INNOVATIVE SOFTWARE FOR FLEXIBLE PAVEMENT DESIGN ...
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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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