nonlinear elastic solution.

- Viscoelastic solutions

3. Rigid Pavement Analysis: stresses and deflections in rigid pavements:

- Stresses due to curling

- Stresses and deflections due to loading

- Stresses due to friction

- Design of dowels and joints

Reference

1. Yang H. Huang, Pavement Analysis and Design, second Edition, Pernctile Hall Inc.

USA, 1993.

2. Yoder, E. J. and M. W. Witczak, “ Principles of Pavement Design”, A Wiley-

Interscience Publication, John Wiley & Sons Inc.,USA 1975

3. A.T. Papagiannakis and E.A Masad, “Pavement Design and Materials”

4. AASHTO Guide for design of pavement structures 1993, AASHTO, American

Association of State Highways and Transportation Officials, USA. 1993.

5. Fred L. Mannering and Walter P. Kilarsk, 1998, “Principles of Highways Engineering

and Traffic Analysis” second edition, John Wiley & Sons Inc., USA, 1998

6. Oglesby Clarkson H., “ Highway Engineering”, John Wiley & Sons Inc., USA, 1975

7. Nicholas J. Garber and Lester A. Hotel, “Traffic and Highway engineering”

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Introduction

The physical components of a highway include the right of way, pavements,

shoulders, signs, signals and marking.

Pavement and shoulders represent the most costly items associated with highway

construction and maintenance. The heavy traffic load requires constructing of

effective pavement surface. In addition, the surface needs maintenance to ensure

the effectiveness of the pavement surfaces in all weather conditions and avoid

permanent deformation. Therefore, it is important for highway engineers to have

a basic understanding of pavement analysis and design principles.

Functions of Pavement

Pavement provides two basic functions:

1. It helps to guide the driver and delineate the roadway by giving a visual

perspective of the travelled path. Consequently, pavement gives the driver

information about the driving tasks and the steering control of the vehicle.

2. It supports the vehicle loads

The second function is mainly considered in the courses of analysis and

design of pavements

The University of Mustansiriyah Faculty of Engineering Highway and Transportation Engineering Department

Subject: Pavement Structural Analysis 2021-2022 3

rd Year Stage

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Pavement Types

There are three major types of pavements: flexible or asphalt pavements, rigid or

concrete pavements, and composite pavements.

1.1 Flexible Pavements:

Flexible pavement can be defined as the one consisting of a mixture of asphaltic

or bituminous material and aggregates placed on a bed of compacted granular

material of appropriate quality in layers over the subgrade. Water bound

macadam roads and stabilized soil roads with or without asphaltic toppings are

examples of flexible pavements.

The design of flexible pavement is based on the principle that for a load of any

magnitude, the intensity of a load diminishes as the load is transmitted

downwards from the surface by virtue of spreading over an increasingly larger

area, by carrying it deep enough into the ground through successive layers of

granular material

Figure (1.1): Flexible Pavement Cross-section



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Thus for flexible pavement, there can be grading in the quality of materials used,

the materials with high degree of strength is used at or near the surface. Thus the

strength of subgrade primarily influences the thickness of the flexible pavement

Hot Mix Asphalt (HMA)Surface

Tack coat

Prime Coat

Prime Coat

Hot Mix Asphalt (HMA) Binder Course

Stabilised Base Course

Granular Subbase Course

Natural Subgrade

Figure (1.2) Typical flexible pavement configuration: High Traffic Volume

Prime Coat

Figure (1.3) Typical flexible pavement configuration: Low Traffic Volume

Hot Mix Asphalt (HMA) Surface Course

Granular Base Course

Natural Subgrade



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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.

Figure 1.4 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

subgrade. The use of the various courses is based on either necessity or economy,

and some of the courses may be omitted.

Figure 1.4: Typical cross section of a conventional flexible pavement (1 in. = 25 .4 mm).



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Full - depth asphalt pavements:

Full-depth asphalt pavements are constructed by placing one or more layers of

Hot Mix Asphalt (HMA) directly on the subgrade or improved subgrade. This

concept was conceived by the Asphalt Institute in 1960 and is generally

considered the most cost-effective and dependable type of asphalt pavement for

heavy traffic. This type of construction is quite popular in areas where local

materials are not available. It is more convenient to purchase only one material,

i.e., HMA, rather than several materials from different sources, thus minimizing

the administration and equipment costs.

Figure 1.5 shows the typical cross section for a full-depth asphalt pavement. 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.

Figure 1.5: Typical cross-section of a full-depth asphalt pavement (1 in. = 25 .4 mm).

According to the Asphalt Institute (AI, 1987), full-depth asphalt pavements have the

following advantages:

1. They have no permeable granular layers to entrap water and impair

performance.



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2. Time required for construction is reduced. On widening projects, where

adjacent traffic flow must usually be maintained, full-depth asphalt can be

especially advantageous.

3. When placed in a thick lift of 4 in. (102 mm) or more, construction seasons

may be extended.

4. They provide and retain uniformity in the pavement structure.

5. They are less affected by moisture or frost.

According to limited studies, moisture contents do not build up in subgrades

under full-depth asphalt pavement structures as they do under pavements with

granular bases. Thus, there is little or no reduction in subgrade strength.

Typical layers of a flexible pavement

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 where the aggregate in the surface course could be

polished by traffic and become slippery.

Tack Coat: Tack coat is a very light application of asphalt, usually asphalt

emulsion diluted with water. It provides proper bonding between two layers of

binder course and must be thin, uniformly cover the entire surface, and set very

fast. It does not require penetrating into the underlying course.

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.



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Surface course: is the layer directly in contact with traffic loads and generally

contains superior quality materials. They are 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 excessive quantities 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.

Figure (1.6) surface course



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Binder course

This layer provides the bulk of the asphalt concrete structure. Its chief purpose is

to distribute load to the base course the binder course generally consists of

aggregates having less asphalt and doesn't require quality 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.

Figure (1.7) Binder course


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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, improve drainage, 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.

Figure (1.8) Subbase course

Sub-grade

The top soil or sub-grade is a layer of natural soil prepared to receive 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.

Figure (1.9) Subgrade course


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Failure of flexible pavements

The major flexible pavement failures are fatigue cracking, rutting, and thermal

cracking. The fatigue cracking of flexible pavement is due to horizontal tensile

strain at the bottom of the asphaltic concrete. The failure criterion relates

allowable number of load repetitions to tensile strain and this relation can be

determined in the laboratory fatigue test on asphaltic concrete specimens. Rutting

occurs only on flexible pavements as indicated by permanent deformation or rut

depth along wheel load path. Two design methods have been used to control

rutting: one to limit the vertical compressive strain on the top of subgrade and

other to limit rutting to a tolerable amount (12 mm normally). Thermal cracking

includes both low-temperature cracking and thermal fatigue cracking.

1.2 Rigid Pavements:

A rigid pavement is constructed from cement concrete or reinforced concrete

slabs. Grouted concrete roads are in the category of semi-rigid pavements. Figure

1.10 shows a typical cross section for rigid pavements. In contrast to flexible

pavements, rigid pavements are placed either directly on the prepared subgrade or

on a single layer of granular or stabilized material. Because there is only one

layer of material under the concrete and above the subgrade, some call it a base

course, others a subbase.

Figure 1.10: Typical cross section of a rigid pavement (1 in. = 25.4 mm).



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Use of Base Course

Early concrete pavements were constructed directly on the subgrade without a

base course. As the weight and volume of traffic increased, pumping began to

occur, and the use of a granular base course became quite popular. When

pavements are subject to a large number of very heavy wheel loads with free

water on top of the base course, even granular materials can be eroded by the

pulsative action of water. For heavily traveled pavements, the use of a cement-

treated or asphalt-treated base course has now become a common practice.

Although the use of a base course can reduce the critical stress in the concrete, it

is uneconomical to build a base course for the purpose of reducing the concrete

stress.

Because the strength of concrete is much greater than that of the base course, the

same critical stress in the concrete slab can be obtained without a base course by

slightly increasing the concrete thickness. The following reasons have been

frequently cited for using a base course.

a) Control of Pumping

Pumping is defined as the ejection of water and subgrade soil through joints

and cracks and along the edges of pavements, caused by downward slab

movements due to heavy axle loads. The sequence of events leading to

pumping includes the creation of void space under the pavement caused by

the temperature curling of the slab and the plastic deformation of the

subgrade, the entrance of water, the ejection of muddy water, the enlargement

of void space, and finally the faulting and cracking of the leading slab ahead

of traffic. Pumping occurs under the leading slab when the trailing slab

rebounds, which creates a vacuum and sucks the fine material from

underneath the leading slab, as shown in Figure 1.11. The corrective



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measures for pumping include joint sealing, under sealing with asphalt

cements, and muck jacking with soil cement.

Figure 1.11: Pumping of rigid pavement.

Three factors must exist simultaneously to produce pumping:

1. The material under the concrete slab must be saturated with free water. If

the material is well drained, no pumping will occur. Therefore, good

drainage is one of the most efficient ways to prevent pumping.

2. There must be frequent passage of heavy wheel loads. Pumping will take

place only under heavy wheel loads with large slab deflections. Even under

very heavy loads, pumping will occur only after a large number of load

repetitions.

The material under the concrete slab must be erodible. The credibility of a

material depends on the hydrodynamic forces created by the dynamic

action of moving wheel loads. Any untreated granular materials, and even

some weakly cemented materials, are erodible because the large

hydrodynamic pressure will transport the fine particles in the subbase or

subgrade to the surface. These fine particles will go into suspension and

cause pumping



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b) Control of Frost Action:

Frost action is detrimental to pavement performance. It results in frost heave,

which causes concrete slabs to break and softens the subgrade during the

frost-melt period. In northern climates, frost heave can reach several inches or

more than one foot. The increase in volume of 9% when water becomes

frozen is not the real cause of frost heave. For example, if a soil has a porosity

of 0.5 and is subjected to a frost penetration of 3 ft (0.91 m), the amount of

heave due to 9% increase in volume is 0.09 x 3 x 0.5 = 0.135 ft or 1.62 in. (41

mm), which is much smaller than the 6 in. (152 mm) or more of heave

experienced in such climate.

Frost heave is caused by the formation and continuing expansion of ice

lenses. After a period of freezing weather, frost penetrates into the pavement

and subgrade, as indicated by the depth of frost penetration in Figure 1.12.

Above the frost line, the temperature is below the ordinary freezing point for

water. The water will freeze in the larger voids but not in the smaller voids

where the freezing point may be depressed as low as 23°F(-5°C) .

When water freezes in the larger voids, the amount of liquid water at that

point decreases. The moisture deficiency and the lower temperature in the

freezing zone increase the capillary tension and induce flow toward the newly

formed ice. The adjacent small voids are still unfrozen and act as conduits to

deliver the water to the ice. If there is no water table or if the subgrade is

above the capillary zone, only scattered and small ice lenses can be formed. If

the subgrade is above the frost line and within the capillary fringe of the

groundwater table, the capillary tension induced by freezing sucks up water

from the water table below. The result is a great increase in the amount of

water in the freezing zone and the segregation of water into ice lenses. The

amount of heave is at least as much as the combined lens thicknesses.



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Figure 1.12: Formation of ice lenses, due to frost action.

Three factors must be present simultaneously to produce frost action:

1. The soil within the depth of frost penetration must be frost susceptible. It

should be recognized that silt is more frost susceptible than clay because it

has both high capillarity and high permeability. Although clay has a very

high capillarity, its permeability is so low that very little water can be

attracted from the water table to form ice lenses during the freezing period.

Soils with more than 3% finer than 0.02 mm are generally frost

susceptible, except that uniform fine sands with more than 10% finer than

0 .02 mm are frost susceptible .

2. There must be a supply of water. A high water table can provide a

continuous supply of water to the freezing zone by capillary action.

Lowering the water table by subsurface drainage is an effective method to

minimize frost action.

3. The temperature must remain freezing for a sufficient period of time. Due

to the very low permeability of frost-susceptible soils, it takes time for the

capillary water to flow from the water table to the location where the ice

lenses are formed. A quick freeze does not have sufficient time to form ice

lenses of any significant size.



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c) Improvement of Drainage:

When the water table is high and close to the ground surface, a base course can

raise the pavement to a desirable elevation above the water table. When water

seeps through pavement cracks and joints, an open-graded base course can

carry it away to the road side. Cedergren (1988) recommends the use of an

open-graded base course under every important pavement to provide an

internal drainage system capable of rapidly removing all water that enters.

d) Control of Shrinkage and Swell:

When moisture changes cause the subgrade to shrink and swell, the base course

can serve as a surcharge load to reduce the amount of shrinkage and swell. A

dense-graded or stabilized base course can serve as a water proofing layer, and

an open-graded base course can serve as a drainage layer. Thus, the reduction

of water entering the subgrade further reduces the shrinkage and swell

potentials.

e) Expedition of Construction:

A base course can be used as a working platform for heavy construction

equipment. Under inclement weather conditions, a base course can keep the

surface clean and dry and facilitate the construction work. As can be seen from

the above reasoning, there is always a necessity to build a base course.

Consequently, base courses have been widely used for rigid pavements.



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Types of Rigid Pavements

Rigid pavements can be classified into four types:

1. Jointed Plain Concrete Pavement: are plain cement concrete pavements

constructed with closely spaced contraction joints. Dowel bars or aggregate

interlocks are normally used for load transfer across joints. They normally have

a joint spacing of 5 to 10m.

2. Jointed Reinforced Concrete Pavement: Although reinforcements do not

improve the structural capacity significantly, they can drastically increase the

joint spacing to 10 to 30m. Dowel bars are required for load transfer.

Reinforcement's help to keep the slab together even after cracks.

3. Continuous Reinforced Concrete Pavement: Complete elimination of joints

is achieved by reinforcement.

4. Prestressed Concrete Pavements: Concrete is weak in tension but strong in

compression. The thickness of concrete pavement required is governed by its

modulus of rupture, which varies with the tensile strength of the concrete. The

preapplication of a compressive stress to the concrete greatly reduces the tensile

stress caused by the traffic loads and thus decreases the thickness of concrete

required. The prestressed concrete pavements have less probability of cracking

and fewer transverse joints and therefore result in less maintenance and longer

pavement life.



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Figure 1.13: Four types of concrete pavements (1 ft= 0.305 m) .

Failure criteria of rigid pavements

Traditionally fatigue cracking has been considered as the major or only criterion

for rigid pavement design. The allowable number of load repetitions to cause

fatigue cracking depends on the stress ratio between flexural tensile stress and

concrete modulus of rupture. Of late, pumping is identified as an important

failure criterion.

Pumping is the ejection of soil slurry through the joints and cracks of cement

concrete pavement, caused during the downward movement of slab under the

heavy wheel loads. Other major types of distress in rigid pavements include

faulting, spalling, and deterioration.



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1.3 Composite Pavements:

Composite pavement is composed of both HMA and PCC. The use of PCC as a

bottom layer and HMA as a top layer results in an ideal pavement with the most

desirable characteristics. The PCC provides a strong base and the HMA provides

a smooth and non- reflective surface. However, this type of pavement is very

expensive and is rarely used as a new construction. As of 2001, there are about

97,000 miles (155,000 km) of composite pavements in the United States,

practically all of which are the rehabilitation of concrete pavements using asphalt

overlays.

Figure 1.14: Two different cross-sections for composite pavements (1 in. = 25 .4 mm).



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Difference between Flexible Pavements and Rigid Pavements:



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Figure 1.15: Flexible and rigid pavements

Figure 1.16: layers of flexible pavements

PAVEMENT STRUCTURAL ANALYSIS

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