An Experimental Plan for Validation of an Endurance Limit for HMA Pavements (2009)

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NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements 1. INTRODUCTION AND RESEARCH APPROACH 1.1 Introduction This report documents the research completed in National Cooperative Highway Research Program (NCHRP) Project 9-44, Developing a Plan for Validating an Endurance Limit for HMA Pavements. For hot mix asphalt (HMA) pavements, the endurance limit has been defined as a level of strain below which there is no cumulative damage over an indefinite number of load cycles (1). The endurance limit is an important concept in the design of long life flexible pavements that is gaining increasing acceptance worldwide. Appropriate application of the endurance limit in flexible pavement design will lead to more effective pavement sections with significant benefit and cost savings to the public. 1.2 Problem Statement and Research Objective 1.2.1 Problem Statement The endurance limit, as applied to HMA and flexible pavement design, is a strain level below which the fatigue life of the HMA is infinite and the pavement will not experience bottom-up fatigue cracking. Current mechanistic-empirical fatigue criteria for HMA, including the field calibrated criterion in the Mechanistic Empirical Pavement Design Guide (MEPDG), assume the fatigue life of HMA to be a power function of the tensile strain at the bottom of the asphalt layer. These criteria do not include the provision for an endurance limit. There is mounting evidence that an endurance limit for asphalt concrete does exist. It has been observed in laboratory studies of fatigue at low strain levels, and several documented cases studies indicate that bottom-up fatigue cracking is almost non-existent in properly constructed, thick asphalt concrete pavements. A concentrated research effort, however, is needed to validate the endurance limit concept, and to devise effective methods for incorporating it in mechanistic-empirical pavement design methods. 1.2.2 Objective The objective of NCHRP Project 9-44 was to prepare a research plan and associated cost estimate for a future study to validate the endurance limit for HMA and to improve mechanistic- 1

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements empirical pavement design. To be successful, the research plan must address the following: 1. Validation of the existence of an endurance limit for HMA in pavements through an analysis of laboratory and field data; 2. Potential differences in the endurance limit measured in the laboratory and observed in field performance; and 3. Identification of a recommended methodology for incorporating an asphalt concrete endurance limit in mechanistic-empirical pavement design. 1.3 Research Approach The research approach taken in NCHRP Project 9-44 was to synthesize information gathered from a review of relevant research and a workshop with invited experts to develop a comprehensive work plan and budget for a future project to validate the endurance limit for HMA and improve mechanistic-empirical pavement. NCHRP Project 9-44 included six major tasks, which are briefly described below. 1.3.1 Task 1. Review Relevant Research. In this task, published research associated with the endurance limit and the design of flexible pavements and HMA mixtures to resist fatigue cracking was reviewed. Information obtained in Task 1 was used to select topics for the facilitated workshop that was conducted in Task 2 and to develop the overall approach for incorporating the endurance limit in mechanistic-empirical pavement design. This review focused on the following key topics: • Laboratory endurance limit studies, • Alternative forms for fatigue testing, • Approaches for incremental damage analysis, • Laboratory studies on healing and damage tolerance, • Field studies of measured strains in thick flexible pavements, • Case studies of long life flexible pavements. 2

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements 1.3.2 Task 2. Conduct Facilitated Workshop. Task 2 included the planning, execution, and documentation of a facilitated workshop directed at evaluating various methodologies for HMA fatigue characterization, and strategies for incorporating an endurance limit in mechanistic-empirical design. Recommendations from the workshop shaped the research plan produced in NCHRP Project 9-44. The HMA Endurance Limit Workshop was held in Washington, D.C. on August 1 and 2, 2007. Participants included members of the NCHRP Project 9-44 panel and research team, key researchers and consultants with extensive experience in HMA fatigue analysis, and engineers from highway agencies who are responsible for designing, constructing, and maintaining flexible pavements. Thirty-four individuals were invited to attend, including four members of the research team, the NCHRP Project 9-44 panel members and Project Manager, and 22 invited experts. There was a high degree of interest in the workshop with 82 percent of the invitees participating. Table 1 presents summary information about the participants. The HMA Endurance Limit Workshop was facilitated by Mr. Charles Markert, a Certified Professional Facilitator. The agenda for the workshop was developed jointly by Mr. Markert and Dr. Ramon Bonaquist, Principal Investigator for NCHRP Project 9-44. A copy of the agenda is reproduced as Figure 1. The key element of the workshop was a series of discussion sessions focusing on four major topics considered important to validating an endurance limit for HMA pavements: • Endurance limit and other important fatigue effects, • Methodologies for HMA fatigue characterization, • Strategies for incorporating an endurance limit in flexible pavement damage analysis, and • Approaches for calibrating and validating pavement analysis methods that include an endurance limit. A summary report documenting the HMA Endurance Limit Workshop was prepared. This report in included as Appendix A. 3

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements Table 1. HMA Endurance Limit Workshop Invitees and Attendees. No Name Affiliation Role Attend 1 Dr. David Anderson Consultant Panel Member Y 2 Dr. Ramon Bonaquist Advanced Asphalt Technologies, LLC Research Team Y 3 Dr. Stephen Brown University of Nottingham Invited Expert N 4 Dr. William Buttlar University of Illinois Invited Expert N 5 Dr. Samuel Carpenter University of Illinois Invited Expert Y 6 Dr. Donald Christensen Advanced Asphalt Technologies, LLC Research Team Y 7 Mr. Danny Dawood Pennsylvania Department of Transportation Panel Member N 8 Dr. Herve Di Benedetto Ecole Nat. des TPE Invited Expert Y 9 Mr. Bruce Dietrich Florida Department of Transportation Panel Member Y 10 Dr. Jon Epps Granite Construction Company, Inc. Invited Expert N 11 Mr. Kenneth Fults KWF Pavement Consulting Invited Expert Y 12 Mr. Roger Green Ohio Department of Transportation Panel Member Y 13 Dr. Kevin Hall University of Arkansas Invited Expert Y 14 Dr. Edward Harrigan National Cooperative Highway Research Program Panel Member Y 15 Mr. Frederick Hejl Transportation Research Board TRB Liaison N 16 Dr. Richard Kim North Carolina State University Invited Expert Y 17 Dr. Dallas Little Texas A&M University Invited Expert Y 18 Dr. Robert Lytton Texas A&M University Invited Expert N 19 Dr. Leslie Ann McCarthy Federal Highway Administration Panel Member Y 20 Mr. Charles Markert Dynamic Leadership Consulting Group Research Team Y 21 Dr. Andre Molenaar Delft University Invited Expert Y 22 Professor Carl Monismith University of California Berkeley Invited Expert Y 23 Dr. David Newcomb National Asphalt Pavement Association Invited Expert Y 24 Dr. Michael Nunn Lane One Limited Invited Expert Y 25 Ms. Linda Pierce Washington Department of Transportation Invited Expert N 26 Dr. Brian Prowell Advanced Material Services, LLC Invited Expert Y 27 Dr. Rey Roque University of Florida Invited Expert Y 28 Ms. Amy Schutzbach Illinois Department of Transportation Panel Member Y 29 Mr. Darin Tedford Nevada Department of Transportation Panel Member N 30 Dr. Jacob Uzan Technion University Invited Expert Y 31 Mr. Harold Von Quintus Applied Research Associates Invited Expert Y 32 Dr. Linbing Wang Virginia Polytechnic and State University Panel Member Y 33 Ms. Rane Wagner Rane Wagner and Associates Research Team Y 34 Dr. Matthew Witczak Arizona State University Invited Expert Y 4

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements AGENDA Item Questions to be answered Sponsor Welcome, Facilitator Opening, Introductions Personal Expectations What are your expectations for this session? Briefing 1 Fatigue in the MEDPG How are fatigue and the endurance limit addressed in the MEPDG? Briefing 2 NCHRP 9-38 Laboratory Evaluation of Endurance Limit What was found about the endurance limit in NCHRP Project 9-38? Briefing 3 A Review of UK Pavement Design What approach is taken in the UK? Purpose Discussion What is the purpose of this session? Discussion: Existence of Fatigue Endurance Limit Does a Fatigue Endurance- Limit Exist? Continue Discussion Does a Fatigue Endurance- Limit Exist? Issues Related to Fatigue -Endurance- Limit “What are the other issues?” Identify Major Issues Which of these possibilities go on the Short List? Place your dots – one per card. What is the Meaning of Each Major Issues? Discussion on the top few. Discussion: Alternative Methodologies for Characterizing Fatigue Do we need alternative to Beam Fatigue? Can they address endurance limit? Are they implementable? Flexible Pavement Damage Analysis How can we improve flexible pavement damage analysis? What important issues are not currently addressed? Plus/Delta Adjourn DAY TWO Review Agenda/Progress/Issues Strategies for Incorporating Endurance Limit in Flexible Pavement Damage Analysis What are the possible strategies for incorporating Endurance Limit? Identify Barriers to Success? What are the barriers to success in this effort? Identify Countermeasures? What are some countermeasures? Identify Simplifying Assumptions Can we identify some simplifying assumptions that will help? Calibration/Verification Is calibration necessary? How should it be done? Field sections, accelerated pavement testing, etc? Suggest Data Evaluation Approaches What are your suggestions for calibration/verification? Identify Potential Action for NCHRP 9-44 What should be included in the workplan for future research developed in NCHRP 9-44? Silver Bullet Actions Which suggestions from Potential Actions can be addressed in a 3-year project? Recommendations, Findings & Conclusions What are your recommendations, findings & conclusions as a group? Closing ADJOURN Figure 1. HMA Endurance Limit Workshop Agenda. 5

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements 6 1.3.3 Task 3. Identify Data Requirements A major part of Task 3 was the development of a framework for designing pavements to resist bottom initiated fatigue cracking that considers the effects of an endurance limit. The framework was developed to identify specific laboratory studies needed to fully develop the design procedure and the types of pavement test section data needed for the validation. The framework of the design procedure that was developed is based on the following research hypothesis that emanated from the HMA Endurance Limit Workshop. The endurance limit for HMA does not reflect an absence of load induced damage in the HMA. It is the result of a balance of damage caused by loading and healing or damage recovery that occurs during rest periods. Under this hypothesis the primary objective in designing a flexible pavement to resist bottom initiated fatigue cracking is to make sure that the damage induced by loading remains small enough so that healing occurs and there is no accumulation of damage over the life of the pavement. This is a significant departure from current cumulative or incremental damage models, which assume that no healing occurs and that each load cycle uses up a portion of the finite fatigue life of the HMA. A number of approaches for designing pavements to resist bottom initiated fatigue cracking were identified in Task 1. Table 2 briefly summarizes the approaches that were considered. These range from relatively simple modifications of traditional mechanistic-empirical fatigue algorithms to sophisticated finite element models based on damage mechanics and fracture mechanics. The major deficiency of the more practical approaches is that they do not account for the beneficial effects of healing. In the HMA Endurance Limit Workshop, healing was identified as a significant factor affecting the endurance limit in HMA (1). The sophisticated approaches can account for healing, but are not practical at this time for use in routine pavement design. Since an acceptable existing design procedure could not be identified, the framework for a new design procedure was developed. It is based on limiting strains at the bottom of the lowest asphalt bound layer to those that will permit full healing to occur between traffic loads. This approach results in lower allowable strains for conditions that result in less healing: higher traffic volumes and colder temperatures. Chapter 2 includes a description of the framework for the new design procedure.

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements Table 2. Summary of Existing Pavement Analysis Approaches Considered. Approach Key Elements Selected References Advantages Disadvantages Strain Limit Assume fatigue life is infinite at damage levels below the endurance limit. Use Miner’s law for strain levels above the endurance limit. Timm and Young (2) Witczak (3) Thompson and Carpenter (4) Easy to implement in existing M-E design. Compatible with layered elastic analysis used in MEPDG. Does not consider the beneficial effect of rest periods. Relies on Miners law for strains above the endurance limit. Above endurance limit fatigue life of HMA is predefined. Crack Initiation Limit strain level to that causing crack initiation in laboratory fatigue tests. Sidess and Uzan (5) Easy to implement in existing M-E design. Compatible with layered elastic analysis used in MEPDG. Rational basis for design. Does not consider the beneficial effect of rest periods. Relies on Miners law. Cycles to crack initiation are predefined. Strain Limit-Crack Initiation Assume fatigue life is infinite at damage levels below the endurance limit. Use Miner’s law for strain levels above the endurance limit. The endurance limit is estimated from the indirect tensile strength test and is dependent on the modulus of the mixture. Von Quintus (6, 7) Relatively easy to implement in existing M-E based methods. Compatible with layered elastic analysis used in the MEPDG. Value is dependent on the temperature (modulus), and volumetric properties of the mixture. Does not consider the beneficial effect of rest periods. Relies on Miner’s law for strains above the endurance limit. Key property used to estimate endurance limit is highly variable. Recursive Miner’s Law Modify fatigue life of HMA to account for the strength loss of a pavement structure as a function of traffic loading. Tsai, et al., (8) Easy to implement in existing M-E design. Compatible with layered elastic analysis used in MEPDG. Accounts for changes in fatigue life of HMA with traffic. Assumes that HMA fatigue life deteriorates with traffic loading. Does not consider the beneficial effect of rest periods. Visco-Elastic Continuum Damage Model the evolution of damage in a viscoelastic continuum. Mun, et al., (9) Can be used to predict crack initiation. Directly accounts for damage accumulation and healing. Computationally intensive. Not compatible with layered elastic analysis used in MEPDG. Fracture Mechanics Model responses at the crack tip and the propagation of cracks. Roque, et al. (10) Predict crack growth. Requires crack initiation model. Computationally intensive. Not compatible with layered elastic analysis used in MEPDG. 7

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements Five experiments were identified to full develop the design procedure incorporating an HMA endurance limit. Table 3 summarizes the laboratory experiments that are needed. The experiments are briefly described below. Details of these experiments are included in the HMA Endurance Limit Validation Study Research Plan that is presented in Appendix B. Table 3. Summary of Proposed Laboratory Experiments. Experiment Topic Factors 1 Mixture Compositional Factors Affecting Healing in HMA • Binder Type • Binder Age • Effective Binder Content • Air Voids • Design Compaction • Gradation • Filler Content 2 Effect of Applied Strain on Healing • Strain Level • Healing Rate From Experiment 1 3 Effect of Temperature and Rest Period Duration on Healing • Temperature • Rest Period Duration 4 Development of Testing and Analysis Procedures to Determine Allowable Strain Levels • Healing Rate From Experiment 1 • Mixtures From NCHRP 9-38 5 Estimation of Allowable Strain Levels from Mixture Composition • Mix Compositional Factors Affecting Damage Accumulation • Significant Factors From Experiment 1 • Temperature • Rest Period Duration Experiment 1 is a screening study to identify the mixture compositional factors that affect healing and therefore, the allowable strain levels in HMA. The results from this experiment will be used in the remaining experiments. Experiment 2 addresses a major assumption that was made in developing the allowable strain limit procedure, that is, the healing rate is independent of the applied strain level. In this experiment healing rates will be determined using different strain levels. This experiment will be conducted on mixtures from Experiment 1 that have high and low healing rates. Experiment 3 is a study to verify the applicability of time-temperature superposition to healing in HMA. This was the second major assumption included in the development of the allowable strain limit procedure. Experiment 3 will be conducted on a mixture from Experiment 1 that exhibits a moderate healing rate. Testing and analysis methods 8

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements for determining allowable strain limits that result in complete healing will be developed in Experiment 4. This experiment will include testing and analysis of selected mixtures from Experiment 1 and mixtures used in the endurance limit testing completed in NCHRP Project 9- 38. This experiment will generate the Level 1 test procedure for use with a future modified version of the MEPDG. In the last experiment, Experiment 5, a wide range of mixtures will be tested using the methods developed in Experiment 4 to develop predictive models relating the allowable strain limits to mixture compositional factors. This last experiment will generate the relationships between allowable strain and easily measured mixture compositional properties that will be used in calibrating the procedure and thus verifying the endurance limit for HMA. These relationships will also provide the Level 2 and 3 analyses for a future modified version of the MEPDG. 1.3.4 Task 4. Identify Applicable Projects. Task 4 consisted of assessing the usefulness of various field projects, both accelerated pavement tests and in-service pavement sections for use in validating an endurance limit for HMA. Results from accelerated pavement tests can be used to tests critical elements of the framework developed in Task 3. These include the effects of temperature, applied strain, and material properties on the allowable strain levels. The accelerated pavement tests recommended for consideration were: • Fatigue tests conducted during the Superpave validation study at the FHWA Pavement Test Facility (11). • Sections at the NCAT Test Track that have remained in service from the first cycle through the current cycle (12). • Sections from the WesTrack experiment containing mixtures with different composition (13). • Sections from the structural design experiment performed at the NCAT Test Track (14, 15). • Selected sections from the MNRoad project (16). 9

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements The allowable strain limit design procedure will be calibrated and validated using in-service pavement sections. It is important to recognize that the allowable strain limit design procedure is not intended to be a tool for predicting the extent of bottom initiated cracking with time and traffic like the MEPDG fatigue model. Its purpose is to identify design features that minimize the possibility of bottom initiated fatigue cracking. Thus, field calibration of the allowable strain limit design procedure will be easier and likely more precise than the calibration that was completed for the MEPDG fatigue model. Sections from the LTPP program (17) and pavements that have received perpetual pavement awards from the Asphalt Pavement Alliance (18) were considered for use in the calibration and validation. The LTPP sections were selected because these sections have received extensive monitoring over a number of years, and distress, deflection, and material property data are available from the LTPP database (17). Since sufficient sections for the analysis are available from the LTPP program, only these sections were included in the research plan. Table 4 presents the test matrix for using LTPP sections to calibrate and validate the allowable strain limit design procedure. Since the procedure is not intended for prediction of the extent of cracking in a pavement section, but rather as a tool to identify design features to minimize the potential for bottom initiated fatigue cracking, an extremely large data set is not required. The recommended matrix includes a total of 32 pavement sections: 16 not exhibiting alligator cracking and 16 exhibiting low to moderate amounts of alligator cracking. An equal number of sections from the four environmental zones are included in the matrix. Only pavements with HMA thicknesses exceeding 8 inches are included. Subgrade deformation becomes an important consideration in thinner HMA pavements. Simultaneous calibration and validation can be performed on this data set using jackknifing as described in Research Results Digest Number 283 (19). Jackknifing allows the assessment of model accuracy without separating the 32 sections into calibration and validation subsets. Jackknifing is performed by systematically removing one of the sections, calibrating the model using the remaining sections, then predicting the value of the section that was removed. For the section that was removed, the model error is computed as the difference between the predicted and measured values. The process of withholding, calibrating, and determining the error is repeated until each section has been removed. This process produces n values of the error from which the jackknifing goodness of fit statistics can be computed. The 10

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements advantage of jackknifing is the goodness of fit statistics are based on predictions of measurements that are not included in the calibration. They are, therefore, better estimates of the accuracy of future predictions than goodness of fit statistics based on calibration using the full data set. Table 4. Matrix for Field Calibration of the Allowable Strain Limit Design Procedure. Environment HMA Thickness, in No Alligator Cracking Low Alligator Cracking 8 to 12 2 2 Wet Freeze >12 2 2 8 to 12 2 2 Wet No Freeze >12 2 2 8 to 12 2 2 Dry Freeze >12 2 2 8 to 12 2 2 Dry No Freeze >12 2 2 1.3.5 Task 5. Prepare Detailed Work Plan. The HMA Endurance Limit Validation Study Research Plan was prepared in Task 5. The research plan is a comprehensive document describing the research that must be completed to successfully incorporate the concept of an endurance limit for HMA into a fatigue algorithm for bottom initiated fatigue cracking and to validate the resulting procedure using full-scale pavement sections. It includes four major parts. The first is a summary that briefly describes the proposed research and presents overall cost estimates and time requirements. The second part is a description of the required research tasks. This section includes detailed information for each task and subtask, including (1) a description of the work to be performed, (2) preliminary experimental designs when appropriate, (3) a list of milestones related to the task, (4) labor hour estimates, and (5) a listing of pertinent data and reference material that will be needed to accomplish the task. The third part is a detailed schedule for the project. The schedule addresses the sequence of the research tasks and the interactions between tasks. Finally, the fourth part presents the proposed budget for the project. The budget includes detailed estimates of labor and other costs associated with each task and subtask. 11

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements 1.3.6 Task 6. Prepare Final Report. The final project task was the preparation of this final report documenting the work performed in NCHRP Project 9-44. The report was prepared in the format required by NCHRP. It includes the workshop summary and research plan as stand-alone appendices. 12