Design Optimization Of Rigid Pavement: Comparing Existing Flexible And Rigid Pavement

ABSTRACT

Road construction projects have been implemented all over Nigeria as part of the national development plan. Roads are one of the country’s basic infrastructural facilities where high amounts of budget allocated every fiscal year planning period. Since the cost comprises of a large portion of government investment, a careful evaluation of the alternatives is utmost importance to make the right choice for a particular project. In the history of Nigeria road development program, almost all of the road pavements are flexible, and it demands high foreign currency for asphalt material importing from abroad. In addition, flexible pavement needs to be maintained and rehabilitated within a few years after its initial construction. In view of the emerging cement factories and the availability of cement in Nigeria, it is practical to consider rigid pavement as one of the alternatives.

This work presents a method for optimizing the design rigid pavements according to PCA method. The objective function includes the costs of concrete, formwork, prepared subgrade soil support and at the end there will be a comparison between rigid and flexible pavement design. All the constraints functions are set to meet design requirements of PCA and current practices rules. The optimization process is developed through the use of the Generalized Reduced Gradient algorithm. An example is considered in order to illustrate the applicability of the proposed design model and solution methodology. It is concluded that this approach is economically more effective comparing to conventional design methods used by designers and engineers

TABLE OF CONTENTS

 TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWELDGEMENT

ABSTRACT

TABLE OF CONTENT

CHAPTER ONE

• INTRODUCTION
• BACKGROUND OF THE PROJECT
• PROBLEM STATEMENT
• OBJECTIVE OF THE STUDY
• SCOPE OF THE STUDY
• SIGNIFICANCE OF THE STUDY
• PROJECT ORGANISATION

CHAPTER TWO

LITERATURE REVIEW

2.0      LITERATURE REVIEW
2.1      CONCEPTUAL REVIEW OF THE STUDY

2.2     THEORETICAL REVIEW

2.3    OVERVIEW OF FLEXIBLE AND RIGID PAVEMENT

CHAPTER THREE

3.0      METHODOLOGY

3.1      DESIGN OF HIGHWAY PAVEMENTS

3.2      PCA DESIGN METHOD FOR RIGID PAVEMENTS

3.3      PCA FATIGUE MODEL BASED ON TENSILE STRESS DUE TO EDGE LOADS

3.4      PCA EROSION MODEL BASED ON DEFLECTIONS DUE TO CORNER LOADS

3.5     OPTIMIZATION BASED ON MINIMUM COST DESIGN OF CONCRETE PAVEMENTS

3.6    SOLUTION METHODOLOGY: GENERALIZED REDUCED GRADIENT METHOD

3.7    COMPARISON BETWEEN RIGID AND FLEXIBLE PAVEMENT

3.8    COST ANALYSIS OF RIGID AND FLEXIBLE PAVEMENT

CHAPTER FOUR

4.0       RESULT ANALYSIS

4.1      NUMERICAL RESULTS AND DISCUSSION

4.2      COMPARISON BETWEEN THE CLASSICAL SOLUTION AND OPTIMAL SOLUTION

CHAPTER FIVE

• CONCLUSIONS AND SUMMARY

5.3     REFERENCES

CHAPTER ONE

1.0                                                                              INTRODUCTION

Transportation has been one of the essential components of the civil engineering profession since its early days. From time immemorial, the building of roads, bridges, pipelines, tunnels, canals, railroads, ports, and harbors has shaped the profession and defined much of its public image. As cities grew, civil engineers became involved in developing, building, and operating transit facilities, including street railways and elevated and underground systems [1]. The role of civil engineers is to providing transportation infrastructure to accommodate a growing population. The transportation by road is the only road which could give maximum service to one all. This mode has also the maximum flexibility for travel with reference to route, direction, time and sped of travel. It is possible to provide door to door service only by road transport [2].

Pavement is the actual travel surface especially made durable and serviceable to withstand the traffic load commuting upon it. Pavement grants friction for the vehicles thus providing comfort to the driver and transfers the traffic load from the upper surface to the natural soil. In earlier times before the vehicular traffic became most regular, cobblestone paths were much familiar for animal carts and on foot traffic load. Pavements are primarily to be used by vehicles and pedestrians. Storm water drainage and environmental conditions are a major concern in the designing of a pavement. The first of the constructed roads date back to 4000 BC and consisted of stone paved streets or timber roads. A highway pavement is a structure consisting of superimposed layers of processed materials above the natural soil sub-grade, whose primary function is to distribute the applied vehicle loads to the sub-grade [3]. The pavement structure should be able to provide a surface of acceptable riding quality, adequate skid resistance, favorable light reflecting characteristics, and low noise pollution. The ultimate aim is to ensure that the transmitted stresses due to wheel load are sufficiently reduced, so that they will not exceed bearing capacity of the sub grade. The roads of the earlier times depended solely on stone, gravel and sand for construction and water was used as a binding agent to level and give a finished look to the surface. All hard road pavements usually fall into two broad categories namely

1. Flexible Pavement
2. Rigid Pavement

Rigid pavement with and without base course are used in many countries all around the world. It has a large number of advantages such as long life spans, negligible maintenance, user and environment friendly and lower cost if they are designed correctly and constructed well. Because of its rigidity and high tensile strength, a rigid pavement tends to distribute the load over a relatively wide area of sub-grade, and a major portion of the structural capacity is supplied by the slab itself. Concrete pavement has good design period vs bituminous. It is generally better able to cope with unexpected loads and fuel spillages in industrial estates and service areas. Concrete road is generally able to maintain an adequate skid resistance under heavy traffic for longer than bituminous surfacing [1, 2, 3, 4].

The pavements were designed under different traffic conditions and different soil parameters. Pavement design methods are generally grouped into two major types, namely purely empirical approach and mechanistic-empirical approach. The Portland Cement Association’s thickness design procedure (PCA method) is the most well-known, widely-adopted, and mechanically based procedure for the thickness design of jointed concrete pavements. The PCA thickness design criteria are to limit the number of load repetitions based on both fatigue analysis and erosion analysis. Cumulative damage concept is used for the fatigue analysis to prevent the first crack initiation due to critical edge stresses, whereas the principal consideration of erosion analysis is to prevent pavement failures such as pumping, erosion of foundation, and joint faulting due to critical corner deflections during the design period. The design factors considered by the PCA method include the design period, the flexural strength of concrete (or the concrete modulus of rupture), the modulus of sub-base-subgrade reaction, design traffic (including load safety factor, axle load distribution), with or without doweled joints and a tied concrete shoulder. The presence of doweled joints will affect the erosion analysis while the presence of concrete shoulder will affect both fatigue and erosion analysis [15].

Structural designers have traditionally the task to develop designs that provides safety. Structural optimization on the other hand deals with the design of structural elements and systems employed in several engineering fields. One of the most common structural design methods involves decisions making based experience and intuition. The design of the structures both buildings, bridges and roads are often governed mostly by cost rather than by weight considerations. A better design is achieved if an appropriate cost or objective function can be reduced. The structural performances depend on the optimization techniques. The numerical optimization is one of the tools that helps provide the desired results in a timely and economical fashion[16, 17, 18, 19]. The effectiveness of the optimization method depends on both the algorithm and the software in use. Many algorithms have been developed and evaluated for practical optimization. The use of an algorithm that provides reliable results for the problem of interest is important. The use of constrained minimization methods necessitates for the design variables to be modified successively during the design process by moving in the design space from one design point to another. Constrained optimization is a very active field of research and many algorithms have been developed. Nonlinear structural analysis is an example where optimization can be used to solve a nonlinear cost minimization problem. It was shown that the minimum cost optimum design of simple structural elements could be stated as a nonlinear mathematical programming problem in a design variables space. Using numerical optimization as a design tool has several advantages: optimization techniques can greatly reduce the design time and yield improved, efficient and economical designs. Advances in numerical optimization methods, computer based numerical tools for analysis and design of structures and availability of powerful computing hardware have significantly helped the design process to ascertain the optimum design namely. During the seventies a large number of design problems were solved using these optimization techniques. It is clear that there is a need to develop optimum design equations for rigid pavements. Pavement engineers have identified the importance of taking advantage of the available routines for the optimum design of pavement structures[24]

This work presents a method for optimizing rigid pavements design according to PCA method. The objective function includes the costs of concrete, formwork and prepared subgrade soil support. All the constraints functions are set to meet design requirements of PCA and current practices rules. The optimization process is developed through the use of the Generalized Reduced Gradient algorithm. An example is considered in order to illustrate the applicability of the proposed design model and solution methodology. It is concluded that this approach is economically more effective comparing to conventional design methods used by designers and engineers. At the end of this work, the cost of constructing and using the two types of pavement are compared in terms of suitability and durability.

1.1                                                              BACKGROUND OF THE STUDY

In many countries with developed road networks, new road construction typically accounts of more or less 50% of the road budget. While the remainder of national road budgets is spent on maintenance and rehabilitation of existing roads. Long-life Pavements (LLP) project is approved if the costs of future maintenance, rehabilitation and the resulting road user delay costs are economically justified [8].

There has been historically difference of opinion as to whether Hot Mix Asphalt (flexible) pavements are more economical or less economical over time, than Portland Cement Concrete (rigid) pavements. Even experienced state highway agencies and highway engineers disagree on the subject [9].

Nigerian has been undertaking massive development programs to eradicate the country’s poverty problems and to bring up the country to the level of middle income countries in 2025 G.C. Aware of the road infrastructure development as the backbone and the blood artery for all economic, social progress, due emphasis has been given to the implementation of the Road Sector Development program (RSDP) since 1997.

Initial cost is generally the major factor in deciding the type of the pavement in design. The planners often think that the flexible pavement is cheaper than the rigid pavements. In fact this is not always the case. In the last decade the price of bitumen which is the main ingredient of flexible pavement has increased because of the increase in crude oil prices.

Government executives frequently have to choose between concrete and asphalt paving materials for roads and highways. In the last few years in asphalt which caused a dramatic escalation in asphalt prices – reflected in a 250% increase during 2005-2008. It is likely that once the economic recovery gains traction, large shortages may reappear, oil prices will rise and asphalt prices will resume their upward climb. From 2003 to 2008 oil prices increased nearly 300%. During the same period, liquid asphalt increased 250%. The increases in asphalt prices during this period were not only a result of rising oil prices, but also by changes in oil refining practices which has led to a reduction in heavy crude production and reduced supply.

Global economic weakness has resulted in a 50% decline in oil prices during the past year. Despite reduced paving demand and lower oil prices, asphalt prices have declined only 12% from record high levels during the same period.

1.2                                                  PROBLEM STATEMENT

Concrete pavement is often used as a superstructure element as road slab for highways, airports, industrial grounds, streets, parking areas. Because of many disadvantages noticed on flexible pavement that the use of rigid pavement was considered. Rigid have advantages such as long life spans, negligible maintenance, user and environment friendly and lower cost if they are designed correctly and constructed well

1.3                                               OBJECTIVE OF THE STUDY

The research work had been focused on the specific objectives to determine and compare the durability, cost, life span of rigid and flexible pavements and to investigate all other qualitative merits of rigid and flexible pavement.

1.4                                                   SCOPE OF THE STUDY

In this paper, the economic analysis has been done on both flexible and rigid pavements. These cost analyses play a great role on the decision-making stages in selection of pavement types. In this analysis, the durability, cost, life span of rigid and flexible pavements have been determined for both flexible pavements and rigid pavements according to design guideline.

Relative to this, the research project was conducted with the main objective of identifying the cost of rigid and flexible pavements at Nigeria roads. The research work had been focused on the specific objectives to determine and compare the life cycle costs of rigid and flexible pavements and to investigate all other qualitative merits of rigid and flexible pavement.

1.5                                           SIGNIFICANCE OF THE STUDY

This study helps designers and builders to explore innovative and cost effective products to satisfy increasing demand that would economize the construction as well as increase durability.

It also help engineers to know the suitability of pavement depending on various parameters such as material, loading, longer life, cost effectiveness etc.