Part3_Chapter3_Flexible Design

Figure 3.3.2. Illustration of possible asphalt pavement layered systems.

In addition to the trial design, the designer must provide inputs for the project site conditions including subgrade properties (including presence of bedrock), traffic, and climatic data. There are also several design inputs related to construction such as the initial smoothness (initial IRI), estimated month of construction, and estimated month that the pavement will be opened to traffic.

A major difficulty in obtaining adequate design inputs is that the desired project specific information is not generally available at the design stage and must often be estimated several years in advance of construction. The actual materials used in a project may not

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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 3—DESIGN ANALYSIS

CHAPTER 3

DESIGN OF NEW AND RECONSTRUCTED FLEXIBLE PAVEMENTS

3.3.1 INTRODUCTION

This chapter describes the mechanistic-empirical design procedures for new and reconstructed flexible pavements. For the purposes of this guide, new and reconstructed flexible pavements are defined as pavement systems having asphalt concrete surfaces. These include the following:

• Conventional flexible pavements—i.e., thin hot-mix asphalt (HMA) layer over granular base/subbase materials.
• Deep strength HMA pavements—i.e., thick HMA layers over granular layers.
• Full-depth HMA pavements—i.e., sections consisting only of HMA layers (HMA surface and HMA stabilized bases).
• "Semi-rigid" pavements—i.e., sections having some type of a chemically stabilized layer as a base layer below the HMA layer.

The allowable pavement cross sections include both conventional layering (decreasing material quality with depth) and sandwich structures (unbound aggregate layer placed between two stabilized layers).

The procedures described in this chapter are most applicable when designing pavements constructed with conventional dense-graded HMA mixtures. The performance models developed are valid for pavement constructed with such mixtures. For flexible pavements constructed with materials such as stone matrix asphalt (SMA) mixtures, polymer modified asphalt (PMA) mixtures, and asphalt mixtures containing recycled asphalt pavement (RAP) materials, the tools described in this chapter could also be used but primarily in the analysis mode in conjunction with level 1 mixture related inputs. Caution should be exercised when drawing design related inferences because the calibrated performance equations included as part of the design guide did not include a significant percentage of pavements sections with such mixtures. Generally speaking, the methodology described in the design guide is applicable for modified mixtures. However, a more comprehensive dataset will be eventually required to develop the necessary calibrated performance models for use in design.

The mechanistic-empirical design of new and reconstructed flexible pavements requires an iterative hands-on approach by the designer. The designer must select a trial design and then analyze the design in detail to determine if it meets the performance criteria established by the designer. The flexible pavement performance measures considered in this guide include permanent deformation (rutting), fatigue cracking (both bottom-up and top-down), thermal cracking, and smoothness (International Roughness Index or IRI). If the trial design does not satisfy the performance criteria, the design is modified and

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3.3.2 OVERVIEW OF FLEXIBLE PAVEMENT DESIGN PROCESS

reanalyzed until the design does satisfy all criteria. The designs that meet the applicable performance criteria are then considered feasible from a structural and functional viewpoint and can be further considered for other evaluations such as life cycle cost analysis.

This chapter provides a detailed description of the design procedure for flexible pavement systems. The contents of this chapter are as follows:

1. Introduction (section 3.3.1).
2. An overview of the design procedure (section 3.3.2).
3. Design inputs for new flexible pavement design (section 3.3.3).
4. Flexible pavement design procedure (section 3.3.4).
5. Special loading considerations (section 3.3.5)
6. Calibration to local conditions (section 3.3.6).

The procedures described in this chapter can be used for design of flexible pavements on new alignments or for reconstruction of existing pavements. There are no fundamental differences in the way pavements are designed for new alignment or reconstruction. However, one practical aspect is that for reconstruction, the potential usage of the materials from the existing pavement structure is an important issue that should be considered. The effects of using the materials recycled from the existing pavement structure in various modified layers of the new pavement can be evaluated using the procedures described in this Guide.

3.3.2 OVERVIEW OF FLEXIBLE PAVEMENT DESIGN PROCESS

The overall iterative design process for asphalt pavements is illustrated in figure 3.3.1. The term "asphalt pavement" refers to any new, reconstructed, or rehabilitated pavement system that has asphalt concrete as the surface layer. The main steps in the design process include the following:

1. Assemble a trial design for specific site conditions—define subgrade support, asphalt concrete and other paving material properties, traffic loads, climate, pavement type and design and construction features.
2. Establish criteria for acceptable pavement performance at the end of the design period (i.e., acceptable levels of rutting, fatigue cracking, thermal cracking, and IRI).
3. Select the desired level of reliability for each of the applicable performance indicators (e.g., select reliability levels for rutting, cracking, and IRI).
4. Process input to obtain monthly values of traffic inputs and seasonal variations of material and climatic inputs needed in the design evaluations for the entire design period.
5. Compute structural responses (stresses and strains) using multilayer elastic theory or finite element based pavement response models for each axle type and load and for each damage-calculation increment throughout the design period.

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3.3.2.1 Design Inputs

Figure 3.3.1. Overall design process for flexible pavements.

6. Calculate accumulated distress and/or damage at the end of each analysis period for the entire design period.

7. Predict key distresses (rutting, bottom-up/top-down fatigue cracking, thermal cracking) at the end of each analysis period throughout the design life using the calibrated mechanistic-empirical performance models provided in the Guide.

8. Predict smoothness (IRI) as a function of initial IRI, distresses that accumulate over time, and site factors at the end of each analysis increment.

9. Evaluate the expected performance of the trial design at the given reliability level.

10. If the trial design does not meet the performance criteria, modify the design and repeat the steps 4 through 9 above until the design does meet the criteria.

Guidelines on the design process are provided in section 3.3.4. The designs that satisfy performance criteria are considered feasible from a structural and functional viewpoint and can be further considered for other evaluations, such as life cycle cost analysis.

Trial Design Inputs and Site Conditions

The design procedure offers the capability to consider a wide range of structural sections as illustrated in figure 3.3.2. An acceptable design is determined by iteratively analyzing and modifying trial designs until all performance criteria (e.g., rutting, fatigue cracking, thermal cracking, IRI) are satisfied over the analysis period. A trial design includes all details needed to perform design evaluations using the procedures prescribed in this Guide.

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Figure 3.3.2. Illustration of possible asphalt pavement layered systems.

In addition to the trial design, the designer must provide inputs for the project site conditions including subgrade properties (including presence of bedrock), traffic, and climatic data. There are also several design inputs related to construction such as the initial smoothness (initial IRI), estimated month of construction, and estimated month that the pavement will be opened to traffic.

A major difficulty in obtaining adequate design inputs is that the desired project specific information is not generally available at the design stage and must often be estimated several years in advance of construction. The actual materials used in a project may not

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