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REVISED INTERIM REPORT:

CHAPTERS 1 AND 2

Submitted to the

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM

(NCHRP)

For Project NCHRP 1-42: Top-Down Fatigue Cracking of Hot-Mix

Asphalt Layers

LIMITED USE DOCUMENT

This Interim Report is furnished only for review by

members of the NCHRP project panel and is regarded as

fully privileged. Dissemination of information included

herein must be approved by the NCHRP.

From

Advanced Asphalt Technologies, LLC

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ACKNOWLEDGEMENT OF SPONSORSHIP

The work was sponsored by the American Association of State Highway and

Transportation Officials, in cooperation with the Federal Highway Administration,

and was conducted in the National Cooperative Highway Research Program, which

is administered by the Transportation Research Board of the National Research

council.

DISCLAIMER

This is an uncorrected draft as submitted by the research agency. 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

ACKNOWLEDGMENT OF SPONSORSHIP...

i

DISCLAIMER ... i

LIST OF FIGURES ... iii

LIST OF TABLES ...

iv

ACKNOWLEDGMENTS ... v

ABSTRACT... v

SUMMARY OF FINDINGS ... v

1. INTRODUCTION ... 1

RESEARCH PROBLEM... 1 RESEARCH OBJECTIVES ... 2 SCOPE ... 2

2. BACKGROUND ... 8

REVIEW OF LITERATURE AND CURRENT PRACTICE... 8

PRELIMINARY FINDINGS CONCERNING TOP-DOWN CRACKING OF ASPHALT CONCRETE PAVEMENTS ... 94

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LIST OF FIGURES

1. Top Down Cracking in Colorado... 9 2. Transverse Contact Stresses Predicted at Pavement Surface with Pavement

Thickness = 10 cm, E1 = 1,400 MPa, E2 = 300 MPa ... 10

3. Stress Intensity Factors as a Function of Crack Length at Various Distances

from Applied Load... 15 4. Octahedral Shear Stress in an Asphalt Concrete Pavement as a Function of

Depth, at Different Radial Distances from the Applied Load ... 16 5. Shear Stresses as a Function of Depth for a 200 mm-Thick Flexible

Pavement; Rigid and Flexible Loading, with and without a Temperature

Gradient... 18 6. Maximum Surface Tensile Stresses in Flexible Pavements in Michigan, as

Calculated Using Michpave, a Layered Elastic Analysis Software Program ... 24 7. Effect of Segregation on IDT Strengths of HMAC Mixtures in Michigan ... 26 8. Segregation in Hot-Mix Asphalt Pavement Exhibiting Top-Down Cracking ... 26 9. Diagram of Relationship between Slat Conveyors and Typical Location of

Segregation and Top Down Cracks in Colorado ... 27 10. Relationship Between Corrected and Uncorrected IDT Strength, Showing

Regression Line, 95 % Prediction Intervals for New Observations, and d2s

Error Bars for Measured Data... 35 11. IDT Creep and Strength Test ... 35 12. Diagram Illustrating Concept of Dissipated Creep Strain Energy... 38 13. Comparison of Measured |E*| Values and Those Predicted Using the Hirsch

Model; R2 = 98 % ... 52 14. Predicted and Measured |G*| Values for SHRP Core Asphalt Binders, Using

Christensen’s Two Point System and an Assumed Glassy Modulus of 1.0

GPa... 56 15. Estimated Viscosities after Age-Hardening as Estimated Using the

Mirza-Witczak Global Aging System, and as Estimated Using the Proposed

Modification of this System... 60 16. Predicted Aged Binder Complex Modulus Values as a Function of

Temperature ... 61 17. Predicted Aged Binder Complex Modulus Values as a Function of Depth ... 61 18. Comparison of Mixture Compliance Values as Measured Using the IDT

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19. Plot of Failure Envelopes Predicted by Maximum Normal Stress (Maximum Principal Stress), Maximum Shearing Stress and Maximum Distortion

Energy Theories ... 66 20. Plots of Damage And Stress in a 17.8-cm Thick Asphalt Concrete Pavement

under a Non-Uniform 40 kN Load... 69 21. Continuum Damage Analysis of Flexural Fatigue... 75 22. Predicted Damage Modulus Values for VA Limestone Mixture/Fine

Gradation/Optimum Binder Content, at the Conclusion of Flexural Fatigue

Test... 78 23. Predicted Complex Modulus in Tension/Compression Compared with

Measured Flexural Complex Modulus for SHRP Mixtures ... 78 24. Predicted and Observed Values for Continuum Damage Fatigue Constant C2

for SHRP Flexural Fatigue Data and NCHRP Uniaxial Fatigue Data... 80 25. Predicted and Measured Log Cycles to Failure for SHRP Flexural Fatigue

Data, with d2s Confidence Limits ... 81 26. Percent Observed Cracking as a Function of Calculated Damage for

WesTrack Fatigue Experiment ... 83 27. Healing Rate of Asphalt Mixtures Made with Five Different SHRP Core

Binders ... 87

LIST OF TABLES

1. Summary of Research on Top-Down Cracking... 29 2. Merrill’s Summary of Features of Some Layered Elastic Analysis Packages

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ACKNOWLEDGMENTS

This Revised Interim Report presents the work accomplished on NCHRP Project 1-42

through May 25, 2004. Advanced Asphalt Technologies, LLC (AAT), is the prime contractor for this project, and Abatech, Inc. (Abatech) is the only subcontractor. The Principal Investigator for this project is Dr. Donald Christensen of AAT. The effort at Abatech is being performed under the supervision of Dr. Geoff Rowe. This report was primarily compiled by Dr. Christensen, with input and assistance by Dr. Ray Bonaquist of AAT and Dr. Geoff Rowe of Abatech.

ABSTRACT

This Report is a partial summary of the interim results of NCHRP Project 1-42: Top-Down

Fatigue Cracking of Hot-Mix Asphalt Layers. It includes a review of literature; the revised work

plan for Phase II of the project is not included in this abbreviated report. The primary findings of the literature review are that top-down cracking in flexible pavements is primarily caused by traffic-associated fatigue and/or thermal stresses. Top-down cracking is significantly affected by the interaction of many factors, including poor compaction and segregation during construction, pavement structure, modulus gradients within the pavement, age hardening of the pavement surface, moisture damage and mixture fracture toughness.

SUMMARY OF FINDINGS

This Report is a partial summary of the interim results of NCHRP Project 1-42: Top-Down

Fatigue Cracking of Hot-Mix Asphalt Layers. It includes a review of literature, but not a revised

work plan for Phase II of the project. The literature review consists of two components—a review of recent research publications dealing with top-down cracking in asphalt concrete, and the refinement of several models as needed for the proposed Phase II work plan. These models will also be useful to other pavement engineers analyzing top-down cracking and related phenomena in flexible pavements. The following list summarized the findings are made based upon this literature review and associated analysis. For the convenience of the reader, it has been organized according to the four project objectives, each finding being listed under the objective to which it most closely pertains.

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Objective 1. Identify the Mechanisms that govern the initiation and propagation of top-down cracking in HMA layers

• Accumulated damage associated with repeated traffic loading is the primary

mechanism of top-down cracking in asphalt concrete pavements. It is also likely that thermal stresses contribute significantly to this form of distress.

• Both tire-contact surface stresses and shear stresses contribute to top-down cracking. Contact stresses are probably more important in the initiation of top-down cracks, while propagation of these cracks to significant depth in the pavement primarily results from shear stresses.

Objective 2. Identify or develop method(s) of laboratory testing HMA mixtures for determining susceptibility of the HMA surface layer to this cracking

• The most effective means for reducing top-down cracking in asphalt concrete

pavements at this time involve for the most part mix design—top-down cracking can be reduced by ensuring that HMAC mixtures have adequate resistance to

low-temperature cracking, high fracture toughness, and good resistance to moisture damage and age hardening.

• For most pavement design and analysis problems, creep compliance and complex modulus values can be estimated with adequate accuracy using the Hirsch model and associated procedures.

• For evaluating new or unusual material, for forensic studies and for research purposes—when laboratory testing is needed—creep compliance should be

determined using procedures given in AASHTO TP-9, and complex modulus should be determined using dynamic uniaxial compression, as developed in projects NCHRP 9-19 and 9-29.

• The dissipated creep strain energy (DCSE) approach to estimating fracture toughness, as developed by Roque et al., appears to be very promising for evaluating the

resistance of asphalt concrete mixtures to crack propagation, an important aspect of resistance to top-down cracking. However, further evaluation of this concept is needed, and is proposed in the Revised Phase II Work Plan for NCHRP Project 1-42.

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Objective 3. Determine the significant factors associated with the occurrence of top-down fatigue cracking.

• Mixture properties probably affect top-down cracking in many different complex ways. However, it is clear that increasing fracture toughness will reduce the extent of top-down cracking in flexible pavements. Increasing resistance to age hardening and moisture damage will also probably reduce the incidence of top-down cracking. • Increased traffic loading, increasing magnitude of tire stresses, and complex

tire-pavement contact stresses are all likely to increase the incidence of top-down cracking. Traffic loading contributes to top-down cracking not only through contact stresses, but also through shear stresses extending deep into the pavement.

• Pavement age hardening and moisture damage contribute to top-down cracking in asphalt concrete pavements.

• Pavement thickness probably affects the occurrence of top-down cracking, but there is not clear agreement among researchers concerning how pavement thickness affects this form of distress. Stiffness gradients in pavements, arising from binder grade selection, mix design, age hardening and/or temperature gradients, also probably contribute to top-down cracking, but the nature and magnitude of this contribution is not yet clear.

• Segregation during construction, particularly slat-conveyor segregation near the wheel paths, can cause or exacerbate top-down cracking in asphalt concrete pavements.

Objective 4. Identify promising models for predicting the initiation and propagation of top-down cracking.

• In the long term, the best approach to analyzing stresses and strains potentially leading to top-down cracking in flexible pavements is the three-dimensional finite element method. However, for this approach to be practical, it must be possible to perform thousands of analyses in a short period of time—as will be done, for

example, in the parametric study included in the Revised Phase II Work Plan. Three-dimensional finite element analyses cannot yet be performed and interpreted quickly enough to perform such a large number of analyses; to date, most studies using this method have been limited to a few dozen analyses of idealized pavements.

• The effective evaluation of a pavement design and analysis method must involve the calculation of stresses, strains and damage over a period of at least several years, using realistically variable conditions of temperature, asphalt concrete modulus, sub grade stiffness, age hardening, and other factors; the results must then be compared to measured deflections and observed distress to determine if the design and analysis method is accurate. Such evaluations require thousands of separate analyses, and so

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cannot be performed at this time on techniques involving three-dimensional finite element analysis. Therefore, further evaluations of models for top-down cracking that involve 3-dimenional finite element analysis cannot be performed during Phase II of NCHRP Project 1-42.

• Thorough parametric studies of factors effecting top-down cracking—including pavement thickness, binder grade, asphalt concrete composition, sub grade stiffness, segregation and age hardening—also involve thousands of individual analyses. Therefore, three-dimensional finite element methods cannot at this time be used to perform extensive parametric studies of top-down cracking and similar phenomena. • Because three-dimensional finite element methods are not yet practical for

performing the thousands of analyses needed for a thorough parametric study of the factors that potentially contribute to top-down cracking in asphalt concrete

pavements, layered elastic analysis must be used for this purpose in Phase II of NCHRP Project 1-42.

• Pavement engineers and researchers should however continue to develop and refine three-dimensional finite element methods for pavement design and analysis, and associated engineering tools for using the results of these analyses, including

continuum damage theory and computational fracture mechanics. These approaches to pavement design and analysis will probably become practical in about 10 years. • Layered elastic analysis, properly executed, represents an exact closed form solution

involving the rigorous application of mechanics, and is not an approximate method. Some specific implementations of layered elastic theory are simplified, and as a result cannot accurately deal with horizontal surface stresses in flexible pavements; such programs are not suitable for use in evaluating top-down cracking. A thorough comparison of the results of layered elastic analyses and finite element methods has however not been performed and reported on in the literature. Such a study is included in the revised Phase II Work Plan, and should clarify the performance of these methods of pavement analysis.

• Regardless of the method used for stress-strain analysis of flexible pavements, it is essential that the analysis use accurate models for predicting pavement temperature as a function of time and depth, asphalt concrete modulus, strength and/or fracture toughness, the accumulation of fatigue damage, and age hardening. Accurate approaches for estimating these properties have been identified and will be used in Phase II of NCHRP Project 1-42.

• Because the stresses at the surface of a flexible pavement subject to traffic loading are complex, involving both normal and shear stresses, evaluation of surface stresses must involve application of an appropriate failure theory. The most effective such theory appears to be octahedral shear stress theory, because it is suitable for ductile materials, is mathematically direct and has been applied to asphalt concrete

pavements in the past.

• Accurate continuum-damage fatigue equations have been developed and validated by Christensen and Bonaquist, and appear the most effective approach for routine pavement design and analysis, including application to top-down cracking. For cases

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when laboratory testing is needed, uniaxial fatigue testing, analyzed using continuum damage principals, is probably the most efficient and effective procedure for

characterizing the fatigue resistance of asphalt concrete mixtures.

• Ideally, continuum damage theory should be implemented in conjunction with three-dimensional finite element methods to provide an accurate estimate of damage caused by traffic loading and thermal stress; however, as discussed above, three-dimensional finite element methods are not yet practical for routine use; until such methods are available, continuum damage approaches can probably be used with layered elastic analysis to provide reasonable estimates of surface damage and top-down cracking. Christensen and Bonaquist have developed a simple, direct approach to applying continuum damage theory to pavement analysis which can be easily implemented and appears very effective in modeling both uniaxial and flexural laboratory fatigue tests, and in modeling fatigue in the WesTrack project.

• Healing is potentially an important factor in top-down cracking, but no simple and effective models exist for predicting the effects of healing in flexible pavements. It does appear that healing rates can be related to asphalt binder viscosity, a relationship that might be useful in developing practical models for predicting healing rates in asphalt concrete mixtures.

• Ideally, methods for modeling top-down cracking should probably address plastic deformation, in addition to viscoelastic behavior and fatigue damage. There is

evidence that significant plastic deformations can occur in asphalt concrete mixtures, even at low temperatures. As with other types of pavement modeling requiring three-dimensional finite element analysis, visco-elastic-plastic models must await further improvements in computer hardware and software before they become a practical tool, or even before they can be thoroughly evaluated and refined.

• Fracture mechanics is probably useful in modeling some aspects of top-down

cracking, especially the relative resistance of different mixtures to crack propagation, as characterized through fracture toughness. However, computational fracture

mechanics may not be essential in developing practical approaches to modeling the initiation and propagation of top-down cracking in asphalt concrete pavements. Distortional stresses and continuum damage might be adequate for practical modeling of this form of distress, and is much simpler to implement than computational fracture mechanics, which should be regarded for the time being as a research tool and not a procedure for routine use by pavement engineers.

• Thermal stresses probably contribute significantly to top-down cracking. The thermo-viscoelastic analysis developed by Roque et al. is the most widely used and probably the most effective approach to estimating thermally induced stresses in flexible pavements. Stresses estimated in this way can be combined with stresses resulting from traffic loading, through the use of octahedral shear stress, to evaluate

distortional stresses and the resulting damage that are the primary cause of top-down cracking.

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CHAPTER 1.

INTRODUCTION

RESEARCH PROBLEM

The research problem being attacked in NCHRP Project 1-42 is summarized by NCHRP as follows:

Until Recently, load-associated fatigue cracking of hot-mix asphalt (HMA) concrete-surfaced pavement that occur in the wheel path have been thought to always initiate at the bottom of the HMA layer and propagate to the surface... However, recent studies have determined that load-related HMA fatigue cracks can also be initiated at the surface of the pavement and propagate downward through the HMA layer. The penetration of water and other foreign debris into these cracks can further accelerate the propagation of the crack through the HMA surface layer. These studies indicated that environmental conditions, tire-pavement interaction, mixture characteristics, pavement structure, and construction practices are among the factors that influence the occurrence of this cracking. Hypotheses regarding the top-down cracking mechanisms have been suggested; test methods for evaluating HMA mixture susceptibility to cracking have been proposed; and preliminary models for predicting crack initiation and propagation have been developed. However, only limited research has been performed to evaluate and validate these hypotheses, test methods, and models.

Research is needed to address the issues associated with top-down fatigue cracking and to develop guidance for pavement engineers in selecting HMA mixtures and designing flexible pavements.

From “Project 1-42, FY 2003: Top-Down Fatigue Cracking of Hot-Mix Asphalt Layers,” at

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RESEARCH OBJECTIVE

There are four primary objectives to the research discussed in this Interim report. As described in the Research Project Statement (RPS) for NCHRP Project 1-42:

The objectives of this research are to (1) identify the mechanisms that govern the initiation and propagation of top-down fatigue cracking in HMA layers, (2) identify or develop method(s) of laboratory testing of HMA mixtures for determining susceptibility of the HMA surface layer to this cracking, (3) determine the significant factors associated with the occurrence of top-down fatigue cracking, and (4) identify promising models for predicting the

initiation and propagation of top-down cracking.

From “Research Project Statement” for NCHRP Project 1-42.

SCOPE

The work involved in completing NCHRP Project 1-42 has been broken down into seven tasks: • Task 1: Review Literature

• Task 2: Recommend Conceptual Model • Task 3: Refine Phase II Work Plan • Task 4: Prepare Interim Report • Task 5: Perform Testing and Analysis • Task 6: Develop AASHTO Model Standards • Task 7: Develop Validation Plans

• Task 8: Prepare Final Report

At this point, Tasks 1 through 4 have been completed. An initial Interim Report was submitted several months ago and reviewed by the NCHRP Project 1-42 Panel. A panel meeting was held in April 2004, in which the results summarized in the Interim Report were presented and discussed. Several shortcomings in the Interim Report and plans for Phase II of the project were identified by the panel, and are discussed later in this introduction. This Revised Interim Report has been modified in view of this meeting and panel comments received after submission of the initial Interim Report. This introduction is relatively long and detailed, because it is important that several fundamental and important issues

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concerning the overall approach to this project be addressed early in this report, so that the reader has some understanding of these issues while reading the literature review.

This Interim Report is an abbreviated version of the report as initiall submitted to the NCHRP, in that it does not contain a Revised Phase II Work Plan. It contains only two chanpters: (1) Introduction, and (2) Background.

The technical background presented in Chapter 2 includes a review of recent research pertaining top-town cracking in asphalt concrete pavements, and an in-depth technical description of the analytical methods (and major alternatives) proposed for use in Phase II of the project. Details of specific models for estimating the modulus of asphalt binders and asphalt concrete, and for predicting the accumulation of damage during fatigue loading of asphalt concrete mixtures might seem tedious, but these are essential features of any model for predicting top-down cracking and other forms of pavement distress. Unfortunately, the importance of this aspect of pavement design and analysis are often minimized when

researchers focus on developing and evaluating advanced methods of mechanical analysis. It should be common sense among practicing engineers that such techniques will be useless if the fundamental properties of the pavement are not properly modeled. Conversely, layered elastic analysis, although much maligned by pavement researchers, may prove to be much more effective than generally accepted if combined with accurate models for asphalt concrete modulus, accumulation of fatigue damage and age hardening.

1 In understanding the literature review and Revised Phase II Work Plan contained in this report, it is essential to acknowledge that the fourth project objective in the RPS is

“…to…identify promising models for predicting the initiation and propagation of top-down cracking.” This objective does not require the development or refinement of models, or even the thorough evaluation of existing models—only the identification of promising models. Furthermore, the objective uses the plural—models—rather than the singular. Therefore, it is not necessary to identify a single “best” model, but only one or more promising models. Acknowledging these facts is important, because the most promising models for evaluating top-down cracking are at this time in an early stage of development, and thus cannot be

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thoroughly evaluated. Furthermore, such a thorough evaluation would not only require that these models be in a much greater state of refinement than is the case, it would also require a thorough field validation, which is entirely outside the scope of the NCHRP Project 1-42 Research Project Statement (RPS) and the contract documents. It should be clearly stated at this point that the ideal model for predicting top-down cracking should include the following features:

• Computation of stresses and strains in three dimensions using a finite element analysis with a properly defined element types and mesh fineness, with a simple and effective user interface incorporated automated pre- and post-processing • True viscoelastic response (as oppose to the quasi-elastic approach often used in

pavement analysis and design) • Anisotropic material properties • Plastic behavior

• Accumulation of microdamage, most likely through application of continuum damage theory, including an indication of when and where crack initiation is likely to occur

• Modeling of the initial stages of crack propagation (assuming that being able to model the precise behavior of a pavement with substantial, deep cracking is of no value to the practicing engineer)

Although various researchers and engineers have implemented separately all of these features in a variety of studies, the commercial development of such a pavement design and analysis program is still years away. Even proponents of finite element methods (FEM) admit to its shortcomings. Merrill, in his dissertation (which recommended the use of FEM in studying surface cracking) stated “Realistic three-dimensional models can require vast amounts of computation time and storage.” In evaluating his three-dimensional FEM model, Merrill performed a total of 21 analyses using the FEM model, which allowed only limited variation of a few factors (1). In contrast, the more substantial analytical study proposed later in this revised Interim Report includes over 6,000 analyses, and even then does not include as many factors over as wide a range as is ideally needed by engineers studying pavement behavior. Recent research by Myers et al. relied heavily on FEM, but using for the most part a simplified two-dimensional approach, and not three-dimensional analyses, and only

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involved several hundred analyses (2,3). Application of realistic non-linear material models faces similar obstacles. In a recent publication summarizing results of visco-plastic

continuum damage modeling, Chehab and his co-authors presented the results of the

characterization and modeling of a single asphalt concrete mixture (4). This approach could be considered ready for use by practicing engineers if most highway agencies in the United States would agree to use only this mixture in constructing and rehabilitating their

pavements. The authors of this Interim Report believe that this is unlikely to occur.

Computational fracture mechanics involves both three-dimensional FEM and fracture mechanics, with very stringent mesh requirements since accurate modeling of stresses and strains at a crack tip are required. Anderson, in his text on fracture mechanics, makes the following statements in his concluding remarks on computational fracture mechanics (Anderson, 1995):

A numerical fracture simulation of a cracked body can compute crack tip parameters, but such an analysis alone cannot predict when fracture will occur…Fracture can be modeled, but a separate failure criterion is required…Computer simulation of processes such as microcrack nucleation, void growth, and interface fracture should lead to new insights into fracture and damage mechanisms. Such research may then lead to rational failure criteria that can be incorporated into global continuum models of cracked bodies.

T.L. Anderson in Fracture Mechanics: Fundamentals and Applications

In other words, computational fracture mechanics is of limited use in modeling realistic failure processes, but probably can provide useful information for refining more conventional engineering design methods involving continuum mechanics and strength of materials. This is a research tool and not a design method for use by practicing engineers.

These observations concerning the difficulties of three-dimensional finite element analyses, viscoplastic continuum damage modeling, and computational fracture mechanics are not meant to discourage their use and further development by pavement researchers. Indeed, as stated above, a mechanistic approach incorporating most or all of these features

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should prove most accurate in predicting the occurrence and extent of top-down cracking, and will no doubt at some time be incorporated into a nearly completely mechanistic pavement design and analysis approach. However, at this time, such an approach is simply not realistic as a standard method for pavement design and analysis. In fact, because these methods are currently so difficult and time consuming to apply, they cannot be used to evaluate a wide range of realistic pavement systems and make predictions that can be compared with observations. Therefore, these approaches cannot even be effectively compared or evaluated, beyond the simple conclusion that they are theoretically sound features of an ideal model, that must await further development of computer hardware, software and engineering models prior to further evaluation and implementation. In the meantime, pavement researchers should be strongly encouraged to further develop and refine various approaches to three-dimensional FEM, viscoplastic continuum models, continuum damage theory, and computational fracture mechanics.

Although there are significant limitations to these advanced models, Objective 4 of Project 1-42—identification of promising models for predicting the initiation and

propagation of top-down cracking—can still be achieved. In fact, in the statements above, it has already been achieved. What cannot be accomplished is the refinement of these models, or a realistic evaluation and comparison of what they contribute to the effectiveness of a fully mechanistic flexible pavement design and analysis program addressing top-down cracking. Fortunately, this is not included as an objective in the Project 1-42 RPS, and was not contemplated in the proposal submitted by AAT. However, Objectives 1 and 3 of the NCHRP Project 1-42 RPS involve identification of mechanisms governing top-down

cracking, and determination of significant factors associated with the occurrence of top-down cracking in asphalt concrete pavements. Achieving these objectives requires analysis of pavement behavior under a wide range of conditions, including different average

temperatures, different temperature gradients, varying subgrade stiffness values, a range of asphalt concrete compositions, etc. As mentioned above, the analytical experiment included in this Revised Interim Report addresses these issues, and includes over 6,000 separate analyses. The only approach that can be used to perform such a large number of analyses is layered elastic analysis (LEA). There are of course shortcomings to LEA, although a careful

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reading of the pertinent literature shows that for well-designed LEA programs, these

concerns are exaggerated. The primary shortcoming appears to be that LEA programs often do not accurately predict stresses and strains in the top 1 mm (approximately) of the

pavement surface, especially directly under the tire edge. Furthermore, this shortcoming is

apparently not a function of the limitations of LEA per se, but instead is the result of characterizing complex surface loads through a series of discrete circular loads, rather than through a continuous function of horizontal and vertical loads (1). It should be concluded that LEA, properly executed, can be effectively used to study many of the mechanisms and factors affecting top-down cracking, and in any case, is the only available approach that can be used to analyze a realistically broad range of conditions. However, a thorough and careful comparison of LEA and FEM has not yet been conducted—the available comparisons have focused on the very surface of the pavement and have been performed over a limited range of conditions.

Readers should keep in mind that this is an interim report and not a final project report. This is a working document, the purpose of which is to provide clear direction for successful completion of the project, and not to present the final results of the project in a detailed and thoroughly edited format suitable for publication. Although several were weeks were spent addressing as many of the panel members comments as was practical, as with any document, this Interim Report could be improved further by adding more content, performing further revisions and a thorough re-editing. The most important issue to consider at this time,

however, is not whether or not this report is perfect (it is not), or whether it supports the ideas and approaches favored by each panel member (it does not and cannot). This report has been compiled to present the results of a limited review conducted with limited funds and time, and to present an effective plan for completing the balance of NCHRP Project 1-42 and achieving the project objectives, within constraints resulting from the current state of technology in pavement design and analysis, and the limited time and budget allowed for project completion.

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CHAPTER 2.

BACKGROUND

REVIEW OF LITERATURE AND CURRENT PRACTICE

The literature review below has been compiled to provide the necessary technical

background to develop an effective plan for Phase II of NCHRP Project 1-42. The initial part of the review consists of an overview of recent research on top-down cracking in bituminous pavements. This is followed by four sections addressing the four primary objectives of NCHRP Project 1-42: (1) Mechanisms Governing Top-Down Cracking; (2) Factors

Affecting Top-Down Cracking; (3) Laboratory Tests for Evaluating Resistance to Top-Down Cracking; and (4) Modeling Top-Down Cracking in Asphalt Concrete Pavements. To help emphasize the most important aspects of the literature review, each of these sections is followed by a summary of findings. The section on modeling is particularly long and detailed, in order to document procedures proposed for use in Phase II of Project NHCRP 1-42. Chapter 2 concludes with reiteration of the findings concerning top-down cracking in asphalt concrete pavements as presented throughout the literature review. Figure 1 is a photograph of a typical top-down crack, reproduced from a report by Harmelink and Aschenbrenner on top-down cracking in Colorado (5).

Overview of Recent Research

There have been several recent studies on top-down cracking, some of them more-or-less ongoing and encompassing a wide range of issues related to this problem. Most notably Roque and his associates (and former associates) at the University of Florida have published numerous papers and reports on various aspects of top-down cracking in asphalt concrete pavements (6,7,8,9,10,11,12,13,14,15). Their research emphasizes the complexity and importance of stresses generated at the tire-pavement interface and tends to be highly analytical, incorporating relatively advanced methods such as finite element analyses and fracture mechanics (7,8). Figure 2 is a plot of transverse stresses at the tire-pavement interface as estimated by Roque et al. (16). Like many researchers, Roque and associates

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suggest that thermal stresses contribute significantly to the incidence of top-down cracking, though the recent thrust of their research emphasizes the importance of fracture toughness of asphalt concrete in resisting top down cracking (5). Myers et al. have used fracture

mechanics to evaluate the potential effect of various factors on the rate of top-down crack propagation (11,2,3). They found that the driving force in top-down cracks (stress intensity or

K1) increases significantly as stiffness gradients increase (decreasing with depth) in asphalt

pavements. Not surprisingly, they also found that the position of the wheel load relative to the crack and the crack depth also affected stress intensity in top-down cracks. Their research indicated that for Superpave mixtures, fine-graded aggregates will provide pavements better able to resist cracking compared to coarse-graded aggregate blends. These studies are discussed and evaluated in more detail later in this section.

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Figure 2. Transverse Contact Stresses Predicted at Pavement Surface with Pavement Thickness = 10 cm, E1 = 1,400 MPa, E2 = 300 MPa (16).

Svasdisant et al., and Schorsch and Balaldi (researchers at Michigan State University) have published a number of papers and reports on top-down cracking in Michigan (17,18). Their research has been more applied in nature compared to that of Roque’s group. They have used layered elastic analysis and some finite element analysis and have not considered tire-pavement interfacial stresses (17). The Michigan State researchers have stressed the importance of segregation and moisture damage in exacerbating top-down cracking and have relied mostly upon empirical models to characterize this problem (18). In some ways, the efforts of the Michigan State group have been similar to those in Colorado, where a recently published report indicated that top-down cracking in that state is largely the result of

segregation (5). The Colorado study included no pavement analysis or analysis of mixture materials properties, but focused almost entirely on segregation as caused by specific design flaws in most paving machines.

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There has been a wide range of smaller, more theoretical studies dealing with issues related to top-down cracking. Stolarski and his associates at the University of Minnesota have performed relatively advanced analyses on the stresses developed at the pavement surface in both intact and cracked structures (19,20). The results are unfortunately difficult to interpret in practical ways meaningful to pavement engineers. Wang and his co-authors have performed an interesting analysis using micro mechanics, which produced somewhat unusual results in that it demonstrated the importance of shear stresses in top-down cracking, whereas most other studies have focused exclusively on tensile stresses (21).

Top-down cracking has also been observed in Washington State and in the well-known MnROAD study (22,23,24,25). Research in Washington has been along the lines of a survey of the frequency and nature of top-down cracking, without developing new theories or analyses to better understand the problem. One important finding in Washington was that both segregation and moisture damage appear to contribute to top-down cracking (22). No research reports or papers have been published on the top-down cracking at MnROAD, though several brief presentations have been made. In this test road facility, the cracking appears to be partly related to pavement age hardening and partly related to traffic loading. Cracking is more severe for sections made using a harder binder (24,25). An interesting research project in Kenya demonstrated the importance of extreme age hardening at the pavement surface in the occurrence of top-down cracking. Such age hardening has been found in several other research projects and can cause very pronounced stiffness gradients in the pavements, which can increase the magnitude of stresses at the pavement surface (26).

Mechanisms Governing Top-Down Cracking

The literature reviewed to date provides significant insight into the nature of top-down cracking. It is not surprising that asphalt concrete pavements exhibit top-down cracking. The surfaces of flexible pavements are subject to heavy, moving traffic loads that create

significant distortional (shear and tensile) stresses at and near the pavement surface. The surface is also subject to solar radiation, age hardening through oxidation, and moisture damage. What is surprising is that this mode of distress has not been acknowledged earlier.

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This may be in part do the emphasis placed in this country on traditional pavement design methods that address fatigue damage exclusively through maximum tensile strains at the bottom of the bound layers in asphalt concrete pavements. It is also quite possible that recent changes in the binder grades and aggregate gradations have increased the frequency and severity of surface initiated cracking in asphalt concrete pavements.

Because top-down cracking usually occurs in or near the wheel paths in a pavement, traffic-associated fatigue must play an important role in the process. A reasonable hypothesis is that repeated loading degrades the stiffness and/or strength of the HMAC within and adjacent to the wheel paths. Stresses and strains near the pavement surface are very irregular and are concentrated in localized areas because of the nature of tire-pavement stresses; this can focus and accelerate damage in certain areas of the wheel path. Furthermore, recent laboratory studies of the manner in which asphalt concrete modulus decreases under fatigue loads suggests that this degradation probably occurs very rapid during initial loading (27). This initial rapid fatigue damage is not apparent in bottom-up cracking simply because bottom-initiated fatigue damage isn’t visible until it has progressed the entire way through the pavement. It should also be kept in mind that the temperature and temperature gradients at the underside of a flexible pavement are often much different than those at the top, and this might result in an increased potential for damage at the pavement surface even when

distortional stresses are lower at this location.

During construction of an asphalt concrete pavement, the top several millimeters of the wearing course cannot normally be thoroughly compacted, even under the best conditions and with the most careful workmanship. In many cases, conditions and workmanship are not optimal and so the surface of the pavement suffers even more. This results, in many cases, in a relatively weak, porous pavement surface, prone to age hardening and moisture damage. Segregation during construction, often along longitudinal lines defined by details of the paver design, creates weak spots in the pavement where age hardening and moisture damage can reach more quickly and deeply into the pavement. Another source of distress at the pavement surface is thermal fatigue—repeated thermally-induced tensile stresses at the pavement surface that are not large enough to cause sudden thermal cracks, but which can gradually

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damage the pavement and contribute to surface induced cracking. Thus, over a period of several years, traffic-associated fatigue—accelerated by stress concentrations at the tire-pavement interface, along with age hardening, moisture damage and thermal stresses gradually, but significantly weaken the material in and near the wheel paths of flexible pavements.

Ultimately, top-down cracking must occur when the stresses at or near the pavement surface exceed the strength of the material. It is almost common sense that the same source of stresses causing degradation of the pavement surface can at some point cause critical stresses initiating cracks at the pavement surface. Surface cracks in asphalt concrete

pavements must be initiated by stresses caused by traffic loading, temperature changes, or by some combination of these factors. In evaluating thermal cracking and traditional bottom-up cracking, failure is determined when maximum tensile stresses exceed the tensile strength of the asphalt concrete. However, the situation is more complicated where top-down cracking is being analyzed, since the stresses induced at the surface of the pavement involve both normal and shearing components. A failure theory suitable for complex states of stress must be applied in this case. The von Mises failure criteria, as implemented through octahedral shear stress, is such a failure theory and has been successfully applied to asphalt concrete (28,29). The octahedral shear stress theory can be thought of as a different but equivalent formulation of the distortion energy failure theory (28). This failure theory and the manner in which it is proposed for use in Phase II of NCHRP Project 1-42 is discussed in more detail later in this chapter.

Although failure criteria such as the von Mises theory are useful in predicting crack initiation, they will not necessarily relate to crack propagation rates under fatigue loading. Recently Myers et al. have applied finite element analyses to calculate stress intensity factors for surface cracks in flexible pavements under various conditions (11,2,3). The stress

intensity factor, K is a parameter that describes the driving force for a specific crack, and is a complex function of the crack geometry (position, length), the magnitude and geometry of the applied load, and the geometry of the loaded object. Although not a direct indicator of crack propagation rate, for a given material propagation rates should be directly related to K:

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as stress intensity increases, rate of propagation should also increase. Myers and Roque provided rough estimates of crack propagation rates based upon these calculated stress intensity factors and the results of fatigue/crack growth tests performed the IDT geometry. These results have not been verified, and the rates seem very high, ranging from 9 to 390 mm/10,000 loading cycles (11). To further complicate matters, there are three modes of cracking: tensile opening (mode I), in-plane shear (mode II), and out-of-plane shear (mode III). Myers found that mode I stress intensity factors (KI values) were greater than mode II

(KII) values at a 625 mm distance from the wheel load for a variety of simple pavement

structures, suggesting that tensile opening is the dominant fracture mode in surface cracking of flexible pavements. It should however be expected that KII values would be greatest

directly under the edge of the wheel load (and still large in adjacent areas), where shearing stresses are the greatest. Additional efforts should be made to evaluate the magnitude of both

KII and KIII values near the edge of a wheel load to fully understand the relative importance

of the three models of fracture in surface cracking. Furthermore, subsequent work should recognize that the driving force in a propagating crack under mixed mode fracture is a function of all three stress intensity factors—not simply the largest.

The initial work of Myers et al. indicated that the greatest stress intensity factors for short cracks exist under the widest tire rib near the pavement surface (2). This suggests that surface cracks are often initiated in this location. However, KI values nearly as large as these were

found to exist for longer cracks at some distance from the applied wheel load. This initial work was however performed for uniform modulus values through the bound layer in the pavement. Later work include several modulus gradients, and showed much higher KI values

for cracks 20-25 mm in length at a distance of from 625 to 750 mm from the applied load (3). Stress intensity values for surface cracks in flexible pavements were found to be a complex function of crack length, load position and modulus gradients. Myers et al. make several important conclusions based on this research. Temperature and modulus gradients appear to be critical to the initiation and propagation of surface cracks. Furthermore, the driving force in surface cracks will vary depending upon the position of the applied load relative to the crack and the crack length, and these factors should be considered in pavement design and management. For example, a surface crack might initiate and grow relatively quickly to a

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certain depth, and then stabilize for a time. Maintenance to prevent further growth at this point would be unnecessary; crack repair would be more efficiently delayed until the crack reached a length at which more rapid propagation is expected (3). Figure 3 is a plot taken from the initial research publication (2).

Figure 3. Stress Intensity Factors as a Function of Crack Length at Various Distances from Applied Load (2).

Myers et al. stated in their initial study that distortion energy theory (or the equivalent octahedral shear stress theory) could not be applied to surface cracking because it does not predict significant distortional stresses deeper than about 12.5 mm into the pavement. To evaluate this hypothesis, octahedral shear stresses were calculated for a pavement identical in structure to that used by Myers et al. in this study: 200-mm in thickness, with an HMAC modulus of 5,500 MPa and a subgrade modulus of 140 MPa; a tire loaded under 24 kN at 690 kPa pressure was assumed. Layered elastic analysis was used. The octahedral shear stress values were calculated at depths equivalent to the crack lengths used by Myers et al.

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(6.2, 12.5, 19.1, 25.4, and 37.5 mm), and at distances from the tire center of 0, 0.8, 1, 1.2, 1.5, 2 and 4 times the tire radius of 10.6 cm. The results are shown graphically in Figure 4. Contrary to the hypothesis by Myers et al., the octahedral shear stress directly under the edge of the tire increases rapidly to a value of about 280 kPa and remains nearly constant down to the maximum depth evaluated of 37.5 mm. Furthermore, the magnitude of this stress is substantial; Anderson et al. reported an average IDT strength of 1,100 kPa (160 lb/in2) for mixtures made with 12 widely differing binders at temperatures of 4.4 and 15.5 °C. (30) This corresponds to an allowable octahedral shear stress of 520 kPa (75 lb/in2), so the maximum octahedral shear stress, which extends undiminished through much of the pavement, is over 50 % of the allowable stress. It should also be noted that shear failure under the edge of the tire is entirely consistent with the observation that top-down cracking tends to occur near the edges of wheel paths.

0

100

200

300

400

0

10

20

30

40

50

Depth from Surface, mm

O

c

ta

h

e

d

ral

S

h

ear

S

tr

ess,

kP

a

1.0r

4.0r

2.0r

1.5r

0.8r

1.2r

0.0r

Figure 4. Octahedral Shear Stress in an Asphalt Concrete Pavement as a Function of Depth, at Different Radial Distances from the Applied Load (thickness = 200 mm, E1 =

5,500 MPa, E2 = 140 MPa, P = 24 kN, p = 690 kPa).

The source of the disagreement between this analysis and the findings by Myers et al. is not clear. It may be the result of their assumption that surface cracks are initiated and

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propagated solely by tensile stresses, which only exist at the pavement surface. Figure 5 is taken from the initial work by Myers et al. on surface cracking (6); note that this figure also shows that the shear stresses under the wheel edge in flexible pavements are quite large and persist deep into the pavement, and so are in agreement with the analysis presented in Figure 4 above. However, Myers et al. discount the importance of these shear stresses, commenting that assumption of a perfectly rigid circular load exaggerates shear stresses, and that field observations show crack opening inconsistent with shear failure. The first of these

observations is unclear, since Figure 5 includes both rigid and flexible tire loads, and in any case is supposition not supported by analysis or observation. It should be expected that shear cracks in asphalt pavement will open, for a variety of reasons: aggregate particles within the crack sliding past one-another and forcing the crack to open; water within the crack being subjected to hydraulic pressure under traffic loads; ice within the crack expanding (even in Florida, occasionally), thermal stresses; and shrinkage of the asphalt concrete surface due to loss of volatiles and absorption of asphalt binder by the aggregate. The importance of shear stresses in the formation and growth of top-down cracks should not be neglected. The

magnitude of these stresses is easily large enough to cause failure under repeated loading and is in a location consistent with the observed location of top-down cracks. Furthermore, large tensile stresses are produced at the tire-pavement interface in this same location, further contributing to the probability of cracks initiating here and propagating through shear

stresses. It is hypothesized that top-down cracking is initiated through the combined action of tire-pavement stresses and shear stresses, but are propagated deeper into the pavement

largely by the action of high shear stresses under the edge of a tire. Propagation of top-down cracks by this mechanisms probably means that computational fracture mechanics is not necessary for engineering analyses of this type of distress, which is fortunate because this is an extremely difficult and time consuming technique, best used as a research tool for

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Figure 5. Shear Stresses as a Function of Depth for a 200 mm-Thick Flexible Pavement; Rigid and Flexible Loading, with and without a Temperature Gradient (6).

Based upon the literature reviewed to date, the following findings are made concerning the mechanisms likely causing top down cracking:

• Accumulated damage associated with repeated traffic loading is the primary mechanism of top-down cracking in asphalt concrete pavements.

• Both tire-contact surface stresses and shear stresses contribute to top-down cracking. Contact stresses are probably more important in the initiation of top-down cracks, while propagation of these cracks to significant depth in the pavement primarily results from shear stresses.

• Thermal stresses can contribute significantly to the occurrence of top-down cracking.

• Computational fracture mechanics is a useful research tool for improving pavement design and analysis procedures, but is probably neither necessary or appropriate as a standard design tool for use in designing pavements to resist top down cracking.

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Factors Contributing to Top-Down Cracking

Although there is some disagreement among researchers who have examined top-down cracking in flexible pavements, there are many points of agreement. Factors contributing to top-down cracking can be categorized as related to mixture properties, such as mixture stiffness or moisture resistance; load-related factors, such as traffic level and tire-pavement contact stresses; environmental factors such as age hardening and moisture damage;

structural factors, including stiffness gradients and pavement thickness; and construction factors, such as poor compaction and segregation. The discussion below addresses factors identified in the literature that potentially contribute in a significant way to top-down cracking in asphalt concrete pavements.

Mixture Properties

The most commonly cited mixture property contributing to top-down cracking cited by researchers is poor low-temperature properties, although this factor is generally inferred rather than stated directly. Many researchers have identified thermal stress as a contributing factor to top-down cracking and so poor low-temperature properties—excessive stiffness and poor strength—should be expected to contribute to top-down cracking (6,26,31). Researchers in Michigan have pointed out that moisture damage can also contribute to top-down cracking and so poor resistance to such moisture damage is possibly an important contributing factor to top-down cracking (18). Another mixture property indirectly identified as contributing to top-down cracking is poor compactibility, since poor compaction can lead to the high air voids at the pavement surface, identified by several researchers as causing increased age hardening and moisture damage, which then increase the likelihood of top down cracking (17,25,26,31). Implied in many analyses is that mixture modulus as a function of temperature and loading time or frequency must have an indirect but substantial influence on top-down cracking.

Load-Related Factors

There are two primary load-related factors that can potentially affect the occurrence of top-down cracking: overall traffic level, and tire-pavement contact stresses. As with any

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fatigue phenomenon, the extent of damage caused by repeated loads is a function both of the magnitude of the load and the frequency. Matsuno and Nishizawa reported that top-down cracking in asphalt concrete pavements in Japan mostly occurred only on roads with high traffic volumes (32). In current pavement design methods, much use is made of equivalent single axle loads (ESALs), which represent the total number of 80 kN axles estimated to cause an amount of fatigue damage equivalent to that caused by the actual number and type of vehicles crossing a pavement during a given period of time. Newer approaches for

characterizing traffic for pavement design purposes use the concept of load spectrums, where the traffic loading is characterized by its frequency distribution relative to axle load. In either case, the damage caused by the traffic is generally characterized through fatigue models, which are discussed later in this Chapter. A newer, less thoroughly developed approach to modeling damage to pavements subject to traffic loading involves application of fracture mechanics to estimate the rate of growth of cracks in a pavement. This approach is also discussed later in this Chapter.

As discussed previously, Myers and Roque have demonstrated that significant tire-contact stresses exist in flexible pavements, and that these stresses are easily large enough to account for surface crack initiation (6,7,8). In a rigorous and enlightening analytical study applying three-dimensional finite element analysis and continuum damage mechanics, Mun has shown that non-uniform tire pavement contact stresses significantly increase surface damage in flexible pavements (33). However, tire-pavement contact stresses appear to

dissipate very rapidly within the pavement, so surface cracks that extend to significant depths into the pavement are probably the result of both surface contact stresses and shear stresses. In Mun’s study, plots of shear stress throughout the pavement cross section demonstrate the significance of their contribution to top-down cracking, reinforcing the suggestion made earlier in this report that surface stresses alone cannot adequately explain top-down cracking (33). It should be understood that tire-pavement contact stresses, though perhaps similar in magnitude for a selected class of vehicle and axle type, vary considerably depending upon the type of tire (radial vs.bias-ply) and the exact configuration of treads (2,3). In many studies, one or two representative tire types are selected for detailed analysis. An efficient

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method for accounting for tire-contact stress variability in a practical pavement design and analysis procedure has not yet been developed (or even proposed).

Environmental Factors

Three primary environmental factors contributing to top down cracking recur in the studies reviewed to date: thermal stresses, age hardening and moisture damage. Many researchers have agreed that there appears to be a thermal stress component to top down cracking (6,26,31). There is also widespread agreement that severe age hardening at the pavement surface appears necessary before significant top down cracking can occur (17,25,26,31). This age hardening, in essence, causes a thin and relatively brittle “skin” to form over the pavement surface, which is potentially subject to high stresses and subsequent surface cracking. Matsuno and Nishizawa, in their study of top-down cracking in Japan, found that such cracking usually did not occur in pavement sections shaded by overpasses (32). Matsuno and Nishizawa attributed the increased cracking of sunlit sections to elevated temperatures producing a softer, weaker pavement surface. However, their analysis neglected the effects of age hardening—as pointed out by Merrill, it is likely that the shaded pavement sections did not age harden as much as the sunlit ones, and hence the surface of these

pavements was probably more ductile and resistant to cracking (1). The decreases in top-down cracking caused by age hardening is made worse by moisture damage, which can weaken the already embrittled pavement surface (18). Interesting research by Novak et al. has shown that significant pore pressures can be generated in saturated pavements under the action of traffic loading, and that these pore pressures can extend to significant depths into the pavement (34). They found that the magnitude of these stresses increase with increasing mixture permeability, emphasizing the importance of achieving low permeability values in pavement surfaces through thorough compaction. Although this research was somewhat preliminary in nature—the analysis was only two-dimensional in nature and was not verified by comparison with measured pore pressures, common sense suggests that pore pressure and hydraulic stresses in some situations can contribute to top-down cracking.

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Pavement Structure and Related Issues

Some engineers examining top down cracking have concluded that pavement thickness does not have a large effect on this mode of distress (6). Researchers in Michigan however concluded that pavement thickness would affect the magnitude of surface tensile stresses, depending upon the stiffness of the underlying subgrade (17). Figure 6, taken from their report, illustrates this finding; in the analysis represented in this figure, the 3 inch thick pavement exhibited significantly greater surface tensile stress than the 6- or 9-inch thick pavements, which essentially exhibited an equal level of tensile stress. That the two thicker pavements exhibited essentially identical tensile stresses does not change the fact that the stresses in the 3 inch pavement were substantially greater than those observed in the others. A conclusion that pavement thickness affects the magnitude of surface stresses does not mean that this relationship is consistent or simple in nature, or that a change in pavement thickness always results in a change in surface stresses. It only means that altering pavement thickness will frequently result in a change in surface tensile stresses in a pavement. It might be tempting to conclude from Figure 6 that the effect of pavement thickness on pavement surface stresses disappears for thicker pavements, but it should be remembered that this represents only one of many analyses performed by the Michigan State team (space

precludes presenting all of their research results), and the Michigan State researchers made no claim that this pavement thickness effect disappeared or lessened for thicker pavements. Although data from the MnROAD project has not been thoroughly analyzed, it seems to suggest that top-down cracking is more severe in thinner pavement sections (25). As noted earlier, a study in Washington State found that top down cracking dominated in pavements greater than 160 mm thick, while for thinner pavements, cracking was full depth (35). Mun’s study, cited several times previously, found that surface damage increases with increasing pavement thickness, and is also influenced by the stiffness of the base course, but not the stiffness of the wearing course or sub grade (33). In performing a variety of layered elastic and finite element analyses of idealized pavements, Merrill found that surface tensile stresses were greater for thicker pavements (1). However, as pointed out by the author of that study, this does not mean that top down cracking did not occur in the thinner pavements, only that, if it did, it was probably occurring simultaneously with bottom-up cracking. In other words, thicker pavements appear to significantly reduce or even eliminate the occurrence of

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bottom-up fatigue cracking, while not significantly reducing the incidence of top-down cracking. Therefore, as highway agencies have moved towards thicker flexible pavements, top-down cracking has in many cases become the predominant mode of distress.

A primary structural factor that has been cited in most studies as contributing to top-down cracking is the existence of stiffness gradients in flexible pavements (17,22,26,31). These gradients have three different causes: age hardening, temperature gradients and structural design (binder grade selection). Age hardening is perhaps the most important cause of gradients. When a flexible pavement ages, the pavement surface is exposed to sunlight, air and water to a much greater extent than the underlying material. Several researchers have found extreme age hardening in the material recovered from the pavement surface, which then dramatically decreases with depth (26,31). Michigan researchers furthermore have established that the surface of aged pavements exhibited significantly lower tensile strengths (17). Temperature gradients also produce gradients in mixture stiffness, since changes in temperature will affect the modulus of a mixture. On hot summer afternoons, for example, the pavement surface can exceed 60 °C, while the material at the bottom of the bound material will be much cooler. Similar gradients can occur in the winter, but in opposite directions. Many researchers have indicated that the critical gradient for top-down cracking involves a high modulus at the surface and rapidly deceasing modulus with depth. However, Matsuno and Nishizawa have presented convincing arguments that at least in some cases surface cracking is exacerbated by the opposite condition—a low modulus at the surface with increasing modulus with depth (32). These researchers agreed with most other researchers in finding that age hardening of the pavement surface probably contributes to top-down

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Figure 6. Maximum Surface Tensile Stresses in Flexible Pavements in Michigan, as Calculated Using Michpave, a Layered Elastic Analysis Software Program (17).

Unfortunately, none of the research reviewed to date contains specific information on measured or estimated temperature gradients. The combination of age hardening and temperature gradients could create extreme, complex gradients. For example, on summer afternoons, the material near the surface of a pavement could become quite soft because of high temperatures, while the pavement surface itself—because of the extreme age

hardening—might still be relatively stiff. Such conditions might lead to very high tensile and shear stresses under traffic loading that could cause top-down cracking. The third cause of gradients in flexible pavements can be the design of the pavement itself—primarily the binder grade selection (17). In the Superpave system of mix design, engineers are encouraged to use binders of lower performance in base course mixtures. For example, a typical

Superpave pavement might contain a PG 76-22 in the wearing course mixture, a PG 64-22 in an intermediate course mixture and a PG 58-28 in the base course mixture. In a realistic analysis of surface stresses and strains in flexible pavements, all such sources of gradients

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should be considered, in a range of combinations, so that specific conditions leading to the most critical conditions for surface cracking can be identified.

Construction Problems

The two construction-related factors that most researchers have identified as contributing to top-down cracking have been mentioned previously: poor compaction at the pavement surface and segregation. These factors contribute to top-down cracking in a number of significant ways. Poor compaction at the pavement surface will render the pavement more prone to both age hardening and moisture damage and will also weaken the pavement surface. Segregation will cause local weak spots in the pavement, which will become even weaker with age, as they will be sights prone to more rapid age hardening and moisture damage. In studies in Colorado and Michigan, top-down cracking was found most often to originate in segregated areas of the pavement (18,5). However, even in the Colorado study, 33 % of the observed top-down cracks occurred in areas where segregation was not visible. Therefore, segregation should be considered a significant contributing factor to top-down cracking, but not the only cause. If the surface of a pavement gradually hardens and becomes weak and brittle, the combination of traffic stresses and thermal stresses will at some point cause surface cracking. If segregation is present, it is only logical that such cracking will occur first at these points. Reducing the incidence of segregation in flexible pavements will probably increase the time until cracking occurs and will probably also reduce the extent of top-down cracking, but it will likely not entirely eliminate it. Figure 7 shows the effect of segregation on tensile strength and moisture resistance as evaluated in the Michigan study (18). Figure 8 shows mixture segregation at the point of a top-down crack as found in the Colorado study; Figure 9 illustrates the relationship among top-down cracking, segregation and the location of slat conveyors on the paving machine (5). The observed relationship between segregation and slat conveyor location indicates that some manufacturers may need to evaluate and refine their designs for paving machines to eliminate, or at least reduce segregation.

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Figure 7. Effect of Segregation on IDT Strengths of HMAC Mixtures in Michigan (18).

Figure 8. Segregation in Hot-Mix Asphalt Pavement Exhibiting Top-Down Cracking (5).

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Figure 9. Diagram of Relationship between Slat Conveyors and Typical Location of Segregation and Top Down Cracks in Colorado (5).

In the Revised Phase II Work Plan, one of the factors included in the proposed parametric study is segregated vs. non-segregated mixture. The “segregated” mixtures in this experiment will in this case be altered to reflect compositional changes typical of segregation, as reported

References

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