a fatigue endurance limit for highway and airport pavements trb 2003-001428

Carpenter, Ghuzlan, and Shen 1

A Fatigue Endurance Limit for Highway and Airport Pavements

Samuel H. Carpenter Civil and Environmental Engineering Department University of Illinois at Urbana-Champaign

1206 Newmark CE Lab 205 N. Mathews Avenue Urbana, IL 61801 (217) 333-4188 [email protected]

Khalid A. Ghuzlan California Department of Transportation (Caltrans) District 4 9th Floor / Design North Counties 111 Grand Ave. 94612 Oakland, CA 94623-0660 (510)-622-8652 [email protected]

Shihui Shen Civil and Environmental Engineering Department

University of Illinois at Urbana-Champaign 1206 Newmark CE Lab

205 N. Mathews Avenue Urbana, IL 61801

(217) 244-6064

DRAFT - Paper No 03-3428 Not for publication Submitted for Possible Presentation and publication at the Annual Meeting of the Transportation Research Board, January, 2003

TRB 2003 Annual Meeting CD-ROM Paper revised from original submittal.


ABSTRACT The existence of a fatigue endurance limit has been postulated for a considerable time. With the increasing emphasis on Extended Life Hot Mix Asphalt Pavements (ELHMAP), or perpetual pavements (PP) the verification of the existence of this endurance limit, a strain below which none or very little fatigue damage develops, has become a substantial consideration in the design of these new multi-layered full depth pavements. This paper presents fatigue data collected on a surface and binder mixture that were tested for an extended period from 5 to 48 million load repetitions at strain levels down to 70 micro strain. The fatigue results are analyzed in the traditional manner, and using the dissipated energy ratio (DER). This analysis shows that there is a difference in the data at normal strain levels recommended for fatigue testing and at the low strain levels. This difference cannot substantiate an endurance limit using traditional analyses procedures, but the dissipated energy approach clearly shows that there is a distinct change in material behavior at low flexural strain levels that supports the fact that at low strain levels the damage accumulated from each load cycle is disproportionately less than what is predicted from extrapolations of normal strain level fatigue testing which may be attributed to the healing process. The conclusion of this study is that lab testing can verify the existence of a fatigue endurance limit in the range of 90 to 70 micro strain, below which the fatigue life of the mixture is significantly extended relative to normal design considerations.

Key Words: Fatigue, Dissipated Energy, Endurance Limit, Healing, Low-Strain

Word Count: 4000 Figure Count: 8 @ 250 - 2000 Table Count: 2 @ 250 - 500 Total Count: 6500



A Fatigue Endurance Limit for Highway and Airport Pavements INTRODUCTION Figure 1 illustrates two typical fatigue curves for a surface and binder mix as developed using the current AASHTO procedure(1) from the SHRP program(2). Curves such as these represent the essential design approach for asphalt pavements, given suitable field calibration to account for pulse duration differences, rest periods between loads, and structural interactions that cannot be duplicated efficiently in the laboratory. Fatigue data of this nature produce the coefficients in many of the fatigue models used in thickness design. While there are many model forms, a common representation is: Nf = K1(1/)K2 (1) Where: Nf is the number of load cycles to failure is the flexural tensile strain in the beam K1 and K2 are the fatigue coefficients (slope and intercept) This fatigue algorithm or any of the various formulations provide a linear relationship between log of tensile strain and log of load repetitions shown in Figure 1.

This relationship has been validated by any number of studies and is accepted as common for pavement design considerations when suitable field calibrations are applied. The calibrations have varied from 6 to 70 and even higher and reflect differences between field and lab test conditions (3). As such, this form of the fatigue model provides a simple and consistent means of comparing the differences in fatigue behavior of mixtures tested in the laboratory, and is used here.

The data used to prepare the relation shown in Equation 1 do not establish or provide support for the existence of a fatigue endurance limit, a concept that has been postulated for a considerable time. With the increasing emphasis on Extended Life Hot Mix Asphalt Pavements (ELHMAP), or perpetual pavements, the verification of the existence of this endurance limit, a strain below which none or very little fatigue damage develops, has become a substantial consideration in the design of these new multi-layered full depth pavements.

The following sections of this paper will discuss the ELHMAP concept and the impact that a fatigue endurance limit would have on the structural design of these pavements. Data will be provided for several typical mixtures demonstrating that the fatigue endurance limit does exist and that further research is clearly warranted to determine its precise value and the variables which might affect its magnitude. For this study, mixtures that were sampled from the truck as part of QC sampling were reheated and compacted in the laboratory for testing in flexural fatigue to provide data for this study.



y = 0.0037x-0.1823

R2 = 0.9828

y = 0.0048x-0.228

R2 = 0.9504

0.0001

0.001

0.01

100 1000 10000 100000 1000000 1E+07

Load Repetitions

Tens

ile S

trai

n

Mix 64

Mix 6

Figure 1. Traditional Flexural Fatigue Curves for Two Mixtures Investigated in This Study. EXTENDED LIFE PAVEMENTS A new concept for asphalt pavements that has generated a serious amount of interest in recent years is the ELHMAP. This pavement is also termed a perpetual pavement (4). The design philosophy is to construct a structural section that will not fail with a premium surface material that can be easily rehabilitated as needed. A typical cross section is shown in Figure 2. The ELHMAP concept utilizes asphalt mixtures in three layers that are engineered for the performance required of each layer. The surface layer must be highly rut resistant and the bottom layer must be highly fatigue resistant. The inner layer is typically a standard high quality rut resistant binder mix. Fatigue resistance is enhanced by increasing the asphalt content of the bottom layer, termed a rich bottom layer by increasing the design asphalt content 0.5 percent and lowering the air voids to two percent. This combination produces an approximate no change in the modulus of the layer. Structural reductions in thickness will result primarily from any modification to the fatigue algorithm, the K1 and K2 coefficients, resulting from the mix changes. The increase in asphalt content can add to stripping resistance in the bottom layer, contributing to reliability of the design. This layered approach produces distinctly different modulus values for each layer which is a departure from traditional full depth designs where the surface layer is the significantly different layer. While this raises questions regarding the structural design relative to a traditional full depth, the only difference comes in the differing modulus values impact on the tensile strain at the bottom of the rich bottom layer coupled with an improved fatigue algorithm.



Highly Rut Resistant Mixture

Typical Binder or Base Mixture

Binder or Base Mixture with Rich Asphalt Content

Treated Subgrade or Working Platform

Extended Life Hot Mix Asphalt Pavement Figure 2. Typical Cross Section for an Extended Life Hot Mix Asphalt Pavement. MATERIALS TESTED Ten mixtures have been tested to provide fatigue data to be used in investigating the existence of a fatigue endurance limit. These mixtures were sampled from the truck as part of the QC/QA sampling during construction. The mixtures were transported to the laboratory and reheated and compacted to 7 and 4 percent air voids in a rolling wheel compactor. The ten mixtures were part of the total of 79 mixtures tested by Ghuzlan(6). The properties of the mixtures presented here are given in Table 1. All mixtures are Illinois DOT mixtures using neat binders and limestone or dolomite coarse aggregates with manufactured sands. Testing was performed on an Industrial Process Control (IPC) fatigue machine (6) in a temperature controlled cabinet meeting the AASHTO requirements (1). Table 1. Mixture Properties for Fatigue Testing

MIX 6 64 5 7 8-7 8-4 9 17 21 11 20 Asphalt Content

5.4 4.6 4.6 5.3 5.3 5.3 5.4 5.7 4.2 5.7 5.0

Nominal Maximum Size, mm

9.5 19 19 19 9.5 9.5 9.5 9.5 19 12.5 12.5

Percent Passing 4.75 mm

58 36 36 39 53 53 63 62 40 50 49

Percent Passing 0.075 mm

4.5 3.9 4.0 3.9 4.6 4.6 5.2 4.2 4.2 4.7 4.0

Air Voids 4.0 4.0 5.4 7.0 4.0 7.0 7.0 4.0 7.0 7.0 7.0



ENDURANCE LIMIT In conjunction with the introduction of the ELHMAP, the concern has been raised about exactly what fatigue curve should be used for the structural design of these thick pavements. Because of the thicknesses involved with these and traditional full depth pavements used in both highways and airport pavements the strains at the bottom of the layer are extremely low. These low tensile strain levels, typically well below 100 micro strain are used in the structural design through a linear extrapolation of the traditional fatigue data shown in Figure 1 which are developed above 300 micro strain. It has long been felt that fatigue behavior at low strain levels does not follow the same relationship as the material subjected to strains at the normal levels. Indeed, it has even been postulated that there is a strain limit below which there is no fatigue damage.

The existence of a fatigue endurance limit has been postulated in the past by Monismith et. al. (5). The available data indicated that a strain level in the 70 micro strain range appeared to produce an extraordinarily long fatigue life. However, there was not sufficient test data to substantiate this observation, and it has gone largely uninvestigated over the years. The significance of the endurance limit is that such a value would provide a thickness limit for the pavement, given the materials used for construction. Increasing the thickness beyond that established by the endurance limit would provide no increased structural resistance to fatigue damage and would represent an unneeded expense. The verification of a fatigue endurance limit would appear to be answerable merely by conducting fatigue testing at the low strain levels to 70 micro strain and comparing the results with the other strain levels. The following sections illustrate that the traditional laboratory fatigue data, which can clearly indicate a fatigue endurance limit, do not provide sufficient fundamental data that could be used to mechanistically validate the observed phenomenological performance difference between normal strain, and low strain test results. Traditional Fatigue Analysis A traditional fatigue analysis is performed on data collected following AASHTO procedures (1) which require a 10 Hz haversine load with constant strain at 20 C. The number of load repetitions when the initial modulus decreases to 50 percent of the initial value is defined as loads to failure, Nf. This failure value is plotted against the applied tensile strain measured during the initial 50 load cycles to develop one data point for the fatigue curve. The test is repeated at various strain levels from 1000 to 250 micro strain, depending on the material, to develop a complete traditional fatigue curve. The curves shown in Figure 1 are representative of the strain levels utilized in traditional testing programs. The fatigue equations shown are developed from the best fit to these data. The K1 and K2 parameters for the mixtures tested at normal strain levels are given in Table 2.



Table 2. K1 and K2 Fatigue Parameters for Mixtures Tested

K1 K2 Mix 6-7 2.112 E-12 -4.992 Mix 64-7 3.743 E-10 -4.168 Mix 5-7 8.928 E-11 -4.411 Mix 7-7 5.684 E-09 -3.951 Mix 8-7 3.196 E-12 -5.068 Mix 8-4 1.630 E-10 -4.528 Mix 9-7 3.645 E-11 -4.603 Mix 17-7 2.144 E-11 -4.789 Mix 21-4 1.022 E-10 -4.303 Mix 11-7 3.299 E-12 -4.889 Mix 20-7 1.385 E-10 -4.303

-7, or 4 indicates air void level

The structural design of the asphalt pavement is performed using these curves regardless of the number of load repetitions required and the resultant tensile strain level developing in the pavement. This typically requires a linear extrapolation of the curve to lower strain levels for higher load applications. Dissipated Energy Ratio Analysis The dissipated energy ratio analysis was developed by Ghuzlan and Carpenter (6, 7) that refined work done by Carpenter and Jansen (8) and built on the dissipated energy work done by other researchers (2, 9, 10). These other researchers have used the total dissipated energy, or the dissipated energy curve to relate to fatigue failure. Rowe has examined the rate of change in dissipated energy to indicate fatigue performance with good results. Of these approaches, it is the change in dissipated energy examined by Jansen and by Rowe that can provide a mechanistic picture of how damage accumulates in the fatigue test.

However, the rate of change in dissipated energy by itself does not provide for a single unified method to examine failure in different test modes. That is, the precise same variable or procedure is not used in each mode to define failure. Thus, failure is still determined differently for the different fatigue modes. To overcome this difficulty, Ghuzlan and Carpenter (7) examined a ratio of the change in dissipated energy between two cycles divided by the dissipated energy of the first cycle, represented as:

DER = [DEn+1 DEn]/DEn

Where: DER is the Dissipated energy ratio

DEn is the dissipated energy produced in load cycle n DEn+1 is the dissipated energy produced in load cycle n+1 This ratio provides a true indication of the damage being done to the mixture from

one cycle to another as a function of how much dissipated energy was involved in the previous cycle.

Different loading conditions will produce different dissipated energy hysteresis curves (stress versus strain plot). Because damage is the difference in dissipated energy,



this ratio clearly illustrates the percent of the input dissipated energy that goes into damage for a cycle. This representation of damage produces curves similar to that shown schematically in Figure 3. There are three distinct portions to the curve. Of interest here is portion II which is an extended level plateau in the data plot. This plateau value represents a period where there is a constant percent of input energy being turned into damage. This value appears to be a mixture and load/strain input related value. For any one mixture the plateau value is a function of the load inputs, and for similar load inputs, the plateau value is different for different mixtures (6).

Portion III of the curve in Figure 3 represents ultimate failure in the mixture. The upturn in the curve indicates that more damage is being done per load cycle (input energy) with each subsequent load cycle. This represents an unstable condition in the mixture, and ultimately the mixture has no load carrying capability. This failure point, defined by the onset of unstable damage accumulation, occurs for all fatigue modes, providing a means of producing fatigue curves relating the plateau value of the dissipated energy ratio to the number of loads to a true failure in the laboratory.

Log Load Repetitions

Dis

sipa

ted

Ene

rgy

Rat

io

I

II

III

Plateau Value

Log Load Repetitions

Dis

sipa

ted

Ene

rgy

Rat

io

I

II

III

Plateau Value

Figure 3. Typical Dissipated Energy Ratio Plot with Three Behavior Zones

The work by Ghuzlan(7) clearly establishes this approach as a unifying approach between all modes of loading, and types of loads. The analysis procedure produces one unique fatigue curve for a mixture developed from the precise same analysis procedure for all conditions. Additionally, the true failure point from this analysis has been shown to be strongly related to the point of 50 percent stiffness selected by Monismith (11) as representing a value to use that relates to field failure in asphalt pavements. For the 79 different mixtures tested by Ghuzlan(6), the relationship is:

NTF = 21758 + 1.30727(N50) (2)

Where: NTF is the loads to failure defined by the DER approach N50 is the number of loads to reduce stiffness by 50 percent. R2 = 0.91, and SEE = 16,858



The importance of this relationship is that the use of an intermediate point of damage accumulation that has been related to field failure of a pavement, such as 50 percent for unmodified binders or a higher number for modified binders, can be related to a materials true failure point of ultimate damage accumulation. The establishment of such a relationship supports the use of an intermediate number such as the 50 percent stiffness reduction for failure, and eliminates the need for long-term testing to establish the true failure point. The validation of this connection between laboratory true failure and field failure emphasizes the importance to define the material failure point and the field failure point for different mixtures. The mixtures tested in establishing this relationship (6) included PG binders, pen graded binders, viscosity graded binders, and modified binders using SBR and EVA modifiers.

LOW STRAIN FATIGUE TESTING

The dissipated energy ratio procedure provides an easy mechanistic means of examining the energy handling capability of a mixture as it relates to fatigue behavior in a manner not possible with traditional methods, especially important when testing at low strain levels. Flexural fatigue testing at low strain levels is problematic because it is not possible to devote sufficient machine time to take every sample to failure. Thus, there is very little data showing flexural fatigue test results beyond several million load repetitions. Mixture 6 and 64 were tested to 38 and 46 million load repetitions, respectively and failure was extrapolated. The remaining mixtures were tested to 5 or 8 million load repetitions. Even after this abbreviated testing the plateau value is flat, and extrapolation to establish the point of 50 percent stiffness reduction is reasonable. This testing generated the stiffness and dissipated energy ratio vs. load repetition data shown in Figure 4 for mix 6 which are typical of those for all mixtures. The plateau value for the dissipated energy ratio is flat, and very low, indicating that a stable region of damage accumulation has been achieved. The stiffness reduction trend is consistent with normal testing, but is nowhere near reaching the 50 percent reduction defining failure. 8000

7000 5000

. 4000 (a) (b)

6000

0 5,000,000 10,000,000 15,000,000 20,000,000 25,000,000 30,000,000 35,000,000 40,000,000

Number of Load Cycles

Stiff

ness

, Mpa

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08

Load Cycles

DD

E/D

E

Figure 4. Stiffness Reduction (a), and Dissipated Energy Ratio (b) for Mix 6 at 70 Microstrain in Flexure.



Figure 5 shows the complete fatigue data results for normal strain levels plotted in the traditional fatigue analysis manner. The normal strain tests clearly indicate that the fatigue curves are different for each mixture, and must be handled as different materials for structural fatigue design.

100

1000

10000

100 1000 10000 100000 1000000 10000000 100000000

Load Repetitions to Nf

Tens

ile S

train

, MIc

ro

Mix 8-4 Mix21-4 Mix 6-7 Mix 64.7 Mix 7-7 Mix 9-7 Mix 8-7Mix 17-7 Mix 5-7 Mix 11-7 Mix 20-7

Figure 5. Traditional Fatigue Plot Including Low Strain Test Data. Figure 6 presents the fatigue test data with the low strain test data included. This plot indicates that there appears to be a distinctly different performance at the low strains compared to the normal strain levels for all mixes tested. Some mixtures appear to reach the extended life more slowly than others. Comparing mix 11 to mix 64 in Figure 6 it can be seen that at strain levels around 100 micro strain mix 11 will sustain more load repetitions to failure than mix 64. This indicates that the endurance elimit could be different for different mixtures. There is nothing unique about the 70 micro strain level for an endurance limit although it would appear to be a useful value that falls within what could be considered the strain level where an endurance limit would be encountered. More importantly, the phenomenological relationship shown in Figure 6 does not provide a mechanistic connection between behavior of the mixtures at low and normal strain levels. Lacking such a fundamental connection between a mechanistically determined material property and the observed behavior at low strains a valid explanation cannot be developed from this data.



10

100

1000

10000

1.E+01 1.E+06 1.E+11 1.E+16 1.E+21 1.E+26 1.E+31 1.E+36 1.E+41

Load Reps to Nf

Flex

ural

Stra

in, M

icro

Mix 8-4 Mix 21-4 Mix 6-7 Mix 64-7 Mix 7-7 Mix 9-7 Mix 8-7Mix 17-7 Mix 5-7 Mix 11-7 Mix 20-7

Figure 6. Traditional Fatigue Representation With Low Strain Behavior Extrapolated.

The fundamental explanation needed to support a hypothesis for an endurance limit can be obtained from the dissipated energy analysis of the fatigue data. Figure 7 presents the DER plateau value plotted against load repetitions to failure for the mixtures studied here. The plateau values for the low strain tests fall precisely into line with the data from all strain levels establishing a unique relationship between failure and plateau value of the DER. From a mechanistic viewpoint this uniqueness is to be expected when using a fundamental relationship. A change in damage accumulation results in a consistent corresponding change in load repetitions to failure. This consistency indicates that the energy dissipating mechanism of the mixture is consistent over all levels of energy input, resulting in a unique relationship with loads to failure. However, this relationship in itself does not support a fatigue endurance limit, something which requires a discontinuity in behavior.

The discontinuity indicating a change in behavior at low strain levels becomes evident in a plot of the DER plateau value as a function of the tensile strain as shown in Figure 8. At normal strain levels the change in the rate of damage accumulation is proportional to the change in the strain level. However, at low strain levels a non-linearity is introduced, and the proportionality found at normal strain levels is not continued. The amount of damage done per cycle changes at strains below about 100 micro strain, and is extremely evident at 70 micro strain for the mixtures tested here.



1E-411E-381E-351E-321E-291E-261E-231E-201E-171E-141E-111E-081E-05

0.01

1 100000 1E+10 1E+15 1E+20 1E+25 1E+30 1E+35 1E+40

Load Repetitions to Nf

DER

Pla

teau

Val

ue


Figure 7. Dissipated Energy Ratio Plateau Value Plotted Against Load Repetitions to Failure.

10

100

1000

10000

1E-41 1E-36 1E-31 1E-26 1E-21 1E-16 1E-11 1E-06 0.1

Plateau Value, DER

Flex

ural

Stra

in, M

icro


Figure 8. Flexural Strain Plotted Against Plateau Value Illustrating the Differentiated Behavior at Low-Strain Values.



Figure 8 clearly establishes a different energy into damage response in the

mixtures at the lower strain levels compared to normal levels. There is a fundamental change in material behavior at low strain levels where the energy going into damage after a load cycle is decreased substantially in comparison to normal strain results. The data presented support the long held understanding that the strain response is not a fundamental property in relation to failure, but only a convenient phenomenological relationship. Healing A Source of the Fatigue Endurance Limit

At low strain levels there is a decidedly reduced amount of damage being done per cycle compared to normal strain level testing. The data support a gradual trend toward the endurance limit, and not a distinct break point. The recognition of healing and the resultant property changes it produces in a mixture can be proposed to explain this observed non-linearity, lending credence to a physical rationale for a fatigue endurance limit. The work being performed at Texas A&M University provides insight into this phenomenon (17). Although not specifically addressed in this study, healing provides the connection required to have a fatigue endurance limit.

Healing is a continual process that can be thought of as a process that returns energy into the HMA, increasing the load carrying ability of the mixture, and in effect repairing a portion of the damage done by the previous loads. Healing becomes most evident when a rest period is imposed between load cycles and the healing can be seen in the increased modulus after the rest period. In actuality this repair process is continual, and occurs to some extent even during load cycling. At high strain levels the amount of healing energy is relatively small in relation to the damage energy, but at low strain levels the proportion of damage energy is smaller and could approach the energy returned to the mixture by the healing process. Given that an asphalt aggregate combination produces a specific amount of healing potential, there could exist a strain level at which the damage energy was equal to the healing energy, and no damage would accumulate if the load cycles were slow enough and total healing was allowed to occur. Even if loading was continual, there would exist a point at which the kinetics of the healing process would offset the load cycle damage, and little or no damage would accumulate in the HMA, producing an extended fatigue life, a fatigue endurance limit similar to what is shown in the testing presented here.



CONCLUSIONS The data generated from the mechanistic analysis of damage accumulation through the dissipated energy approach clearly provides support for the existence of a fatigue endurance limit. The normal and low strain data can be considered as two distinctly different processes that can be represented by their individual fatigue curves as related to tensile strain, something that cannot be substantiated from the traditional analysis. Although the data set is currently limited to low strain testing at 70 micro strain, the data shows that the trend is toward an extraordinarily extended fatigue life. While the change may be a continuous function rather than a precise lower limit it would appear that an asymptote is being approached at 70 micro strain. The exact limit is very likely mixture/binder specific. Whether or not the 70 micro strain level is accepted as an endurance limit it is apparent that this level is capable of providing a significantly longer fatigue life than would be predicted from normal testing. For practical design considerations this could be considered a limit beyond which life extension becomes extremely long in comparison to traditional designs and load repetitions used. The existence of the fatigue endurance limit proposed here has serious ramification for design of asphalt pavements. There must still be a positive production of sound MHA mixtures with adequate fatigue performance. To date, all mixtures tested have produced appropriate K1 and K2 values which satisfy the unique phenomenological relationship first shown by Myre(12), and verified by Ghuzlan and Carpenter(6,13). Such mixtures have sufficient internal strength to provide satisfactory fatigue resistance to support the existence of an endurance limit.

Given a suitable mixture the structural design for fatigue in an ELHMAP does not require either traffic or a fatigue algorithm. When the materials and the pavement structural section are sufficient to produce a tensile strain of around 70 micro strain, there is no effect of traffic on fatigue life. If the rich bottom layer produces a different modulus, this will change the strain response allowing thickness variations. Material variability will not impact fatigue life as long as the tensile strain remains around 70 micro strain.

Thicker asphalt sections to reduce the strain below 70 micro strain are not necessary to provide any increased factor of safety against fatigue. This concept has a significant value to the practitioner as it provides a very simple means of selecting asphalt layer thickness based only on modulus testing combined with the use of a suitable response model, both of which are becoming standard elements of pavement design. RECOMMENDATIONS This research provides a fundamental mechanistic approach that clearly establishes a difference in performance in asphalt mixtures at low strain levels that indicates a fatigue endurance limit. This endurance limit requires more diverse testing, and should be combined with emerging research that is establishing a tensile strength/strain limit (14, 15) below which fatigue damage does not appear to develop. This is an especially intriguing relationship given the work by Maupin which previously established relationships between tensile strength and fatigue life (16). The testing conducted for this study was performed on standard flexural beam equipment. Further testing at low strain levels using other sample configurations and



strain gages mounted on the specimens should be conducted. This will provide more accurate values of the dissipated energy at low strain levels, and allow monitoring of the specimen to ensure accurate strain at low levels over long times to reduce possibility of creep induced because of sample geometry affecting results. There must be intermediate testing at strain levels below the 250 micro strain level and below the 70 micro strain level to define the nature of the transition from normal behavior to the modified behavior at the low strain levels. The testing in this study was limited primarily to the 70 micro strain level. The existing database of pavements already constructed that produced a design strain level at the bottom of the asphalt layer in the range of 70 micro strain should be carefully examined to establish performance trends from in service pavements. This must be done to establish any complicating factors present in the field that would compromise the endurance limit behavior. The impact of overloads on fatigue life of a pavement designed to the endurance limit approach are currently under investigation to determine any detrimental impact on the extended life performance of the pavement section of brief overload situations. Given that healing has been demonstrated by others as a valid consideration in fatigue life extensions, the impact of healing and asphalt composition on the energy level required for damage accumulation should be investigated. The healing of HMA should be tied to energy which can then be tied to mixture performance. It is likely that fundamental studies of this nature are necessary to define and validate if and how the fatigue endurance limit develops as has been proposed here. ACKNOWLEDGEMENTS The authors would like to gratefully acknowledge the funding for this study through the Federal Aviation Administration Center of Excellence for Airport Technology at the University of Illinois. The center is funded under Research Grant Number 95-C-001, through the Pavement Center at the FAA. Ms. Patricia Watts is the FAA Program Manger for Air Transportation Centers of Excellence, and Dr. Satish Agrawal is the FAA Technical Director for the Pavement Center. DISCLAIMER The contents of this paper reflect the views of the authors who are responsible for the facts and accuracy of the data presented within. The contents do not necessarily reflect the official views and policies of the Federal aviation Administration. This paper does not constitute a standard, specification, or regulation. REFERENCES 1. Standard Test AASHTO Provisional Standards, Standard Test Method for

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3. Thompson, M. R, and F. Hugo, Design Methods, Workshop 2, Proceedings of the Sixth International Conference on the Structural Design of Asphalt Pavements, Vol. 2, Ann Arbor, Michigan, 1987

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Concrete Mixtures, Paper submitted for possible presentation and publication by the Transportation Research Board at Annual Meeting, January 2003.

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Analysis Procedure Using a Viscoelastic Continuum Damage Model, Paper Presented at the 2002 Meeting of the Association of Asphalt Paving Technologists, March 2002

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