Overview of the Development of the Mechanistic Design Guide

An Overview of the Development of the Mechanistic Design Guide for New and Rehabilitated Pavement Structures

Recognizing the need for a uniform set of principles to guide the design of new and rehabilitated pavements, a number of organizations representing both the public and private sectors set out to create a new set of procedures in the mid-1990’s.

Among them were the American Association of State Highway and Transportation Officials (AASHTO), along with the Federal Highway Administration, the National Academy of Science’s Transportation Research Board (TRB), and other stakeholders, including the American Concrete Pavement Association.

Defined Objective
The objective 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, accompanied by the necessary computational software.

The objective is being accomplished through developing:

Supporting the guide will be plans and materials for implementation and training services to facilitate adoption of the design guide and associated software. Public and private sector organizations also will execute strategies to promote national interest and increase acceptance by the transportation community.

Overview: The AASHTO Guide
The AASHTO Guide for the Design of Pavement Structures is the primary document used to design new and rehabilitated highway pavements.

It was determined that gaps existed in the knowledge base, and so mechanistic design methods would have to be supported by empirical relationships. It was also determined that many of the issues relating to the mechanistic-empirical approach needed to be better defined before practical and realistic design procedures can be developed and put into use.

Throughout the highway community, the major pavement-design emphasis is now on rehabilitation, for which empirical design approaches often are inadequate. Because mechanistic approaches more realistically characterize in-service pavements and improve the reliability of designs, it is expected that the next generation of design approaches will rely heavily on mechanistic principles.

It is intended that the mechanistic-emperical design guide will eventually be adopted by AASHTO as the new AASHTO Guide for the Design of New and Rehabilitated Pavement Structures.

Evolution and Origins
AASHTO’s Joint Task Force on Pavements (JTFP) is responsible for the development and implementation of pavement design technologies. This charge has been pursued by the Task Force since the days of its forerunner organization, AASHO, first developed the AASHO Road Test, which ultimately led to the development of the 1993 Guide.

Subsequently, the JTFP initiated an effort to develop an improved guide. As part of this effort, a workshop was convened on March 24, 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 loadings, foundations, materials characterization, pavement performance, and environment to help determine the technology best suited for inclusion in the new guide.

Oversight of the new guide was assigned to the National Cooperative Highway Research Program, which was created in 1962 to conduct research in acute problem areas that affect highway planning, design, construction, operation, and maintenance nationwide. Administered by the Transportation Research Board and sponsored by the individual state departments of transportation represented by AASHTO, the NCHRP works in close cooperation with the FHWA on key issues.

Under the aegis of NCHRP Panel C1-37A, work began in earnest in the late 1990’s to address the need for updated pavement design principles. Its goals were to:

Although the original timeline was 2002, there has been a tremendous groundswell to provide a scientific, practical, and objective set of pavement design principles. At this writing, the public and private sectors are staging for implementation of the guide, which holds great promise for providing better, more objective information to those who specify and design pavements.