MPC
Research Projects (2008-09)

Project Title

Implementation of the Mechanistic-Empirical Pavement Design Guide, Year 1

University

University of Wyoming

Project Investigator

Khaled Ksaibati

Description of Project Abstract

In the past, pavement design was performed on an experience only basis. Today, an empirical approach is used to design pavements. The empirical methodology is the statistical modeling of pavement performance. The new guide is aimed at using a Mechanistic – Empirical approach. This methodology uses calculations of pavement responses such as stress, strains, and deformations (Mechanistic) and then adjusts accordingly based on performance models (Empirical). The ultimate goal for the future is to have pavement designed on a mechanistic approach only (AASHTO).

The empirical design came about with the AASHTO Road Test in 1958. The design parameters of created by AASHTO from the Road Test include pavement serviceability, supporting value of the sub-grade, quantity of the predicted traffic, quality of the construction materials and climate. Design equations were based off of the conditions at the AASHTO Road Test in which multiple surfacing sections were tested with loaded trucks. The soil support plus traffic plus climate is equal to the serviceability or pavement condition. By 1972, the AASHTO Guide for pavement design came out. The design guide was rational based on the experience of the Pavement Engineers and their knowledge of how to avoid structural failures (AASHTO).

The 1972 AASHTO Guide was limited due to the fact it was based on the AASHTO Road Test. The Road Test only included one climate, one sub-grade, two years duration, limited cross sections and 1950's materials and specifications. It also only focused on new construction carrying vehicles (tires, suspension, weight, etc.) from the 1950's, traffic volumes from the 1950's and the construction methods of the 1950's. Due to these limitations a dilemma of how to project beyond the AASHTO Road Test limits came about (AASHTO).

The AASHTO Guide was updated in 1986 and 1993, but in the mid 1990's AASHTO took about research for a new guide to pavement design. A 2003 survey showed three DOT's used the 1972 Guide, two used the 1986 Guide, 26 used the 1993 Guide and 17 used the agency's own guide or a combo of the AASHTO and agency's guides (Wagner). The critical items for the new design guide were mechanistic, empirically calibrated, allow for user calibration, include existing theory and models, create software and provide a rational engineering approach. This became the Mechanistic-Empirical approach to pavement design and is known as the NCHRP 1-37A project (AASHTO).

Figure 1 shows the Mechanistic-Empirical design process in a basic flow chart (Wagner).

The Mechanistic-Empirical design process contains more than 100 total inputs with 35+ for flexible pavement and 25+ for PCC. This can be compared to the 1993 AASHTO Guide which contains 5 inputs for flexible pavement and 10 inputs for ridged pavements (AASHTO). The Mechanical-Empirical inputs come from climate, material and traffic properties. Material factors come from modulus values and thermal properties of the specific materials. Climate factors are based site specific climate considerations. The Mechanistic-Empirical design process uses 800+ weather sites to narrow these factors to the specific site. The AASHTO guide uses extrapolation from the Road Test site in Ottawa, Illinois. Traffic inputs come from local data collected and will be the number of axles by type and weight, no more ESAL's (Wagner).

It can be seen why the Mechanistic-Empirical approach will be great for pavement design. A more reliable design will be created and there is no longer a dependence on extrapolation of empirical relationships. Also, it allows for calibration nationally, regionally or to local performance data for materials, climate and traffic (Wagner).

Unfortunately, the Mechanistic-Empirical design process is not yet an AASHTO Design Guide. With so many inputs and factors, it is expected for problems to arise. These troubles are coming from the lack of ability to collect the desired inputs and the lack of research. It is in these critical inputs in which the desired performance models are created. For example, the Integrated Climatic Model (ICM) for climate factors uses temperature and moisture inputs to run the model. For the Mechanistic-Empirical performance models of pavement materials, inputs come from modulus values, thermal properties and strength properties (AASHTO). More time and equipment are needed for the DOT's in order to be able to collect the necessary data to create the required inputs inputs. Also, calibration and sensitivity efforts are an ongoing process.

Through the means of surveys to the DOT's in the MPC region, the specific problems occurring in each state will be identified. These problems will be summarized with the goal of determining the necessary equipment and/ or research that is needed. Specific recommendations will be made for needed regional research. It is through these recommendations that the facilitation of the implementation of the MEPDG throughout the region will be performed in order to fulfill the goal of complete implementation of the Mechanistic-Empirical design process.

Project Objective

At the March Mountain-Plains Consortium (MPC) Pavement Research Summit in Denver, Colorado, a roadmap for future pavement – related research studies was laid out. During this workshop it was decided that the top priority will be the implementation of the Mechanistic-Empirical Pavement Design Guide (MEPDG). The represented agencies at the workshop included: WYDOT, CDOT, SDDOT, NDDOT, SDLTAP, SDSU, FHWA, Colorado State University, North Dakota State University, South Dakota State University, University of Utah, and University of Wyoming.

This study seeks to address the necessary means needed to facilitate the implementation of the MEPDG for the MPC region. This study will seek to obtain information from the DOT's throughout the Region, process the gathered data and provide an approach to help with the implementation of the MEPDG.

Project Approach/Methods

The following tasks will be performed in this one year project:

• Task 1 - Conducting a Literature Review-A comprehensive review of the literature will be performed, focusing on the performance of the MEPDG. Results of the literature review will be included in the final report.
• Task 2 - Preparing the Survey- A survey will be prepared to be sent out to all of the DOT's in the MPC Region. The survey will request information about who is responsible for use of the MEPDG in the state and how far along the process is. The survey will allow the DOT's to raise specific issues related to the implementation of the MEPDG, what is needed, and where further research is required. The survey will be reviewed after communicating with WYDOT's Pavement Engineer.
• Task 3 - Mailing the Surveys - Once a final draft of the survey has been agreed upon, it will be sent out to the DOT's in the MPC Region.
• Task 4 - Collecting and Analyzing Results - As the surveys are returned, the results will be analyzed and organized
• Task 5 - Identifying Research Needs - The survey results and the findings from the literature search will be summarized in a final report. The report will include information on the needed areas of research to insure region wide implementation of the MEPDG.

MPC Critical Issues Addressed by the Research

Low Volume Roads and Bridges

Technology Transfer Benefits

This project will be very beneficial to all DOTs dealing with the MEPDG in the region. Identifying the needed areas of research will help insure the implementation of the MEPDG.

Time Duration

July 1, 2008- June 30, 2009

Total Project Cost

$60,526

MPC Funds Requested

$30,190

TRB Keywords

Mechanistic–empirical, pavement design guide, pavement design, implementation, MEPDG

References

1. AASHTO. "M-E Design: Implementation of a Mechanistic-Empirical Pavement Design." Washington, DC: AASHTO.
2. Wagner, Chris. "MEPDG: Where Are We Now?" Louisiana Transportation Engineering Conference. Baton Rouge, LA: FHWA - Resource Center, Feb. 12, 2007.