Roadbuilders have long recognized the need for a system that links all the variables that go into designing a pavement and then, provide data to support selection and optimization of pavement design features.

The Mechanistic-Empirical Pavement Design Guide (M-E PDG) represents more than 10 years of research, testing, reviews, and revisions aimed at improving the pavement design process, and ultimately, resulting in more uniform standards from agency to agency, project to project, and so forth.

This issue of CONCRETE PAVEMENT PROGRESS seeks to provide a point-in-time understanding of what the M-E PDG is; how it has been developed; and both its advantages and limitations.

This issue also covers what's next for the M-E PDG and the resources available for officials looking to implement it. The articles in this newsletter will also focus predominantly on the concrete pavement features and attributes of the program.

The M-E PDG is a state-of-the-practice tool that automates and standardizes the process of pavement analysis and design.

It represents a drastic departure from the empirically-based procedure currently in use by most agencies in both the level of required inputs and the mode of calculation.

It also integrates pavement design, materials selection, construction, and pavement management systems, according to the Federal Highway Administration.

It offers an easier, more objective way for designers and others to estimate pavement performance and potential distresses. In the near future, officials also anticipate the ability to customize the program with their specific traffic, materials and environmental conditions.

Currently, the guide is available as research-grade software available online through the Transportation Research Board. Click here to access it.

Getting Where We Are

Although the formal process of developing the M-E PDG only dates back to about 1998, there were other important milestones that came before then. For example, the Portland Cement Association proposed an early form of mechanistic pavement design. Individual states also developed early versions. Here's a snapshot of some milestones in empirical, mechanistic, and mechanistic-empirical pavement design.

• In the late 1950’s, the American Association of State Highway Officials (then known as AASHO) conducted an extensive pavement performance experiment near Ottawa, Ill., that established the basic pavement design methodology still in use. Test results were intended to help designers account for traffic’s affect on pavement performance  as they set out to build the United State’s infrastructure. They also set the precedent for pavement design guidelines officials used as the highway and roadway network grew.
  
• Officials recognized that updates were necessary to the existing guidelines to account for increased traffic volumes and weights. This provided the impetus for various updates, as well as a major rewrites in 1986 and  1993. The end result was the AASHTO 1993 Guide for Design of Pavement Structures, which was more adaptable than earlier versions but was still limited in scope due to it’s empirical nature. The guide in its various forms was widely accepted and implemented across states nationwide.
  
• In 1996, transportation officials representing the public sector, industry, and academia began to pursue a mechanistic approach for uniform pavement design guidelines throughout the United States.

• To complete the task, AASHTO Joint Task Force on Pavements (JTFP) called on the National Cooperative Highway Research Project to develop an updated guide for the design of new and rehabilitated pavement structures. To accomplish this, NCHRP moved forward with development of a mechanistic-empirical design procedure through the 1-37A project.
• In 1998, at the collective recommendation of agency, industry, and academia, NCHRP awarded the contract to  develop a guide based as fully as possible on existing mechanistic-empirical principles. The team behind the project, the largest of its kind in NCHRP's 40-year history, delivered the Mechanistic-Empirical Pavement Design Guide (M-E PDG) in 2004.The prime contractor for NCHRP 1-37A was Applied Research Associates, Inc. (ARA), and the subcontractor was Arizona State University (ASU). 

Michael I. Darter, Ph.D., P. E., Principal Engineer at ARA, lead the effortto develop the concrete pavement components of the M-E PDG. Matthew W. Witczak, Ph.D., Professor in the Department of Civil and Environmental Engineering at ASU, led the asphalt design development. Amir Hanna, Ph.D., NCHRP Senior Program Officer with Transportation Research Board (TRB), led the NCHRP 1-37A effort, while Ed Harrigan, Ph.D. also a NCHRP Senior Program Officer at TRB, led the 1-40 project.

• Following completion of the 1-37A project, NCHRP conducted an independent review of the Guide and began implementation efforts through the 1-40 project.  The Guide was reviewed for soundness of the approach and design/analysis methodology by a team of independent subject matter experts.
• A Comprehensive User Manual and Local Calibration Guide for the M-E PDG and Software were also developed as part of the 1-40 study.  These projects were considered critical resources to assist states in their implementation efforts.
• Most recently, the M-E PDG was  approved as an Interim Pavement Design Procedure by AASHTO in late 2007. This marked the beginning of widespread implementation among states and also set the stage for next steps in refinement and calibration of the
  M-E PDG.

Because using the M-E PDG is so different from what states currently practice, the biggest challenge facing it is implementation. Stakeholders, recognizing this barrier, are working on several different measures to simplify the implementation process:

• Localized Guidance: Local and regional environmental, materials and traffic factors are important to consider when designing a pavement. Therefore, Industry and agencies are working to calibrate the nationally calibrated models included in the M-E PDG. In the end, these locally calibrated models are expected to aid states in inputting their specific information for more accurate and customized results.

Click here to learn more about what the concrete pavement industry is doing to provide local models.

• User-Friendly Software: AASHTO is working on securing funding and proposals for a second version of the M-E PDG software. At present, it is a research-grade program that is difficult for new users to navigate. Darwin-ME is anticipated for release in 2010 and will include a more user-friendly interface, updated models, and enhanced features.

• Implementation Efforts: Throughout the various stages necessary to approve the guide for implementation, stakeholders have agreed that implementation among most states is a component, albeit a challenging one, to its success. Therefore, many are offering technology transfer, outreach, and educational programs aimed at making the process easier. Click here to learn more. Many states have undertaken calibration studies as they move towards implementation of the Guide, Ayers said. For example, Missouri has already used it for a number of projects.

For more information, contact Mike Ayers at 217-621-3438.

ACPA is conducting extensive trial runs of the Mechanistic-Empirical Pavement Design Guide (M-E PDG), recently approved for implementation by the American Association of State Highway and Transportation Officials.

ACPA is recognized as one of the first stakeholder organizations to take this step to help users better understand and implement the M-E PDG as a tool for designing and optimizing concrete pavements.

Anticipated products from ACPA’s trial runs are:

• A sensitivity analysis of features accounted for in the design guide. These will identify the key variables the concrete pavement industry should pay attention to and influence during construction. This analysis also will help the concrete pavement industry identify how the recommended design will affect construction practices.
• Graphs (based on the trial runs) to show users how specific conditions will affect pavement thickness. The graphs will represent a catalog of features, which is an attempt to put the guide’s recommendations into easy-to-understand terms, said Michael Ayers, Ph.D., ACPA’s Director of Pavement Technology.

For more information, contact Michael Ayers at 217-621-3438.

Looking for more information? A number and range of resources are available to help decision-makers fully understand and implement the M-E PDG. The following represents some of these technology transfer and training activities offered by various organizations:

• Industry: ACPA has a number of different opportunities for learning how to use the M-E PDG for new and rehabilitated pavements. These include:
.....- Web-based seminars (webinars) as a means of transferring knowledge to customers via teleconferencing capabilities. The Association has thus far presented a basic intro to M-E PDG. Additional training is planned. Click here for more information.
.....- An updated version of Concrete Pavements 101, to include an M-E PDG training module. This course provides an introduction to design and construction of concrete pavements. The next course is April 8 to 10 in Tampa Bay, Fla. Click here to download a flyer, which includes registration information.
.....- Concrete Pavements 201 is intended primarily for pavement design engineers, traffic and materials engineers, contractors, consultants, and others requiring in-depth knowledge of the M-E PDG. Click here for more information about this course.
....- (For University professors and instructors) ACPA's Professor's Seminar also has been updated to include training on the M-E PDG. This course provides university instructors with the knowledge and resources to incorporate concrete pavement into their engineering classes. The 2008 seminar will be held June 16 to 19 in Chicago, Ill. Click here for a flyer.

• FHWA 's Design Guide Implementation Team (DGIT) and Lead States Group conduct regular workshops and other efforts to promote and facilitate the refinement, implementation, and evolution of the M-E PDG. These efforts are often conducted with AASHTO, NCHRP, and FHWA, according to the group's website.  Additionally, the National Concrete Pavement Technology Center (CP Tech Center) has assisted the Lead States group in coordinating these activities and has discussed continuing this support in the future, according to Sharon Prochnow, Program Coordinator, CP Tech Center. For more information about DGIT activities, contact Chris Wagner at 404-562-3693.

• FHWA's National Highway Institute also offers a training course on M-E PDG. The course, FHWA-NHI-131064, is an introduction to mechanistic design for new and rehabilitated pavements. The course is intended for pavement design engineers, materials engineers, and pavement management practitioners from government transportation agencies and the paving industry, and design consultants. Click here for more information.

• Some state DOT's also sponsor training on topic. For more information, contact your respective state DOT training personnel to learn more.

For more information about the M-E PDG, or for details about related training courses, contact Mike Ayers at 217-621-3438.

Additional resources on the M-E PDG are available online.
Click here to read the AASHTO Memorandum on Project 1-37A.
Click here for TRB's Introduction to the Guide.