Part1_Chapter1_ Introduction

does not exist in a large portion of the country. Direct consideration of climatic effects at a project site will lead to improved pavement performance and reliability.

• Subgrade deficiencies: One type of subgrade was used for all test sections at the Road Test, but many types exist nationally that result in different performance of highway pavements. More adequate characterization and consideration of subgrade support will make it possible to improve design of the pavement structure resulting in improved performance and reliability.
• Surfacing materials deficiencies: Only one hot mix asphalt (HMA) mixture and one portland cement concrete (PCC) mixture were used at the Road Test. Today, there exist many different hot mix asphalt concrete (HMAC) and PCC mixtures (e.g., Superpave, stone-mastic asphalt, high-strength PCC) whose effects cannot be fully considered.
• Base course deficiencies: Only two unbound dense granular base/subbase materials were included in the main flexible and rigid pavement sections of the AASHO Road Test (limited testing of stabilized bases was included for flexible pavements). These exhibited significant loss of modulus due to frost and erosion. Today, various stabilized types of higher quality are used routinely, especially for heavier traffic loadings.
• Truck characterization deficiencies: Vehicle suspension, axle configurations, and tire types and pressures were representative of the types used in the late 1950's. Many of these are outmoded (tire pressures of 80 psi versus 120 psi today), resulting in deficient pavement designs to carry these loadings.
• Construction and drainage deficiencies: Pavement designs, materials, and construction were representative of those used at the time of the Road Test. No subdrainage was included in the Road Test sections, but positive subdrainage has become common in today's highways.
• Design life deficiencies: Because of the short duration of the Road Test, the long-term effects of climate and aging of materials were not addressed. The AASHO Road Test was conducted over 2 years, while the design lives for many of today's pavements are 20 to 50 years. Direct consideration of the cyclic effect on materials response and aging will lead to improved design life reliability.
• Performance deficiencies: Earlier AASHTO procedures relate the thickness of the pavement surface layers (asphalt layers or concrete slab) to serviceability. However, research and observations have shown that many pavements need rehabilitation for reasons that are not related directly to pavement thickness (e.g., rutting, thermal cracking, faulting). These failure modes are not considered directly in previous versions of the AASHTO Guide, which may be leading to more premature failures.
• Reliability deficiencies: The 1986 AASHTO Guide included a procedure for considering design reliability that has never been fully validated. This procedure resulted in a large multiplier of design traffic loadings to achieve a desired reliability level (e.g., a pavement designed for 50 million equivalent single axle loads [ESALs] was actually designed for 228 million). The multiplier increased greatly with design level of reliability and may result in excessive layer thicknesses for heavier trafficked pavements that may not be warranted but are commonly being designed today.

The primary measure of pavement performance in the earlier procedures is present serviceability, and the dominant factor affecting serviceability is longitudinal profile. Yet, in many cases, pavement managers find that distress factors other than ride—such as cracking, rutting, and joint

1.1.4

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ACKNOWLEDGMENT OF SPONSORSHIP

This work was sponsored by the American Association of State Highway and Transportation Officials, in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program, which is administered by the Transportation Research Board of the National Research Council.

DISCLAIMER

This is the final draft as submitted by the research agency. The opinions and conclusions expressed or implied in the report are those of the research agency. They are not necessarily those of the Transportation Research Board, the National Research Council, the Federal Highway Administration, the American Association of State Highway and Transportation Officials, or the individual states participating in the National Cooperative Highway Research Program.

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PART 1—INTRODUCTION

CHAPTER 1

BACKGROUND, SCOPE, AND OVERVIEW

1.1.1 BACKGROUND

1.1.1.1 Objective of the Design Guide

The overall objective of the Guide for the Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures (referred to hereinafter as the Design Guide) is to provide the highway community with a state-of-the-practice tool for the design of new and rehabilitated pavement structures, based on mechanistic-empirical principles. This objective was accomplished through developing the following:

• The Design Guide itself, which is based on comprehensive pavement design procedures that use existing mechanistic-empirical technologies.
• User-oriented computational software and documentation based on the Design Guide procedure.

The Design Guide represents a major change in the way pavement design is performed. The designer first considers site conditions (traffic, climate, subgrade, existing pavement condition for rehabilitation) and construction conditions in proposing a trial design for a new pavement or rehabilitation. The trial design is then evaluated for adequacy through the prediction of key distresses and smoothness. If the design does not meet desired performance criteria, it is revised and the evaluation process repeated as necessary. Thus, the designer is fully involved in the design process and has the flexibility to consider different design features and materials for the prevailing site conditions. This approach makes it possible to optimize the design and to more fully insure that specific distress types will not develop.

The mechanistic-empirical (M-E) format of the Design Guide provides a framework for future continuous improvement to keep up with changes in trucking, materials, construction, design concepts, computers, and so on. In addition, guidelines for implementation and staff training have been prepared to facilitate use of the new design procedure, as well as strategies to maximize acceptance by the transportation community.

1.1.1.2 Economic Justification for a Revised and Improved Design Guide

The nation's highways reached an estimated 2.7 trillion vehicle-miles in 2000. This is four times the 1960 level. This amounts to 7.4 billion vehicle-miles of travel every day. Truck travel (single-unit and combinations) has increased 231 percent since 1970. Combination truck travel has increased 285 percent over 1970 levels and now accounts for 4.9 percent of total annual vehicle-miles of travel versus 3.2% in 1970. (1) The 4 million miles of U.S. roadways (with 2 million miles of paved roads) have been constructed, rehabilitated, and maintained over the previous century, and they represent a huge national investment that has provided a safe and

1.1.1

comfortable means of transportation for both private and commercial vehicles. Highways have contributed significantly to the economic growth of the nation.

Pavement structures wear down and deteriorate under heavy axle loadings and exposure to the elements (very hot and very cold temperatures, freezing and thawing, precipitation). Therefore, they must be maintained and improved on a regular basis. This requires a very significant commitment of resources on the part of the nation's highway agencies (State, Federal, and local). Figure 1.1.1 illustrates the magnitude and increasing level of highway expenditures by function from 1980 to 2000.

Figure 1.1.1 The magnitude and increasing level of highway expenditures by function from 1980 to 2000. (1)

Total highway expenditure by all units of government in 2000 was $126.7 billion, a 203 percent increase compared to 1980 (average annual increase of 10 percent). Note from figure 1.1.1 that the annual level of expenditure is clearly accelerating over time. (1) The 2000 total disbursement by State highway agencies was $89.8 billion, of which 53.1 percent went to capital outlays which includes 10.5 percent for new highway construction and 42.6 percent for improvements on existing highways (1). Approximately one-half of the capital outlay goes to pavement related work. The sheer magnitude of annual expenditures on highway pavements justifies the application of the best available design procedures to optimize the use of highway funds. Any improvements in design of new or rehabilitated pavement structures will have significant and sizeable implications in reducing the cost of maintaining these highway pavements.

As of the publication of this Design Guide, the AASHTO Guide for Design of Pavement Structures is the primary document used to design new and rehabilitated highway pavements. The Federal Highway Administration's 1995-1997 National Pavement Design Review found that some 80 percent of States use the 1972, 1986, or 1993 AASHTO Guides (2, 3, 4). All those versions are empirically based on performance equations developed using 1950's AASHO Road Test data. The 1986 and 1993 AASHTO Guides contain some refinements in materials input parameters, design reliability, and empirical procedures for rehabilitation design (3, 4).

1.1.2

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1.1.1.3 Need for the Design Guide

Since the AASHTO Road Test, the AASHTO Joint Task Force on Pavements (JTFP) has been responsible for the development and implementation of pavement design technologies. This charge has led to many significant initiatives, including the development of every revision of the AASHTO Guide. More recently, and in recognition of the limitations of the AASHTO Guide, the JTFP initiated an effort to develop an improved Design Guide. As part of this effort, a workshop was convened on March 24-26, 1996, in Irvine, California, to develop a framework for improving the Guide. The workshop attendees—pavement experts from public and private agencies, industry, and academia—addressed the areas of traffic loading, foundations, materials characterization, pavement performance, and environment to help determine the technologies best suited for the new Design Guide. At the conclusion of that workshop, a major long-term goal identified by the JTFP was the development of a design guide based as fully as possible on mechanistic principles. This Design Guide is the end result of that goal.

The various editions of the AASHTO Guide for Design of Pavement Structures have served well for several decades; nevertheless, many serious limitations exist for their continued use as the nation's primary pavement design procedures:

• Traffic loading deficiencies: Heavy truck traffic design volume levels have increased tremendously (about 10 to 20 times) since the design of the pavements used in the Interstate system in the 1960's. The original Interstate pavements were designed for 5 to 15 million trucks, whereas today these same pavements must be designed for 50 to 200 million trucks and more over an even longer design life (e.g. 30-40 years versus 20 years). The existing AASHTO Guide cannot be used reliably to design for this level of traffic. Road Test pavements sustained slightly over 1 million axle load applications—less than the traffic carried by many modern pavements within the first few years of their use—and the equations forming the basis of the earlier procedures were based on regression analyses of the AASHTO Road Test data. Thus, application of the procedure to modern traffic streams means the designer often must extrapolate the design methodology far beyond the data and experience providing the basis for the procedure. Clearly, the result is that designers have been working "in the dark" on highly trafficked projects. Such projects may well have been either "under-designed" or "over-designed," with the result of significant economic loss.
• Rehabilitation deficiencies: Pavement rehabilitation design procedures were not considered at the AASHTO Road Test. Procedures in the 1993 Guide are completely empirical and very limited, especially in consideration of heavy traffic. Major economic losses will continue unless improved capabilities for rehabilitation design are provided to meet today's highway traffic needs, as most projects today include rehabilitation design. Improved rehabilitation designs will lead to longer-lasting and more cost-effective rehabilitated pavements.
• Climatic effects deficiencies: Because the AASHTO Road Test was conducted at one specific geographic location, it is impossible to address the effects of different climatic conditions on pavement performance. For example, at the Road Test a significant amount of distress occurred in the pavements during the spring thaw, a condition that

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does not exist in a large portion of the country. Direct consideration of climatic effects at a project site will lead to improved pavement performance and reliability.

• Subgrade deficiencies: One type of subgrade was used for all test sections at the Road Test, but many types exist nationally that result in different performance of highway pavements. More adequate characterization and consideration of subgrade support will make it possible to improve design of the pavement structure resulting in improved performance and reliability.
• Surfacing materials deficiencies: Only one hot mix asphalt (HMA) mixture and one portland cement concrete (PCC) mixture were used at the Road Test. Today, there exist many different hot mix asphalt concrete (HMAC) and PCC mixtures (e.g., Superpave, stone-mastic asphalt, high-strength PCC) whose effects cannot be fully considered.
• Base course deficiencies: Only two unbound dense granular base/subbase materials were included in the main flexible and rigid pavement sections of the AASHO Road Test (limited testing of stabilized bases was included for flexible pavements). These exhibited significant loss of modulus due to frost and erosion. Today, various stabilized types of higher quality are used routinely, especially for heavier traffic loadings.
• Truck characterization deficiencies: Vehicle suspension, axle configurations, and tire types and pressures were representative of the types used in the late 1950's. Many of these are outmoded (tire pressures of 80 psi versus 120 psi today), resulting in deficient pavement designs to carry these loadings.
• Construction and drainage deficiencies: Pavement designs, materials, and construction were representative of those used at the time of the Road Test. No subdrainage was included in the Road Test sections, but positive subdrainage has become common in today's highways.
• Design life deficiencies: Because of the short duration of the Road Test, the long-term effects of climate and aging of materials were not addressed. The AASHO Road Test was conducted over 2 years, while the design lives for many of today's pavements are 20 to 50 years. Direct consideration of the cyclic effect on materials response and aging will lead to improved design life reliability.
• Performance deficiencies: Earlier AASHTO procedures relate the thickness of the pavement surface layers (asphalt layers or concrete slab) to serviceability. However, research and observations have shown that many pavements need rehabilitation for reasons that are not related directly to pavement thickness (e.g., rutting, thermal cracking, faulting). These failure modes are not considered directly in previous versions of the AASHTO Guide, which may be leading to more premature failures.
• Reliability deficiencies: The 1986 AASHTO Guide included a procedure for considering design reliability that has never been fully validated. This procedure resulted in a large multiplier of design traffic loadings to achieve a desired reliability level (e.g., a pavement designed for 50 million equivalent single axle loads [ESALs] was actually designed for 228 million). The multiplier increased greatly with design level of reliability and may result in excessive layer thicknesses for heavier trafficked pavements that may not be warranted but are commonly being designed today.

The primary measure of pavement performance in the earlier procedures is present serviceability, and the dominant factor affecting serviceability is longitudinal profile. Yet, in many cases, pavement managers find that distress factors other than ride—such as cracking, rutting, and joint

1.1.4

6