Fatigue Evaluation of Warm-Mix Asphalt Mixtures

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<ul><li><p>26</p><p>Temperatures used at pavement production plants have a hugeand direct impact on binder aging. Research over the past decadeshas shown that binder aging reflects on the fatigue cracking resis-tance of asphalt pavement mixes. WMA mixtures are produced andplaced by using temperatures 30F to 100F lower than those ofHMA mixtures. WMA production temperatures cause less binderoxidation. The effect of this phenomenon on the mixes fatigue crackresistance is still not well defined (3).</p><p>Several tests are available to evaluate the fatigue characteristics ofasphalt mixtures. The cyclic, four-point bending beam tests have beenused over the years to test laboratory or field samples for the purposeof studying pavement fatigue cracking characteristics. Four-pointbending beam tests are performed by using large samples prepared inthe lab or on slabs trenched from the field (4, 5).</p><p>A uniaxial, cyclic fatigue pushpull test can be performed oncylindrical asphalt samples, similar to samples used for dynamicmodulus testing, and they offer an easier alternative to the four-point bending beam test. Advantages of this type of testing over thecyclic, four-point bending beams are that samples can be easily pre-pared by using the Superpave gyratory compactor. Also, the testcan be easily performed by using the new asphalt mixture perfor-mance tester. Advanced mathematical theories and the viscoelasticcontinuum damage (VECD) approach can be easily used to analyzethe data (3, 6, 7 ).</p><p>The objectives of this study were (a) to evaluate the impact of threeWMA technologies on the fatigue cracking resistance of asphalt mix-tures by using a uniaxial, cyclic, direct tension compression test and(b) to analyze the data by taking a VECD approach.</p><p>EXPERIMENTAL PLAN AND METHOD</p><p>To achieve the objectives of this study, an experimental plan wasdeveloped (Figure 1).</p><p>WMA asphalt should have a strong impact on asphalt mixture per-formance because mixing and compaction temperatures are lowerthan conventional HMA temperatures. The binder is less aged; there-fore it is less oxidized. The proposed research evaluated the fatiguecracking resistance of WMA mixtures as compared with that of HMAmixtures by using a simplified VECD approach.</p><p>Two of the mixtures used in the NCHRP Project 9-43 mix designexperiment were used in this study as control mixtures (3). A uni-axial, cyclic, direct tension compression fatigue test was performedon the two control mixtures, as well as on the control mixtures pro-duced, by using three WMA technologies: Advera, Evotherm 3G,and Sasobit. All mixtures were produced by using a PG 64-22 virginbinder. The dosage rate of the Advera was 0.25% by weight of the</p><p>Fatigue Evaluation of Warm-Mix Asphalt MixturesUse of Uniaxial, Cyclic, Direct Tension Compression Test</p><p>Mohamed Monir Haggag, Walaa S. Mogawer, and Ramon Bonaquist</p><p>Warm-mix asphalt (WMA) is the generic term used to refer to a groupof technologies that are used to produce asphalt pavement mixtures attemperatures lower than those of traditional hot-mix asphalt (HMA).One of the potential benefits of WMA is that it provides better fatiguecracking characteristics than does HMA. The lower temperaturesshould reduce the aging of asphalt binders that occur during produc-tion. The reduced aging of asphalt binders should lead to improvedfatigue characteristics of asphalt mixtures. The research reported hereaddressed two main objectives. The first was to study the impacts ofthree WMA technologies on the fatigue cracking resistance of HMA byusing one asphalt binder and two aggregate sources. The fatigue char-acteristics were measured by using a uniaxial, cyclic, direct tension com-pression test. The second objective was to analyze the data produced bythe test by using the simplified viscoelastic continuum damage approachproposed in the NCHRP 9-43 Phase I report. Three WMA technologieswere used: Advera, Evotherm 3G, and Sasobit. All mixtures were pro-duced by using a PG 64-22 virgin binder. Data showed no significant dif-ference between HMA mixtures and WMA mixtures for each mix exceptfor the Advera.</p><p>Warm-mix asphalt (WMA) refers to a group of technologies that isused to produce asphalt pavement mixes at temperatures lower thanthose of traditional hot-mix asphalt (HMA) (1). Many new tech-nologies are now in use (e.g., foaming, organic, and chemical WMAtechnologies) and more may be coming soon in the asphalt pave-ment market. European countries that have used WMA technolo-gies for a few years cite numerous economical, environmental,and potential mechanical advantages of the WMA over HMA andcold asphalt mixtures (2). In the United States, state agencies havefocused their efforts on the development of standard specificationsfor the use of WMA technologies. To assist in the endeavor, a stan-dard WMA mix design is needed. NCHRP has developed Project 9-43 to address the issue of having a standard mix design procedurefor WMA (3).</p><p>M. M. Haggag, Department of Civil and Environmental Engineering, Highway Sus-tainability Research Center, University of Massachusetts, Dartmouth, 151 Mar-tine Street, Room 131, Fall River, MA 02723. W. S. Mogawer, Department ofCivil and Environmental Engineering, Highway Sustainability Research Center,University of Massachusetts, Dartmouth, 285 Old Westport Road, North Dart-mouth, MA 02747. R. Bonaquist, Advanced Asphalt Technologies, LLC, 108Powers Court, Suite 100, Sterling, VA 20166. Corresponding author: W. S.Mogawer, wmogawer@umassd.edu.</p><p>Transportation Research Record: Journal of the Transportation Research Board,No. 2208, Transportation Research Board of the National Academies, Washington,D.C., 2011, pp. 2632.DOI: 10.3141/2208-04</p></li><li><p>mixture. The remaining technologies were dosed by the weight ofbinder in the mixture. The Evotherm 3G dose was 0.5%, and forSasobit it was 1.5%.</p><p>The control HMA mixtures were prepared at the recommendedviscosity-based mixing and compaction temperatures (320F and310F). WMA mixtures were prepared at the midpoint of the tem-peratures used in the mix design experiment of NCHRP Project 9-43 (250F and 240F). All produced samples were long-term,oven-aged, according to AASHTO R30, to simulate the effect oflong-term aging. Continuum damage theory was used to charac-terize the fatigue resistance of the eight mixtures. Analysis of vari-ance was used to decide whether or not there were significantperformance differences between the WMA and HMA mixtures.Table 1 is a summary of the two mixtures used in the proposedstudy.</p><p>REDUCED CYCLES ANALYSES PROCEDURE</p><p>Data from the uniaxial, cyclic, direct tension compression fatiguetest were analyzed by using the concept of reduced cycles andeffective strain, which Christensen and Bonaquist developed (6).The main aim of the proposed approach was to get plots of theratio of damaged modulus to the undamaged modulus as a func-tion of the damage parameter S collapse by choosing the appro-priate value for the continuum damage constant . Reducedcycles could be calculated accurately and without the integrationprocesses that might be difficult for many practicing engineers. Itcan be shown theoretically that reduced cycles are equivalent to thecontinuum damage parameter S (3). Reduced cycles are defined byEquation 1:</p><p>Haggag, Mogawer, and Bonaquist 27</p><p>where</p><p>NR = reduced cycles;NR-ini = initial value of reduced cycles, prior to the selected</p><p>loading period;N = actual loading cycles;fo = reference frequency (10 Hz suggested);f = actual test frequency;</p><p>E*LVE = undamaged [linear viscoelastic (LVE)] dynamic mod-ulus under given conditions, lb/in.2;</p><p>E*LVE/o = reference initial (LVE) dynamic modulus, lb/in2 (theLVE modulus at 20C is suggested);</p><p> = continuum damage material constant with a typical valueof about 2.0;</p><p>N N N ffE</p><p>ER Ro</p><p>o</p><p>E</p><p>= +</p><p>-ini</p><p>LVE</p><p>LVE</p><p>*</p><p>*</p><p>2</p><p>EE</p><p>o</p><p>TT</p><p>21 1</p><p>( )</p><p>HighAbsorptionAggregates</p><p>LowAbsorptionAggregates</p><p>1) No WMA (Control)2) 0.25% Advera3) 1.5% Sasobit4) 0.5% Evotherm 3G</p><p>Warm MixAsphalt(WMA)</p><p>Technology</p><p>9.5 mmMixtureDesign</p><p>DesignGyrations</p><p>Ndesign = 75</p><p>DesignGyrations</p><p>Ndesign = 100</p><p>PG64-22Virgin Binder</p><p>Fatigue TestUniaxial Cyclic DirectTension-Compression</p><p>Test</p><p>Data AnalysisSimplified ViscoelasticContinuum Damage -</p><p>Reduced Cycles &amp;Effective Strain</p><p>Compare FatiguePerformance of WMA</p><p>and HMA Mixtures</p><p>FIGURE 1 Experimental plan.</p><p>TABLE 1 Design Properties for Fatigue Mixtures</p><p>Control ControlProperty Mixture 1 Mixture 2</p><p>Mix number 4 6Design gyrations 75 100Aggregate water absorption (%) 1.6 1.3RAP No NoNMAS (mm) 9.5 9.5Aggregate source</p><p>Coarse PA gravel VA diabaseFine PA limestone VA diabase</p><p>PA gravel Natural sandRAP None None</p><p>GradationSieve size (mm)</p><p>12.5 100a 100a9.5 98a 98a4.75 63a 53a2.36 44a 40a1.18 32a 31a0.6 22a 22a0.3 12a 12a0.15 5a 7a0.075 3.0a 4.8a</p><p>Aggregate property (%)FAA 43.5 48.3CAA 98/95b 100/100bFlat and elongated 7.4 7.6Sand equivalent 80.2 76.7</p><p>Binder content (wt %) 6.3 5.7Effective binder content (wt %) 5.3 4.7Air voids (vol %) 4.3 3.7Voids in mineral aggregate (vol %) 16.3 15.1Effective binder content (vol %) 12.0 11.4Voids filled with asphalt (%) 73.6 75.5Dust-to-effective-asphalt ratio 0.6 1.0</p><p>NOTE: PA = Pennsylvania, VA = Virginia, RAP = reclaimed asphalt pavement,NMAS = nominal maximum aggregate sizes, CAA = coarse aggregate angular-ity, FAA = fine aggregate angularity.aPercent passing sieve.bNumbers indicate that 98% (100%) of CAA has one or more fractured facesand 95% (100%) has two or more fractured faces.</p></li><li><p>E = effective applied strain level, applied strain minus theendurance limit strain;</p><p>oE</p><p>= reference effective strain level (0.0002 suggested); and= shift factor at test temperature T relative to reference</p><p>temperature To.</p><p>The use of the concept of reduced cycles and effective strain isaccompanied by many advantages compared with other continuumdamage approaches (3). Here are some of the advantages:</p><p>1. Reduced cycles are shown to be theoretically equivalent to thecontinuum damage parameter S.</p><p>2. Reduced cycles can be calculated accurately by using the pre-viously discussed equation.</p><p>3. Numerical integration is avoided, and assumptions are usedthat concern the relationship between damage ratio C and S.</p><p>4. The concept of reduced cycles is much easier for many prac-ticing engineers to understand.</p><p>5. The outcome of the proposed approach can be used for directfatigue performance comparisons between asphalt mixtures.</p><p>6. The proposed analysis provides information on the endurancelimits of mixtures tested.</p><p>The following procedure describes the analysis of uniaxial fatiguedata by using continuum damage, reduced cycles, and an effectivestrain approach (3). The same procedure was followed in this study.</p><p>1. Choose the reference conditions. A strain of 0.000200, peak topeak, and a temperature of 68F (20C) are suggested to be used asreference strain and temperature values, respectively. The undam-aged modulus or LVE modulus at 68F (20C) should be used as thereference modulus. A frequency of 10 Hz should be used as the ref-erence frequency (most commonly used frequency in fatigue tests).</p><p>2. Determine timetemperature shift factors through the mastercurves that resulted from performance of dynamic modulus on twosamples that were produced by using the target mix.</p><p>3. Test a total of four to eight samples by using the uniaxial pushpull test, with two to four samples at both 39.2F (4C) and 68F(20C). Different temperatures can be used, although some difficul-ties might arise from the use of higher or lower temperatures andstrains and the running of the proposed analysis.</p><p>4. Calculate C and the reduced cycles for each test with the aid ofa spreadsheet. C can be calculated by using Equation 2:</p><p>where E*n is damaged modulus at cycle n.Reduced cycles can be used with Equation 1. A value of two can</p><p>be used for the continuum damage constant and an endurancelimit strain of zero. Variation of strain values during the test can becompensated for by splitting the data into segments, and by calcu-lating reduced cycles for each segment separately. Add this value tothe value preceding it.</p><p>The LVE modulus can be estimated by visual examination of aplot of E* as a function of loading cycles at the lowest tested strainlevel. The estimated LVE modulus should be within a few percentagepoints of the highest observed value.</p><p>In some cases, macro damage (localization) might take place. Theproposed continuum damage approach is not applicable after the</p><p>CE</p><p>En</p><p>=</p><p>*</p><p>*( )</p><p>LVE</p><p>2</p><p>TTo</p><p>28 Transportation Research Record 2208</p><p>localization point. During the test, the modulus should decreasesmoothly otherwise. Data after macro damage should be discardedfrom the analysis.</p><p>5. Fit Equation 3 to the C versus NR data.</p><p>where K1 = cycles to 50% damage (the fatigue half-life) and K2 = fitting parameter.</p><p>Linear regression can be used for the fitting by performing alogarithmic transformation of Equation 3 to produce the following:</p><p>where A = K2 (ln K1) and B = K2.While the proposed continuum damage approach is applied at low</p><p>strains, the measured sample modulus can approach the LVE, whichcauses noisy data when Equation 4 is used. A weighted, least-squares approach to linear regression should be used for this reason,with a weight of NR0.5. With this approach, little weight is given todata with little or no damage, while more weight is given to data pointsthat present more damage. Less-weight data will have negligible effecton function that relates C and NR and, also, better distribution of theresiduals.</p><p>6. Use trial and error, and keep the continuum damage constant at 2.00, to adjust the endurance limit strain for the data at 68F(20C) until the R2 value for the regression is maximized. Thenadjust the endurance limit strain value for the data at 39.2F(4C); again, until the R2 value for the regression is maximized.It is recommended not to change the value of to decrease the number of variables. If all trials did not result in complete convergence, try other values, which might result in completeconvergence.</p><p>7. Determine the value of the fatigue half-life K1 and appropriatelimits; 2-s limits are suggested for standard presentation of data.The fatigue response of two materials for which these confidencelimits do not overlap should be considered statistically different.This is a conservative approach.</p><p>A proposed standard practice for continuum damage fatigueanalysis, on the basis of the improved procedure, was developedunder the Asphalt Research Consortium. It included a document inthe form of an AASHTO standard practice and an accompanyingMicrosoft Excel workbook to perform the analysis.</p><p>RESULTS AND DATA ANALYSIS</p><p>Figure 2 illustrates the E* master curve for the control and theWMA mixtures for the mixtures designed by using 100 gyrations.It showed no significant difference in the stiffness of the controlmixture and the three WMA mixtures at all temperatures and frequencies.</p><p>The trend of the control mixture and the two WMA mixtures,Advera and Evotherm 3G, which showed no significant difference,can be seen in Figure 3. The control, Advera, and Evotherm 3G</p><p>ln ln ( )1 1 4C</p><p>A B NR</p><p> = +</p><p>CNK...</p></li></ul>

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