Comparison of Bitumen Fatigue Testing Procedures Measured in Shear and Correlations with Four-Point Bending Mixture Fatigue

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1 2 nd Workshop on Four Point Bending, Pais (ed.), University of Minho. ISBN Comparison of Bitumen Fatigue Testing Procedures Measured in Shear and Correlations with Four-Point Bending Mixture Fatigue C.M. Johnson, H.U. Bahia, & A. Coenen University of Wisconsin, Madison, Wisconsin, USA ABSTRACT: Characterizing the contribution of bitumen fatigue resistance to asphalt pavement performance remains a hurdle to implementing a unified bitumen fatigue criterion in specifications. This study is intended to explore preliminary validation of the most promising bitumen fatigue testing procedures using four-point flexural bending of asphalt mixtures. One mixture gradation and compaction level was used to isolate the relative fatigue performance of three polymer-modified bitumens. It is anticipated that testing of bitumen using the Dynamic Shear Rheometer (DSR) can be used as a fatigue performance indicator, thereby reducing the need for costly mixture fatigue testing. Three bitumen test procedures are highlighted in this study; one mimics the cyclic loading of conventional fatigue, the second is a monotonic test procedure that measures the ability of the material to withstand loading energy prior to damage, and the third is a sweep test in which the strain is increased continuously with number of cycles. The comparison of the different bitumen testing procedures to flexural fatigue is presented herein. The results show that there are no simple correlations, and depending on the loading conditions selected, the ranking of binders could vary by test method. 1 INTRODUCTION There is currently a significant amount of effort being focused on the development of bitumen test procedures that can more accurately determine the critical material failure properties associated with pavement distresses such as rutting, fatigue and thermal cracking. Fatigue damage in particular has proven quite troublesome to evaluate in the bitumen alone, as it typically requires multiple repeated load cycles over a testing period that can last for hours. However, there is little argument that the bitumen/mastic phase of asphalt concrete is most critical for resisting fatigue damage, and thus must be evaluated for fatigue performance. This study hopes to evaluate new test methods for determining the damage resistance and fatigue performance of different bitumens by comparing their results with four-point bending fatigue test data on mixtures prepared using the same bitumens. Currently, the SuperPave performance grading specification relies on a parameter derived from the linear viscoelastic dissipated energy (W) under cyclic shear loading, defined by Equation 1: ( 2 W = π γ G* sinδ ) where γ 0 is the applied strain, G* is the complex shear modulus, and δ is the phase angle. As the values of applied strain are within the linear visco-elastic limit for the test, the G* and δ are independent of the strain, thus the parameter G* sin δ. A maximum value was set as the specification limit in order to limit dissipated energy, as it was assumed that large values could indicate energy being dissipated into more damage accumulation. However, recent studies have (1)

2 shown that measurement of the undamaged linear viscoelastic behavior does not accurately describe the fatigue performance of the material because some energy dissipation in cyclic loading is due to hysteresis (visco-elastic damping), which is not damage (Bahia et al. 2001). A better method for characterizing fatigue failure is by measuring the rate of damage growth in the material (Bahia et al. 2001; Deacon 1997; Delgadillo et al. 2005; Tsai et al. 2005). As a result, a cyclic fatigue test (known as a time sweep test) which can be performed on bitumen specimens using the Dynamic Shear Rheometer (DSR) was proposed by Bahia et al. The new test was to mimic mixture testing and, although developed independently, has its basis in work done in the early 1960 s by Pell and coworkers (Pell 1962). The main benefit to this test is a direct application of fatigue-type loading, and if performed at sufficient stiffness levels, relevant fatigue performance indicators can be measured. However, the suitability of this test for use in specification is questionable due to the possibility of long testing times. Hence, recent work on bitumen fatigue has focused on the search for test procedures that can be used as accelerated fatigue tests (Johnson et al. 2009). Regarding the characterization of damage growth in mixtures, the concept of using monotonic testing to evaluate damage resistance of asphalt mixtures showed promise when the damage performance was related to cyclic test results (Daniel and Kim 2002; Kim and Wen 2002; Roque et al. 2004). The obvious benefit of relating monotonic test results to cyclic results is that monotonic tests typically take significantly less time to perform. Additionally, monotonic tests have at times been referred to as high-amplitude one-cycle cyclic tests in attempts to draw valid comparisons. For this reason, two bitumen testing procedures are currently under development by the authors. The test procedures require less time and utilize the advantages of the Visco- Elastic Continuum Damage (VECD) concept to effectively take into account damage accumulation behavior and incorporate various loading conditions. One test, known as the Binder Yield Energy Test (BYET) (Johnson et al. 2009), uses a constant shear rate in the DSR to apply a monotonic load to 8mm-diameter by 2mm-thick specimens at intermediate temperatures typically associated with fatigue damage. The second test is a cyclic test in which the strain amplitude is increased incrementally until the sample is damaged. This test is called the strain amplitude sweep and has been developed to include a linear ramping of strain with a specific number of cycles for each strain level. This study analyzes test results for three bitumens, by comparing results from the three aforementioned tests with results from four-point bending of mixtures prepared using the same bitumens. The goal is to compare fatigue properties of bitumen to the fatigue performance of asphalt concrete in order to determine the suitability of bitumen fatigue testing to ultimately predict pavement performance. 2 MATERIALS AND EXPERIMENTAL DESIGN 2.1 Materials The materials used for this study are a subset of materials previously investigated for low temperature cracking performance (Marasteanu et al. 2007), and consist of commercially produced polymer-modified bitumens, as well as mixture slab specimens consisting of limestone aggregates. Details of the materials are shown in Tables 1 and 2. Table 1. Description of Bitumens Bitumen PG Grade Modification Type Styrene-Butadiene- Styrene Rubber Ethylene Ter-polymer Ethylene Ter-polymer 134

3 Table 2. Description of Mixture Sieve Size Percent Passing (mm) Pb 6.9% Percent Air Voids 4% The mixture design meets the Wisconsin Superpave Requirements for roads with 3,000,000 to 30,000,000 ESAL s. More information on the preparation of the specimens can be seen described by Marasteanu et al. (2007). 2.2 Testing Plan In order to simulate the state of bitumen aging in the mixture specimens, the testing of bitumens was conducted after they were aged in the Rolling Thin Film Oven (RTFO) in accordance with AASHTO T240. The details of the tests performed are as follows. 1. Bitumen Frequency Sweep Test In order to determine an appropriate testing temperature, a frequency sweep test was run on each bitumen at temperatures of 7, 13, 19, 25, and 28 C to cover the range of intermediate pavement temperatures. The testing was conducted using a 0.1% applied strain loading over a range of frequencies from 0.1 to 30 Hz. Master curves were then constructed using the models of (Zeng et al. 2002). The Superpave fatigue specification requires the value of G* sin δ to be less than or equal to a value of 5000 kpa at 1.59 Hz. The master curves were used to find the temperature for each bitumen where this condition is met (known as continuous grading), shown in Table 3. An iso-stiffness temperature is used to isolate the effect of bitumen stiffness from damage characteristics, as explained in previous studies (Santagata et al. 2009; Shenoy 2002). In this study, instead of using iso-stiffness temperature, the temperatures at equal dissipated energy were targeted. This temperature is important because one of the fatigue failure criteria for the time sweep tests is the ratio of the dissipated energy at a given cycle during cyclic testing to the initial dissipated energy, also called dissipated energy ratio (DER). Table 3. Temperature at G* sin δ = 5000 kpa Bitumen Testing Temperature [ C] SBS ET ET Bitumen Time Sweep Test The cyclic fatigue testing of the bitumen specimens was performed in strain-control mode at 10 Hz for consistency with the asphalt mixture beam testing. According to previous work on the relationship between global mixture strain and strain in the binder phase, an approximate scale factor of 1 to 100 times is used to translate from 135

4 mixture to bitumen phase (Masad et al. 2001). In this study, the 500 micro-strain applied load on the beams/mixtures was multiplied by 50 and 100 to arrive at bitumen time sweep applied amplitudes of 2.5 and 5.0% strain, respectively. All tests were conducted at the temperatures listed in Table 3. In order to define failure, the DER concept was employed consistent with work on bitumen cyclic fatigue (Bahia et al. 2001; Santagata et al. 2008). The parameter N P20 was used because it has shown to compare favorably with mixture performance, as well as provide valuable insight into the growth of damage during cyclic fatigue testing. In order to calculate N P20, one must first calculate the DER at each data point, taken as DER n i= = 1 W W n i (2) where W n is the dissipated energy from Equation 1 at a given cycle n. The parameter N P20 is determined from the number of cycles at which the value of (DER N) / N is equal to 20%. 3. Binder Yield Energy Test Monotonic constant shear rate loading was applied consistent with the procedure described by (Johnson et al. 2009), along with Yield Energy and Strain at Peak Stress (Gamma at Tau Max, or GTM) being recorded in the same manner (as shown in Figure 1). All tests were conducted at the temperatures listed in Table E+05 Binder Yield Energy Test 1.6E+05 Shear Stress (Pa) 1.2E E E E+00 YIELD ENERGY γ τ max Strain Figure 1. Typical BYET results and representation of measured parameters. 4. Strain Sweep Test Cyclic amplitude sweep testing can be thought of as an accelerated cyclic fatigue test where the applied loading amplitude is incrementally increased until material failure. Previous work in this area has focused on stress-controlled testing (Martono and Bahia 2008), but recently a strain-controlled version of the test has been under investigation. All tests were run at 10 Hz frequency and temperatures listed in Table 3, with an initial 100 cycles applied at 0.1% strain to determine undamaged linear viscoelastic properties. Each subsequent load step consisted of 100 cycles at a rate of increase of 1% applied strain per step for 20 steps, beginning at 1% and ending at 20% applied strain. 136

5 5. Asphalt Mixture Flexural Beam Fatigue Test The specimens that were prepared for four-point bending (4PB) were trimmed from slab compacted samples to a vertical cross-sectional area of 63mm thick by 50mm wide with an overall length of 380mm. The freely rotating clamps of the 4PB apparatus hold the specimen at positions of 119mm center-to-center spacing along the entire length of the specimen. All beam fatigue testing was performed using a servo-hydraulic testing machine controlled by software from SHRP Equipment Corp. (ATS v4.1). Testing procedures were run in accordance with the ASTM D and AASHTO T In order to meet the criteria of both standards, specimens were tested until a reduction of 65% from the initial stiffness was measured, with initial stiffness defined as the stiffness of the specimen at the 50 th cycle of loading. Each test was conducted to a minimum of 10,000 loading cycles under strain-control with amplitude of 500 micro-strain. The testing procedure was programmed for a maximum of five million cycles, thus any Number of Cycles to Failure,, greater than five million are reported as 5,000,000 in the recorded data. Specimens were tested in an environmental chamber to ensure accurately maintained desired temperatures. Temperatures were adjusted to bring the specimen to the desired temperature within a period of one hour and then remained at test temperature for an additional hour to ensure equilibration prior to application of load. Specimens maintained an internal temperature within +0.1 o C and an external temperature within +0.3 o C, monitored throughout the duration of the each test with a secondary sample equipped with two thermocouples, one internal and one external (both reporting temperatures to accuracy of 0.1 o C). A position corresponding to zero-load on the central portion of the frame, as read by the load cell, was determined prior to placement of the asphalt concrete specimen into the 4PB apparatus. Hydraulic power ensured that this position was accurately maintained during clamping of the specimen and placement of the LVDT on the target located at the center of the specimen. Zero-load was maintained during equilibration time, though displacement readings were non-zero. It was assumed that measured displacement during equilibration time was more significantly influenced by thermal equilibration of equipment than physical change to the specimen. For this reason, LVDT readings were reset after equilibration of the specimen, prior to application of load to allow both stress and strain to be nil upon commencement of cyclic loading. The terminating criterion of 65% reduction from initial stiffness allows for data collection beyond the defined failure point. The failure point for this set of testing is defined as the peak value achieved on a graphical representation of Normalized Complex Modulus x Cycles versus Cycles, with the base of normalization as the 50 th cycle stiffness value. 3 RESULTS 3.1 Bitumen Test Results The bitumen time sweep test is the most direct corollary to the four-point flexural fatigue testing. Results of time sweeps for the three binders are shown in the plots of Figure 2 where the complex modulus is shown as a function of number of loading cycles. In this testing the applied strain was kept at 2.5% strain and at 5% strain at edge of the parallel plate geometry in the DSR. Based on the dissipated energy ratio, the cycle at which 20 % decrease in the ratio is observed is identified as N P20. The values are given in Table

6 Figure 2. Plot of time sweep results for bitumen at 2.5% applied strain. Table 4. Bitumen Time Sweep Test Results Bitumen N P20-2.5% strain N P20-5.0% strain SBS 360,000* 123, ET 163,500 15, ET 175,800 32,400 *Note: At 2.5% applied strain, the SBS had reached only a 17.6% reduction in DER after the preset limit of 10 hrs of testing. The SBS-modified bitumen shows a substantially higher resistance to fatigue damage than the other two binders. It is also important to note how much increasing applied strain reduces the cycles to failure. The ranking of the three binders does not appear to be affected by the change in strain of testing. Recent work involving accelerated pavement testing has shown that the type of polymer modification to bitumens can change the fatigue performance of asphalt mixtures (Kutay et al. 2008). However, determining the specific material characteristic for this increase in performance has proved challenging. It is known that elastomeric polymers such as those used in this study can affect bitumen response to loading by simultaneously changing modulus and phase angle, both of which are significant in calculating dissipated energy. Criticality of the results is that although testing is conducted at equal dissipated energy, the cycles to failure are 2 to 7 times different between bitumens depending on type of polymer, PG grade and strain. 3.2 Binder Yield Energy Test Results Figure 3 depicts the stress-strain response of the three binders tested. Though applicability of the BYET to fatigue performance indication is still under investigation, it provides a novel ability to gain further insight into the mechanical response of polymer modified bitumen. For example, as shown in Figure 3, the stress-strain response for the SBS material shows a distinct double-peak behavior, which is not shown in the bitumens modified with ethylene terpolymer. This trend of response is problematic because it complicates the modeling of the undamaged response using linear viscoelastic (LVE) constitutive equations, as discussed previously in Johnson et al. (2009). The modeling is necessary to perform continuum damage analysis which then allows estimation of binder contribution to fatigue life of pavements. 138

7 Figure 3. BYET results at 1% shear strain per second. The BYET test, however, may still be employed for possible empirical insight into fatigue behavior. While not a cyclic test procedure, it can be hypothesized that an energy threshold exists, at which point the material loses its ability to resist damage. The ultimate goal of this test is to relate the propensity to absorb applied energy, as denoted by the area under the curve to the maximum value. Yield Energy values from two replicates for each of the bitumens are shown in Table 5. The data includes the average yield energy, the maximum point of the stress-strain curve (YE, Pa), and the shear strain at the maximum stress (GTM%). Unlike the time sweep results, the 64-34ET appears to offer the highest yield energy and thus the best resistance to fatigue. However, the strain at maximum stress (GTM%) gives a similar ranking to the time sweep test results; with respect to the SBS modified binder showing the highest strain at failure. Observing the plot in Figure 3, it is apparent that the response beyond the maximum peak varies significantly between the binders and, depending on the strain level selected, the binders show different rankings of the stress levels. In fact, comparing Figure 2 with Figure 3, one can observe some similarity in that the SBS modified binder is showing a higher resistance to cyclic loading in Figure 2 at large number of cycles and also showing higher stress at higher strain in Figure 3. This similarity may indicate that interpretation of the BYET test should consider the behavior beyond the peak stress, and perhaps the post peak behavior is better related to mixture fatigue under cyclic loading. Similar findings were noted by (Jimenez et al. 2003) as presented at the 6 th International RILEM Symposium. Table 5. Binder Y ield Energy Test Results Bitumen Average YE, Pa Average GTM, % SBS 2,617,000 1,559% ET 2,568,636 1,184% ET 4,024,517 1,093% 139

8 3.3 Flexural Beam Fatigue Results The first step towards validation of bitumen fatigue should involve mixture testing, and fourpoint bending was employed for this study. Due to limited supply of mixture slab specimens, two replicate specimens were prepared for each bitumen investigated, and each specimen was tested at an applied load of 500 micro-strain. Number of cycles to failure as calculated per AASHTO T321 are reported in Table 6. Table 6. 4PB Fatigue Results (AASHTO T321) Bitumen Used in Mixture Replicate 1 Replicate 2 Average SBS 230, , , ET 5,000,000 5,000,000 5,000, ET 580, , ,000 Specimen ran for 100,000 cycles with no failure before premature test termination; subsequently re-started and withstood 480,000 additional cycles. Data not used to calculate average. Due to the lack of defined failure per AASHTO T321 after 5 million cycles for PG58-34 ET beams, alternate failure criteria were explored. Both 35% and 50% reduction in stiffness (relative to the stiffness after 50 cycles) were explored, as shown in Tables 7 and 8. As a 35% stiffness reduction was achieved by all sets of beams, 35% is the maximum reduction used for further comparison to bitumen test results. Table 7. 4PB Fatigue Results (50% Stiffness Reduction) Bitumen Used in Mixture Replicate 1 Replicate 2 Average SBS 145, , , ET 5,000,000 3,000,000 N/A ET N/A 660, ,000 Table 8. 4PB Fatigue Results (35% Stiffness Reduction) Bitumen Used in Mixture Replicate 1 Replicate 2 Average SBS 25,000 40,000 32, ET 330,000 85, , ET N/A 35,000 35, Strain Sweep Results The stress versus strain plots are shown in Figure 4 for the amplitude sweep test results, but determination of a failure criterion remains challenging for this type of test. Recent work at UW-Madison has shown that VECD analysis of time sweep data can successfully indicate the time sweep performance of a given material at different applied strain levels (Wen and Bahia 2009). By calculating the damage accumulation as the change in dissipated energy with respect to time using the equations shown by Kim et al. (2006), one can plot the reduction in modulus as a function of change in dissipated energy to generate a characteristic damage curve. The benefit of this approach is that it is theoretically independent of the loading conditions, so the characteristic damage curve can be generated from testing at any applied load level. The research team decided to employ this analysis method to model the damage growth for the strain sweep, since the multiple strain levels applied during each test could be simultaneously analyzed to form a singular damage curve for each material, the failure criterion was selected as the amount of damage at a predetermined percentage of initial undamaged LVE complex modulus. Specifically, the VECD damage parameter D at 25% of the initial complex shear 140

9 modulus was chosen to ensure a substantial level of damage had occurred in the specimens. The basic idea behind this criterion is that a good-performing material will require higher levels of damage before it reaches 25% of its initial modulus. Results are given in Figure 5 and Table 9. Figure 4. Stress versus applied strain from strain sweep test. Figure 5. Plot of normalized modulus versus damage intensity using VECD analysis for strain sweep. 141

10 Table 9. Results of VECD analysis of strain sweep data. Binder Damage Parameter at 25% G* SBS 18, ET 21, ET 18,836 The results of the strain sweep are not encouraging as they show all three binders to have very similar trends in the strain sweep and in the VECD analysis results listed in Table 9. These results raise questions about similarities between damage under cyclic loading and under quasi-static loading, such as the monotonic loading scheme. Comparison to mixture data is contained in the next section. 4 ANALYSIS & COMPARISON OF TEST METHODS 4.1 BYET versus Mixture Beam Fatigue As shown in Figure 6, initial observation shows little relation between the parameters derived from the BYET and beam fatigue cycles to failure. However, upon further investigation into data presented in Figure 3, it appears that material behavior after peak stress may play an important role. At roughly 1,000% strain, there is a clear ranking with the ET giving the highest strength. However, as the deformation increases, the ability of that material to withstand additional stress decreases more rapidly in comparison to the other two bitumens. At higher strain levels approaching the end of the test at 3,600% strain, the ranking of the binders based on the BYET results matches that from the beam fatigue results. This observation is consistent with the findings shown by Jimenez et al. (2003) which stipulates that post peak behavior could be a better indicator of fatigue behvior. Figure 6. Plot of BYET results versus beam fatigue values. 4.2 Binder Time Sweep versus Mixture Beam Fatigue Initially, the time sweep data did not appear to have any relation to beam fatigue results. As shown in Figure 7, the bitumens appear to follow the same trends at 2.5% and 5.0% loading, with neither strain level having an improved relationship with the mixture data. However, time sweep number of cycles to failure was plotted against the initial dissipated energy for each bi- 142

11 tumen (Figure 8), to build a basic fatigue law for each material using the following classic fatigue relationship: N P20 k = k Wi 1 2 (3) Using the developed relationship between initial dissipated energy and cycles to failure, one can determine the level of input energy needed to obtain the same ranking as the beam fatigue results. Values of k 1 and k 2 for each binder are given in Table 10. Figure 7. Comparison of time sweep N P20 versus beam fatigue values. Figure 8. Initial dissipated energy versus number of cycles to failure 143

12 Table 10. Values of k 1 and k 2 from fatigue plots. Binder k 1 k SBS 4.85E ET 5.10E ET 2.89E By setting the input Wi value at 7,500 Pa (approximately % applied strain for these materials and testing conditions), a favorable correlation between binder time sweep and mixture beam fatigue can be achieved, as shown in Table 11 (ranking shown in parentheses). However, when the initial Wi is increased by an order of magnitude to 75,000 Pa, one can see how the ranking changes and results in mismatch with mixture results. Table 11. Predicted Time Sweep NP20 vs. Beam Fatigue Nf (A,B,C = Ranking) Bitumen Predicted Time Sweep NP20 at Wi = 7,500 Pa Beam Fatigue Nf Predicted Time Sweep NP20 at Wi = 75,000 Pa SBS 942,265 (C) 32,499 (C) 103,837 (A) ET 2,255,774 (A) 207,499 (A) 15,758 (C) ET 966,260 (B) 34,999 (B) 37,324 (B) 4.3 Strain Sweep versus Beam Fatigue As can be seen in Figure 5, the binders show very similar damage trends based on the VECD analysis. However using a larger scale at higher damage intensity levels, there appears to be some variations. In fact by selecting 25% of the initial undamaged modulus as the failure criterion, there is a consistent ranking with beam fatigue results, as shown in Figure 9. Figure 9. Plot of damage intensity from 20 30% of initial undamaged modulus. Whether the level of damage chosen here is appropriate will require further investigation, but this underscores the fact that one must be careful when selecting a failure criterion. 144

13 4.4 Summary of Rankings for Fatigue Performance The relative ranking of each material for the test methods employed in this study are shown in Table 12. It is clear that depending on chosen specification parameters and failure criteria, the rankings vary greatly. This clearly illustrates the need for further investigation into the fundamental mechanisms behind fatigue failure. Without this, failure criterion can be selected in such a fashion that suits the given data, and may not accurately represent the true material properties. Table 12. Ranking of Bitumens Using Various Fatigue Tests Time Time Sweep Bitumen Used Beam Sweep Predicted in Mixture Fatigue 2.5 & at Wi = 5.0% 7,500 Pa BYET Yield Energy BYET GTM Strain Sweep D@0.25M SBS C A C B A C ET A C A C B A ET B B B A C B 5 CONCLUSIONS AND RECOMMENDATIONS The mechanism by which bitumen contributes to mixture fatigue is a critical factor in predicting the life span of asphalt pavements. Presently, there appears to be a benefit to including polymer modification to a bitumen in order to enhance its fatigue performance, but it is not readily clear why this happens or how to measure it in a laboratory setting such that binder designers (or producers) can select the best binder and/or best modifier. Multiple methods were evaluated in this study with the following main findings based on the results collected and analyzed: Time sweep testing of binders is the most direct analog to beam fatigue testing, and yet preliminary results showed no simple correlation between the results of the two tests. However, by building a relationship between fatigue life and applied load, the relative ranking of the materials could be aligned for the two test methods by selecting the appropriate applied load. Hopefully this can provide some insight into appropriate bitumen testing conditions to measure fatigue performance. Monotonic testing of bitumens using the BYET coupled with VECD analysis can give researchers and practitioners another tool for evaluating the differences in mechanical behavior between various types of modified materials. However, capturing the contribution of certain polymer additives using the LVE constitutive equations used in the VECD does not appear to be adequate. Without a method to predict the undamaged behavior of these materials, quantifying the fundamental nature of damage accumulation from the BYET will continue to be difficult. There appears to be a need to consider the post stress peak behavior for a simplistic evaluation of binder contributions to mixture resistance of fatigue. Strain-controlled amplitude sweep testing gives researchers the ability to employ VECD concepts for bitumen characterization. In addition to matching the cyclic nature of fatigue damage, the test procedure takes significantly less time than other methods. However, definition of the most appropriate failure criterion for this methodology is not readily apparent. As was shown in this study, the relative performance of the materials depends heavily on how the failure criterion is selected. Based on the limited data collected, it is seen that each binder test method can be analyzed to give different ranking. Since fatigue of binders and mixtures depend highly on loading conditions; and since fatigue failure can be defined in many ways, one can 145

14 always find a correlation that fits the data. However, the true meaning of such correlations between binder and mixture performance is questionable, to say the least. Future work will continue to expand the testing plan to include more binders, as well as additional loading amplitudes and testing temperatures. Further investigation into the fundamental mechanisms behind these prospective test procedures is absolutely essential for incorporating them into regular practice, such that pavement engineers can apply the results of these tests with confidence in the design and production of long-lasting asphalt roadway. 6 ACKNOWLEDGEMENTS The authors would like to thank the Federal Highway Administration (USA) for funding part of this research through the Asphalt Research Consortium. Additionally, UW-Madison would like to thank its partners in National Pooled Fund Study TPF-5(080), including University of Minnesota, University of Illinois Urbana/Champaign, and Iowa State University. 7 REFERENCES Bahia, H. U., Hanson, D. I., Zeng, M., Zhai, H., Khatri, M. A., and Anderson, R. M. (2001). Characterization of Modified Asphalt Binders in Superpave Mix Design, NCHRP Report 459, National Academy Press, Washington, D.C. Daniel, J. S., and Kim, Y. R. (2002). "Development of a simplified fatigue test and analysis procedure using a viscoelastic, continuum damage model." J. Assn. Asphalt Paving Technologists, v71, Deacon, J. A. (1997). "Influence of binder loss modulus on the fatigue performance of asphalt concrete pavements." J. Assn. Asphalt Paving Technologists, v66. Delgadillo, R., Bahia, H., You, Z., and Rowe, G. (2005). "Rational fatigue limits for asphalt binders derived from pavement analysis." J. Assn. Asphalt Paving Technologists, v74, Jimenez, F. P., Recasens, R. M., and Aldape, J. C. (2003). "Analysis of fatigue performance of asphalt mixtures: Relationship between toughness and fatigue resistance." Proceedings from the 6 th International RILEM Symposium, Johnson, C. M., Bahia, H. U., and Wen, H. (2009). "Practical Application of Viscoelastic Continuum Damage Theory to Asphalt Binder Fatigue Characterization." J. Assn. Asphalt Paving Technologists, v78. Kim, Y., Lee, H. J., Little, D. N., and Kim, Y. R. (2006). "A simple testing method to evaluate fatigue fracture and damage performance of asphalt mixtures." J. Assn. Asphalt Paving Technologists, v75, Kim, Y. R., and Wen, H. (2002). "Fracture energy from indirect tension testing." J. Assn. Asphalt Paving Technologists, v71, Kutay, M. E., Gibson, N., and Youtcheff, J. (2008). "Conventional and viscoelastic continuum damage (VECD) based fatigue analysis of polymer modified asphalt pavements." J. Assn. Asphalt Paving Technologists, v77. Marasteanu, M., Zofka, A., Turos, M., Li, X., Velasquez, R., Li, X., Buttlar, W., Paulino, G., Braham, A., Dave, E., Ojo, J., Bahia, H., Williams, C., Bausano, J., Gallistel, A., and McGraw, J. (2007). "Investigation of Low Temperature Cracking in Asphalt Pavements." National Pooled Fund Study 776, Minnesota Department of Transportation. Martono, W., and Bahia, H. U. (2008). "Developing a surrogate test for fatigue of asphalt binders." Proceedings from the 87th Annual Meeting of the Transportation Research Board. Masad, E., Somadevan, N., Bahia, H. U., and Kose, S. (2001). "Modeling and experimental measurements of strain distribution in asphalt mixes." Journal of Transportation Engineering, 127(6), Pell, P. S. (1962). "Fatigue Characteristics of Bitumen and Bituminous Mixes." Proceedings, International Conference on the Structural Design of Asphalt Pavements, v1,

15 Roque, R., Birgisson, B., Drakos, C., and Dietrich, B. (2004). "Development and field evaluation of energy-based criteria for top-down cracking performance of hot mix asphalt." J. Assn. Asphalt Paving Technologists, v73, Santagata, E., Baglieri, O., and Dalmazzo, D. (2008). "Experimental investigation on the fatigue damage behaviour of modified bituminous binders and mastics." J. Assn. Asphalt Paving Technologists, v77, Santagata, E., Baglieri, O., Dalmazzo, D., and Tsantilis, L. (2009). "Rheological and Chemical Investigation on the Damage and Healing Properties of Bituminous Binders." J. Assn. Asphalt Paving Technologists, v78. Shenoy, A. (2002). "Fatigue testing and evaluation of asphalt binders using the dynamic shear rheometer." Journal of Testing and Evaluation, 30(4), Tsai, B.-W., Monismith, C. L., Dunning, M., Gibson, N., D'Angelo, J., Leahy, R., King, G., Christensen, D., Anderson, D., Davis, R., and Jones, D. (2005). "Influence of asphalt binder properties on the fatigue performance of asphalt concrete pavements." J. Assn. Asphalt Paving Technologists, v74, Wen, H., and Bahia, H. U. (2009). "Characterizing Fatigue of Asphalt Binders Using Continuum Damage Mechanics." Transportation Research Board 88th Annual Meeting. Zeng, M., Bahia, H. U., Zhai, H., Anderson, M. R., and Turner, P. (2002). "Rheological modeling of modified asphalt binders and mixtures." J. Assn. Asphalt Paving Technologists, v70,