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Road and Transport Engineering Thesis

2020-03-15

INNOVATIVE SOFTWARE FOR

FLEXIBLE PAVEMENT DESIGN,

MAINTENANCE AND REHABILITATION

Firew, Haimanot

http://hdl.handle.net/123456789/10295

Downloaded from DSpace Repository, DSpace Institution's institutional repository

BAHIR DAR UNIVERSITY

BAHIR DAR INSTITUTE OF TECHNOLOGY

SCHOOL OF RESEARCH AND GRADUATE STUDIES

FACULTY OF CIVIL AND WATER RESOURCES ENGINEERING

INNOVATIVE SOFTWARE FOR FLEXIBLE PAVEMENT DESIGN,

MAINTENANCE AND REHABILITATION

By: Haimanot Firew Minale

Bahir Dar, Ethiopia

June, 2019

INNOVATIVE SOFTWARE FOR FLEXIBLE PAVEMENT DESIGN, MAINTENANCE AND

REHABILITATION

By: Haimanot Firew

A thesis submitted to the school of Research and Graduate Studies of Bahir Dar

Institute of Technology, BDU in partial fulfillment of the requirements for the degree of

Master of Science in the Road and Transport Engineering in the School of Civil and Water

Resource Engineering.

Advisor: DrHabtamuMelese

Bahir Dar, Ethiopia

May, 2019

i

DECLARATION

ii

iii

ACKNOLEDGEMNTS

Above all, I would like to thank God for making this happen. My deepest gratitude goes

to my beloved families who have been with me all those ways to get here. Without them I

was nothing; they not only assisted me financially but also extended their support morally

and emotionally.

I would like to pay special thankfulness, warmth and appreciation to my advisor, Dr.

HabtamuMelese,who made my thesis successful and assisted me at every point to cherish

my goal. His encouragement made it possible to achieve the goal.

I must express my gratitude to TakeleTesfa, and Andrea de Lucia. Your encouragement

when the times got rough are much appreciated and duly notedfor providing me with

unfailing support and continuous encouragement throughout the process of doing this

thesis. This accomplishment would not have been possible without them.

Finally, I would also like to acknowledge with much appreciation the crucial role of my

friends, all the faculty staff members, colleagues, and Ethiopian Roads Authority (ERA)

consultantswho have contributed in countless ways and whose assistance turned my

research a success.

iv

ABSRTACT

The design of thickness of the flexible pavement has taken the backbone place in

determining the overall performance and providing high level of serviceability of the

pavement structure for the heavy traffic loads under the adverse climatic conditions,

during the expected design period.

In Ethiopia there is no software package for flexible pavement design and life cycle cost

determination. And it is not common to use software-based design of flexible pavements

rather the design agencies practice the manual method. It is quite very difficult to achieve

efficiency and reliability using manual work. The pavement design procedure by

American Association of State Highway and Transportation Officials (AASHTO) using

nomographs could be inconsistent as different results could be obtained by different users

for the same input parameters.

This research aims to provide a software package for flexible pavement design using

Ethiopian Roads Authority (ERA, 2013) and American Association of State Highway and

Transportation Officials (AASHTO, 1993) design methods by develop Flexible Pavement

Software (FPS). The software determines the layer thicknesses of flexible pavement

structure using both Ethiopian Roads Authority (ERA, 2013) and American Association

of State Highway and Transportation Officials (AASHTO, 1993) methods. It also

determines the life cycle cost of the project based on Net Present Value (NPV) method.

The software was validated and the results obtained were found absolutely accurate.

Flexible Pavement Software (FPS) is very important as it makes the design process very

easy, accurate and saves a lot of precious time and cost, and determines the life cycle cost

of the project. Also, increase the value to client by delivering more design alternatives in

less time. So, the application of this software will be of great help by avoiding the

precision errors that could result in a conservative design or an under design.

KEY WORDS: ERA Flexible Pavement Design, AASHTO Flexible Pavement Design,

Net Present Value, FPS

v

TABLE OF CONTENTS

DECLARATION............................................................................................................................ i

ACKNOLEDGEMNTS ................................................................................................................ ii

ABSRTACT .................................................................................................................................. iv

TABLE OF CONTENTS ............................................................................................................. v

LIST OF FIGURES .................................................................................................................... vii

LIST OF TABLES ....................................................................................................................... ix

ABBREVIATIONS ....................................................................................................................... x

1. INTRODUCTION..................................................................................................................... 1

1.1 Background ............................................................................................................... 1

1.2 Problem Statement .................................................................................................... 3

1.3 Objective of the Study ............................................................................................... 3

1.3.1 General Objective ............................................................................................... 3

1.3.2 Specific Objective ............................................................................................... 4

1.4 Scope of the study ..................................................................................................... 4

1.5 Significant of the Study ............................................................................................. 4

2. LITRATURE REVIEW ........................................................................................................... 5

2.1 Flexible Pavement ..................................................................................................... 5

2.1.1 Flexible Pavement Layers .................................................................................. 6

2.2 Flexible Pavement Design ......................................................................................... 7

2.2.1 AASHTO Design Method .................................................................................. 8

2.2.2 ERA Design Method ........................................................................................ 15

2.3 Life Cycle Cost Analysis (LCCA) .......................................................................... 21

2.3.1 Purpose of LCCA ............................................................................................. 21

2.3.2 LCCA Procedures ............................................................................................. 22

2.3.3 Net Present Value (NPV) ................................................................................. 22

2.3.4 Estimate Agency Costs ..................................................................................... 25

2.3.5 Discount Rate ................................................................................................... 26

2.3.6 Analysis Period ................................................................................................. 26

2.3.7 Salvage Value ................................................................................................... 27

2.3.8 Maintenance and Rehabilitation Alternatives................................................... 27

vi

2.3.9 Performance Periods and Activity Timing ....................................................... 28

3. METHODOLOGY ................................................................................................................. 29

3.1 Introduction ............................................................................................................. 29

3.2 Design Procedures of AASHTO, 1993 ................................................................... 29

3.2.1 Design Input Parameters for AASHTO Method .............................................. 29

3.3 Design Procedures of ERA, 2013 ........................................................................... 36

3.3.1 Design Process .................................................................................................. 36

3.3 Life cycle cost Analysis (LCCA) ............................................................................ 37

3.4.1 Net Present Value (NPV) ................................................................................. 38

3.4.2 LCCA Procedure .............................................................................................. 38

3.6 Research Methods ................................................................................................... 41

4. RESULTS AND DISCUSSION ............................................................................................. 43

4.1 Introduction ............................................................................................................. 43

4.2 Results and Discussion ............................................................................................ 43

4.2.1 Getting Started Flexible Pavement Software (FPS) ......................................... 43

4.2.2 Design based on ERA, 2013 ............................................................................. 45

4.2.3 Design based on AASHTO, 1993 ..................................................................... 56

4.3 Validation of Flexible Pavement Software (FPS) ................................................... 63

5. CONCLUSIONS AND RECOMMENDATIONS ................................................................ 66

5.1 Conclusion ............................................................................................................... 66

5.2 Recommendation ..................................................................................................... 66

REFERENCES ............................................................................................................................ 68

APPENDIX .................................................................................................................................. 70

Appendix 1: Sample Code on Flexible Pavement Software (FPS) ............................... 70

Appendix 2: Design Example 1 .................................................................................... 72

Appendix 3: Design Example 2 .................................................................................... 74

vii

LIST OF FIGURES

Figure 1: AASHTO nomograph for Flexible Pavement Design ....................................... 10

Figure 2: Pavement Design with Empirical AASHTO Design Equation. ........................ 12

Figure 3: Selection of thicknesses..................................................................................... 14

Figure 4: Ethiopian Roads Authority (ERA, 2013) structural catalogue sample. ............ 20

Figure 5: Material definition sample for structural catalogue (ERA, 2013). .................... 21

Figure 6: Example of expenditure stream diagram. ......................................................... 23

Figure 7: Maintenance and Rehabilitation activities during analysis period. ................... 27

Figure 8: Flow chart of the VB.NET code. ....................................................................... 41

Figure 9: Progress bar appeared when running the software ............................................ 44

Figure 10: Tip of the day dialog box. ............................................................................... 44

Figure 11: Main Window of Software .............................................................................. 44

Figure 12: Dropdown list of File menu............................................................................. 45

Figure 13: Select the desired design method. ................................................................... 45

Figure 14: Select ERA 2013 from dropdown list of Main graphical user interface. ........ 46

Figure 15: Input general data for all sections. ................................................................... 47

Figure 16: Selecting a specific homogeneous section that we want to do. ....................... 47

Figure 17: Input data for a specific homogeneous section. .............................................. 48

Figure 18: Input traffic related data for each vehicle category. ........................................ 49

Figure 19: Vehicle classification according to ERA, 2013 manual .................................. 49

Figure 20: Input cost estimation parameters. .................................................................... 50

Figure 21: Input cost estimation parameters for each alternative. .................................... 51

Figure 22: Select each alternatives and input cost estimation parameters. ....................... 51

Figure 23: Description of materials for each layer. .......................................................... 52

Figure 24: Description for charts. ..................................................................................... 52

Figure 25: Select ERA 2013 from output dropdown list. ................................................. 53

Figure 26: Life cycle cost and layer thicknesses of each section. .................................... 54

Figure 27: Layer thicknesses and life cycle cost. ............................................................. 54

Figure 28: Select chart for ERA 2013 from output dropdown list. .................................. 55

Figure 29: Chart for life cycle cost. .................................................................................. 55

Figure 30: Select ERA 2013 from output dropdown list. ................................................. 56

viii

Figure 31: Input general data of all sections. ................................................................... 57

Figure 32: Input data for a specific homogeneous section. .............................................. 57

Figure 33: Input cost estimation parameters for AASHTO, 1993 method. ..................... 58

Figure 34: Input treatment type and the corresponding treatment cost. ........................... 59

Figure 35: Select AASHTO 1993 from output dropdown list. ......................................... 60

Figure 36: Layer thicknesses and life cycle cost of a specific homogeneous section. ..... 60

Figure 37: Layer thickness and life cycle cost of each sections. ...................................... 61

Figure 38: Drawing for layer thicknesses of each homogeneous section. ........................ 62

Figure 39: Select chart for AASHTO 1993 from output dropdown list. .......................... 62

Figure 40: Chart for layer thicknesses and life cycle cost. ............................................... 63

ix

LIST OF TABLES

Table 1: Minimum thickness (inches) ............................................................................... 13

Table 2: Design Period (ERA, 2013) ................................................................................ 17

Table 3: Traffic Classes for Flexible Pavement Design (ERA, 2013) .............................. 19

Table 4: Subgrade Strength Classes (ERA, 2013) ............................................................ 20

Table 5: 18-kip ESAL in design lane ................................................................................ 30

Table 6: Suggested level of Reliabilities (AASHTO, 1993)............................................. 31

Table 7: Standard Normal Deviates (AASHTO, 1993) ................................................... 32

Table 8: Recommended values for Modifying Layer Coefficients .................................. 35

Table 9: Validation of Flexible Pavement Software (FPS) for thickness design. ............ 64

Table 10: Validation for Life Cycle Cost (LCC) determination ....................................... 64

Table 11: Summary of FPS validation using AASHTO, 1993 ......................................... 65

Table 12: Summary of FPS validation using Michael S. Mamlouk ................................. 65

Table 13: Summary of FPS validation for Net Present Value (NPV) .............................. 65

x

ABBREVIATIONS

a1, a2, a3– Layer coefficients of surface course, base course and subbase

courserespectively

AADT– Average Annual Daily Traffic

AASHO–American Association of State Highway Officials

AASHTO – American Association of State Highway and Transportation Officials

AC – Asphalt Concrete

CBR – California Bearing Ratio

D1, D2, D3–Actual thicknesses (in inches) of surface, base and sub base courses

respectively

DD –Directional Distribution factor

DL –Lane Distribution factor

EALF –Equivalent Axle Load Factor

ERA–Ethiopian Roads Authority

ESA – Equivalent Standard Axle

FPS – Flexible Pavement Software

LCC – Life Cycle Cost

LCCA – Life Cycle Cost Analysis

m2, m3– Drainage coefficients for base and subbase layers respectively

MR–Resilient Modulus (psi)

NPV – Net Present Value

Po –Initial serviceability index

Pt–Terminal serviceability index

xi

SN – Structural Number

So–Combined standard error of traffic prediction

W18–Predicted number of 18kip traffic load application (ESAL)

ZR–Standard normal deviate

ΔPSI –Serviceability change during the design period

1

1. INTRODUCTION

1.1 Background

Roads are the arteries through which the economy pulses. Roads make a crucial

contribution to economic development and growth and bring important social benefits.

They are of vital importance in order to make a nation grow and develop. In addition,

providing access to employment, social, health and education services makes a road

network crucial in fighting against poverty. Roads open up more areas and stimulate

economic and social development. For those reasons, road infrastructure is the most

important of all public assets.

Nowadays, the number and type of traffic increases from day to day throughout the world

and in a country like Ethiopia the change is alarming. This leads to the construction of

road infrastructures which needs economical and safe design of roads. The most common

type of pavement used in Ethiopia is flexible pavement.

The design of thickness of the flexible pavement has taken the backbone place in

determining the overall performance and providing high level of serviceability of the

pavement structure for the heavy traffic loads under the adverse climatic conditions,

during the expected design period. Pavement design is the process of developing the most

economical combination of pavement layers with respect to both material type and

thickness to suit the soil foundation and the traffic load during the design period.

There are different methods for the design of pavement structures. The design of flexible

pavements in our country is based on the prevailing condition of soil and materials report

using the Ethiopian Roads Authority (ERA) Pavement Design Manual, Volume 1 for

Flexible Pavements and Gravel Roads, where the results obtained will be compared with

that of the AASHTO Structural Design of Flexible Pavements manual. Finally, the

thickness obtained using both design guides will be compared (Ethiopian Roads

Authority, 2013).

In this thesis the design procedure by AASHTO and ERA are implemented by Visual

Basic.NET. AASHTO design procedure is a result of empirical equations which were

developed as a result of AASHO Road Test which was performed from 1956 to 1960 in

2

Ottawa, IL. An empirical approach is dependent on experiments and experience. This

means that the relationship between the input variables and the design thicknesses are

arrived at through experiment, experience or a combination of the two. Therefore, the

AASHTO design procedure is limited to the set of conditions and material types which

were implemented during the AASHO Road Test.

The prime factor influencing the structural design of a pavement is the load-carrying

capacity required. The thickness of pavement necessary to provide the desired load-

carrying capacity is a function of vehicle wheel load or axle load, configuration of

vehicle wheels or tracks, volume of traffic during the design life of pavement, Soil

strength and modulus of rupture (flexural strength) for concrete pavements.

Surveys show that adequately maintaining road infrastructure is very essential. But a

backlog of outstanding maintenance has caused irreversible deterioration of the road

network. If insufficient maintenance is carried out, roads can need replacing or major

repairs after just a few years. That deterioration spread across a road system very quickly

results in soaring costs and a major financial impact on the economy and citizens. With

this in mind, the importance of maintenance needs to be recognized by decision-makers.

An economic analysis process known as Life-Cycle Cost Analysis (LCCA) is used to

evaluate the cost-efficiency of alternatives based on the Net Present Value (NPV)

concept. It is essential to consider maintenance and rehabilitation costs in addition to

initial construction cost in order to obtain optimum pavement life-cycle costs. LCC refers

to all costs related to a highway over the life cycle of the pavement structure. These cost

components include capital costs, maintenance costs, rehabilitation including overlay

costs, as well as residual value, and user costs. User costs are generally more difficult to

quantify compared to the other input costs, and for the purposes of this research, user

costs are not generally included in the analysis.

Highway pavements can be designed with many possible combinations of construction

and maintenance and rehabilitation (M&R) strategies. It is desirable to find the optimal

pavement structure, in terms of minimum cost while satisfying the engineering

constraints, by computer technology. Thus, there is a need to develop new computerized

3

method to provide highway agencies with a better and more efficient decision-aid tool for

pavement design and management.

The aim of this research is to develop a software package for flexible pavement design

and life cycle cost determination. The software can provide cost comparison for different

alternatives based on life cycle cost (LCC) in addition to design layer thicknesses of

flexible pavement.

1.2 Problem Statement

The design of the flexible pavement requires the field tests and survey, and besides these

requires lots of tedious calculations with the use of various graphs and tables, which

makes it a difficult job to do and there occurs lots of errors and mistakes during the

design stage, which results into the failure of the road (Rafi Ullah Khan et. al, 2012).

In Ethiopia there is no software package for flexible pavement design, maintenance and

rehabilitation. It is not common to use software-based design of flexible pavements.

Therefore, human mistake and error cannot be fully avoided in the design which

influences the quality of the design and development of the science (Amare Setegn,

2012).

The pavement design procedure by American Association of State Highway and

Transportation Officials (AASHTO) using nomographs could be inconsistent as different

results could be obtained by different users for the same input parameters.

1.3 Objective of the Study

1.3.1 General Objective

This research aims to provide a package for flexible pavement design based on Ethiopian

Roads Authority (ERA, 2013) and American Association of State Highway and

Transportation Officials (AASHTO, 1993) design methods by develop a software

program. The software is used to simplify the design process, determine life cycle cost,

save the precious time, reduce errors and increase the value to client by delivering more

design alternatives in less time.

4

1.3.2 Specific Objective

The specific objectives are to develop software that could be utilized to;

1. Design a pavement layer thicknesses based on ERA, 2013 method.

2. Design the pavement layer thicknesses based on AASHTO, 1993 method.

3. Estimate the life cycle cost of the project.

1.4 Scope of the study

The study focuses on developing Flexible Pavement Software (FPS) for the flexible

pavement thickness design by using Ethiopian Roads Authority (ERA 2013) and

American Association of State Highway and Transportation Officials (AASHTO 1993)

design manuals. Furthermore, determine life cycle cost of flexible pavement based on Net

Present Value (NPV) method. The study includes validation analysis by comparing

Flexible Pavement Software (FPS) outputs and the manual design outputs.

1.5 Significant of the Study

In Ethiopia there is no software package for flexible pavement design and life cycle cost

determination. The development of software for the flexible pavement design is very

important for our country as it provides a package and makes the tedious design process

very easy, accurate and saves a lot of precious time. Also, increase the value to client by

delivering more design alternatives in less time. So, the application of this software will

be of great help by avoiding the precision errors that could result in a conservative design

or an under design.

5

2. LITRATURE REVIEW

Pavement is the structure which consists of the superimposed layers of the processed

materials that keep apart the tyres of vehicles from the materials used as foundation and

the soil subgrade which distributes the load coming from vehicles, and protects it from

failure (Rafi Ullah Khan et al., 2012).

The primary function of the pavement structure is to reduce and distribute the surface

stresses (contact tire pressure) to an acceptable level at the subgrade (to a level that

prevents permanent deformation) (K. Ozbayet al., 2003).

The purpose of a pavement is to carry traffic safely, conveniently and economically over

its extended life. The pavement must provide smooth riding quality with adequate skid

resistance and have adequate thickness to ensure that traffic loads are distributed over an

area so that the stresses and strains at all levels in the pavement and subgrade are within

the capabilities of the materials at each level. The performance of the pavement therefore

related to its ability to serve traffic over a period of time. From the day it is opened to

traffic, a pavement will suffer progressive structural deterioration. It is possible that the

pavement may not fulfill its intended function of carrying a projected amount of traffic

during its design life, because the degree of deterioration is such that reconstruction or

major structural repair is necessitated before the end of the design life (K. Ozbayet al.,

2003).

2.1 Flexible Pavement

Flexible pavements are those which are surfaced with bituminous (or asphalt) materials.

These types of pavements are called “flexible” since the total pavement structure “bends”

or “deflects” due to traffic loads. A flexible pavement structure is generally composed of

several layers of materials which can accommodate this “flexing” (K. Ozbayet al.,2003).

A flexible pavement reduces the stresses by distributing the traffic wheel loads over

greater and greater areas, through the individual layers, until the stress at the subgrade is

at an acceptably low level. The traffic loads are transmitted to the subgrade by aggregate-

to-aggregate particle contact. Confining pressures (lateral forces due to material weight)

in the subbase and base layers increase the bearing strength of these materials. A cone of

6

distributed loads reduces and spreads the stresses to the subgrade as sited in (K. Ozbay, et

al., 2003).

Each layer of a flexible pavement structure receives loads from the above layer, spreads

them out, and passes on these loads to the next layer below. Thus, the stresses will be

reduced, which are maximum at the top layer and minimum on the top of subgrade. In

order to take maximum advantage of this property, layers are usually arranged in the

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) on the bottom.

2.1.1 Flexible Pavement Layers

Flexible pavements generally consist of a prepared road bed underlying layers of

subbase, base, and surface courses. In some cases, the sub base and/ base will be

stabilized to maximize the use of local materials.

2.1.1.1 Surface Course

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

superior quality materials. It is usually constructed with dense graded asphalt concrete

(AC). 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.

2.1.1.2 Base course

The base course is the portion of the pavement structure immediately beneath the surface

course. It is constructed on the subbase course, or, if no subbase is used, directly on the

road bed soil. Its major function in the pavement is structural support (AASHTO, 1993).

It provides additional load distribution and contributes to the sub-surface drainage.

7

2.1.1.3 Subbase Course

The subbase course is the portion of the flexible pavement structure between the roadbed

soil and the base course. It usually consists of a compacted layer of granular

material,either treated or untreated, or a layer of soil treated with a suitable admixture. In

addition to its position in the pavement, it is usually distinguished from the base course

material by less stringent specification requirements for strength, plasticity, and

gradation. The subbase material should be of significantly better quality than the roadbed

soil. For reasons of economy, the subbase is often omitted if roadbed soils are of high

quality. Because lower quality materials may be used in the lower layers of a flexible

pavement structure, the use of a subbase course is often the most economical solution for

construction of pavements over poor roadbed soils (American Association of State

Highway Transportation Officials, 1993).

2.1.1.4 Prepared Roadbed

The prepared roadbed is a layer of compacted roadbed soil or select borrow material

which has been compacted to a specified density (AASHTO, 1993).

2.2 Flexible Pavement Design

Pavement design is the process of developing the most economical combination of

pavement layers with respect to both material type and thickness to suit the soil

foundation and the traffic load during the design period.

Effective pavement design is one of the important aspects of project design. The

pavement is the portion of the highway, which is most obvious to the motorist. The

condition and adequacy of the highway are often judged by the smoothness or roughness

of the pavement. Deficient pavement conditions can result in increased user costs and

travel delays, braking and fuel consumption, vehicle maintenance repairs and the

probability of increased crashes. The pavement life is substantially affected by the

number of heavy load repetitions applied, such as single, tandem, tridem and quad axle

trucks, buses, tractor trailers and equipment. A properly designed pavement structure will

take into account the applied loading (Project Development & Design Guide, 2006).

8

Road pavements are designed to limit the stress created at the subgrade level by the

traffic travelling on the pavement surface so that the subgrade is not subject to significant

deformations. The pavement spreads the concentrated loads of the vehicle wheels over a

sufficiently large area at subgrade level. At the same time, the pavement materials

themselves should not deteriorate to any serious extent within a specified period of time

(ERA, 2013).

However, it is inevitable that road pavements will deteriorate with time and traffic,

therefore, the goal of pavement design is to limit, during the period considered, the

deterioration which affects the riding quality of the road, such as rutting, cracking,

potholes and other such surface distresses, to acceptable levels (Flintsch and Kuttesch,

2004).

At the end of the design period, a strengthening overlay would normally be required but

other remedial treatments, such as major rehabilitation or reconstruction, may be needed.

The design method aims at producing a pavement which will reach a relatively low level

of deterioration at the end of the design period, assuming that routine and periodic

maintenance are performed during that period (Flintsch and Kuttesch, 2004).

The design of flexible pavements involves a study of soils and paving materials, their

behavior under load and the design of the pavement structure to carry that load under all

climatic conditions. Additionally, in the cases of new construction, the thickness of the

designed pavement structure is a necessary input into the geometric design of the

roadway.

2.2.1 AASHTO Design Method

The design method developed by American Association of State Highway and

Transportation officials (AASHTO) is an empirical method based on the tests results

conducted in Ottawa and Illinois (Rafi Ullah Khanet al., 2012). An empirical approach is

dependent on experiments and experience. This means that the relationship between the

input variables and the design thicknesses are arrived at through experiment, experience

or a combination of the two. Therefore, the AASHTO design procedure is limited to the

set of conditions and material types which were implemented during the AASHO Road

Test.

9

The AASHTO flexible design procedure solely depends on the design equations

developed after the road test, and a series of nomographs. The pavement has to be

designed for the traffic loading and the stresses caused by the temperature and moisture

variations, incorporating various design variables and time constraints.

2.2.1.1 The AASHTO Design Procedure

The AASHTO design procedure for the flexible pavement design is based on an

empirical equation. The AASHO road test established a correlation between soil

condition, traffic, change in pavement condition and pavement structure. This

relationship is shown in equation 2.1. The equation incorporates a term called Structural

Number (SN). It can be defined as „‟an abstract number expressing the structural strength

of a pavement structure required for a given combination of soil support (MR), traffic

expressed in equivalent single 18 kips axle (ESAL), final serviceability and

environment‟‟ (AASHTO, 1993).

ESALs are represented by the W18 term. The ZR and So terms are reliability and

variability factors not originally part of the AASHTO design procedure but added later to

describe the ability of the pavement to function under the design conditions, essentially

acting as factors of safety. The other quantities in the equation are regression coefficients

that provided the best match between the independent variables (SN, ∆PSI, MR) and the

performance of the pavement section as qualified by ESALs (David H.et al.,2014).

( ) ( ) (

)

( )

( ) Equation 2.1

Where the variables are defined as:

W18 = predicted number of 18kip traffic load application (ESAL).

ZR = standard normal deviate.

So = combined standard error of traffic prediction.

SN = structural number.

ΔPSI = serviceability change during the design period.

10

MR = resilient modulus (psi).

While the purpose of equation 2.1 is to determine the required structural number of a

proposed pavement section, it is written to compute ESALs (W18), and solving

algebraically for SN is a daunting task. Once all the input parameters for a specific

pavement design are determined, one can compute the corresponding thickness of the

structure for a design value of traffic load by iteration of the empirical equation. To

alleviate this problem, AASHTO published a design nomograph (Figure 1) that solves for

SN given the other inputs. Notice that W18 (ESALs) is treated as another input with the

nomograph solving toward SN (David H. et al., 2014).

Figure 1: AASHTO nomograph for Flexible Pavement Design

The AASHTO design equation (Equation 2.1 or Figure 1) is to be used successively for

each layer in a multilayer pavement structure to determine the required pavement

thickness. As described by AASHTO, this operation is performed in a top – down fashion

as depicted in Figure 2. The design begins by finding the required structural number

above the granular base (SN1) using the granular base modulus and other input

parameters in the design equation or nomograph. By definition, this structural number is

the product of the structural coefficient and thickness of layer one, and is used to solve

for the thickness of the first layer. This product is followed for each of the subsequent

11

layers, as shown in Figure 2, to arrive at a unique set of pavement layer thicknesses

(David H. et al., 2014).

The thickness of each layer is computed by Layered Design Analysis in which the

structural number for each layer is first computed and then the corresponding thicknesses.

2.2.1.2 Layered Design Analysis

It should be recognized that, for flexible pavements, the structure is a layered system and

should be designed accordingly. The structure should be designed in accordance with the

principles shown in Figure 2. First, the structural number required over the road bed soil

should be computed. In the same way, the structural number required over the sub base

layer and the base layer should also be computed, using the applicable strength values for

each. By working with differences between the computed structural numbers required

over each layer, the maximum allowable thickness of any given layer can be computed.

For example, the maximum allowable structural number for the sub base material would

be equal to the structural number required over the sub base subtracted from the structural

number required over the road base soil. In a like manner, the structural number of the

other layers may be computed. The thicknesses for the respective layers may then be

determined as indicated on Figure 2 (AASHTO, 1993).

The AASHTO method utilizes a step by step method of analyzing the layer thicknesses

by first iterating the structural number. Once the design structural number (SN) for an

initial pavement structure is determined, it is necessary to identify a set of pavement layer

thicknesses which, when combined, will provide the load carrying capacity

corresponding to the design structural number. The structural number for each layer is

converted into the corresponding thicknesses by means of appropriate layer coefficients

and drainage coefficients. The following equation provides the basis for converting

structural number in to layer thicknesses (David H. et al.,2014):

SN = a1 * D1 + a2 * D2 * m2 + a3 * D3 * m3Equation 2.2

Where:

a1, a2, a3 = layer coefficients representative of surface, base and sub base layers,

12

respectively

D1, D2, D3 = actual thicknesses (in inches) of surface, base and sub base courses

respectively

m2, m3 = drainage coefficients of base and sub base courses respectively

Figure 2: Pavement Design with Empirical AASHTO Design Equation.

It should be recognized that this procedure should not be applied to determine the

structural number required above sub base or base materials having a modulus greater

than 40,000 psi. For such cases, layer thickness of materials above the “high” modulus

layer should be established based on cost effectiveness and minimum practical thickness

considerations.

13

The structural number equation does not have a single unique solution; i.e., there are

many combinations of layer thicknesses that are satisfactory solutions. The thickness of

the flexible pavement layers should be rounded to the nearest ½ inch. When selecting

appropriate values for the layer thicknesses, it is necessary to consider their cost

effectiveness along with the construction and maintenance constraints in order to avoid

the possibility of producing an impractical design (AASHTO, 1993).

From a cost-effective view, if the ratio of costs for layer 1 to layer 2 is less than the

corresponding ratio of layer coefficients times the drainage coefficient, then the optimum

economical design is one where the minimum base thickness is used. Since it is generally

impractical and uneconomical to place surface, base, or sub base courses of less than

some minimum thickness, the following are provided as minimum practical thicknesses

for each pavement course (Table 1)(AASHTO, 1993):

Table 1: Minimum thickness (inches)

Traffic, ESAL‟s Asphalt Concrete Aggregate Base

Less than 50,000 1.0 (or surface treatment) 4

50,001 – 150,000 2.0 4

150,001 – 500,000 2.5 4

500,001 – 2,000,000 3.0 6

2,000,001 – 7,000,000 3.5 6

Greater than 7,000,000 4.0 6

Because such minimums depend somewhat on local practices and conditions, individual

design agencies may find it desirable to modify the above minimum thicknesses for their

own use. Individual agencies should also establish the effective thicknesses and layer

coefficients of both single and double surface treatments. The thickness of the surface

treatment layer may be neglectable in computing structural number, but its effect on the

base and subbase properties may be large due to reductions in surface water entry

(AASHTO, 1993).

General Procedure

The procedure for thickness design is usually started from the top, as shown in Figure and

described as follows (Yang H. Huang, 2004):

14

Figure 3: Selection of thicknesses

1. Use E2 as MR, determine the structural number SN2 required to protect the base,

and compute the thickness of layer 1 from:

D1 ≥ SN1 / a1 Equation2.3

2. Using E3 as MR, determine the structural SN2 required to protect the subbase,

and compute the thickness of layer 2 from:

D2 ≥ (SN2 – a1D1) / (a2m2) Equation 2.4

3. Based on the roadbed soil resilient modulus MR, determine the total structural

number SN3 required, and compute the thickness of layer 3 from:

D3 ≥ (SN3 – a1D1 – a2D2m2 / (a3m3) Equation 2.5

2.2.1.3 AASHTO Empirical Design Limitations

It is extremely important to know the equation's limitations and basic assumptions when

using the 1993 AASHTO Guide empirical equation, otherwise, this can lead to invalid

results at the least and incorrect results at the worst.

Though the empirical AASHTO design procedure has been used since the 1960‟s, there

are many factors that limit its continued use and provide motivation for developing and

implementing more modern methods. Most notably among these factors is the very

nature of the method itself: empirical. This term means that, the design equations

described above are strictly limited to the conditions of the original road test. This

includes all the coefficients in equation 2.1, the structural coefficients (ai), drainage

coefficients (mi), ESAL equations and so forth. Any deviation from these conditions

results in an unknown extrapolation. The limitations of the AASHO road test are

numerous. The experiment had one soil type, one climate, one type of asphalt mix (pre-

marshal mix design), limited load applications and tires inflated to 70psi. Any deviation

from these factors in modern design means extrapolation, which can lead to under or over

15

– design. Most designs conducted today are extrapolations beyond the original

experimental conditions.

When using the 1993 AASHTO Guide empirical equation, it is extremely important to

know the equation‟s limitations and basic assumptions. Otherwise, it is quite easy to use

an equation with conditions and materials for which it was never intended. This can lead

to invalid results at the least and incorrect results at the worst. From the AASHO Road

Test, equations were developed which related loss in serviceability, traffic, and pavement

thickness. Because they were developed for the specific conditions of the AASHO Road

Test, these equations have some significant limitations:

The equations were developed based on the specific pavement materials and

roadbed soil present at the AASHO Road Test.

The equations were developed based on the environment at the AASHO Road

Test only.

The equations are based on an accelerated two-year testing period rather than a

longer, more typical 20+ year pavement life. Therefore, environmental factors

were difficult if not impossible to extrapolate out to a longer period.

The loads used to develop the equations were operating vehicles with identical

axle loads and configurations, as opposed to mixed traffic.

In order to apply the equations developed as a result of the AASHO Road Test, some

basic assumptions are needed:

The characterization of subgrade support may be extended to other subgrade soils

by an abstract soil support scale.

Loading can be applied to mixed traffic by use of ESALs.

Material characterizations may be applied to other surfaces, bases, and subbases

by assigning appropriate layer coefficients.

The accelerated testing done at the AASHO Road Test (2-year period) can be

extended to a longer design period.

2.2.2 ERA Design Method

2.2.2.1 Ethiopian Roads Authority (ERA 2013) Manual

16

The manual gives recommendations for the structural design of „flexible‟ pavements in

Ethiopia. The manual is appropriate for roads which are required to carry up to 80 million

cumulative equivalent standard axles in one direction. This upper limit is suitable at

present for the most heavily trafficked roads in Ethiopia (ERA, 2013).

To resist deterioration, a flexible pavement must satisfy some requirements. The principal

structural requirements are as follows:

1. The subgrade should be able to sustain traffic loading without excessive

deformation; this is controlled by the vertical compressive stress or strain at this

level.

2. Bituminous materials and cement-bound materials used in road base design

should not crack under the influence of traffic; this is controlled by the horizontal

tensile stress or strain at the bottom of the bound layer.

3. The road base is often the main structural layer of the pavement, required to

distribute the applied traffic loading so that the underlying materials are not

overstressed. It must be able to sustain the stress and strain generated within itself

without excessive or rapid deterioration of any kind.

4. In pavements containing bituminous materials, the internal deformation of these

materials must be limited.

5. The load spreading ability of granular sub-base and capping layers must be

adequate to provide a satisfactory construction platform.

When some of the above criteria are not satisfied, distress or failure will occur. For

instance, rutting may be the result of excessive internal deformation within bituminous

materials, or excessive deformation at the subgrade level (or within granular layers

above) (ERA, 2013).

2.2.2.2 Design based on Ethiopian Roads Authority (ERA 2013)

The design of flexible pavements is based on the catalogue of pavement structures

published in TRL‟s Overseas Road Note 31. Thus, ERA manual structural catalogue had

been produced in order to design the flexible pavement thickness design based on the

17

traffic and subgrade strength classes‟ requirement (ERA, 2013). Therefore, the

thicknesses of each layers are determined from traffic class and subgrade class.

Design Period

Determining an appropriate design period is the first step towards pavement design.

Many factors may influence this decision, including budget constraints. However, the

designer should follow certain guidelines in choosing an appropriate design period,

taking into account the conditions governing the project. Some of the points to consider

include:

i) Functional importance of the road

ii) Traffic volume

iii) Location and terrain of the project

iv) Financial constraints

v) Difficulty in forecasting traffic

Usually it is economical to construct roads with longer design periods for important roads

and for roads with high traffic volume. Where rehabilitation would cause major

inconvenience to road users, a longer period may be used. For roads in difficult locations

and terrain where regular maintenance proves to be costly and time consuming because

of poor access and non-availability of nearby construction material sources, a longer

design period is also appropriate.

Difficulties in traffic forecasting may also influence the design period. When accurate

traffic estimates cannot be made, it may be advisable to reduce the design period to avoid

costly overdesign and to adopt a stage construction strategy to cater for unexpected traffic

growth. Table 2 shows the general guidelines:

Table 2: Design Period (ERA, 2013)

Road Classification Design Period (Years)

Trunk Road 20

Link Road 20

Main Access Road 15

Other Roads 10

Traffic Forecast

18

Even with stable economic conditions, traffic forecasting is an uncertain process.

Although the pavement design engineer may often receive help from specialized

professionals at this stage ofthe traffic evaluation, some general remarks are in order.To

forecast traffic growth, it is usually necessary to separate traffic into the following three

categories:

Normal traffic: Traffic which would pass along the existing road or track even if no new

pavement were provided.

Diverted traffic:Traffic that changes from another route (or mode of transport) to the

project road because of the improved pavement, but still travels between the same origin

and destination.

Generated traffic:Additional traffic which occurs in response to the provision or

improvement of the road.

Traffic Classes for Flexible Pavement Design

Accurate estimates of cumulative traffic are difficult to achieve due to errors in the

surveys and uncertainties with regard to traffic growth, axle loads and axle equivalencies.

To a reasonable extent, however, pavement thickness design is not very sensitive to

cumulative axle loads and the method recommended in this manual provides fixed

structures of paved roads for ranges of traffic as shown in Table 3.

As long as the estimate of cumulative equivalent standard axles is close to the center of

one of the ranges, any errors are unlikely to affect the choice of pavement design.

However, if estimates of cumulative traffic are close to the boundaries of the traffic

ranges, then the basic traffic data and forecasts should be re-evaluated and sensitivity

analyses carried out to ensure that the choice of traffic class is appropriate. Depending on

the degree of accuracy achieved, if in doubt, selecting the next higher traffic class may be

appropriate (ERA, 2013).

19

Table 3: Traffic Classes for Flexible Pavement Design (ERA, 2013)

Traffic Classes Range of ESAs (millions)

LV1 <0.01

LV2 0.01 – 0.1

T1/LV3 (see note) 0.1 – 0.3

T2/LV4 (see note) 0.3 – 0.5

T2/LV5 (see note) 0.5 – 0.7

T3 0.7 – 1.5

T4 1.5 – 3.0

T5 3.0 – 6.0

T6 6.0 – 10

T7 10 – 17

T8 17 – 30

T9 30 – 50

T10* 50 – 80

T11 >80 Notes. There are more options available for the Low Volume classes which use granular

unbound road bases and sub-bases (i.e. Chart A). These are dealt with in the ERA

LVR Design Manual.

*T10 is suitable for traffic up to 80 mesas. At this level the pavement is expected

to be „long-life and suitable for higher traffic levels.

Design Subgrade Strength

To determine the subgrade strength for the design of the road pavement, it is necessary to

first determine the density-moisture content-strength relationship(s) specific to the

subgrade soil(s) encountered along the road under study. It is then necessary to select the

density which will be representative of the subgrade once compacted and to estimate the

subgrade moisture content that will ultimately govern the design, i.e. the moisture content

after construction.

20

Design Subgrade Strength Class

The structural catalogue given in the manual (Figure 4) requires that the subgrade

strength for design be assigned to one of six strength classes reflecting the sensitivity of

thickness design to subgrade strength. Sample definition of material for structural catalog

is shown in Figure 5. The classes are defined in Table 4. For subgrades with CBRs less

than 2, special treatment is required.

Table 4: Subgrade Strength Classes (ERA, 2013)

Subgrade Class CBR Range (%)

S1 <3

S2 3,4

S3 5,6,7

S4 8-14

S5 15-30

S6 >30

Figure 4: Ethiopian Roads Authority (ERA, 2013) structural catalogue sample.

21

Figure 5: Material definition sample for structural catalogue (ERA, 2013).

2.3 Life Cycle Cost Analysis (LCCA)

Too often in the past, design alternatives have considered only structural sections or

design strategies which are expected to last the entire predicted service life or selected

performance period. The life – cycle economics and the interaction of initial construction

and subsequent overlay were often not included in past design analysis.

Life-cycle cost analysis is a tool that can help evaluate the long-term benefit of structures.

However, it must be correctly used and the data used in conducting LCCA must be

derived from existing records that accurately reflect the expectations for the initial cost,

rehabilitation timing and costs, salvage value, and discount rate (ERA, 2013).

LCC refer to all costs related to a highway over the life cycle of the pavement structure.

These cost components include capital costs, maintenance costs, rehabilitation including

overlay costs, as well as residual value, and user costs (Khaled A. andAbaza, 2002). User

costs are generally more difficult to quantify compared to the other input costs, and for

the purposes of this research, user costs are not generally included in the analysis.

2.3.1 Purpose of LCCA

LCCA is an analysis technique that builds on the well-founded principles of economic

analysis to evaluate the over-all-long-term economic efficiency between competing

alternative investment options. It does not address equity issues. It incorporates initial and

discounted future agency, user, and other relevant costs over the life of alternative

investments. It attempts to identify the best value (the lowest long-term cost that satisfies

22

the performance objective being sought) for investment expenditures (James Walls III

and Michael R. Smith, 1998).

2.3.2 LCCA Procedures

Life Cycle Cost (LCC) analysis should be conducted as early in the project development

cycle as possible. For pavement design, the appropriate time for conducting the LCCA is

during the project design stage. The LCCA level of detail should be consistent with the

level of investment. Typical LCCA models based on primary pavement management

strategies can be used to reduce unnecessarily repetitive analyses (James Walls III and

Michael R. Smith, 1998).

LCCA need only consider differential cost among alternatives. Costs common to all

alternatives cancel out, are generally so noted in the text, and are not included in LCCA

calculations. Inclusion of all potential LCCA factors in every analysis is

counterproductive; however, all LCCA factors and assumptions should be addressed,

even if only limited to an explanation of the rationale for not including eliminated factors

in detail. Sunk costs, which are irrelevant to the decision at hand, should not be included.

There are several economic models applicable to the evaluation and comparison of

alternative pavement design/rehabilitation strategies, all of which incorporate to varying

degrees, future costs and/or benefits. A very basic method utilized by many transportation

agencies, is the present worth method, using costs alone.

2.3.3 Net Present Value (NPV)

The Net Present Value (NPV) is defined as the present value of the benefits minus the

present value of operating costs. This method discounts all future sums to the present

using an appropriate discount rate. NPV is determined by discounting all project costs to

the base, or present, year (usually the present year, year of construction or year of

authorization). Thus, the entire project can be expressed as a single base year, or present

year, cost. Alternatives are then compared by comparing these base year costs.

Performance periods for individual pavement designs and rehabilitation strategies have a

significant impact on analysis results. Longer performance periods for individual

23

pavement designs require fewer rehabilitation projects and associated agency and work

zones user costs (James Walls III and Michael R. Smith, 1998).

Routine, reactive type annual maintenance costs have only a marginal effect on NPV.

They are hard to obtain, generally very small in comparison to initial construction and

rehabilitation costs, and differentials between competing pavement strategies are usually

very small, particularly when discounted over 30- to 40-year analysis periods.

Figure 6: Example of expenditure stream diagram.

The projected value in terms of the present value of money is used for the initial costs,

maintenance and rehabilitation costs and salvage value being used, as shown by the

expenditure stream diagram in Figure 6. The discount rate factor is then applied to

calculate the time value of money. LCC analysis requires the following inputs:

2.3.3.1 Initial Capital Costs

Initial capital costs are the total sum of the investments to design and construct a highway

or a highway improvement. The initial construction cost is presented in unit prices from

bid records of projects constructed in previous years and only representative prices must

be used. Unit prices may be taken out from the overall cost of previous projects if the

representative costs are not available. The start-up cost can be taken into consideration as

well as part of the LCCA (Fugro-BRE,2000). The most significant items which are

included in initial capital costs are grading, base course, surfacing, right-of-way,

engineering, signing, and signals (Khaled A. andAbaza, 2002).

24

The basis for cost of the initial construction should be unit prices from bid records of

projects constructed over the last two or three years, and only representative prices should

be included. For example, very small projects or projects where paving is only a minor

component of the total cost may cause unit prices to be skewed.

It is realistic to consider the initial cost both by itself and as part of the life-cycle cost

analysis. This recognizes that the agency is constrained by an annual budget, and needs to

examine the short-term ramifications of expenditures as well as the long-term impact of

pavement type decisions.

2.3.3.2 Maintenance and rehabilitation costs

Maintenance and Rehabilitation (M&R) is another matter that requires attention.

Preventive maintenance strategies appear to be much more cost effective compared to

conventional maintenance strategies (C.Wei and S.Tighe, 2004). It is difficult to

determine maintenance costs because there is usually an absence of efficient record

keeping and differentiation between maintenance actions cannot be achieved. Hence,

tools to help users define the effects of preventive maintenance are required (Flintsch

andKuttesch, 2004). Compared to the initial construction and rehabilitation costs, the

maintenance cost of an LCCA has limited effect. Historical records of the actual

pavement costs and activities must be utilized if these costs are present in the LCCA

procedure (Pavement Management and Pavement Design Manual, 2008). An artificial

increase in LCC would take place if there were unsuitable and frequent maintenance

activities like rehabilitation (Fugro-BRE,2000).

Maintenance Costs

Maintenance costs are those costs that are essential to maintain a pavement investment at

a specified level of service or at a specified rate of deterioration. For LCC analysis, items

directly affecting pavement surface performance include crack filling, crack repair,

grinding, and patching.

Maintenance costs are frequently difficult to define because of either a lack of record

keeping or accounting that does not appropriately discriminate between different types of

maintenance activities. Maintenance costs in a life-cycle cost analysis usually have

25

minimal impact when compared to the initial and first rehabilitation costs. If maintenance

costs are used within an LCCA procedure, then historical documentation of actual

pavement activities and expenditures should be used. As with rehabilitation,

unrealistically frequent or inappropriate maintenance activities can artificially increase

life-cycle cost (David H. et al, 2014).

Rehabilitation Costs

Rehabilitation means restoring or rebuilding an existing highway facility which is in a

state of disrepair. Rehabilitation activities are capable of maximizing the life expectancy

of the facility while minimizing the agency and facility user costs. The life-cycle benefit

cost analysis needs to consider at least one rehabilitation activity, if the analysis period is

well defined. The costs for rehabilitation could be derived from the historical project

data, but deciding when to implement rehabilitation is affected by many factors, such as

the type, condition, age of the facility, traffic condition, and so on (David H. et al,2014).

Rehabilitation costs are associated with upgrading or overlaying a pavement when the

riding quality or serviceability decreases to a minimum level of acceptability. Items

include overlays, recycling, seal coats, and reconstruction.

2.3.4 Estimate Agency Costs

Construction quantities and costs are directly related to the initial design and subsequent

rehabilitation strategy. The first step in estimating agency costs is to determine

construction quantities/unit prices. Unit prices can be determined from historical data on

previously bid jobs of comparable scale.

LCCA comparisons are always made between mutually exclusive competing alternatives.

LCCA need only consider differential costs between alternatives. Costs common to all

alternatives cancel out, these cost factors are generally noted and excluded from LCCA

calculations (James Walls III and Michael R. Smith, 1998).

Agency costs include all costs incurred directly by the agency over the life of the project.

They typically include initial preliminary engineering, contract administration,

26

construction supervision and construction costs, as well as future routine and preventive

maintenance, resurfacing and rehabilitation cost, and the associated administrative cost.

Routine reactive-type maintenance cost data are normally not available except on a very

general, areawide cost per lane mile. Fortunately, routine reactive-type maintenance costs

generally are not very high, primarily because of the relatively high-performance levels

maintained on major highway facilities. When discounted to the present, small reactive

maintenance cost differences have negligible effect on NPV and can generally be ignored

(James Walls III and Michael R. Smith, 1998).

Agency costs also include maintenance of traffic cost and can include operating cost such

as pump station energy costs, tunnel lighting, and ventilation. At times, the salvage value,

the remaining value of the investment at the end of the analysis period, is included as a

negative cost (James Walls III and Michael R. Smith, 1998).

2.3.5 Discount Rate

When long-term public investments are being analyzed, costs are compared at several

points of time for which discount is necessary (David H. et al, 2014). A dollar spent in

the future is considered of lesser worth than a dollar spent today, which is why it is said

that time, has money value. Hence, it is essential to convert the costs and benefits stated

at different points of time to the costs and benefits that would happen at a common time

(Project Development & Design Guide, 2006). The discount rate is the rate of interest

used to adjust future values to present values, normally taken as the difference between

the prime interest rate and the rate of inflation.

The selection of a discount rate in life-cycle costing can be contentious because there is a

great deal of uncertainty associated with future interest rates and inflation. An

unreasonably low or negative discount rate essentially means that it would not matter

financially if a project were to be constructed today or 10 years from now. This would

overemphasize the influence of uncertain future costs. Too high a discount rate would

overemphasize the importance of the initial cost (David H. et al, 2014).

2.3.6 Analysis Period

The analysis period is the time period over which the economic analysis is conducted. It

is the final component to be established before performing an LCC analysis is to select an

27

appropriate comparison time period. The analysis period is the total length of time the

facility is expected to serve its intended function or the time frame before the component

in question requires replacement or upgrade. This period may contain several

maintenance and rehabilitation activities. Figure 7 illustrates an example of these

activities for pavement performance (Pavement Management and Pavement Design

Manual, 2008).

Figure 7: Maintenance and Rehabilitation activities during analysis period.

2.3.7 Salvage Value

Because some or all of the pavement structure continues to serve its purposes beyond the

analysis period, it is important to account for its condition at the end of the analysis

period. The salvage value is the value of the investment or capital outlay remaining at the

end of the analysis period. For a valid LCC analysis, the residual value must be included

(David H. et al, 2014).

Salvage value should be based on the remaining life of an alternative at the end of the

analysis period as a prorated share of the last rehabilitation cost (James Walls III and

Michael R. Smith, 1998).

2.3.8Maintenance and Rehabilitation Alternatives

The primary purpose of an LCCA is to quantify the long-term implication of initial

pavement design decisions on the future cost of maintenance and rehabilitation activities

28

necessary to maintain some preestablished minimum acceptable level of service for some

specified time (James Walls III and Michael R. Smith, 1998).

A Pavement Design Strategy is the combination of initial pavement design and necessary

supporting maintenance and rehabilitation activities. Analysis Period is the time horizon

over which future cost are evaluated. The first step in conducting an LCCA of alternative

pavement designs is to identify the alternative pavement design strategies for the analysis

period under consideration (James Walls III and Michael R. Smith, 1998).

2.3.9 Performance Periods and Activity Timing

Performance life for the initial pavement design and subsequent rehabilitation activities

has a major impact on LCCA results. It directly affects the frequency of agency

intervention on the highway facility, which in turn affects agency cost as well as user

costs during periods of construction and maintenance activities (James Walls III and

Michael R. Smith, 1998).

29

3. METHODOLOGY

3.1 Introduction

This section will review the design procedures of flexible pavement by AASHTO, 1993

and Ethiopian Roads Authority, 2013, procedures for determination of life cycle cost, the

framework of the software development and research methods.

3.2 Design Procedures of AASHTO, 1993

3.2.1 Design Input Parameters for AASHTO Method

3.2.1.1 Traffic

The design procedures for both highways and low volume roads are all based on

cumulative expected 18-kip equivalent single axle loads (ESAL) during the analysis

period (ẁ18). For any design situation in which the initial pavement structure is expected

to last, the analysis period without any rehabilitation or resurfacing, all that is required is

the total traffic over the analysis period(AASHTO, 1993).

Traffic-related data, which includes axle loads, axle configurations and number of

applications, are required for both new construction and rehabilitation pavement

structural design. Cars and light truck traffic produce only small stresses in normal

pavement structures and therefore truck traffic is the major consideration in the structural

design of pavements. The project design ESALs are expressed as the cumulative

Equivalent Single Axle Loads (ESALs) in the design lane for the design period. The

results of the AASHO Road Test indicated that the damaging effect on the pavement

structure of an axle load of any mass can be represented by a number of 80 KN ESALs

(Khaled A. and Abaza, 2002).

30

The predicted traffic furnished by the planning group is generally the cumulative 18-

kip ESAL axle applications expected on the highway, whereas the designer requires the

axle applications in the design lane. Thus, unless specifically furnished, the designer must

factor the design traffic by direction and then by lanes (if more than two) (C. Wei and S.

Tighe, 2004).The following equation may be used to determine the traffic (w18) in the

design lane:

W18 = DD * DL * ẁ18 Equation 3.1

Where:

DD = a directional distribution factor, expressed as a ratio, that accounts for the

distribution of ESAL units by direction, e.g, east – west, north – south. e.t.c.

DL = a lane distribution factor, expressed as a ratio, that accounts for distribution of

traffic when two or more lanes are available in one direction, and

ẁ18 = the cumulative two directional 18-kip ESAL units predicted for a specific section

of highway during the analysis period (from the planning group).

Although the DD factor is generally 0.5 (50 percent) for most road ways, there are

instances where more weight may be moving in one direction than the other. Thus, the

side with heavier vehicles should be designed for a greater number of ESAL units.

Experience has shown that DD may vary from 0.3 to 0.7, depending on which direction is

“loaded” and which is “unloaded”. For the DL factor, Table 5 may be used as a guide:

Table 5: 18-kip ESAL in design lane

Number of lanes in each direction Percent of 18-kip ESAL in design

lane

1 100

2 80 - 100

3 60 - 80

4 50 - 75

3.2.1.2 Traffic Load

31

In AASHTO design procedure traffic is characterized by the number of single axle 18-kip

expected during the design period. This is also referred to as Equivalent Single Axle

Load or ESAL. It is denoted as W18 in the basic design equation. The designer should

first convert all the mix of traffic on the pavement to be designed in to an equivalent

single axle load of 18-kip (ESALs). AASHTO offers a method by which different axle

loads and axle configurations can be converted in to the ESAL. An equivalent axle load

factor (EALF) defines the damage per pass to a pavement by the axle in question relative

to the damage per pass of a standard axle load, in this case 18-kip (80-KN) single axle

load. ESAL is therefore computed by

∑ Equation 3.2

in which k is the number of axle load groups, Fi is the EALF for the ith-axle load group,

and ni is the number of passes of the ith-axle load group during the design period. EALF

depends on the type of pavements, thickness or structural capacity and the terminal

conditions at which the pavement is considered failed.

3.2.1.3 Reliability (R)

Basically, it is a means of incorporating some degree of certainty in to the design process

to ensure that the various design alternatives will last the analysis period.Since 1993 the

AASHTO procedure incorporates a reliability coefficient to address the different levels of

importance of a road way. The more important its design is, the higher the reliability

should be. It also depends on the traffic volume. As the traffic volume gets larger, the

reliability should be increased. This allows the designer to set the level of certainty in the

design. Table 6 shows the recommended values reliability level for different types of road

as originally presented in the manual (AASHTO, 1993). Table 7 shows the corresponding

ZR values for the different reliability levels.

Table 6: Suggested level of Reliabilities (AASHTO, 1993)

Functional Classification Recommended Level

of Reliability for Urban

Recommended Level

of Reliability for

Rural

Interstate and Other Freeways 85 - 99.9 80 - 99.9

Principal Arterials 80 - 99 75 - 95

Collectors 80 - 95 75 - 95

32

Local 50 - 80 50 - 80

Table 7: Standard Normal Deviates (AASHTO, 1993)

Reliability (%)

Standard Normal Deviate (ZR)

Reliability (%)

Standard Normal Deviate

(ZR)

50 0 93 -1.476

60 -0.253 94 -1.55

70 -0.524 95 -1.645

75 -0.674 96 -1.751

80 -0.841 97 -1.881

85 -1.037 98 -2.054

90 -1.282 99 -2.327

91 -1.340 99.9 -3.090

92 -1.405 99.99 -3.750

Generally, as the volume of traffic, difficulty of diverting traffic, and public expectation

of availability increases, the risk of not performing to expectations must be minimized.

This is accomplished by selecting higher levels of reliability for various functional

classifications.

3.2.1.4 Overall Standard Normal Deviation

The design procedure has a probabilistic approach and assumes all the input parameters

to be their corresponding mean values. The fact that there could be uncertainties in the

local traffic prediction makes it important that the overall Standard Deviation (So) to be

included in the design. Table 7 shows Standard Normal Deviates for various levels of

reliability (AASHTO, 1993).

3.2.1.5 Serviceability

33

The serviceability of a pavement is defined as its ability to serve the type of traffic

(automobiles and trucks) which use the facility. In the AASHTO system the roughness

scale for ride quality ranges from 5 to 0. For design it is necessary to select both an initial

and terminal serviceability index. An initial serviceability index of 4.2 is suggested to

reflect a newly constructed pavement. A terminal serviceability index of 2.5 is suggested

to be used in the design of major highways. The design serviceability loss, ∆PSI, is the

difference between the newly constructed pavement serviceability and that tolerated

before rehabilitation(Khaled A. and Abaza, 2002).

Selection of the lowest allowable PSI or terminal serviceability index (Pt) is based on the

lowest index that will be tolerated before rehabilitation, resurfacing, or reconstruction

becomes necessary. An index of 2.5 or higher is suggested for highways with lesser

traffic volume. One criterion for identifying a minimum level of serviceability may be

established on the basis of public acceptance.

For relatively minor highways where economics dictates that the initial capital outlay be

kept at a minimum, it is suggested that this be accomplished by reducing the design

period or the total traffic volume, rather than by designing for a terminal serviceability

less than 2.0.

Once Po and Pt are established, the following equation should be applied to define the

total change in serviceability index:

∆PSI = Po – Pt Equation 3.3

Where: Po = initial serviceability index and Pt = terminal serviceability index

3.2.1.6 Layer Material Properties

Resilient Modulus (MR)

The resilient modulus is a measure of the elastic property of soil recognizing certain

nonlinear characteristics. It is recognized that many agencies do not have equipment for

performing the resilient modulus test. Therefore, suitable factors are reported which can

be used to estimate MR from standard CBR, R-value and soil index test results or values.

The development of these factors is based on state of the knowledge correlations. It is

34

strongly recommended that user agencies acquire the necessary equipment to measure

MR. In any case, a well-planned experiment design is essential in order to obtain reliable

correlations.

Heukelom and Klomp(Khaled A. andAbaza, 2002) have reported correlations between

the corps of Engineers CBR value, using dynamic compaction, and the in-situ modulus of

soil. The correlation is given by the following relationship:

MR (Psi) = 1,500 * CBR Equation

3.4

The data from which correlation was developed ranged from 750 to 3,000 times CBR.

This relationship has been used extensively by design agencies and researchers and is

considered reasonable for fine-grained soil with a soaked CBR of 10 or less.

3.2.1.7 Layer Coefficients

A layer coefficient ai of a unit thickness of material is a measure of its relative ability to

function as a structural component of the pavement. Layer coefficients can also be

determined from test roads or from correlation with resilient modulus of the material.

Research and field studies indicate that layer coefficients depend on different factors such

as pavement thickness, underlying support and position in the whole pavement structure.

The values of the layer coefficients are determined from the charts presented in

AASHTO (Design of Pavement Structures Manual, 1993). In the guide equations relating

resilient moduli and corresponding values of layer coefficients are given only for granular

base and sub base materials. For AC course, cement-treated and bituminous-treated

bases, values from the corresponding charts were taken and linear logarithmic regression

is used to determine the correlation.The equations used for each type of layer are as

follow:

a1 = 0.169 ln (E1) Equation3.5

a2, granular = 0.249 log (E2) – 0.997 Equation3.6

a2, cement = 0.2139 ln (E2) – 2.6921

Equation3.7

35

a2, bituminous = 0.1317 ln (E2) – 0.3877

Equation3.8

a3 = 0.227 log (E3) – 0.839 Equation

3.9

Where: a1 = layer coefficient for the AC layer

a2, granular = layer coefficient for a granular base layer

a2, cement = layer coefficient for cement treated base layer

a2, bituminous = layer coefficient for bituminous treated base layer

a3 = layer coefficient for granular sub base layer

Ei = resilient modulus of the corresponding layer

The values presented in the design guide are based on the AASHO road test. It is

important to note that whenever a more precise value for a specific layer is available, one

should use that information. The user is required to determine a reasonable value for the

layer coefficients and input them as input variables.

3.2.1.8 Drainage Coefficient

Drainage is an important factor in the structural design of pavements. The factor used for

modifying the layer coefficients for drainage is called drainage coefficient, m. The

quality of drainage is measured by the length of time for water to be removed from bases

and subbases and depends primarily on their permeability. The percentage of time during

which the pavement structure is exposed levels approaching saturation depends on the

average yearly rainfall and the prevailing drainage coefficients (Yang H. Huang, 2004). It

is up to the design engineer to identify what level of drainage is achieved under a certain

set of drainage conditions. Depending on the quality of drainage and the availability of

moisture, drainage coefficients m2 and m3 should be applied to granular bases and

subbases to modify the layer coefficients.

Table 8: Recommended values for Modifying Layer Coefficients

36

Percent of Time Pavement Structure is Exposed to Moisture levels

Approaching Saturation

Quality of Less than Greater than

Drainage 1 % 1-5 % 5-25 % 25 %

Excellent 1.40-1.35 1.35-1.30 1.30-1.20 1.20

Good 1.35-1.25 1.25-1.15 1.15-1.00 1.00

Fair 1.25-1.15 1.15-1.05 1.00-0.80 0.80

Poor 1.15-1.05 1.05-0.80 0.80-0.60 0.60

Very Poor 1.05-0.95 0.95-0.75 0.75-0.40 0.40

Table 8 shows recommended values of m provided in the design guide (AASHTO, 1993)

for different drainage levels. One general rule can be to choose a higher value of drainage

coefficient for an improved drainage condition. It is also important to note that the values

of drainage coefficients provided in the table are applicable only to untreated base and

sub base materials. Drainage coefficient value for AC layer is considered in the design

manual to be 1.

3.3 Design Procedures of ERA, 2013

3.3.1 Design Process

The design stage of Ethiopian Roads Authority (ERA) was divided in to 3 main parts as

shown below:

1. Estimate the amounts of traffic and cumulative number of equivalent standard axles

over the design life of the road. The ESA obtained will be used to identify the traffic

classes.

2. Determine the subgrade strength classes based on CBR value.

3. Select the combination of pavement material and thickness from the structural

catalogue that will meet the satisfactory of pavement service and design life based on

traffic class and subgrade class values.

Determination of Cumulative Traffic Volumes

In order to determine the cumulative number of vehicles over the design period of the

road, the following procedure should be followed(ERA, 2013):

37

1. Determine the initial traffic volume, AADT(m)0, of each traffic class (m) using the

results of the traffic survey and any other recent traffic count information that is

available.

2. Estimate the annual growth rate “i” expressed as a decimal fraction, and the anticipated

number of years “n” between the traffic survey and the opening of the road.

3. For each vehicle class, estimate the traffic in the first year that the road is opened to

traffic. For normal traffic this is given by:

AADT(m)1 = AADT(m)0 (1+i)n Equation 3.10

4. For each vehicle class, add the estimate for diverted traffic and for generated traffic if

any are anticipated.

For structural pavement design the cumulative traffic loading of each of the motorized

vehicle classes over the design life of the road in one direction is required. For a given

class, m, this is given by the following equation:

T(m) = 0.5 x 365 x AADT(m)0 [(1+i/100)N – 1]/(i/100) Equation 3.11

Where: T(m) = The cumulative traffic of traffic class m

AADT(m)1 = The AADT of traffic class m in the first year

N = The design period in years

i = The annual growth rate of traffic in percent

3.3 Life cycle cost Analysis (LCCA)

Pavement Life-Cycle Cost Analysis (LCCA) is known as a technique helping pavement

designers make better decisions that balance initial construction cost and projected future

cost of a project. The future costs may include maintenance and rehabilitation (M&R)

costs (Changmo Kim et al, 2015).

LCCA results are just one of many factors that influence the ultimate selection of a

pavement design strategy. The final decision may include a number of additional factors

outside the LCCA process, such as local politics, availability of funding, industry

38

capability to perform the required construction, and agency experience with a particular

pavement type, as well as the accuracy of the pavement design and rehabilitation models

(Walls and Smith, 1998).

Many assumptions, estimates, and projections feed the LCCA process. The variability

associated with these inputs can have a major influence on the confidence the analyst can

place in LCCA results. It all depends on the accuracy of the inputs used. The accuracy of

LCCA results depends directly on the analyst‟s ability to accurately forecast such

variables as future costs, pavement performance, and traffic for more than 30 years into

the future (Walls and Smith, 1998).

3.4.1 Net Present Value (NPV)

Net Present Value (NPV) is the economic efficiency indicator of choice. The Uniform

Equivalent Annual Cost (UEAC) indicator is also acceptable, but should be derived from

NPV. Computation of Benefit/Cost (B/C) ratios are generally not recommended because

of the difficulty in sorting out cost and benefits for use in the B/C ratios (James Walls III

and Michael R. Smith, 1998).

NPV is determined by discounting all project costs to the base, or present, year (usually

the present year, year of construction or year of authorization). Thus, the entire project

can be expressed as a single base year, or present year, cost. Alternatives are then

compared by comparing these base year costs. NPV is a common economic calculation

and, its equation can be rewritten as:

∑ ( ) ( ) Equation 3.12

( ) Equation 3.13

Where: pwfi =present worth factor of costs incurring at year i and d = discount rate

3.4.2 LCCA Procedure

The LCCA structured approach can be outlined in the following steps:

1. Define project‟s alternatives.

2. Choose general economic parameters: Discount Rate, Analysis Period.

39

3. Establish expenditure stream for each alternative:

Estimate differential agency costs.

Design maintenance strategies and their timings.

Design rehabilitation strategies and their timings.

4. Compute Net Present Value for each alternative.

5. Compare and interpret results.

6. Re-evaluate design strategies if needed.

3.4.2.1 Define project‟s alternatives

This is the first step in the LCCA procedure. Experts and experienced professionals

suggest potential life cycle strategies for the project. Each pavement design strategy

specifies initial design and performance, time-dependent rehabilitation/treatment

activities, and the timings of these rehabilitation activities and respective performances.

At this stage, common costs between different strategies can be identified (David H. et al,

2014).

3.4.2.2 Choose general economic parameters

General economic parameters are the discount rate and the analysis periods. Both

parameters should be equal for all options (Flintsch andKuttesch, 2004).

3.4.2.3 Establish expenditure stream for each alternative

Expenditure stream diagrams can be constructed as shown in Figure 4 of chapter 2. These

diagrams lay out the design strategies, including scope, and timing for each activity, with

associated agency, and user costs shown in real dollars for each year of the analysis

period (Pavement Management and Pavement Design Manual, 2008). But for the case of

this thesis only agency costs were considered.

3.4.2.4 Compute Net Present Value for each alternative

Once the performance period, activity timing, and costs associated with each alternative

have been established, they must be compared over the chosen analysis period. This is

typically done in net present value (NPV).After constructing the expenditure stream,

40

computing the Net Present Value of each alternative becomes a straightforward

calculation using Equations 3.12 and 3.13.

3.4.2.5 Compare and interpret results

Once NPV for each alternative is computed, with agency, user, and societal costs

presented distinctively, interpretation of these results can be made. Generally, an

alternative is preferred if its NPV is a minimum of 10 percent less than the NPV of other

competing alternatives. If the difference between the NPV of alternatives is less than 10

percent, then such alternatives are considered similar or equivalent (David H. Timm et

al., 2014). The most significant parameters that should be tested for computing

alternatives are: the discount rates, timing of future rehabilitation activities, traffic growth

rate, unit costs of the major construction components and analysis period.

3.4.2.6 Re-evaluate design strategies if needed

Presenting results and analyzing them help the process of re-assessing the design

strategies,

whether in regards to scope, timing, or other factors. Sometimes minor alterations of the

design strategies can lead to a better choice for the project.

3.3 Flow chart of the Flexible Pavement Software on VB.NET code

41

Figure 8: Flow chart of the VB.NET code.

3.6 Research Methods

Before the main design equations can be used, all the design input parameters should be

manipulated to fit in the equations. Some of the parameters are chosen by the designer

and inputted and utilized in the code as they are. These parameters include the Drainage

coefficient (m), Reliability (R), Design ESAL, and Resilient Moduli (MR) for each layer

for a particular season with the number of days that the season lasts and Change in

serviceability (ΔPSI) for AASHTO, 1993 method; Design period, Construction period,

42

CBR value, Directional distribution factor, Lane distribution factor, Counted traffic,

Generated traffic, Diverted traffic, Growth rate, and Equivalency factor of each direction

for ERA, 2013 method; Road length, Road width, Discount rate, Treatment type, Initial

cost, Salvage value, Number of years between initial construction and the corresponding

treatment, and treatment cost for life cycle cost determination.. The other types of the

parameters are those which need to be calculated within the program itself.

The thicknesses of each layer based on the Layered Design Analysis and life cycle cost of

the flexible pavement based on Net Present Value method are finally calculated. As

AASHTO, 1993 design guide recommends, all values of the thicknesses are rounded to

the nearest ½ inch. Minimum requirement for construction according to AASHTO

recommendation have also been incorporated in design thickness calculation. The

thicknesses of each layer based on AASHTO, 1993 and ERA, 2013 methods and life

cycle cost are taken as an output data and saved in output file. Figure 8 shows the flow

chart of the Visual Basic.NET program.

The overview of the methodology adapted in carrying out this research is as that, first the

algorithm was prepared that was necessary for developing the software. All the graphs

that are used in the manual design process of both of the two methods (AASHTO and

ERA) were digitized and made the required corrections. Then using these digitized

graphs and rest of the data software was developed for the design process encompassing

both the two mentioned methods, using the visual basic.net computer programming

language.

Once the software was developed, then input data was collected to test the software for its

accuracy and authenticity. The data collected was put in the software and results were

obtained (Rafi Ullah Khan et al., 2012). The same data was used in the design through

manual and conventional methods and the results were compared with those obtained

from the software, in case of any error or mistakes necessary measures were taken and

changes made to remove the errors. Again, both of the results were taken and compared,

giving the satisfactory and accurate results finally. Finally, the software developed

through the research carried out was found absolutely accurate and authentic, hence can

43

be used for education, research, and design in field giving fully accurate and trustworthy

results.

4. RESULTS AND DISCUSSION

4.1 Introduction

This research was performed to provide a package for flexible pavement design based on

Ethiopian Roads Authority (ERA, 2013) and American Association of State Highway and

Transportation Officials (AASHTO, 1993) design methods by develop a software

program using Visual basic.Net programming language. All the tables and graphs are

incorporated to codes so that to design the pavement with ease. This section describes

how the user input the required variables and analyze for both flexible pavement design

methods.

4.2 Results and Discussion

4.2.1 Getting Started Flexible Pavement Software (FPS)

When running the software, the progress bar dialog box will appear as shown in Figure 9

and consequently some information about the software will be provided in tip of the day

dialogue box, as shown in Figure 10. The Tip of the day dialog box is designed to

introduce the designer to the concepts and facilities of Flexible Pavement Software

(FPS). Further, it provides introduction, direction and improve the designers

understanding about Flexible Pavement Software (FPS).

44

Figure 9: Progress bar appeared when running the software

Figure 10: Tip of the day dialog box.

After reading the information on Tip of the day dialog box, click on>Ok. Then the main

graphical user interface appears as shown in Figure 11. Which contains the menu bar and

provides the designer with the choice to select the design method.

Figure 11: Main Window of Software

45

Figure 12: Dropdown list of File menu.

To begin a new project, click on >New from the dropdown list (Figure 12). Then Select

Design Manual dialog box appears as shown in Figure 13. Which provides the designer

to select the design method he/she want to use.

4.2.2 Design based on ERA, 2013

To design a flexible pavement using ERA, 2013method, select >ERA 2013 from Select

Design Manual dialog box, then save the project by clicking on >Save button.

Figure 13: Select the desired design method.

46

4.2.2.1 Input for ERA, 2013 design method

Now the designer must input all the necessary parameters which are used to design

flexible pavement based on ERA, 2013 method, so click on >Insert from menu bar of

main graphical user interface and select >ERA 3013 data from dropdown list (Figure

14). Then, Input for flexible pavement design (ERA 2013) dialog box will be appeared,

as shown in Figure 15. Then from General tab, insert all the data which are the same for

all homogenous sections such as project title, project description, design period and

construction period. Also, the designer must insert stations of origin-destination of each

homogeneous section, by writing their name or their chainage and click on >Add.

Figure 14: Select ERA 2013 from dropdown list of Main graphical user interface.

When click on >Data for homogenous sections tab,all homogenous sections will be

appeared automatically. Then, select the homogenous section that the designer wants to

do for (Figure 16) and insert the necessary data for the corresponding homogenous

section. The data which inserted for a specific homogeneous section includes directional

distribution factor, lane distribution factor, length and width of homogenous section,

number of vehicle types to be considered and CBR value (Figure 17).

47

Figure 15: Input general data for all sections.

Figure 16: Selecting a specific homogeneous section that we want to do.

48

Figure 17: Input data for a specific homogeneous section.

The dialog box, Traffic data for each vehicle category will be appeared as shown in

Figure 18, by clicking on the button >Traffic data. Then insert traffic related parameters

for the corresponding homogeneous section (Figure 18). The necessary parameters will

be inserted for the vehicle categories that the designer wants to consider. Therefore, the

designer has to insert traffic related parameters which are counted traffic (normal traffic),

generated traffic, diverted traffic, growth rate and equivalency factor for both directions,

for each selected vehicle categories. Click on >More button, to get a further information

about vehicle classification according to ERA, 2013 manual (Figure 19). Finally, click on

Save>Save All.

49

Figure 18: Input traffic related data for each vehicle category.

Figure 19: Vehicle classification according to ERA, 2013 manual

50

Now click on the next tab, Economic analysis data, then the designer will get the traffic

class and subgrade class automatically which are analyzed from the traffic and material

property data inserted before. From this tab, cost estimation parameters such as average

lane width, analysis period and discount rate will be inserted as shown in Figure 20.

Figure 20: Input cost estimation parameters.

Available alternatives from structural catalog for the corresponding traffic class and

subgrade class will be appeared on Cost estimation input dialog box using the input data

by clicking on >Cost estimation for each alternativebutton. Then, the designer has to

insert parameters used for cost estimation for each alternative. Cost estimation parameters

include initial cost, treatment type, number of years between initial construction and the

corresponding expenditure, treatment cost and salvage value (Figure 21). Then click on

>Save and repeat the same process for each alternative as shown in Figure 22. When each

alternative is selected the picture of layered structure will be changed automatically.To

get a description on materials and charts, click >Key to materials and Charts (Figure 23

and Figure 24 respectively).

51

Figure 21: Input cost estimation parameters for each alternative.

Figure 22: Select each alternatives and input cost estimation parameters.

52

Figure 23: Description of materials for each layer.

Figure 24: Description for charts.

53

4.2.2.2 Output for ERA, 2013 design method

Total cost (NPV) dialog box appears (Figure 26) when the designer selects Output from

menu bar of main graphical user interface and click on >Layer thickness and

LCC>ERA 2013(Figure 25). As shown in Figure 26, the designer can see the life cycle

cost of each alternative.Click on >Show structure button to see materials and

thicknesses of each layer of the most economical alternative. Then, click on >Savebutton.

Figure 25: Select ERA 2013 from output dropdown list.

Repeat the same procedure by selecting the name or chainage of the next homogeneous

section. After inserting all the necessary parameters for all homogeneous sections,

Preferable design and estimated cost for each sections dialog box (Figure 27) will be

appeared by clicking on >View preferable design for all sections button. The layer

thicknesses and materials of each homogenous section, life cycle cost of each

homogeneous section and total cost of the project will be obtained from Preferable

design and estimated cost for each sections dialog box.

54

Figure 26: Life cycle cost and layer thicknesses of each section.

Figure 27: Layer thicknesses and life cycle cost.

55

Click on >Output from main graphical user interface menu bar (Figure 28) and select

Generate chart>ERA 2013 to see the chart which shows life cycle cost of total project

and the comparison of life cycle cost of homogeneous sections (Figure 29).

Figure 28: Select chart for ERA 2013 from output dropdown list.

Figure 29: Chart for life cycle cost.

56

4.2.3 Design based on AASHTO, 1993

The AASHTO Guide for Design of Pavement Structures requires some statistical values

that are used to estimate the probability that the pavement will survive the design period

with a pavement serviceability level greater than the terminal serviceability level. The

software provides default values for these statistical values, which may be overridden if

the designer desires.

To begin a new project, click on >New from the dropdown list of main graphical user

interface. Then Design Manual Selection dialog box appears (Figure 13). Select

AASHTO 1993 and save the project.

4.2.3.1 Input for AASHTO, 1993 design method

Now input all the necessary parameters which are used to design flexible pavement based

on AASHTO, 1993 method, so click >Insert from menu bar of main graphical user

interface and select >AASHTO 1993 data from dropdown list (Figure 30). Then, Input

for flexible pavement design (AASHTO 1993) dialog box will be appeared, as shown in

Figure 31. Then from General tab, insert all the data which are the same for all

homogenous sections such as project title, project description and design period. Also,

the designer must insert stations of origin-destination of each homogeneous section, by

writing their name or their chainage and clickon >Add button.

Figure 30: Select ERA 2013 from output dropdown list.

57

Figure 31: Input general data of all sections.

When click on >Design data tab,all homogenous sections will be appeared

automatically. Then, select the homogenous section that the designer wants to do for and

insert the necessary data for the corresponding homogenous section.

Figure 32: Input data for a specific homogeneous section.

58

The data which inserted for a specific homogeneous section includes estimated future

traffic (W18), standard normal deviate (ZR), overall standard deviation (So), effective

resilient modulus of road bed material (MR), serviceability loss (∆PSI), layer coefficients

(ai) and drainage coefficients (mi) (Figure 32).

Click on >Save button and go to the next tab by clicking on >Cost estimation tab. From

this tab, cost estimation parameters such as average lane width, analysis period, discount

rate, initial cost of each layer and salvage value will be inserted as shown in Figure 33.

Estimated treatment cost dialog box (Figure 34) will be appeared when the designer

clicks on >Estimated treatment cost button. Insert parameters such as treatment type

that the designer wants to consider, number of years between initial construction and the

corresponding expenditure, and treatment cost.

Figure 33: Input cost estimation parameters for AASHTO, 1993 method.

59

Figure 34: Input treatment type and the corresponding treatment cost.

4.2.3.2 Output for AASHTO, 1993 design method

Layer thickness and total cost as per AASHTO method dialog box (Figure 36) will be

appeared when the designer selects Output from menu bar of main graphical user

interface and click on >Layer thickness and LCC>AASHTO 1993(Figure 35). From

this dialog box the designer can see layer thicknesses and life cycle cost of a specific

homogeneous section. Click on >Save button and repeat the same procedure for other

homogeneous sections.

60

Figure 35: Select AASHTO 1993 from output dropdown list.

Figure 36: Layer thicknesses and life cycle cost of a specific homogeneous section.

61

Layer thicknesses and total costs for each sections dialog box appears (Figure 37) by

clicking on >Show output of all sections button. On this dialog box layer thicknesses

and life cycle costs of each homogeneous section and total project cost are shown. Finally

the cross sectional drawing will be appeared (Figure 38) on main user interface by

clicking on >Save button.

Click on >Output from main graphical user interface menu bar and select Generate

chart>AASHTO 1993 (Figure 39)to see the chart which shows layer thicknesses of each

homogeneous section, life cycle cost of total project and the comparison of life cycle cost

of homogeneous sections (Figure 40).

Figure 37: Layer thickness and life cycle cost of each sections.

62

Figure 38: Drawing for layer thicknesses of each homogeneous section.

Figure 39: Select chart for AASHTO 1993 from output dropdown list.

63

Figure 40: Chart for layer thicknesses and life cycle cost.

The software is brimming with quick help buttons to assist the designer with obtaining

and properly inputting the necessary information.

4.3 Validation of Flexible Pavement Software (FPS)

Software validation is particularly important for ensuring the accuracy of outputs of the

software compared with the results obtained from manual design. The outputs of Flexible

Pavement Software (FPS) have been validated against manual calculations based on the

design examples from AASHTO guide for design of pavement structures 1993 appendix

H, The Handbook of Highway Engineering page 8-25 by Michael S. Mamlouk and

pavement design report of real project. The results obtained meet the accuracy of

requirement as shown in Table 9 and Table 10.

64

Table 9: Validation of Flexible Pavement Software (FPS) for thickness design.

Input and Output variables AASHTO 1993 Michael S. Mamlouk

Cumulative ESAL 18-kip (10^6) 18.6 7

Reliability (%) 95 95

Standard Deviation (So) 0.35 0.45

Loss in PSI (∆PSI) 2.1 1.6

Elastic Modulus of Asphalt Concrete, EAC (psi) 400,000 450,000

Elastic Modulus of Base Course, EBS (psi) 30,000 40,000

Elastic Modulus of Subbase Course, ESB (psi) 11,000 20,000

Effective Roadbed Resilient Modulus, MR (psi) 5,700 7,000

Layer Coefficient of Asphalt Concrete, a1 0.42 0.44

Layer Coefficient of Base Course, a2 0.14 0.17

Layer Coefficient of Subbase Course, a3 0.08 0.14

Drainage Coefficient of Base, m2 1.2 1.1

Drainage Coefficient of Subbase, m3 1.2 1.1

SN1 by manual calculation 3.2 2.7

SN1 by FPS 3.2 2.7

SN2 by manual calculation 4.5 3.5

SN2 by FPS 4.51 3.5

SN3 by manual calculation 5.6 5.2

SN3 by FPS 5.6 5.2

D1(in) by manual calculation 8 6.5

D1(in) by FPS 8 6.5

D2(in) by manual calculation 7 6

D2(in) by FPS 7 6

D3(in) by manual calculation 11 8

D3(in) by FPS 11 8

Table 10: Validation for Life Cycle Cost (LCC) determination

Input and Output costs Zelelew (2008)

Costs in ETB

Flexible Pavement Software (FPS)

Costs in ETB

Initial cost 426175 426175

Crack Seal 31652 31652

Patching 78172 78172

Micro Slurry 86794 86794

Crack Seal 66785 66785

Patching 199426 199426

Micro Slurry 110773 110773

Crack Seal 115464 115464

Patching 381712 381712

Micro Slurry 141378 141378

Net Present Value (NPV) 1286200 1286200

65

The formula for calculating absolute error:

( ) ( ) ( )

( ) Equation 4.1

= 0 %

( ) ( ) ( )

( ) Equation 4.2

= 0 %

( ) ( ) ( )

( ) Equation 4.3

= 0 %

Table 11: Summary of FPS validation using AASHTO, 1993

Layer Thickness AASHTO 1993 FPS Associated error (%)

D1 (in) 8 8 0

D2 (in) 7 7 0

D3 (in) 11 11 0

Table 12: Summary of FPS validation using Michael S. Mamlouk

Layer Thickness Michael S. Mamlouk FPS Associated error (%)

D1 (in) 6.5 6.5 0

D2 (in) 6 6 0

D3 (in) 8 8 0

Table 13: Summary of FPS validation for Net Present Value (NPV)

Life Cycle Cost (LCC) Zelelew (2008)

Costs in ETB

FPSCosts in

ETB

Associated

Error (%)

Net Present Value

(NPV)

1286200 1286200 0

66

5. CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusion

The research carried out to develop the software is successfully accomplished and the

results it gives are as accurate as expected. The developed software can be used to design

the flexible pavement by ERA and AASHTO methods. The software can determine the

layer thicknesses of flexible pavement structure using ERA and AASHTO methods and

the life cycle cost of the project based on Net Present Value (NPV) method. The results

obtained by each of the method using the software were compared with results obtained

from the manual design and were found absolutely accurate.

The manual flexible pavement design method practiced in Ethiopia, has a drawback in

doing comparison of many alternatives as flexible pavement design involves different

charts, tables and formulas so it is a cumbersome and time taking practice which may

result in unsafe and/or uneconomical design. The pavement design procedure by

AASHTO using nomographs could be inconsistent as different results could be obtained

by different users for the same input parameters. So, the application of this software will

be of great help by avoiding the precision errors that could result in a conservative design

or an under design.

The development of software for the flexible pavement design is very important as it

makes the design process very easy and accurate and saves a lot of precious time. Also,

increase the value to client by delivering more design alternatives in less time. Hence the

design process can be done in a very short time and accurately avoiding the

computational and calculation errors of the conventional manual design method.

5.2 Recommendation

It has been shown that a software package for flexible pavement design is a very essential

tool for Ethiopia. Therefore, it needs attention and further study. This research was

conducted in short time, thus, there are still several improvements that can be made. In

particular the following suggestions may be considered in future study:

The software is limited to flexible pavement design based on ERA and AASHTO

methods so it can further be extended for other design methods.

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The software is limited to the flexible pavement design so it can further for the

rigid pavement design.

Further study is needed to incorporate rehabilitation design /asphalt overlay

design/ in addition to new road design.

The software is limited to the structural design of pavement (carriage way) but it

can also involve road shoulder design.

Further study is needed to consider road widening at curves and high fill sections.

Life cycle cost can be analyzed in a more detailed way. In this research only

agency costs are considered, so user costs can be involved in life cycle cost

analysis.

The use of this software needs a good understanding of the manual design methods,

hence to use this software for education, research, and designing in field; one has to be

fully aware of the manual design methods.

Using Flexible Pavement Software (FPS) (V1.0) is very important for design agencies,

consultants, clients and researchers as it saves the precious time in addition to making the

design process very easy and accurate.

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REFERENCES

American Association of State Highway and Transportation Officials. (1993). AASHTO

Guide for Design of Pavement Structures. Washington, D.C.

Amare Setegn. (2012). Software development for AASHTO and ERA Flexible Pavement

design methods

C. Wei, S. Tighe.(2004). Development of Preventive Maintenance Decision Trees Based

on Cost-Effectiveness Analysis an Ontario Case Study, 83rd Annual TRB Meeting,

Washington, DC

Changmo Kim, Eul-Bum Lee, John T. Harvey, Amy Fong and Ray Lott. (2015).

AutomatedSequence Selectionand Cost Calculation forMaintenance and

Rehabilitationin Highway Life-Cycle CostAnalysis (LCCA)

David H. Timm, Mary M. Robbins, Nam Tran, Carolina Rodezno. (2014). Pavement Management and Pavement Design Manual

Ethiopian Roads Authority.(2013). ERA pavement design manual. Addis Ababa,

Ethiopia.

Flintsch, Kuttesch. (2004). Application of Engineering Economic Analysis Tools for

PavementManagement, 83rd Annual TRB Meeting, Washington, DC

Fugro-BRE.(2000).FHWA Performance Trends of Rehabilitated AC Pavements Tech

BriefNo.FHWA-RD-00-165 Federal Highway Administration, Washington. DC

James Walls III and Michael R. Smith.(1998). Life-Cycle Cost Analysis in Pavement

Design

K. Ozbay, N.A. Parker, D. Jawad, S. Hussain. (2003). Guidelines for Life Cycle Cost

AnalysisFinal Report, Report No FHWA-NJ-2003-012, Trenton, NJ

Khaled A. Abaza. (2002).Optimum Flexible Pavement Life-Cycle Analysis Model, J.

Transp. Eng.

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Michael S. Mamlouk. (2006). The handbook of highway engineering, Design of flexible

pavements. Arizona State University Tempe, AZ, U.S.A.

Pavement Management and Pavement Design Manual, 2008

Project Development & Design Guide.(2006). Massachusetts Highway Department

Rafi Ullah Khan, Muhammad Imran Khan and AfedUllah Khan. (2012).Software

Development (PAKPAVE) for Flexible Pavement Design.

Yang H. Huang.(2004). Pavement Analysis and Design, Second Edition. University of

Kentucky, Pearson Prentice Hall, Upper Saddle River.

Zelelew, (2008).

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APPENDIX

Appendix 1: Sample Code on Flexible Pavement Software (FPS)

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Appendix 2: Design Example1

Design Example taken from the Handbook of Highway Engineering by Michael S.

Mamlouk for validation of Flexible Pavement Software

Design the pavement for an expressway consisting of an asphalt concrete surface, a crushed stone base, and

granular subbase using the 1993 AASHTO design chart (Figure 8.22).The cumulative ESAL in the design

lane for a design period of 15 years in 7 X 106. The area has good quality drainage with 10% of the time the

moisture level is approaching saturation. The effective roadbed soil resilient modulus is 7 ksi, the subbase

has a CBR value of 80, the resilient modulus of the base is 40 Lb, and the resilient modulus of asphalt

concrete is 4.5 X 105 psi. Assume a reliability level of 95% and So of 0.45.

Solution

Step 1

Reliability (R) = 95% (Given)

Step 2

Overall standard deviation (So) = 045 (Given)

Step 3

W18 = 7 x 106 (Given)

Step 4

Effective road-bed soil resilient modulus = 7 ksi (Given)

Step 5

Resilient modulus of subbase = 20 ksi (Figure 8.25)

Resilient modulus of base = 40 ksi (Given)

Resilient modulus of subbase concrete surface = 450 ksi (Given)

Step 6

Assume initial serviceability index (po) = 4.6

Assume terminal serviceability index (pt) = 3.0

∆PSI = 4.6 – 3.0 = 1.6

Step 7

SN3 = 5.2 (Using Figure 8.22 and subgrade MR of 7 ksi)

SN2 = 3.5(Using Figure 8.22 and subbase MR of 20 ksi)

SN1 = 2.7(Using Figure 8.22 and base MR of 40 ksi)

Step 8

a3 = 0.14 (Figure 8.25)

a2 = 0.17 (Figure 8.25)

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a1 = 0.44 (Figure 8.25)

Step 9

Drainage coefficient = m2 = m3 = 1.1 (Table 8.4)

Step 10

Equation 8.11: 2.7 ≤ 0.44 D1

D1 = 6.1 in. (Round to 6.5 in.)

Equation 8.12: 3.5 ≤ 0.44 X 6.5 + 0.17 X D2 X 1.1

D2 = 3.4 in. (Use a minimum value of 6 in.) (Table 8.7)

Equation 8.13: 5.2 ≤ 0.44 X 6.5 + 0.17 X 6 X 1.1 + 0.14 X D3 X 1.1

D3 = 7.9 in. (Round to 8 in.)

Step 11

No information given on Freeze – thaw or swelling

Step 12

No information given on costs.

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Appendix 3: Design Example 2

Design Example taken from AASHTO, 1993 design guide for validation of

FlexiblePavement Software

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