Guide for Mechanistic-Empirical Pavement Design of New and Rehabilitated Pavement Structures

The Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures is a product resulting from the efforts initiated by the American Association of State Highway and Transportation Officials (AASHTO) Joint Task Force on Pavements and the National Cooperative Highway Research Program (NCHRP) to enhance and improve existing pavement design procedures. Pavement and materials engineers need the best available tools to help them provide the most cost effective pavement design to the public. The release of the mechanistic empirical pavement design guide (M-E PDG) through the NCHRP provides such a tool.

Currently, the most widely used pavement design procedure is the AASHTO Guide for the Design of Pavement Structures. The original AASHTO Guide is based solely upon empirical observations of the AASHO road tests performed between 1958 and 1960. At the time, the AASHTO Guide represented the "most comprehensive development of the relationships between performance, structural thickness, and traffic loadings, the results are limited by the scope of the test and the conditions under which the test was conducted." AASHTO has long recognized the need to pursue "a more theoretical or 'rational' method for structural design of highway pavements" through NCHRP.

Benefits

Increased Pavement Life
Ability to Evaluate New Materials and Designs
Excellent Pavement Forensic Analysis Tool
More Economic Pavement Designs (Lower Life-Cycle Cost)
Ability to Design and Calibrate for Local Conditions

The M-E PDG represents a significant advancement in pavement design and includes sound engineering theory and mechanistic principles to determine both the structural response and predict the performance over the lifetime of a pavement structure. The mechanistic theory is balanced with over 525 empirical observations from the Long Term Pavement Performance database that represent a wide range of both material and climatic conditions. The use of both the mechanistic theory and a wide range of empirical observations make the M-E PDG a robust pavement analysis and design procedure.

The M-E PDG can be considered a 40-year step forward in pavement design—more theoretically and mathematically based, strongly bolstered by engineering principles. The M-E PDG is built upon sound and fundamental engineering principles that make it readily useful to academia, researchers, and practitioners of pavement analysis and design.

The M-E PDG process shown integrating pavement design with materials selection, construction quality, and pavement management in a circular system The M-E PDG provides significant potential benefits over the AASHTO Guide in achieving cost-effective pavement designs and rehabilitation strategies. Most importantly, its user-oriented computational software implements an integrated analysis approach for predicting pavement condition over the lifetime of the pavement structure. This analysis considers the complex interaction between traffic loadings, climatic conditions, and pavement structure. (The M-E PDG process integrates pavement design with materials selection, construction quality, and pavement management. Materials selection and construction quality have a direct influence on predicted pavement performance. Pavement management is used for local calibration.)

The M-E PDG also benefits innovative contracting. The software and accompanying manual are an excellent resource for pavement forensic analysis and can be an integral tool in innovative contracting procedures, such as the following:

Warranty
Performance-related contracts
Life-cycle cost analysis
Design/Build
Dispute resolution analysis

The M-E PDG represents a major change in the way pavement design is performed. The ability for a Pavement Engineer to evaluate the as-built properties against the predicted performance creates the link to performance-related contracts. 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 evaluated for adequacy by comparing the desired maximum levels of distress to that predicted by the guide. If the design does not meet desired performance criteria, the designer revises and repeats the evaluation process 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.

Mechanistic design is fundamental, that is, the concepts used are transferable over time, environmental conditions, material properties, and different locales. From an engineering point of view, there is much to be desired in a mechanistic approach to pavement design. "Mechanistic" refers to the application of the principles of engineering mechanics, which leads to a rational design process. Some of the "empirical" part of mechanistic-empirical design relates to the characterization of materials or to traffic, environment, or other inputs to the design process. Other empirical parts of this guide relate to field performance data used to correlate to accumulated damage. This "transfer" function, as it is sometimes called, relates the theoretical computation of "damage" (which is, in turn, a function of pavement deflection, strain, or stress responses) at some critical location with measured distress, completing the full mechanistic-empirical loop of the pavement design.

The M-E PDG procedure will improve the pavement community's understanding of pavement performance and help the community produce high-quality, longer-lasting pavements using the appropriate level of expenditures. Using the new procedure will enable the pavement design community to better predict and address the critical performance elements of pavement structures. In addition, the new procedures may lead to the development and use of new materials that perform better, last longer, and are more cost effective than today's pavements.

Additional Resources
The M-E PDG and associated documentation can be downloaded from the Transportation Research Board Web site. Additional information can be found at the FHWA Design Guide Implementation Team Web site.

Mechanistic-Empirical Pavement Design: Lead States Group

Definition of a Lead State

One of the first States to pursue implementation of the design guide and obtain upper management support.
A champion for implementation.
Knows the political, funding, and internal hurdles that need to be addressed.
Able to compare pavement design/analysis technologies to determine which is most advantageous for a given project.
Able to focus on advanced technologies and/or refinements.
Advertises both successes and failures.
Shares funding success stories.
Develops short- and long-term plans for implementation.
Becomes an expert in the implementation process.

Lead States

As of January 2006, the following 17 States had joined the Lead States Group:

Arizona
California
Florida
Kentucky
Maryland
Minnesota
Mississippi
Missouri
Montana
New Jersey
New Mexico
Pennsylvania
Texas
Utah
Virginia
Washington
Wisconsin

Lead States Group Working Task Groups

Promotion

Develop and maintain on-line Lead States Group newsletters.
Periodically, mail out articles related to the implementation of M-E pavement design.
Chronicle the benefits of DOTs moving to M-E pavement design.
Conduct an annual benchmarking survey.

Implementation

Follow any research work ongoing in NCHRP projects related to design (e.g., NCHRP 1-40, 1-42, 1-39, etc.).
Develop a synthesis of existing implementation plans and identify best practices.
Establish Lead State Group distress limits based upon each agency's pavement management system (PMS) practice, based on project distress surveys on sections that have been prioritized by their PMS.

Communication

Establish channels of communication through formal links with industry (NCAT, NAPA, AI, ACPA, ACI, etc.), local agencies, and academia.
Identify States, other than the Lead States, that have formal implementation plans and share this information through nationwide workshops and web-casts.
Provide reports or presentations at TRB annual meetings and other venues.
Act as the clearinghouse for information developed by other agencies outside of the Lead States Group.

Knowledge

Document State calibration efforts and utilize nationwide lessons learned, as well as use the LTPP sites to help in the individual State calibration efforts and work with the FHWA's DGIT to document best practices by the States in test equipment and protocols.
Identify the gaps in the knowledge base related to the design guide, considering enhancements to the program such as:
1. uncoupling various modules so that individual input and output can be reviewed,
2. considering allowing batch mode input routines, and
3. documenting the source code.
Work with the FHWA to identify and establish a pool of national experts.
Strive to provide equipment and other financial support to universities, in order to develop faculty expertise and course work in theoretical and practical pavement design.

Additional Resources
For more information, visit the Lead States Web site.

Additional information on the M-E PDG can be found at the FHWA Design Guide Implementation Team Web site.

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