Appendix G Facilitated Workshop Final...

August 2010

NCHRP 9-30A

CALIBRATION OF RUTTING MODELS FOR HMA STRUCTURAL AND MIXTURE DESIGN

APPENDIX G FACILITATED WORKSHOP ON RUT DEPTH TRANSFER

FUNCTIONS: EXECUTIVE SUMMARY & MINUTES

Prepared for: National Cooperative Highway Research Program

Transportation Research Board National Research Council of National Academies

Washington, DC

Prepared by: Mr. Harold L. Von Quintus, P.E., ARA (Principal Investigator)

Mr. Jagannath Mallela, ARA (Project Manager) Dr. Ramond Bonaquist, P.E., AAT (Co-Principal Investigator)

Dr. Charles W. Schwartz, P.E., UMd (Co-Principal Investigator) Mr. Regis L. Carvalho, UMd

Submitted by: Applied Research Associates, Inc. 2003 North Mays Street, Suite 105

Round Rock, TX 78664 (512) 218-5088

August 2010

NCHRP 9-30A August 2010 Appendix G: Two-Day Facilitated Workshop Selection of Rut Depth Transfer Functions

Applied Research Associates, Inc. University of Maryland Advanced Asphalt Technologies, LTD.

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ACKNOWLEDGEMENT 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 through the National Cooperative Highway Research Program, which is administered by the Transportation Research Board of the National Academies.

DISCLAIMER 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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TABLE OF CONTENTS

Page No. Abbreviations Used In Appendix G .......................................................................................... G-iv Acknowledgements ............................................................................................................... G-vi Abstract .............................................................................................................. G-vii G WORKSHOP SUMMARY ......................................................................................... G-1 G.1 Introduction ................................................................................................................ G-1 G.2 Other Rut Depth Transfer Function Recommended for Consideration ......................... G-2 G.3 Guidance for Comparing Rut Depth Transfer Functions ............................................... G-4 G.4 Problem Statement Topics for Future RFPs and Research Statements ......................... G-4 ATTACHMENT A – MINUTES FROM TWO-DAY WORKHSOP ................................. G-6 A.1 Agenda ................................................................................................................ G-6 A.2 Meeting Participants ...................................................................................................... G-8 A.3 Day 1 of Workshop ........................................................................................................ G-9 A.3.1 Opening Business............................................................................................... G-9 A.3.2 Models for the Future Session ......................................................................... G-10 A.3.3 Model Evaluation Criteria Session .................................................................. G-17 A.3.4 Current Model – MEPDG Session ................................................................... G-18 A.3.5 Review of Potential Alternative Models Session ............................................. G-20 A.4 Day 2 of Workshop ...................................................................................................... G-22 A.4.1 Review of Agenda for Day 2 ........................................................................... G-22 A.4.2 Review of Potential Alternative Models - Continued ...................................... G-22 A.4.3 Group Discussions ........................................................................................... G-26 A.4.4 Closing Remarks .............................................................................................. G-37 A.4.5 Summary of Discussion ................................................................................... G-37 ATTACHMENT B – MODEL SUMMARIES PERSENTEED AT THE TWO-DAY WORKSHOP .............................................................................................................. G-38 B.1 Independent Review of the Recommended Mechanistic-Empirical Design Guide

and Software: Reliability and Design of Composite (Rehabilitated) Pavements ....... G-38 B.2 NCHRP 1-37A (NCHRP, 2004) .................................................................................. G-47 B.3 Leahy (1989) .............................................................................................................. G-55 B.4 WesTrack (Deacon et al., 2002; Monismith, Popescu, and Harvey 2006) .................. G-59 B.5 Verstraeten (Verstraeten et al., 1977; D’Apuzzo et al., 2004) ..................................... G-64 B.6 VESYS Model (Kenis et al., 1977, 1982, 1988, 1997; Chen et al., 2004) .................. G-66 B.7 Local Calibration Adjustments for the HMA Distress Prediction Models in the M-

E Design Guide Software, NCHRP 1-40B .................................................................. G-71 B.8 Wang: An Integrated Approach for Structural Analysis, Materials

Characterization and Mix Design of Asphalt Concrete Pavements ............................. G-79



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ABBREVIATIONS USED IN APPENDIX G AH – Adam Hand AI – Asphalt Institute ALF – Accelerated Load Facility APT – Accelerated Pavement Testing ASU – Arizona State University BC – Bouzid Choubane CAPA – Computer Aided Pavement Analysis CM – Carl Monismith CR – Cheryl Richter CS – Charles Schwartz DEM – Distinct Element Model DOT – Department of Transportation EH – Edward Harrigan ETG – Expert Task Group FE – Finite Element FEM – Finite Element Model FEP – Finite Element Program FHWA – Federal Highway Administration FN – Flow Number FT – Flow Time HVQ – Harold Von Quintus HVS – Heavy Vehicle Simulator HMA – Hot Mix Asphalt ISAP – International Society of Asphalt Pavements JH – John Haddock JM – Jagannath Mallela JN – Julie Nodes (now Julie Kliewer) LM – Leslie Myers LS – Laura Scott LTPP – Long Term Pavement Performance LW – Lingbing Wang MC – Monte Carlo M-E – Mechanistic Empirical MEPDG – Mechanistic-Empirical Pavement Design Guide MLE – Multi-Layer Elastic MLET – Multi-Layer Elastic Theory MP – Murari Pradhan MRL – Materials Reference Library NCAT – National Center for Asphalt Technology NCHRP – National Cooperative Highway Research Program NCSU – North Carolina State University OOE – Office of Organizational Effectiveness PCC – Portland Cement Concrete PD – Permanent Deformation



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PG – Performance Grade PI – Principal Investigator PMA – Polymer Modified Asphalt PRS – Performance Related Specifications PU – Per Ullidtz RB – Ramond Bonaquist RFP – Request for Proposal RK – Richard Kim RR – Rey Roque RSST-CH – Repeated Simple Shear Test at Constant Height SD – Shangtao Dai SHRP – Strategic Highway Research Program SMA – Stone Mastic Asphalt SPS – Special Pavement Studies SPT – Simple Performance Test SW – Schmuel Weissman TRB – Transportation Research Board TS – Tom Scarpas TSRST – Thermal Stress Restrained Specimen Test VEPCD – VisoElastoPlastic Continuum Damage WRI – Wyoming Research Institute



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ACKNOWLEDGEMENTS The research described herein was performed under NCHRP Project 9-30A by the Transportation Sector of Applied Research Associates (ARA), Inc. Mr. Harold L. Von Quintus served as the Principal Investigator on the project. Mr. Von Quintus was assisted by Mr. Jagannath Mallela as the Project Manager and Engineer on the team. Other management team members and subcontractors included Dr. Charles Schwartz, P.E. of the University of Maryland, and Dr. Ramon Bonaquist of Advanced Asphalt Technologies, LCC. Both Drs. Schwartz and Bonaquist served as Co-Principal Investigators on the project. One of the major efforts of this study was the facilitated workshop to select rut depth transfer functions that were to be calibrated and evaluated within the project. The project management team was supported by individuals who assisted in the facilitated workshop and are listed below.

Laura Scott with the Office of Occupational Effectiveness with the University of Maryland, who served as the official meeting facilitator.

Ms. Lorina Popescu with the University of California at Berkley, who assisted the team with conducting the workshop and preparation of the meeting minutes.

The team greatly appreciates attendees of the workshop for their time in planning for and attending the workshop. Individuals that participated in this workshop, other than the team members, included: Dr. Carl Monismith and Ms. Lorina Popescu with the University of California at Berkley; John Bukowski, Katherine Petros, and Cheryl Richter with the FHWA; Julie Kliewer and Murari Pradhan with the Arizona DOT; Dr. Steve Brown (consultant), Leslie Ann McCarthy (Vanderbilt University), Bouzid Choubane (Florida DOT), Shongtao Dai (Minnesota DOT), Danny Dawood (The Transtec Group), Adam Hand (Granite Construction, Inc.), John Haddock (Purdue University), Shmuel Weissman (University of California at Berkeley), Tom Scarpas (Delft University), Per Ullitz (Dynatest), Rey Roque (University of Florida), Linbing Wang (Virginia Tech), and Mohamed El Basyouny (Arizona State University). All members of the research team also acknowledge and greatly appreciate the support and effort for helping bring this project to completion by Dr. Edward Harrigan with NCHRP and the NCHRP panel members.



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ABSTRACT The objective of NCHRP Project 9-30A was to recommend revisions to the hot mix asphalt (HMA) rut depth transfer function in the Mechanistic-Empirical Pavement Design Guide Software. The recommended revisions were based on the calibration and validation of rut depth transfer functions with laboratory measured mixture properties and data from existing field and other full-scale pavement sections that incorporate modified as well as unmodified asphalt binders. As part of this objective, a facilitated workshop was held to identify potential rut depth transfer functions to improve on the prediction of rutting. The documentation for NCHRP Project 9-30A consists of a research report and eleven appendices (Appendix A through K). This appendix (Appendix G) documents the facilitated workshop, and summarizes the discussions and recommendations to identify rut depth transfer functions to be considered for use in the detailed comparisons included in latter tasks of NCHRP Project 9-30A.



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National Cooperative Highway Research Program NCHRP 9-30A, FY-2005

Calibration of Rutting Models for HMA Structural and Mix Design

Appendix G FACILITATED WORKSHOP ON RUT DEPTH TRANSFER

FUNCTIONS: EXECTYUVE SUMMARY & MINUTES

G WORKSHOP SUMMARY G.1 Introduction A two-day facilitated workshop was held on December 6 and 7, 2005 at the William M. Keck Center, National Academy of Sciences in Washington, DC, as part of NCHRP project 9-30A. The purpose of the workshop was to identify the rut depth prediction models or transfer functions and other issues that should be considered within the production test program for NCHRP project 9-30A. The three objectives of the workshop are listed below.

1. Identify other rut depth transfer functions to be evaluated in task 7 of Phase III of NCHRP project 9-30A. This objective was the primary focus of the workshop. The other two objectives are an outcome from this primary objective.

2. Develop guidance on two key issues affecting the use of rutting transfer functions in the current Mechanistic-Empirical Pavement Design Guide (MEPDG) software developed under NCHRP project 1-37A.

a. The maximum prediction error allowable for the M-E based rutting model. b. The design limit or failure criteria for rutting in HMA layers.

3. Identify problem statements and/or statements of work for future Request for Proposals (RFPs) or statement or work.

a. Identify short-term research efforts to evaluate the use of rutting models in HMA structural design, mixture design, and performance-based specifications.

NOTE: This appendix summarizes the results from a two-day facilitated workshop to identify

rut depth transfer functions to be considered for use in the detailed comparisons included in latter tasks of NCHRP Project 9-30A. The discussions and workshop

minutes are included as Attachment A to Appendix G. In addition, a summary of the models presented and discussed at the workshop are included as Attachment B.



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b. Identify key elements of a long-term research program to develop more fundamental mechanistic modeling systems for rutting based on work performed under NCHRP 9-19, Tasks F and G and other projects.

NCHRP project 9-30A engaged a professional meeting facilitator, Ms. Laura Scott with the Office of Organizational Effectiveness (OOE) at the University of Maryland, to achieve maximum productivity and benefit from the workshop. Ms. Laura Scott was the professional facilitator for the workshop. Attachment A lists all participants of the workshop. The format for the workshop was organized into five segments for each major topic/workshop objective. These segments are listed below.

1. Presentation – Introduction of topic as it relates to NCHRP project 9-30A; these were prepared discussions prior to the workshop and were given by the research team members, workshop participants, or facilitator.

2. Discussion of Key Issues – The discussion of key issues was led by the facilitator for each presentation.

3. Debate of Critical Issues – Continued discussion of critical issues, again led by the facilitator.

4. Voting – For critical issues where consensus could not be reached, ballots were planned for use as a strategy to reach consensus, if needed. In the end, the balloting process was not needed to achieve consensus on any item.

5. Summary – The facilitator or research team member summarized the consensus reached on each critical issue and workshop objective.

In addition to the five segments listed above, two breakout sessions were used towards the end of the workshop to discuss some of the key questions and issues that were identified during the workshop presentations and discussions. Attachment A includes the meeting minutes for the workshop and a summary of the discussions that occurred during the workshop. The remainder of the Volume 2 report summarizes the consensus reached relative to each of the workshop objectives. G.2 Other Rut Depth Transfer Functions Recommended for

Consideration The rut depth transfer functions recommended for detailed analyses and comparisons are summarized in Attachment B of this appendix and included in the NCHRP Project 9-30A research report. The following identifies and briefly defines those transfer functions. The NCHRP 9-30A research team planned to consider both M-E based and

mechanistic-based prediction models. From the workshop presentations and discussions of the fully mechanistic-based prediction models, none of those mechanistic models were judged to be sufficiently developed to predict rutting along actual test sections. It was the consensus of the participants, however, that



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development of fully mechanistic-based prediction models is a worthy pursuit, but one for the long-term given the time and cost of development and validation.

The consensus from the workshop participants regarding direction for the project was to focus on the enhancement of M-E based rut depth prediction models that could replace the current model in the MEPDG software. It was also agreed that the NCHRP 9-30A research team should identify a small subset of sections having the highest quality data in the M-E_DPM database that might serve as good test cases for future mechanistic modeling of HMA rutting. Appendix H is a User’s Manual for the M-E_DPM database. The project team will characterize the HMA mixtures (both neat and polymer modified) at these selected sites via a series of advanced laboratory tests appropriate for providing material inputs to mechanistic-based prediction methods. The specific tests were defined by the research team and approved by NCHRP, and were described in the research report for this project. Results from these tests were entered in the M-E_DPM database (refer to Appendix H) for use in future research.

As described in the NCHRP project 9-30A statement of work, the primary focus

of the project was to enhance the MEPDG HMA rutting transfer function and the measurement of all associated material property inputs via level 1 laboratory testing. There was some debate during the workshop as to whether the current MEPDG rut depth transfer function should be abandoned. A range of opinions were expressed during this debate. The final consensus was that the current MEPDG transfer function should not be dropped, but that mechanistic-based response methods should be used to confirm some of its underlying assumptions and to devise more rational bases for any enhancements.

The WesTrack rut depth transfer function was the consensus selection for the

primary alternate model for comparison to the MEPDG transfer function. Material property inputs required for this transfer function will be measured in the laboratory for the experimental sites, similar to the testing for the enhanced MEPDG model, if not already available from other projects. The rutting predictions using the WesTrack transfer function will be compared against the predictions from the original and the enhanced MEPDG rutting model. If warranted, potential enhancements to the WesTrack rutting transfer function will also be explored.

Three additional enhancements to the current MEPDG HMA rutting model were

identified for examination during Phase III, which are listed below. 1. The mixture adjustment factors based on volumetric and gradation properties,

as used in NCHRP project 1-40B. 2. The addition of a stress term within the MEPDG rut depth transfer function.

This enhancement would be in addition to the resilient strain term already included in the rutting model. The two specific suggestions for further examination were the Verstraeten transfer function and Leahy’s original plastic-to-elastic strain formulation that include deviator stress. The



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consensus was to give priority to the Verstraeten transfer function and to evaluate the Leady transfer function, if time and funds permit.

3. The inclusion of measured mixture-specific permanent deformation constants, as HMA layer properties. Repeated load permanent deformation tests are included within the test program for each experimental site. [NOTE: The independent review team also made this suggestion as part of NCHRP project 1-40A. Current revisions to the MEPDG software are to allow permanent deformation constants to be entered for individual HMA layers.]

It was the consensus of the participants that multiple rut depth transfer functions,

in addition to the original MEPDG transfer function (the baseline model) should be calibrated under the project. These transfer functions include:

o The WesTrack shear strain, shear stress transfer function o The Vertstraten deviator stress-based transfer function o One or more enhanced versions of the MEPDG software.

The rutting predictions from all of these transfer functions will be compared against each other. To the extent possible, any “nearly-mechanistic” predictions using the advanced HMA material characterization for the select subset of experimental sites will also be compared against the M-E predictions.

G.3 Guidance for Comparing Rut Depth Transfer Functions As noted and summarized above, the rut depth predictions using the MEPDG should be computed with and without the mixture adjustment factors based on volumetric properties, as reported from NCHRP project 1-40B. The other M-E based rut depth transfer functions used in the comparisons include the WesTrack transfer function and a stress-based transfer function or inclusion of a stress term within the prediction model. The two transfer functions suggested for use were the original Leahy plastic-to-elastic axial strain relationship that includes deviator stress, and the Verstraeten plastic axial strain relationship based on deviator stress. The important criteria to be considered in the models evaluation process are listed

below in order of importance. o Model accuracy (consensus selection as the most important factor). o Sensitivity to HMA mixture volumetric properties (binder type, aggregate

blend, etc.). o Sensitivity to temperature. o Robustness.

[NOTE: Testing expense was one of the items discussed, because it was included in the amplified work plan. It was the consensus of the group, however, that this was not an important issue.]

It was the consensus of the participants that the rut depth predictions should be as

accurate as possible. The group did not reach consensus regarding any quantification of “acceptable” accuracy. [In other words, a specific value as being good enough, in terms of accuracy, was not the consensus of the group.]



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It was also the consensus of the participants that the rut depth model should have

the capability to predict rut depths over the entire range of values. [In other words, the participants agreed that the rutting comparisons should not be limited to values below some maximum rut depth.]

It was the consensus of the participants that both the evolution and maximum rut

depth at the end of the design life were important. The consensus of the group was that one was not more important than the other.

G.4 Problem Statement Topics for Future RFPs and Research

Statements Multiple topics were identified during the workshop for future research initiatives. Most of these were condensed into three basic areas, which are listed below. Future research topics that are recommended to improve HMA mixture performance are included and discussed in the Volume 1 research report for NCHRP project 9-30A. The consensus of the participants was that mechanistic-based rut depth

predictions models should be aggressively pursued. Some of the participants had concern over the amount of time and cost it would take to develop confidence in the mechanistic-based procedure. Results from the comparison of the rut depth transfer functions should be used to provide support for more accurate transfer functions based on the limitations of the simplified rutting relationships. As such, it was agreed that research topic problem statements should be developed and, based on the conclusions from the comparisons of the different M-E based rut depth transfer functions.

There was also consensus on the value of finite element methods or advanced

analysis tools for checking the validity of many of the simplifying assumptions embedded within the MEPDG – regardless of the M-E based rut depth prediction relationships. For example, the micromechanical analysis models could be used to improve on the fundamental understanding of the impact of volumetric properties on rutting – as quantitatively demonstrated through the WesTrack experiment and the volumetric adjustment factors presented and used within NCHRP project 1-40B.

Specimen preparation and geometry continues to be an issue of debate. No

consensus was reached on this issue regarding the ideal specimen geometry or preparation protocols. However, there was agreement that this issue should be addressed in future studies using more sophisticated models and field validation data.


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ATTACHMENT A MINUTES FROM TWO-DAY WORKSHOP IDENTIFICATION OF RUT DEPTH TRANSFER FUNCTIONS OR PREDICTION MODELS A.1 Agenda

DAY 1: Tuesday, December 6 8:00 Opening Business

Welcome & introductions Harrigan, Monismith Project objectives Von Quintus Workshop goals and agenda Scott

9:00 Models of the future

Overall framework of future directions Monismith Presentation of various models Scarpas, Weissman, Ullitdz, Schwartz/Kim Questions/discussion of future models All

11:30 Lunch, provided on site 12:30 Evaluation criteria Schwartz, Scott

Discussion & consensus of criteria All 1:45 Current model – MEPDG

Existing NCHRP 1-37A procedure Schwartz Preliminary findings from NCHRP 1-40A review panel Von Quintus Current work under NCHRP 1-40D El-Basyouny Discussion of experiences with current model All

3:30 Review of potential alternative models

List/presentations of models recommended for consideration Schwartz Discussion of alternative models All

4:30 Adjourn day 1 Dinner On your own


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DAY 2: Tuesday, December 7 8:00 Review agenda for day 2 Scott 8:15 Review of potential alternative models, continued Von Quintus, Schwartz

Continued discussion of models All Evaluation of models using criteria developed on day 1 Scott

11:45 Lunch, provided on site 12:45 Summary & discussion of evaluation tally Scott

Discussion on evaluation & prioritization of potential models All 2:30 Discussion of project work plan Von Quintus, Scott

Questions/discussion regarding model prioritization All 3:15 Closing remarks and next steps Von Quintus, Monismith, Harrigan

Workshop report submittal & review Von Quintus Travel expense reimubursement forms/vouchers Harrigan/Von Quintus

3:30 Adjourn facilitated workshop

3:30 NCHRP 9-30A Panel Executive Session Monismith/Harrigan (PANEL MEMBERS ONLY) 4:00 Adjourn Panel executive session

NCHRP 9-30A December 6 & 7, 2005 Appendix G: Two-Day Facilitated Workshop


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NCHRP 9-30A Workshop Day 1 – Meeting Notes December 6, 2005

A.2 Meeting Attendees NCHRP Staff Representative Edward T. Harrigan Project Panel Chair Carl L. Monismith (University of California, Berkeley) [email protected] Project Panel Members Bouzid Choubane (Florida Department of Transportation) Shongtao Dai (Minnesota Department of Transportation) Adam J. T. Hand (Granite Construction, Inc.) Julie E. Nodes (Arizona Department of Transportation) Murari Man Pradhan (Arizona Department of Transportation) Liaison Representatives Katherine A. Petros (Federal Highway Administration—FHWA) Myers, Leslie Ann Project Team Members Harold Von Quintus (Applied Research Associates, Inc. – ARA), Principal Investigator (PI) Charles Schwartz (University of Maryland) Raymond Bonaquist (Advanced Asphalt Technologies, Inc.) Richard Kim (North Carolina State University) Jagannath Mallela (ARA) Laura Scott (University of Maryland), Facilitator Regis Carvalho (University of Maryland) Invited Experts John Haddock (Purdue University) Cheryl Richter (FHWA) Tom Scarpas (TU Delft) Per Ullidtz (Dynatest) Lin Bing Wang (Virginia Tech) Shmuel Weissman (University of California, Berkeley) Others Lorina Popescu (University of California, Berkeley)



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A.3 Day 1 of Workshop A.3.1 Opening Business A.3.1.1 Welcome & Introductions—Harrigan, Monismith Meeting called to order by NCHRP’s Program Manager Dr. Ed Harrigan (EH). Opening remarks were made by project panel chairman Dr. Carl Monismith (CM). CM provided some background to the project. Key points made: NCHRP 9-30 concluded that rutting evaluation should have a high priority. Jackknifing

was introduced under 9-30. An MS Access database was developed that contains key pavement section and materials information.

NCHRP 9-19 – developed E*, Flow Number (FN) (repeated load permanent deformation model), and Flow Time (FT) (creep test). Improved permanent deformation model (using 3-D FEM) could not be completed.

Project team member and Co-Principal Investigator (PI) Dr. Ray Bonaquist (RB) explained the simple performance test (SPT) developed under NCHRP Project 9-29 for E*, repeated load permanent deformation, and creep testing with and without confinement. RB concluded that these tests should not be treated as research grade tests but rather routine tests. CM concluded that in the new mechanistic empirical pavement design guide (MEPDG), the aspect of permanent deformation was of most concern. Hence the NCHRP 9-30 panel concluded that this is worth looking into. However, the panel felt at the time that limiting to one model form is perhaps not appropriate. Hence the NCHRP 9-30 database included other testing data. CM concluded that the focus in NCHRP 9-30A is on: Looking into the MEPDG rutting model developed under NCHRP 1-37A model and

improving it or adding other models, and Looking into the future and selecting a model that can handle the effects of very heavy

loading anticipated into the future and climatic effects. A.3.1.2 Project Objectives—Von Quintus The project PI, Mr. Harold Von Quintus (HVQ), restated the NCHRP 9-30A project objectives. HVQ stated that there were two primary areas of interest from a project team perspective: Short-term revisions—looking at other permanent deformation models that can be

incorporated within the framework of the existing MEPDG software, and Long-term—bringing into focus a more rigorous permanent deformation model that will

be developed over a longer period of time (i.e., far beyond the project’s schedule). HVQ summarized the project phases and presented the planned work schedule. Some of the highlights of this presentation are: The main activity under Phase I is to maintain and update the MEDPM.



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Under Phase II, of which this workshop is a part of, the experimental plan from 9-30 will be revised. HVQ emphasized the importance of the workshop to accomplish this goal. As part of the deliverables for Phase II, HVQ mentioned that an executive summary of the facilitated workshop will be prepared and submitted to the panel and everyone in attendance. The revised NCHRP 9-30A experimental plan will be submitted as part of the interim report before the end of Quarter 2 of 2006.

Phase III will basically execute the revised experimental plan which will calibrate and validate not only the NCHRP 1-37A model but also other options summarized in this workshop.

HVQ also provided a status update on NCHRP 1-37A software and mentioned that version 0.900 of this software will be released in February, 2006 for AASHTO’s JTCP review. This version is planned to be used in Phase III. A.3.1.3 Workshop Objectives HVQ listed the various desired short- and long-term goals for the facilitated workshop. Short-term outcomes: Recommend other rut depth prediction models – could be a combination of new and old

models. Address issues raised by the 1-40A team. Examine effort needed to use the models – structural design, mix design, and

performance-related specification (PRS) aspects. Long-term outcomes (stated as being perhaps more important): Identify key elements for future rut depth prediction models. Identify future research needs.

The key questions to be addressed in the facilitated workshop were: Are there other test sections/data that the project team has overlooked? Can we include future models in Phase III evaluation? Do we have adequate data? Definition of model improvement—what is being quantified?

A.3.1.4 Workshop Goals and Agenda—Scott The workshop facilitator, Laura Scott (LS), set the tone for the facilitated workshop by stating that the goal of the workshop is to achieve outcomes outlined by HVQ. LS presented the workshop agenda to the participants (a hard copy of the agenda was provided to the attendees). She noted that the discussions will be directed to have structured conversations around these topics. Regarding the discussion item labeled “Criteria for Model Evaluation,” LS clarified that the idea is to develop consensus, i.e., no up or down vote will be taken. A.3.2 Models for the Future Session Four specific models were discussed. Scarpas (TS), Weissman (SW), Ullidtz (PU), Schwartz/Kim (CS/RK). A.3.2.1 Overall Framework of Future Directions—Monismith



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CM opened the discussion on future directions in rut predictions. Key aspects that were highlighted for possible consideration include: Volume change and shape distortion should both be addressed when modeling at

permanent deformation. Proper attention should be given to nonlinear response characteristics of materials to

time of loading, temperature at loading, and load magnitude. Specimen size and shape influence. Laboratory testing should place appropriate emphasis on specimen preparation. Mix

structure and the influence of the method of specimen preparation are important. Small versus large strain behavior should be properly addressed. Stiffness, creep, and repeated-load characteristics are all important. Three-dimensional finite element modeling or other computational mechanics tools

should be used as necessary. The model should consider a three dimensional response. CM discussed approaches to model permanent deformation (elastic, viscoelastic, linear and nonlinear material behavior, etc.) and types and number of test configurations (tension, compression, shear in dynamic, creep, etc.). A.3.2.2 Presentation of Various Models—Scarpas, Weissman, Ullitdz, Schwartz/Kim

Tom Scarpas—TU Delft—“Issues in Computational Simulation of Flexible Pavements” Overview of Presentation

Tom Scarpas (TS) presented the constitutive modeling aspects of CAPA-3D (Computer Aided Pavement Analysis) finite element program and an example application of the tool to modeling permanent deformation. The CAPA-3D code was developed over a decade at Delft and is used in structural analysis, contact mechanics, multiphase media, etc. TS outlined the importance of a model that doesn’t preempt or bias the response; the model should also be able to perform analyses of a material for anisotropic conditions. The model has to be sensitive to capture the effects of changes in confinement, temperature, strain rate. The model should be able to capture the “tri-dimensionality and directionality of tri-dimensionality of the state of stress. Highlights of CAPA-3D include the following: Stress/strain response—addresses 3-D state of stress and also its changes as the load

moves along (i.e., directionality of the stress state). Sensitivity to confinement. Sensitivity to temperature and strain rate. Response surfaces – are not static, i.e., they evolve. Can create flows that change the

response surface. Code simulates lab testing as well as monotonic tests as well as layered surfaces. Both ultimate response and softening response (post-fracture phenomena) are modeled. Computational “dynamic” viscoplasticity is the basis. Experimental program – majority of tests were in the regime of low confinement +

increasing vertical loads. However, stress state near vicinity of the wheel load belongs to the regime of uniaxial compression + uniaxial tension – difficult to test.



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Testing program at Delft included a tension test specimen – hourglass like – to localize failure. Testing showed that we must be careful in analyzing lab data since both tension and compression responses were found. If we use the scalar quantities (e.g., plastic flow/warp) as a measure in lab testing, it could be misleading.

Majority of critical stress states include shear – which can be resolved into compression and tension.

Compressive damage is diffused – can be modeled using plasticity. Tensile damage is directional – cannot be modeled using isotropic theories.

Cracking response simulation in Delft – isotropic plasticity up to failure – leading to directional anisotropy.

Anistotropic damage simulation – Hoffman surface used. TU Delft’s shear testing setup was shown (pure shear is assured by adding rotational

features in the setup). Elastoplasticity and viscoelasticity are combined. Indirect Tension Test was used for the verification of the FE model. Model allows an input that tells it when the cyclic response starts degrading (i.e., tertiary

phase behavior). Rolling wheel load and bouncing wheel loads can be simulated.

A permanent deformation analysis was presented. Cyclic response techniques were used to simulate the pavement response. In summary, TS stated that the fundamental philosophy should be to bring more reality to the models. FE is not a design tool. It is an instructive tool. What is presented is an instructive tool—it is an add-on to what is already there. The model may be too slow at the moment for actual use to aid in design. Discussion Chuck Schwarz (CS): Results shown were for damage. How will it look if permanent deformation was shown? TS: Permanent deformation can be simulated (it was shown in the plots).

Schmuel Weissman—“A Framework for Future Models for Asphalt Concrete Mixes” Overview of Presentation Schmuel Weissman (SW) presented his approach to the modeling permanent deformation. At the outset he stated that a long-term goal should be to do away with field calibration by modeling the entire evolution of ruts. Cracks and ruts are together in pavements – a combined model is preferable (ideal). But the discussion presented was only on the rutting model. SW stated that small strain deformation theory is not representative of rutting. Finite deformation is a more viable candidate. Example model highlights: A theoretical model was developed for Caltrans. Did not have any lab testing to calibrate

the material model. Maxwell model in parallel (several series used). The model has two components – elastoplastic solid and viscoelastic fluid.



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Capability to model plasticity during unloading – Bauschinger effect can be handled. Incorporates a yield surface. Hardening response. Yield depends on hydrostatic component of the back stress.

Model applications were described in two stages: First use in direct predictions – Monte Carlo simulations and large deformation FE

analysis followed by a risk analysis. Monte Carlo based simulations using various variables (temp, moisture, loading, etc). Rut depth versus number of load applications (N) is the output. Useful since it does away with field calibration.

Second use of the model in improving laboratory testing— Laboratory test parameters can be identified. Sensitivity analysis using the constitutive law to identify critical properties and identify gold-standard tests for evaluation of SPTs.

The steps listed in the described framework purported that the field experience comes in as an input rather than output eliminating the need for field calibration.

SW also quickly presented the idea of a multi-scale approach to model material response. However, not many details were presented. Discussion Rey Roque (RR): Explain how one does not need to perform field calibration. SW: Replace field calibration with direct prediction through risk analysis. The parameter selection in the prediction is based on field experience.

Per Ullidtz—Dynatest, Inc.—“Distinct Element Model” Overview of Presentation Per Ullidtz (PU) discussed a distinct element model (Dem2D) to model permanent deformation: The model is a two-dimensional representation of a particulate (aggregate) structure. Each analysis step is 40 nano secs large. Forces are known at the beginning of each increment (normal and shear) around each

element/particle. Calculate movement of each element due to the applied forces. Contact forces are

modeled using the F- relationship. Sliding of individual aggregates is incorporated. New forces are computed and the program steps into next time increment.

Elements are described by shape (angularity) and gradation. Can simulate various test setups. Static and dynamic loads can be simulated. Model can also handle tension between particles (to simulate binder). Contact points are viscoelastic (Kelvin model).

In summary, PU concluded that DEM is a good candidate for response and performance models. Computational time is an issue (very long run times needed—approximately 2 days to produce a simulation lasting for only a few seconds for the example presented in the workshop). At the present time, it is useful to primarily obtain insights into the behavior of asphalt mixtures.



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Chuck Schwartz (University of Maryland) and Richard Kim (North Carolina State University) Overview of Presentation Project team members CS and Richard Kim (RK) tag-teamed to present this discussion. The topic was HMA Constitutive Modeling. CS started the discussion and stated that their approach is supposed to be a starting point for the future permanent deformation models and was initially included under NCHRP 9-19 but could not be completed. He talk was titled “NCHRP 9-19+ HMA Constitutive Modeling.” Highlights of the 9-19+ approach: Objective: a constitutive model that captures all the key features of the pavement

behavior. Approach: a single model that is able to capture the temperature dependence, loading rate

dependence, stress condition changes, etc. and multiple responses. Basic premise – Schapery’s continuum damage. Captures viscoelasticity and

degradation. CS/RK added viscoplasticity to the model. Uses small deformation theory. Needs linear viscoelasticity parameters which can be obtained from the storage modulus

part of the E* master curve obtained from E* testing. Relaxation/creep compliance can be obtained from here.

CS presented the compression part of model testing performed at the University of Maryland. Needs uniaxial viscoplasticity model. Low cycle creep testing. Needs uniaxial continuum damage – followed Schapery. Issue of time-temp superposition for small strain response – how far can we push into

large strain realm? Creep and recovery tests results were analyzed. Different temperatures, loading rates, and stress conditions were employed in the testing. Time-temperature superposition shown to remain valid well into the post-peak region. For uniaxial loading, good results were obtained.

Constant strain rate tests were also validated (constant temperature 25 C). Independent validation testing (Permanent strain versus N) was done at ASU. Hardening

response could not be simulated well. Suspected that the viscoplasticity model was not working as intended.

The viscoplasticity model was reformulated which gave more realism to the hardening response (flow surface similar to HiSS yield function in Tom Scarpas’s model).

Validation was done using monotonic constant strain rate test – post-peak response differs slightly.

Random load test comparisons were improved with the Perzyna-HiSS viscoplasticity model.

Conclusions: o Perzyna-Hiss viscoplasticity model shows promise o A different calibration testing program seems more appropriate in hindsight.

CS observed that compression and tension should be coupled as in Tom Scarpas’s model. RK presented the tension part of the model developed at NCSU along with other information. His presentation was titled “Asphalt Pavement Performance Prediction Using VEPCD-FEP++”:



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Model was code named VEPCD-FEP ++ = ViscoElastoPlastic Continuum Damage Finite Element Program C++. It is a three-dimensional program developed under the funding from FHWA. It is a public domain pavement analysis code.

Test method used is Triaxial HMA and stress-state dependent model for unbound materials.

Can predict fatigue cracking, rutting, and thermal cracking simultaneously with consideration of stiffness reduction due to damage growth.

Currently is used to bridge the VEPCD model and the micromechanics model in tension. Damage Characteristic Curve (VE continuum damage model) – pseudo stiffness-

continuum damage. Single relation between damage and stiffness. RK states that a single characteristic curve exists between these two parameters.

Viscoplasticity model taken from NCHRP 9-19 (p-q-Y model). Laboratory testing requirement for VEPCD model for tension part – E*, monotonic test at

5 and 40 C (testing schedule takes approximately 4 days). Verification of the VEPCD Model for Tension (Cracking) includes:

o Monotonic tests at varying temperatures (5 and 40°C) and rates. o Random loading tests with varying frequencies and stress levels. o TSRST tests with three different cooling rates.

Verification for Compression (Rutting) includes: o Characterization Test Methods: Variable Time test and Variable Load test. o Verification Test Methods: Flow Number Test and Random Load test. o Test conditions: 500 kPa confining pressure and 55°C.

Random load predictions were shown along with measured strain for FHWA ALF mixes. Prediction of Flow Number in the secondary region is adequate (more deviation in

primary region). Effect of confining pressure on E* – need to account for interlocking in compression

model. Overall, the model can account for crack density evolution, stiffness reduction, damage

evolution. RK also presented an overview and example simulation of laboratory specimen using ViscoPave that models and displays pavement behavior under compression and tension. Discussion SW: How to marry local behavior of an individual element in the finite element idealization with global behavior obtained from laboratory testing. For a truly 3-D constitutive model, there is a need to relate axial strain and lateral strain. CS: This has to be looked into. TS: There are ways to do it. SW: For each parameter you may need a test that isolates what you are looking for and then you can develop a test from there. CM: Hollow Cylinder Testing is a good option.



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A.3.2.3 Questions/Discussion on Future Models Given below is a paraphrased dialogue among workshop attendees in response to the following question: Is moving toward fully mechanistic prediction of pavement rutting a worthwhile goal? Good Reasons Provides a process for dealing with a large number of unknowns and how to deal with

variability of the unknowns and other input parameters. Provides a road map/tool to explain what’s going on within the pavement structure and

mixture; it can directly tie structural design to mixture design. Provides a tool for establishing what properties and tests are more relevant; and provides

a means to develop or do more simple testing techniques without loosing accuracy. Provides a tool for recognizing that pavements are engineered structures and for

developing performance-related specifications. Traffic has been increasing and is becoming more critical, particularly on heavy traffic

roads (truck routes, LCVs, “platoons on rails” etc.) and these effects must be captured in the future models.

Degree certainty of predictions increases. Increased ability to deal with materials other than what is known today. With the use of fully mechanistic prediction models “We don’t need calibration.” Fully mechanistic models can be used as an aid to develop better and simpler design

procedures. Can be useful during the mix design stage. Pavement design is separate from mix design within many state agencies. Fully

mechanistic prediction models become valuable in developing an understanding to improve mixes to perform properly.

Develop better performance-based specifications; and provides a tool allowing agencies to set the limits and incentives/disincentives that are based on simulations rather than experience.

Connections with market focus/warranties.( Agencies/Contractors coming back to research due to pavement warranty requirements).

Concerns Implementation will be an issue; local agencies don’t see the use of fully mechanistic

models as an important issue. Industry sees this as mixture and materials problem and not a structural design problem.

Many local agencies do not agree with the M-E based design procedure. Large number of unknowns and dealing with the variability of the required parameters. Is not/should not be viewed as a design method by pavement engineers. Complicated testing issues and the length of time to complete the testing program to

support fully mechanistic prediction models. More importantly, many of the required properties are not known or available during the mix design stage.

Separating rutting from other distresses (cracking, etc.) Pavement design separated from mix design at local agencies. Change mentality of designers – pavements are engineered structures – consideration

needs to be equal to bridge or building engineering. The different approaches that exist are not in agreement.



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Education Can’t separate materials and design. Need to effectively communicate to user community. Need diversity of opinions and funding to support that.

A.3.3 Model Evaluation Criteria Session A.3.3.1 Evaluation Criteria—Schwartz, Scott CS brought back the room to the present (i.e., things we need to address within NCHRP 9-30A). He stated that the project team is also charged to improve the rutting model in the MEPDG as well as look at alternative candidate models (1 to 2) under Phase III. CS made a short presentation on criteria to evaluate alternate models for possible development under NCHRP 9-30A. He presented the project team’s unranked and non-prioritized criteria (hardcopies of this information were provided to the attendees). Stated that we need a model that fits within the framework of the current MEPDG. RR: Do we want to stay within the framework of MEPDG? This can limit the development. CS: This is open to discussion. Jagannath Mallela (JM): Clarified that staying within the framework does not necessarily restrict you in a big way other than perhaps the way traffic or climate are characterized. Things like materials parameters and analysis methodologies can be changed. There was some discussion the room about Level 1, 2, 3 analysis methods just like we have Level 1, 2, 3 inputs. JM: Clarified that there is a provision in the Guide right now to handle not only different input levels but also analysis levels (e.g., Level 1 subgrade inputs switch the analysis mode from layer elastic to finite element, thermal cracking uses a different analysis method compared to other distresses, etc.). Rey Roque (RR): What is the purpose of the model? What are the criteria are for model development? Who is the end user? This dictates how you develop the model. Monismith: The MEPDG has 3 different levels. Most states will use Level 2 and many others Level 3. Our obligation (under this project) is to make the model that is used as sound as possible.

Testing Discussion SW: Use a model that allows for non-linear analysis. It is better than multi-layer elastic theory (MLET). JM: It may need more properties. Cheryl Richter (CR): We need to strive for inputs that give us a better solution. SW: What does type of tests mean? For the analytical model or for performance prediction?



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CS: For performance.

Calibration Discussion RR: Perhaps we should not look at models. Perhaps we should look at systems? Models are restricted to the inference space over which they are developed. Use the term system rather than model. RR gave examples of model application for PRS purposes. Leslie Myers (LM): The system approach for PRS will be addressed in 9-22 which is on hold until after 1-40A is completed. CS: We are looking at the whole system even under 9-30A.

Open Floor Discussion Julie Nodes (JN): The model needs to respond to parameters that the Contractor has control over. This comment has more to do with PRS. From a state highway agency perspective, the controllable mixture properties have to be reflected in the model otherwise model won’t be useful. CS: Would be acceptable to use say E* as a surrogate to contractor parameters. JN: That is ok but it becomes an issue if something goes awry. How do they fix E*? Using E* means that the agencies should have adequate knowledge to advise the Contractor on how to adjust mix parameters if something goes wrong. CM: We have the ability with the new MEPDG to figure out effects of contractor controlled variables (e.g., air voids, binder contents, etc). A.3.4 Current Model—MEPDG Session A.3.4.1 Existing NCHRP 1-37A Procedure—Schwartz CS presented an overview of the 1-37A rutting model. Presented a history of development and perceived key limitations (e.g., the apparent arbitrary use of the depth function).

Model History Leahy model. Updates of Leahy’s model by Ayres. Updates of Leahy’s and Ayers’s models by Kaloush – basis for the Guide.

Details of the Current Model Pavement is divided into sub-layers. Rutting for each sub-layer including the unbound layer using a separate model is

calculated. Uses summation of sub-layer permanent deformations to determine total permanent deformation.

The time hardening scheme is built into the model. The asphalt aging model considers the increase in binder stiffness as a function of depth

and time; doesn’t affect the rutting model as much as the fatigue performance model. Considers traffic wander.



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Considers temperature distributions within sub-seasons. The field calibration coefficients were determined from LTPP sections. One calibrated rutting equation applies to all types of asphalt mixes; the only parameter

specific to each layer is the E*; Depth function: An empirical function derived from Mn/Road test. It was included as a

correction factor for the model’s main equation which was predicting most of the rutting happening at a depth between 3 to 6 in – this is contradictory to what is observed in the field.

Caveat planned to be addressed in Phase III: since LTPP did not conduct trenching studies, it was not possible to quantify the contribution of each layer’s rutting to the total measured rutting; the allocation of rutting to the various layers is assumed in the current model.

A.3.4.2 Preliminary findings from NCHRP 1-40A Review Panel—Von Quintus HVQ presented interim findings and recommendations of the NCHRP 1-40A review team with regard to the permanent deformation model. Only one set of calibration constants was provided for all different types of asphalt

mixes. Only one design speed is used in calculations which induce some unrealistic pulse

durations and frequency predictions which leads to inaccurate estimates of the dynamic modulus which leads to inaccurate prediction of permanent strain.

Replace the rut depth prediction model. Introduce allowable stresses levels for granular layers and foundation soils (threshold

concept). A.3.4.3 Current Work Under NCHRP 1-40D—Von Quintus HVQ presented work being conducted under NCHRP 1-40D(001) with regard to the permanent deformation model. MEPDG software recommended corrections:

o Include changes that account us volumetric mix properties to adjust calibration coefficients of the permanent deformation and fatigue cracking models (findings of NCHRP 1-40B).

o Add updated E* equation. o Add findings from NCHRP 9-23 – soil-water curve; water content predictions of

unbound layers. o Recalibrate rut prediction model.

Note: Further corrections will include better description of the slippage between layers. A.3.4.4 Discussion of Experiences with NCHRP 1-37A Model LS opened the floor for participants to share their personal experiences with the existing MEPDG rutting model.

Leslie Myers 25 different mixes were evaluated and E* testing and repeated load testing tests were

performed.



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After the completion of tests it was shown that E* works well in intermediate temperature ranges but not in high temperature ranges for predicting characteristics of different mixes.

At high temperatures, the repeated load test was able to discriminate between different mixes (a polymer modified vs. non-polymer modified mix).

Need to double check how the mix is characterized at higher temperature. One question that was raised was if it is possible to use the repeated load test data in the

model prediction. LM stated that is was not possible to figure out at this stage what is a software bug and what is a model issue. Concluded that we need to look at better characterizing mixes at high temperatures.

Other observations Model overestimates the contribution of the unbound layers to rutting.

RK: Can we use FN test results to modify model coefficients? CS: Tried that for Westrack and MnRoad. But was difficult because did not have the material properties at the correct temperature and stress condition. CM: Can we use actual temperatures and loads in the model rather than the defaults. JM: It is possible to do that. TS: Questioned the 1-37A rutting prediction approach. Asked if plastic strain can be predicted from resilient modulus? Does the relationship exist? Why is it related to rutting? Not convinced that the relationship exists and that it is related to rutting. Leahy’s model which includes a stress term is a better approach. TS: Can we use E* for linear elastic analysis? TS: Stated that the p/r relationship may be a good way to represent lab behavior but not sure if it can be fed to a rutting model. A.3.5 Review of Potential Alternative Models Session A.3.5.1 List/Presentation of Models Recommended for Consideration—Schwartz CS presented models that have been identified as “ready to go” to be considered for use under NCHRP 9-30A. Although he discussed empirical as well as other M-E models, he focused more on the latter set of models. Broadly, the M-E models were classified as: Permanent axial strain models (e.g., Leahy, Asphalt Institute (AI), VESYS, Verstraeten,

Uzan). Permanent shear strain models (Westrack).

CS discussed the possibility of using an enhanced 1-37A model (to be enhanced as part of 9-30A), the NCHRP 1-40B Option A model, etc.

Discussion of Alternative Models



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CS asked the workshop participants if there were any other alternate models that the project team may have overlooked that need to be considered. However, this did not generate much response. RK: Stated that NCSU has developed a new model to account for air voids. TS: Strain ratio – does it exist? If so, why is it relevant to rutting? TS: Can E* be really used for a linear elastic analysis? BC: Can the criteria used to select from among the various models presented be changed from what CS presented earlier? Project team: Yes



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NCHRP 9-30A Workshop Day 2 – Meeting Notes

December 7, 2005

A.4 Day 2 of Workshop A.4.1 Review Agenda for Day 2—Scott Focus was changed slightly from the original workshop agenda. LS stated that the original plan was to present alternative models. However, since there are not many significantly different models, not a lot of time will be spent discussing these. The focus was on discussing the criteria to be used for selecting models to be considered in Phase III as well as formulating directions for the future models. HVQ mentioned that the AI plastic deformation/strain and the Westrack models are the two models the project team recommends to be looked at in addition to the future models. These are still open for discussion. CM: The panel feels responsibility to make recommendations to go forward. Sought the invited guests to help in the deliberations. Emphasized the need to look beyond the current models leveraging the experience of all in attendance. Implored everyone to provide input since the decisions from here could set a significant course into the future. A.4.2 Review of Potential Alternative Models – Continued; Monismith, Von Quintus The session kicked off with two presentations by CM and HVQ on the Westrack and 1-40B Option A models, respectively. A.4.2.1 Carl Monismith’s Westrack Model Presentation—“Rut Depth Predictions Using

the Results of SHRP-Developed Simple Shear Test” Basis was the SHRP’s test program—simple shear test (controls the deviatoric component of stresses). Repeated simple shear test at constant height (RSST-CH) was chosen as the primary mode of testing. Highlights of the Westrack derived permanent deformation model: Developed based on Westrack. Went over Westrack’s objectives. Westrack’s original model was empirical. Then progressed to M-E. Recently began exercising the M-E model in conjunction with RSST-CH. 2-in below the edge of the tire was critical location for response computation. Critical

response – shear stress/strain. Densification was eliminated from rutting contribution. Only looked at shear

deformations. For unbound layers, only vertical compressive strain at the top of the subgrade (Shell

approach) was used. Used field cores and made lab specimens for the RSST-CH (initially when the test was

on going used cores).



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For Westrack, 50oC was used as a convenient temperature for the RSST-CH testing (the maximum temperature for Westrack site). A shear stress of 10 psi was used.

Strain at 5% was used an indicator of the behavior of the material. Simple loading – shear strain is expressed as a function of number of loadings. Time

hardening concepts were also used. MLET was used and two computations are made (shear stress/strain at 2” in AC and

vertical comp. strain at the top of the subgrade). Relationship similar to NCHRP 1-37A but using the inelastic shear strain and elastic

shear strain. Model was modified to include resilient shear strain. During SHRP Weisman did an FE analysis which showed that there is a relationship

between rut depth and plastic strain. Rutting is proportional to the plastic strain times a shift factor that depends on the thickness of the HMA layer. Shift factor, K, was developed. This factor is a function of thickness.

A modified AI subgrade strain criterion was used. Model is an M-E approach based on the time hardening principle. Predictions were shown for fine graded, fine plus, and coarse material. Predictions were

good in HMA. Very little deformation below the 3 in level in the HMA (trenching proved this). Most deformation was occurring in the HMA layers.

Model was used to check the California Heavy Vehicle Simulator (HVS) and I-710 (an important urban freeway project – HMA overlay of PCC).

Even HVS tests showed that there is no deformation in the unbound layers. Rut predictions for the HVS test results were good.

CM provided a copy of the paper presented to AAPT which shows more details. In summary, CM concluded the following: M-E model based on time hardening. MLET used. Model parameters a and c obtained

using Westrack and the RSSH-CT testing. PU is working on a different model based on this equation.

Underlying layers do not have much of a contribution. Subgrade strain criterion is adequate.

RSST-CH provides reasonable results (differentiates between conventional and modified materials).

Aggregate structure is important— rolling wheel compaction was used for specimen preparation since it was considered to better represent the real particle orientation in a pavement after compaction.

HVQ: What model should be used in NCHRP 9-30A Phase III? CM’s or PU’s? CM: There will be a definite recommendation to the panel on which model has to be used after evaluating both models carefully. HVQ: What if we cannot measure PD in unbound layers during field investigations because, for example, the amount of deformation is very small?



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CM: Using the AI criterion (or a modification thereof) and apportioning a small contribution (by calculating it) to the subgrade is good enough. However, this needs to be verified as part of the field study. RB: Were different stress levels or temperatures used during the RSST-CH testing? CM: Yes, temperatures were varied from 40 to 60 oC; however, it appears that in this range, temperature does not have a big impact—the coefficients will vary within a narrow range. The coefficient of stress could be arrived based on some guidelines. Use a couple of stress levels and temperatures for large projects. RB: For Phase 3, would you recommend different stress levels and temperatures? CM: Yes. TS: According to the project plan for NCHRP 9-30A, some of the sites are already in existence. What are we simulating if we take cores now (from these sites)? HVQ: The sites we identified have stored materials which can be used to constitute laboratory samples. CM: Like the work at NCAT, we have data such that the effects of aging will not affect results. HVQ: Similarly, APT are also conducted over a short period. A.4.2.2 Harold Von Quintus’s 1-40B Rutting Model Refinement Presentation—“NCHRP

1-40B: Calibration, Refinement, Rut Depth Prediction Model” HVQ presented the efforts undertaken in NCHRP project 1-40B to provide a mixture-specific methodology to adjust the “global” calibration coefficients for rut depth prediction included in the NCHRP 1-37A procedure. Mentioned issues with regard to error and bias in rut depth prediction using the current 1-37A model. The bias and error appeared to be related to mixture volumetric. Highlights of HVQ’s presentation (hardcopies of the presentation were provided to the attendees): Discussed the philosophy of adjusting the k1, k2, and k3 terms in the rutting prediction

model. Based on gradation and volumetric the rut depth model coefficients k1, k2, k3 were revised.

NCHRP 1-40B validation sites – NCAT round 1 (mixture), NCAT round 2 (structural), Westrack, Mn/Road, SPS-1, Florida APT, and FHWA/ALF APT.

Presented results of making local adjustments and predicting distress. The results showed a reduction in bias and error in most cases when compared to the original models. Noted that most of the rutting was assumed to be in the asphalt layer.

Clarified that the calibration factors were derived independent of the sites used in the 1-40B validation (in response to CS’s questions).

Bias exists for PMA mixes even with the adjusted calibration coefficients. Clarified that both bias and error was reduced (in response to RR’s questions). Clarified that time history of rutting development was used in the comparison (in

response to PU’s question).



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Addressed TS’s earlier question on whether there is a relationship with p/r and mentioned that there seems to be a relationship. CS indicated that CM’s Westrack model also includes a stress term (which allows for some additional scaling of stress level effects). HVQ mentioned that under NCHRP 1-40B, the team is planning to perform repeated load permanent deformation tests and use those results in predicting rutting for some selected demonstration projects. RR: In Florida, rut profiles (transverse profiles) are being monitoring carefully. One of the key findings was that rut level is constant in the center of the rut but the profile gradually moves outward (i.e., the rut basin is expanding). Material continues to get shoved out. Is this being handled in the MEPDG? This may explain why the rutting is over predicted. HVQ: That would not be handled. Perhaps could add a factor such as a rut basin width. RR: Is the method used to measure ruts (for the 1-40B data) the same in all instances? HVQ: No it is not the same. The measurement error could be due to the differences in measuring rutting. CS: Adjustments to rutting equation coefficients done in 1-40B were based on laboratory permanent deformation studies. If this is true, then direct lab testing of the mixture is perhaps more useful. HVQ: Discussed components of error. Stated that measurement error cannot be reduced. CS: Will the newly adjusted coefficients distinguish between SMA and HMA? HVQ: Yes. RB: This process is encouraging. CM: Rutting in new pavements will develop in the first few years. Aging may not have a significant impact. The lab mix should be representative of the critical age for rutting. Aging needs to be thought through carefully. CM: Suggest that we concentrate on controlled sections as much as possible such as a NCAT, APT, ALF sections for calibration. Materials and traffic data are missing in “real world” roadway sections, say LTPP. CR: Disagreed that materials data were missing in LTPP. HVQ: Agrees with the notion that controlled sections are good for calibration. However, explained that roadway sections are needed because of the way reliability is being handled in the MEPDG.



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PU: Would be reasonable to use very reliable data to calibrate and validate the model and then use Monte Carlo methods to account for variability. HVQ: The current method in the M-E PDG is not using Monte Carlo. CM: You can build in materials variability using MC. Adam Hand (AH): Clarification sought on LM’s earlier comment that at warmer temps there were some differences noted in the performance of various mixtures. Therefore, we need to need to integrate actual (repeated load permanent deformation) testing data. RK: Are all adjustments based on mixture volumetric in the NCHRP 1-40B calibration factor adjustment approach? HVQ: Based on mixture volumetric and gradation properties. RR: Both APT and roadway sections are extremely valuable and neither should be discarded. Field sections may not be valuable for calibration (if there are large uncertainties). Field sections however bring factors that cannot be handled in HVS/APT. We should continue very much looking at field sections. HVQ: Yes. A.4.3 Group Discussion A.4.3.1 Enhancements to Existing Model – Chuck Schwartz The workshop attendees were split into two groups – a practice-oriented group and a research oriented group. These groups discussions focused on where we want to go from here in terms of rutting model development/improvements and what do we need to do?

Group1: Katherine Petros, Leslie Ann Myers, Lorina Popescu, Cheryl Richter, Schmuel Weissman, Tom Scarpas, Per Ullidtz, Linbing Wang

Group 2: Bouzid Choubane, Shangtao Dai, Adam Hand, Julie Nodes, Murari Pradhan, John Haddock, Rey Roque, Carl Monismith

A.4.3.2 Notes—Group 1 Discussion

Discussion Item # 1 A. Should we pursue fully mechanistic models within this project (do you see a need

or benefit)? B. Should we abandon the current rut depth prediction methodology (p/r) within

this project? C. If fully mechanistic models are to be pursued, what are compelling arguments

for funding and moving forward fully mechanistic models? (a) The group divided this into two questions: “Should we pursue mechanistic modeling?” and

“Should we pursue it within the context of NCHRP 9-30A?” The consensus answer for the



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first was “yes,’ the consensus for the second was “maybe/partial.”

(b) Should we abandon the current axial strain ratio approach? There was some ambiguity in the interpretation of the word “abandon” and a definite lack of consensus within the group. Some opinions voiced: Yes, we should abandon further development. Option A from NCHRP 1-40B is good

enough. The current method/Option A approaches minimize the mechanistic content (because of

the depth function issues). Rename MEPDG as “mEPDG” or just “EPDG” and don’t invest any further development.

No, we should not abandon but rather go back and attempt to modify the method to make it more rational—e.g., find a mechanistic basis for the depth function.

No, we should not abandon development but rather use mechanistic methods to assist in the modification/enhancement of the M-E methods.

(c) Benefits of mechanistic methods: Increased knowledge/understanding of pavement mechanics in general and better insight

into the rutting phenomenon in particular. Can be used to improve/check simpler M-E methods. Capitalize on the many mechanistic components that already exist/have already been

developed—e.g., analysis methods, material modeling advances, instrumented field pavement sections, etc.

Will enable us to reduce reliance on past experience; this is particularly important as new materials, etc. are introduced and for which we cannot afford to wait 20 years to accumulate sufficient empirical data.

Valuable for forensic and other special studies. Impact on/from market forces: If validated mechanistic methods were available, they

might change today’s business model for pavement design/construction; if today’s business model changes—e.g., to more warranty construction—there will be more market pressure on development of better performance models.

Discussion Item # 2 What are the important criteria that have to be considered in the models evaluation process? Please identify and prioritize items from high to low. Items to start with (these are only suggestions):

Not addressed by this breakout group, although there was an extended digression on the significance of compaction technique and the need to simulate field constructed microstructure.

Discussion Item # 3 A. What is more important – predicting the evolution (i.e., rutting versus time), total

magnitude of the rut depth, or both? B. If M-E based prediction models should be pursued, what alternate rut depth

prediction models to be used in Phase III: C. What is an acceptable error term for rutting prediction?



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Not directly addressed by this breakout group.

Discussion Item #4 What should be done within the context of this project to advance fully mechanistic models? (e.g., use in selected projects for comparing measured to predicted rut depths and include mix characterization properties in database).

Formulate “round robin” experiments based on data collected in 9-30A. (This is similar

to what is often done in geotechnical and other engineering subdisciplines). Develop a more comprehensive materials database that could serve as a resource for all

mechanistic model developers. For example, a subset of 9-30A validation sections can be identified for which the data are of particularly high quality. In addition to the material tests required for 9-30A, do a more comprehensive suite of material characterization tests that could be used to calibrate sophisticated constitutive and/or micromechanical models. Some big questions: What tests should be included? How would they be performed (protocols are unlikely to exist for most)? Who will perform the tests—a single lab, multiple labs, etc.?

Establish some type of “models advisory group” to provide input on mechanistic modeling aspects of 9-30A (and, presumably, other projects).

Discussion Item #5 What could be done to apply parts of the future models to this project?

Formulate a “modest” task within 9-30A that could be attacked using mechanistic

approach. This in part would be a “confidence builder” for mechanistic models. Use mechanistic models to understand/refine test procedures. Use mechanistic models to refine/check M-E models.

Discussion Item #6 How to encourage/influence collaboration on or between groups?

“Open source” the MEPDG code so that other researchers/agencies could modify, adapt

to their own purposes. Subsequent discussion clarified that “open source” does not necessarily imply “free” or “public domain.”

The new ETGs being established by FHWA as part of the WRI earmark. Take more advantage of the modeling groups in TRB (Jo Daniels’ subcommittee), ISAP

(Tom Scarpas’ working group), and RILEM Use FHWA web site as a dissemination vehicle for 9-30A and related projects.

A.4.3.3 Notes—Group 2 Discussion

Discussion Item #1 A. Should we pursue fully mechanistic models within this project (do you see a need

or benefit)? BC: Yes. Use of such models could provide a low risk from an owner’s perspective.



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AH: Yes. But in the interim, there’s got to be a product that could be used by practitioners. Can perhaps be handled in the Level 1, 2, 3 type of set up. RR: The word “fully” is not realistic. It will be of great value considering the uncertainty. Multiple approaches should be pursued because they have great divergence in modeling, testing, mixture design, etc. JH: The way we fund projects, it may not be possible to pursue. CM: Panel can have a say in following different paths. JN: Fully mechanistic should be a long term goal. But we need to have something in the interim that is useable. We cannot keep changing drastically what we do. SD: There is no uniform acceptance on testing. This project should not look into fully mechanistic models. MP: Different agencies are going different ways at this time in terms of, for example, testing. We need to go to fully mechanistic but in stages. CM: Shell had a mechanistic design in 1967. Repeated load testing was started in 1953. It takes a long time to get there. RB: The database from this project can be excellent for use when we pursue fully a mechanistic process. RR: I don’t believe that we have a full understanding of rutting or cracking. CM: One good example of an area where we thought we knew most if not everything is the PG binder specification. Group consensus for long-term seems to be >20 years.

B. Should we abandon the current rut depth prediction methodology (p/r) within this project?

AH: Refine and consider others. JH: Not necessarily. JN: Within the timeframe we have, it is good. SD: Modified calibration coefficients approach presented by HVQ from NCHRP 1-40B appears reasonable. AH: PMA predictions are biased. It is good and is progress. But we have an issue with PMA. This model should be improved.



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JH: Will be hard to calibrate a PMA mixture if it does not rut. MP: Input from mixtures are in HVQ’s model. This is good. RB: PMA mixes can be adjusted through calibration or a new model development since the bias is consistent. MP: Can we put in a binder component to improve PMA predictions? CM: This can be addressed in the p/r relationship.

C. If fully mechanistic models are to be pursued, what are compelling arguments for funding and moving forward fully mechanistic models?

Economics Flexibility to adjust to changes in materials, traffic, truck configurations. Traffic prediction is a problem.

Discussion Item # 2 A. What are the important criteria that have to be considered in the models evaluation

process? Please identify and prioritize items from high to low. Items to start with (these are only suggestions):

Model accuracy temperature Mixture influences (binder, volumetric properties, gradation, etc) Stress state Traffic spectra Environmental influences Time-hardening for seasonal effects Rut depth accumulation via individual layers Mixture characterization (small strain properties vs large strain properties) Testing requirements Specimen preparation and geometry Testing expense Robustness of prediction model

High Low Accuracy Rut depth accumulation Mixture influence Testing expense* Temperature Small vs large strain properties Robustness Testing should be “realistic”** Testing Related Discussion. Lots of discussion on whether we should limit ourselves to what can be achieved in a typical DOT laboratory as far as testing is concerned. It was felt by some in the group that in addition to expense of testing, an additional item to consider is also the time and



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resources required to perform testing. It was felt that consideration needs to be given to the importance of the facility when deciding which model to use. CM: How will private industry and major contractors respond to more advanced testing requirements? CM: Hypothesized that for high importance projects, more testing may be viable. RB: In response to CM’s question, stated that his company was born based on the thought that this type of testing may be needed. Stated that there may not be enough market to perform this type of testing. However, mentioned that if the Contractor’s or stakeholder’s bottom line can be improved, advanced testing is paid for. AH: Agreed with RB that it is struggle at this point to convince folks to support advanced testing. MP: Even during the Superpave implementation, it was a struggle. Testing Expense Related Discussion. RR wanted to rephrase the testing expense related question as follows to make the intent more clear: “Should we limit ourselves to a particular type of test because of expense?” RR stated that the answer to this question is in the negative. Several others agreed. Specimen Preparation and Geometry Related Discussion. This issue generated a lot of discussion. However, no clear consensus evolved. Due to the importance of this issue on Phase III work, a decision was made to discuss it further in the general session (after the individual group discussion).

Discussion Item #3 A. What is more important – predicting the evolution (i.e., rutting versus time), total

magnitude of the rut depth, or both? Both. RR pointed out that transverse profile of rutting should be included in predicting rutting magnitude.

B. If M-E based prediction models should be pursued, what alternate rut depth prediction models to be pursued in Phase III?

1. WesTrack; shear strain based model 2. Verstraeten or AI, p based model 3. Others?

AH: All of the above.



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AH: Thin pavements were excluded from the MnRoad according to HVQ in his presentation of NCHRP 1-40B results. Thin pavements in our experience have a significant problem with rutting in underlying layers. Underlying layer rutting should be considered in rutting prediction. RR: Use profile from the surface to predict underlying layer rutting. CM: Subgrade strain approach is good. RR: Worthwhile considering additional mixture related models, e.g., the interstitial surface method from Florida. They can provide inputs at this time and perhaps form a basis for future models. CM and others: Repeated load test data should be incorporated into the model. RB: Should we calibrate more than one model? Group consensus was yes.

C. What is an acceptable error term for rutting prediction? As good as it can get. Discussion took place on what is error. How good are we going to get? Discussed measurement error. BC: More accuracy at the beginning of the prediction period (i.e., right after construction) is desirable. In warranty specifications, this is important.

Discussion Item #4 What should be done within the context of this project to advance fully mechanistic models? (e.g., use in selected projects for comparing measured to predicted rut depths and include mix characterization properties in database).

AH: Populate the database (ME_DPM). BC: Calibrate more than one model. RR: Pursue the identification and better description of other models and have the principals involved present them. Identify other models, describe current status of these models, and flesh out a framework on how to move forward. Classes of models—viscoplasticity based, micromechanics based, and methods in the middle “hybrids.” Have people put together a framework for each of these approaches that can be looked at in a more holistic fashion down the road. JH: Does the user community have a good grasp of mechanistic models? He stated that his experience suggests people think there is not much advantage.



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BC: From a pavement design perspective there is no advantage. From an analysis perspective, it is important. CM: Education is not a part of this project. CM: Panel will decide on what the models are and how the tasks need to be executed.

Discussion Item #5 What could be done to apply parts of the future models to this project?

RR: Some parts of the future models can be used to tweak the current models to improve prediction. Example, PU’s or RR’s parameters can be used to tweak models.

Discussion Item #6 How to encourage/influence collaboration on or between groups?

AH: A fundamental problem. NCHRP should dedicate some percent of the budget to promote collaboration. This workshop is a great example. Difficult issue overall. JM: One step in this direction is the “white papers” on different models suggested earlier. BC: Suggest that the project team prepare a critique of each future model. CM: There is a budget issue within this project. CM: One of the reasons for the workshop is to promote this. A.4.3.4 Open Discussion on Some of the Responses Collected An item-by-item discussion was conducted after the group discussions were completed to exchange notes and see divergence or similarities of opinions. Each group’s spokesperson presented the findings. Highlights from this discussion are presented below.

Discussion Item # 1 A. Should we pursue fully mechanistic models within this project (do you see a need

or benefit)? B. Should we abandon the current rut depth prediction methodology (p/r) within

this project? C. If fully mechanistic models are to be pursued, compelling arguments for funding

and moving forward fully mechanistic models? Item 1A – The groups agreed that fully mechanistic models should be pursued in the long-term. They felt that it may not be possible to fully consider them in this project. (long-term) Item 1B – The groups did not recommend abandoning the current approach. They felt that the current approach can be taken forward with some modifications. Item 1C – The groups agreed that these models should be pursued.



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Discussion Item # 2 A. What are the important criteria that have to be considered in the models evaluation

process? Please identify and prioritize items from high to low. Items to start with (these are only suggestions):

The specific issue discussed was “How should we prepare mix samples for calibration in Phase III?” NCAT is reopening the issue of compaction—gyratory, rolling wheel, field, etc. Perhaps relevant to this project? PU: Use all to see the difference. SW: Different tests may have different sensitivity. CR: To answer question, step back and ask how will the method be used. Design – no field cores. Warranty – field cores, perhaps. Probably, several tests with model tied to compaction and application of model. CM: Different methods provide different degrees of reliability. Should not specify method as a ground rule for project. Should look at alternatives. CS: Can use both methods. Get different parameters. Get different calibration. JM: Feasibility/availability of materials for testing. SW: Correlation between material properties from NCAT project. HVQ: Sample preparation is important issue with NCAT. CM; Do not be constrained to NCAT only. Other agencies, e.g., Florida did some work as well. Work on modified binders?

Discussion Item # 3 A. What is more important – predicting the evolution (i.e., rutting versus time), total

magnitude of the rut depth, or both? B. If M-E based prediction models should be pursued, what alternate rut depth

prediction models to be pursued in Phase III: C. What is an acceptable error term for rutting prediction?

The issue of underlying layer rutting was discussed further. HVQ: Forensic work will include foundation rutting measurement.



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AH: Make sure overall system can predict underlying layer rutting.

Discussion Item # 4 What should be done within the context of this project to advance fully mechanistic

models? (e.g., use in selected projects for comparing measured to predicted rut depths and include mix characterization properties in database).

Partial implementation of a fully mechanistic round-robin experiment. Material testing for future 9-30A testing and suite of tests that other approaches could

use. Models advisory group? Modest task that could use a fully mechanistic approach to build confidence. Testing database could be used by future models. SW suggested storing raw data and not

just processed data. Calibration of more than one model. Summarize how future approaches can be used for rutting prediction. Critical assessment

of these approaches by the project team. RR: Store materials for future testing. Will the MRL remain viable? CR: Funding for MRL: in 2006 FHWA budget. Will be best to do it.

Discussion Item # 5 What could be done to apply parts of the future models to this project? HVQ: Need specific input from workshop attendees on this important item. Time was set aside to discuss this issue in detail in five small subgroups. The findings from each group are given below. Sub-Group 1: Illustrate the use of micromechanics model to show how one might begin to address this issue.

Sub-Group 2: Start with a pavement, simulate it by a sophisticated FE tool, and see which of the assumptions made in the simple models are realistic.

Sub-Group 3: Having a look at other models is a good idea. Where the money comes from is a tough problem. CM stated that a letter will go from the panel to NCHRP and TRB to encourage this. LW: Combine the power of a combination approaches starting from a continuum model to a micromechanics approach in 3-D to understand the impact of volumetric properties. TS: This is a whole new project.

Sub-Group 4:



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Take available models (viscoplasticity, micromechanics) and use them in conjunction with materials with known properties to identify critical mix characteristics, refine M-E models, through sensitivity analysis, etc. Sort of like the “round robin” study with different models having the same material parameters. CM: This is a good idea but costs money. Provided example of Austrian study which did the same thing with very little consequence. Group consensus: It is going to be expensive.

Sub-Group 5: To add some rationality to the parameters of the permanent deformation model by using e.g., using the DEM, particle packing type approach, etc. A.4.3 Closing Remarks Session HVQ distributed pavement sections proposed to be included in 9-30A and said that other sections like the PennDOT test sections will be added. LM stated that other sections from Maine and Wisconsin should also be added. HVQ and CS thanked the audience for participating in the workshop. CM thanked the guests on behalf of the panel and highlighted the importance of their input to 9-30A and the new design guide. LS thanked the audience. A.4.4 Summary of Discussions The main highlights of the discussions held at the workshop are provided below: The NCHRP 1-37A permanent deformation model as modified by the additions

developed during the NCHRP Project 1-40B should be used for the calibration process under NCHRP 9-30A. The project database should be modified to include additional parameters required to support the 1-40B changes.

The depth correction factor in the present NCHRP 1-37A HMA permanent deformation calculation procedure should be examined and corrected as necessary.

As recommended by several Workshop attendees, the NCHRP 1-37A rutting procedure should be modified to include results of the repeated load testing which can be performed using the simple performance test developed under NCHRP Project 9-29.

Some of the Workshop attendees recommended that the stress term in Leahy's original elastic-to-plastic strain relationship used as a basis for the NCHRP 1-37A HMA permanent deformation model be reintroduced in addition to temperature and number of load repetitions, i.e., p/r = f(temperature, stress, and number of load repetitions).

The only additional procedure actually recommended by the Workshop participants for inclusion in the NCHRP 9-30A study was that developed from the WesTrack Project by J. Deacon et al. A summary of this model was presented by CM. This would require additional discussion between the project team and the project panel.



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The Workshop attendees agreed that it would be very desirable to provide funds to permit a number of other investigators to provide working examples of permanent deformation prediction procedures for rutting estimates in HMA pavements. In order for this to be accomplished it was agreed that this should be accomplished with funding necessary to support the organizations/individuals providing the working examples.

A number of participants recommended that the MEPDG include more than one procedure (option) for rutting estimates. It was suggested that the software be programmed to allow the designer the option to select one of a number of alternatives.



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ATTACHMENT B MODEL SUMMARIES PRESENTED AT THE TWO-DAY WORKSHOP

Extract from Draft Final Report to NCHRP on Project 1-40A(03)

B.1 INDEPENDENT REVIEW OF THE RECOMMENDED MECHANISTIC-EMPIRICAL DESIGN GUIDE AND SOFTWARE: RELIABILITY, AND DESIGN OF COMPOSITE (REHABILITATED) PAVEMENTS

Prepared for Scott Wilson Pavement Engineering Ltd, Nottingham, UK by Stephen F Brown

Appendix 2. Rut Depth Prediction A2.1. Procedure used in the Guide A computation procedure is used in the Guide to predict the development of wheel track rutting over the design life of the pavement. It is based on the use of semi-empirical models to determine vertical permanent strain and considers asphalt, granular layers and subgrade separately. The vertical permanent strain at the centre of each structural analysis sub-layer is multiplied by the layer depth to give the permanent deformation for that sub-layer. Summation for all the sub-layers gives the surface deformation. This calculation is repeated for changing load, temperature and moisture regimes during the pavement life to produce a plot showing how rutting accumulates with time. The models used to determine permanent strain all assume that this parameter is directly proportional to resilient strain. Separate models were derived for HMA, unbound granular material and for soils based on earlier research. In each case, a basic model was modified two or three times with a view to improving the prediction of rutting based firstly on laboratory data and, subsequently, on LTPP field data. The R2 values for rut prediction in each material layer were 0.648 (HMA), 0.677 (Granular) and 0.136 (Soils). The accuracy of prediction for total surface rutting after National calibration gave an R2 of 0.399. In undertaking the field calibration exercise, very little data were available about the relative contributions to rutting made by the different layers because no trenching studies were conducted for the LTPP general sections. Reliance was placed on limited data from MnRoad and from the AASHO Road Test and extrapolated to all the sections considered. The Guide Appendix GG, at page GG-1.72, shows that 90% of rutting at MnRoad was in the top 4in of the HMA layer. The principals on which the rut depth prediction method is based assume, for all materials, that the primary stage of permanent strain is predominantly due to volume change, the secondary stage is mainly volume change and some shear, while the tertiary stage is shearing at constant volume. Figure A2.1 (Figure 3.3.13 from the report) illustrates the various stages.

Confidential: This was only provided for review & discussion.



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Advice in the Guide on how to reduce rutting, when trying to improve an initial unsatisfactory design, emphasises the need to increase layer resilient modulus. This is based on the assumption that the resulting reduction in resilient strain will reduce permanent strain because of the direct proportionality between these parameters, which is a feature of the basic material models.

Figure A2.1 The three stages of Permanent Strain in HMA, Soils and Granular Materials under Repeated Loading

A2.2. Review of Procedure The move away from the traditional reliance on use of vertical resilient strain at formation level to limit rutting in mechanistic design is to be welcomed. This was a semi-empirical approach that took no account of the detailed permanent deformation characteristics of materials but relied on a general connection between this strain parameter and the overall rut depth developed during a 20 year period in typical pavements. It was originally derived from AASHO Road Test data. The technique used to calculate rut depth in a pavement by summation of vertical permanent strain with depth is well established, as is the way in which different load and temperature conditions are accommodated, as illustrated in Figure 3.3.19 of the Guide. However, the material models are based on conditions of axial symmetry and so can only be applied to a pavement on the centre line of the wheel load. Since rutting in HMA is considered to be strongly influenced by shearing under the edge of the load, the computations will be inaccurate if applied at this location or at any point away from the axis of symmetry. It is not clear from the wording in the Guide whether computations are limited to the centre line or not. The Guide Appendix GG does not include any literature review of the various approaches to rut depth prediction or to permanent deformation characterisation of materials that have been conducted by research organisations other than those of the Guide development team. A notable omission is the work carried out over many years at the University of California at Berkeley by Monismith and his associates, much of which was initiated under the SHRP program (A2.1).



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In the material models, the assumption that permanent strain is proportional to resilient strain is based on multiple regression statistical analyses of limited laboratory data, for the HMA model, and an assumption, with no theoretical justification, that is built into the soils and granular materials model. There is no theoretical reason why these two strain components should be directly proportional to one another. However, experimental data for well-designed HMA mixtures tends to show that there is a reasonable linear relationship, as illustrated in Figure A2.2, which shows direct shear testing data for HMA specimens from the WesTrack site (A2.2). In this figure, the parameter ‘a’ is proportional to permanent shear strain. A similar conclusion was noted by Brown et al (A2.3) when relating permanent strain at the end of repeated load compression tests with indirect tensile resilient modulus values. These empirical observations apply only to a set of similar mixtures, but conditions could arise under which the relationship would not be appropriate. It is not clear why resilient strain has been introduced to models where the basic dependent variable for permanent strain accumulation is the applied stress. In the context of rut development, the fundamental behavior of HMA or unbound materials in a pavement is such that strains develop as a result of stress application and, in particular, as they are frictional materials, the ratio of shear to normal stress. For unbound materials and soils, this applies to both resilient and permanent strains. It follows that the way to compute permanent strain is to determine the transient wheel load stresses and use a relationship between strain and the stresses for the particular temperature and number of load applications involved. While the Guide method is somewhat like this, it confuses the issue by introducing resilient strain unnecessarily. The JULEA computations produce stresses, from which resilient strains are computed using Hooke’s Law (equation 3.3.14 in the Guide). This step would be unnecessary if the models were simply in terms of stress.

Figure A2.2. Relationship between Resilient and Permanent Strain (proportional to ‘a’) for WesTrack HMA Specimens subjected to Direct Shear Tests at Constant Height (after Monismith et

al, A2.2) For HMA, it has long been recognized that the parameters influencing permanent strain are more numerous than those that influence resilient strain. The former depends on all the characteristics of the aggregate, including grading, shape and surface texture as well as compaction method and compacted state together with the binder type and content. Resilient strain is dominated by the binder properties together with the volumetric proportions of the mixture. The former involves large strains (generally > 0.001) while the latter involves small strains (generally < 0.0001). Figure A2.3 illustrates these points diagrammatically, based on the concepts developed by Shell researchers many years ago as a relationship between binder and mixture stiffness. It indicates that mixtures with poor aggregate characteristics can



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have relatively high stiffness under low strain (resilient) conditions but relatively low stiffness under high strain conditions.

Figure A2.3. General Relationship between Binder Stiffness and Mixture Stiffness for HMA’s with

varying Volumetric Proportions and Aggregate Characteristics. Figure A2.4 shows data (A2.3) indicating that low permanent strain can be associated with low elastic stiffness. All data are for the same mixture but compacted to three levels with the top line relating to compaction to refusal, under which condition, the resilient stiffness is high but the resistance to permanent strain is low. These points having been made, the use of a good mixture design method for HMA, which must involve a test to measure permanent deformation resistance, should rule out the type of mixtures for which the proportionality between resilient and permanent strain is seriously in error. In the Guide, there is no requirement for any laboratory testing in connection with rut depth prediction, other than the resilient properties of the pavement layers but these are used for other aspects of the design process and are not specifically part of the rut depth procedure. It is also a fact that the current Superpave mixture design method does not include any mechanical property testing. For soils and granular materials, the existence of a direct proportionality between the two strain components is much less certain and this aspect of the Guide requires further work. The calibration procedures described in the Guide has taken the models well away from the basic theory of soil behaviour and good quality test data and has resulted in empirical fits to field data of questionable reliability. The overriding assumption relating to the three stages of permanent strain development (Figure A2.1) appears to over-emphasise the role of volume change and to under-play the importance of shear strain development. For a well designed and properly compacted HMA, there will be little volume change under traffic and the principal cause of rutting, if it develops, will be shear strains within the top 4in of the layer beneath the edge of the wheel load. This was a major outcome of the SHRP study on HMA (A2.1), which has not been accommodated within the Guide. Well-compacted HMA under shear is likely to dilate rather than compact in exactly the same way as dense soil or granular material. It should also be recognised that compaction (volume decrease) is brought about by shearing rather than direct compression.

Permanent strain:

Rutting

Resilient behaviour:

Pavement analysis

Binder Stiffness

Mix

ture

Stif

fnes

sPermanent strain:

Rutting

Resilient behaviour:

Pavement analysis

Binder Stiffness

Mix

ture

Stif

fnes

s



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Figure A2.4. Permanent Strain development in Repeated Load Compression tests on HMA mixtures at different states of compaction

A2.3. Proposals The research to be conducted under NCHRP 9-30(A) will be developing concepts for the prediction of permanent deformation in HMA, so some of the suggestions made here are intended to provide input to that project. However, before addressing these longer term issues the following suggestions are made at a more pragmatic level to improve the usefulness of the Guide in the shorter term. The present rut depth prediction method should not be used until improved confidence can be gained in the models (existing or new) for each material type. The suggested way forward is to adopt an allowable stress approach for the lower layers, designed to ensure that they are not subjected to shear stresses which exceed a threshold value. Threshold deviator stress is defined as the magnitude of repeated deviator stress below which the accumulation of plastic shear strain is negligible. The phenomenon has been observed by several investigators over the years and is clearly a very useful concept for design. The idea is not new but has still to be effectively applied in pavement design. Brown (A2.4) has explained the principles recently and suggested that the concept could be applied in practice after some limited additional research. It requires relatively straight forward standard soil tests. The situation for HMA is different and should be dealt with by use of good mixture design, which must include an appropriate laboratory test, correlated to field performance. The idea would be the traditional one of designing to prevent the development of serious rutting rather than attempting to predict the



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amount of rutting. The usual limiting value of about 0.75in could be applied by way of calibration of the laboratory test. At present, there are two principal schools of thought in the US about the testing of HMA for mixture design and rut control. The work at Berkeley, initiated under SHRP, has lead to development of the SST, direct shear test, usually applied under conditions of constant volume. This approach has been validated through application to WesTrack and other field situations (A2.5). However, its use is criticized because the test requires expensive equipment and is somewhat difficult to conduct. The other approach, evolving from NCHRP development work for the ‘Simple Performance Test’, has focused on direct compression testing, either confined or unconfined. The compaction of specimens for permanent deformation testing is a vitally important issue since the field conditions must be reproduced in the laboratory. The permanent strain characteristics of HMA are very dependant on compaction method which affects particle orientation. Two mixtures with the same volumetric proportions can exhibit different permanent strain characteristics if compacted by different methods. The Berkeley approach is to simulate the field situation by use of cores cut from compacted slabs while most other laboratories use gyratory compaction. This has arisen because of the central role played by gyratory compaction in the Superpave mixture design method and because of its speed and simplicity. The equipment has been successfully used for many years in French practice to measure the compactability of HMA but never as a means of producing test specimens for permanent deformation testing. If this is to be the future method, because of convenience, then further work is needed to demonstrate quite clearly that it can reproduce the field compacted state. A further criticism of any molded specimen is that the mould introduces an artificial barrier which affects the compacted state of the material on the outside of the specimen. It follows from the above discussion, the recommendation given here is to use compacted slabs as the source of test specimens for permanent deformation testing of HMA. The question then arises as to the relative merits of the SST and the triaxial test. The SST was introduced to emphasize the role of shear stress and followed finite element analysis that indicated the high level of shear to normal stress induced in the pavement may be difficult to reproduce with the triaxial test (A2.1). More recently criticism has been leveled at the triaxial configuration as being essentially a compression test that does not consider shearing effects. Against this background, the following proposals are made, based on experience at Nottingham with both forms of test on a range of materials, both bound and unbound. The Berkeley philosophy of emphasizing the importance of shear stress for well-compacted, well-designed mixtures is much preferred to that implicit in the Guide, which is based on volume change being the principal cause of rutting. However, the SST as a test method presents a number of problems. The use of a circular specimen and no side platens introduces non-uniform stress conditions. In addition, application of a shear stress and a vertical normal stress without the ability to measure the lateral stresses on the specimen means that the complete stress regime is not known. This makes fundamental interpretation of test results difficult. The same situation arose some years ago in the UK with the Simple Shear Test for soils, even the sophisticated versions developed at Cambridge and at Nottingham (A2.6). The standard SST test is conducted at constant height to eliminate any volume change. However, to achieve this, the normal stress has to be adjusted during testing, which means that the shear to normal stress ratio varies during a test. This is clearly undesirable. It is also an artificial situation relative to how the material will respond in situ. In general, the field stress regime to which an element of HMA is subjected will result in a combination of permanent shear and volumetric strains. The relative magnitude of each component will be a function



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of the detailed applied stress regime and the material characteristics. Consequently, a fundamental test method is required that can apply the correct stresses uniformly and can measure both strain components. This can be done with the triaxial configuration if accurate measurements of both axial and radial deformation are made. Care has to be taken to minimize end effects by use of lubricated platens. These are well-established principles that have long been used in soil mechanics. Brown and Cooper (A2.7) applied them to HMA and produced data that related permanent shear and volumetric strains to the applied shear and normal stresses. The essential equations follow. Applied stresses expressed as mean normal stress (p) and deviator stress (q) are: p = a + 2r q = a - r Where a = axial stress and r = confining stress. The shear (єs) and volumetric (єv) strains are: єs = 2(єa – єr)/3 єv = єa + 2 єr Where єa = axial strain and єr = radial strain. It should be noted that engineering shear strain () used in connection with the SST is twice the pure shear strain (єs) defined above. A fundamental investigation is required to develop constitutive relations for permanent strain in HMA based on use of the above stress and strain invariants. Out of this could develop a simplified approach to routine mixture testing. It is always more reliable to simplify from a position of knowledge than to extrapolate from a position of ignorance. The former approach has been used in the UK based on research at Nottingham (A2.8). A repeated load compression test, either confined or unconfined, depending on circumstances, has been successfully used in HMA design and evaluation for several years. Application of the results from such a test program to the computation of rut depth requires that the stress conditions on any relevant element in the HMA layer, either on the centre line or elsewhere can be used to determine the shear and volumetric strain components. Further analysis could then define the vertical component of permanent strain so that a summation procedure could be applied similar to that used in the Guide. In the Berkeley procedure, background finite element analysis was used to relate the shear strain on an element 2in below the edge of the wheel load to the rut depth (A2.1). The prediction of permanent strain accumulation in soils and granular materials under repeated loading is exceedingly difficult, since so many variables are involved. The only feasible way to do it is through the use of appropriate testing. However, the acquisition of representative field specimens presents difficulties, particularly at the design stage. The best that can be expected is to conduct tests on remoulded soil and candidate granular materials to investigate permanent strain characteristics and how they are affected by changes in moisture and dry density. There is a reasonable relationship between permanent strain development and undrained shear strength. Much further research is required on this subject if it is considered necessary to be able to predict rut depth contributions for the lower layers. The ‘threshold stress’ approach, recommended above, is considered a more credible way forward. There is a philosophical difference in the overall approach to pavement construction and design in the US compared with Europe and the UK which has a direct bearing on this issue. It applies particularly to heavily trafficked pavements but the approach is relevant to all pavements other than those of granular construction with only a surface seal and no HMA. In the UK, unbound bases are no longer used, granular



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materials being confined to sub-base and capping layers so that the construction below the concrete or HMA layers is regarded as the pavement foundation. This requires separate considerations for design. The concept was described by Brown and Dawson (A2.9) and is now used in the official UK design method of the Highways Agency (A2.10). The design considerations for the foundation relate to its ability to support construction traffic including pavers and delivery trucks and to provide a sound platform for satisfactory compaction of the upper layers. In the long term, the foundation must also mobilize an adequate resilient stiffness which has a direct influence on the stresses set up in the road base. An advantage of this approach is that an Effective Foundation Stiffness (Ef) can be part of the construction specification and can be measured with an FWD or Dynamic Plate Test. Stabilization can be used to enhance stiffness when required. In the UK, four foundation classes are defined in terms of Ef. The values are 400, 200, 100 and 50MPa (70,000 to 7,300psi). In order to deal with rutting under construction traffic, the specification relies on proof loading with a standard axle load. For design purposes, unbound materials and soils can either be subjected to repeated load testing using a newly developed simple test, known as the Springbox (A2.11), or the measurement of undrained shear strength, which closely relates to permanent deformation resistance. A2.4. Recommendations

1. The present rut depth prediction procedure should not be implemented in its present form. 2. In the short term, the rutting failure mode should be dealt with through good HMA mixture

design, which must incorporate a suitable test for permanent deformation resistance. In addition, a limiting threshold stress should be used to ensure that the stresses induced in the granular layer and subgrade do not result in any significant contribution to surface rutting.

3. In the long term, the rut depth computation procedure in the Guide should be improved with respect to the modeling and associated computations relating to HMA. Further consideration should be given to the question of rut prediction for the lower layers or whether the threshold stress concept should be used.

A2.5. References

A2.1. Sousa, J. B., J. A. Deacon, S. Weissman, J. T. Harvey, C. L. Monismith, R. B. Leahy, G. Paulsen, and J. S. Coplantz. Permanent Deformation Response of Asphalt-Aggregate Mixes. Report No. SHRP-A-415, Strategic Highway Research Program, National Research Council, Washington, D.C., 1994.

A2.2. Monismith, C. L., J. A. Deacon, and J. T. Harvey. WesTrack: Performance Models for Permanent Deformation and Fatigue. Pavement Research Center, Institute of Transportation Studies, University of California, Berkeley, 2000.

A2.3. Brown, S F, Preston, J N and Cooper, K E, Application of new concepts in asphalt mix design, Journal of AAPT, Vol. 60, 1991, pp 264-286. A2.4. Brown, S F, Design considerations for pavement and rail track foundations, Geotechnics in Pavement and Railway Design and Construction, Ed. Correia and Loizos, Athens, 2004. pp 61-72.

A2.5. Monismith, C L, Popescu, L and Harvey, J T, Rut Depth Estimation for Mechanistic-Empirical Pavement Design Using Simple Shear Test Results, under review, 2005.

A2.6. Ansell, P and Brown, S F, A cyclic simple shear test apparatus for dry granular material, Geotechnical Testing Journal, ASTM, Vol. 1, No. 2, 1978, pp 82-92.



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A2.7. Brown, S F and Cooper, K E, A fundamental study of the stress-strain characteristics of a bituminous material", Journal of Asphalt Technology, Vol. 49, 1980, pp 476 – 496. A2.8. Brown, S F, Practical test procedures for mechanical properties of bituminous materials, Proc. Inst. of Civil Engineers Transport, Vol. 111, 1995, pp 289-297. A2.9. Brown, S F and Dawson, A R, Two-stage approach to asphalt pavement design, Proc. 7th Int. Conf. on Asphalt Pavements, Vol.1, Nottingham, 1992, pp 16-34.

A2.10. Highways Agency, Design Manual for Roads and Bridges, Volume 7: Pavement Design and Maintenance. H M Stationery Office, London. 1999

A2.11. Edwards, J.P, Thom, N.H. and Fleming, P.R. Development of a simplified test for unbound aggregates and weak hydraulically bound materials utilising the NAT. Pavements Unbound. Taylor & Francis, London, pp 3-11, 2004.



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Appendix G Facilitated Workshop Final...

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