Mix Design Practices for Warm-Mix Asphalt (2011) / Chapter Skim
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From page 23... ...
Figures 6 through 10 show the effect of sample reheating on the dynamic modulus master curve for a control HMA and four WMA processes: Aspha-min, Evotherm ET, Sasobit, and LEA. The error bars in these figures represent 95 percent confidence intervals for the mean based on a typical coefficient of variation for the dynamic modulus test of 14 percent and the number of samples that were tested.
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From page 24... ...
Effect of sample reheating on the dynamic modulus of the St. Louis Aspha-min mixture.
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From page 25... ...
Effect of sample reheating on the dynamic modulus of the St. Louis Sasobit mixture.
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From page 26... ...
The relationship shown in Figure 11 was used to estimate the effect of WMA production temperature on the hightemperature properties of the binder. To limit the change in high-temperature grade to one-half of one grade level, divide 3°C by the slope obtained from the binder aging index and the relationship shown in Figure 11.
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From page 27... ...
The data collected in the RTFOT study indicated that the low-temperature grade improvement was approximately 0.035°C per °C reduction in RTFOT temperature for PG XX-28 binders; 0.025°C per °C reduction in RTFOT temperature for PG XX-22 binders; and 0.022°C per °C reduction in RTFOT temperature for PG XX-16 binders. Using these values and typical HMA production temperatures for various binder grades given in Table 20, low-temperature grade improvements for RAP blending chart analyses were developed for some common grades 27 0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Binder Aging Index R at e of C ha ng e of R TF O T Hi gh T em pe ra tu re G ra de W ith R TF O T Te m pe ra tu re , o C/ o C Rate of high-temperature grade change (oC/oC)
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From page 28... ...
For a mixture using PG 64-22 virgin binder and a WMA production temperature of 250°F, the virgin binder lowtemperature continuous grade would be improved 0.6°C to account for the lower WMA production temperature. 3.1.3 Short-Term Oven Conditioning Study An important step in mixture design and analysis is shortterm oven conditioning of laboratory-prepared loose mix prior to compaction.
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From page 29... ...
The error bars in Figure 13 are 95-percent confidence intervals for the average indirect tensile strength. The graphical analysis shown in Figure 13 suggests that 2 h of conditioning at the compaction temperature provides tensile strengths for laboratory-prepared specimens that are approximately equal to those for field mixtures.
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From page 30... ...
In the laboratory mixing experiment, HMA and WMA mixtures incorporating RAP were prepared at different temperatures and short-term conditioned for periods ranging from 0.5 h to 2.0 h. The mixing of the new and recycled binders was quantified by comparing dynamic moduli measured on 30 Figure 14.
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From page 31... ...
/sinδ = 2.20 kPa. 3.1.5 Workability Study The workability study was conducted to identify a workability test to be used in place of viscosity-based mixing and compaction temperatures to directly evaluate workability and compactability of WMA mixtures.
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From page 32... ...
The workability study further found that the number of gyrations to reach 92-percent relative density in the gyratory compactor, shown in Figure 17, had similar sensitivity to temperature and WMA additives. The workability study demonstrated that it is possible to measure differences in the workability and compactability of WMA as compared to HMA.
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From page 33... ...
Coating is evaluated at the planned production temperature. Limits on the gyrations to 92 - percent relative density at the planned field compaction temperature and 54°F below the planned field compaction temperature were added to control compactability.
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From page 34... ...
In the preliminary mixture design and analysis procedure for WMA, it was envisioned that the amount of RAP that could be added to WMA would be limited by the planned WMA production temperature and the compatibility of the new and RAP binders. Since the Phase I RAP study showed that substantial mixing of the RAP and new binders does occur at WMA process temperatures, the limitations on RAP usage included in the preliminary procedure were removed.
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From page 35... ...
Average difference in design VMA (WMA-HMA) from the mix design study (error bars are ± 95-percent one-sided confidence intervals)
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From page 36... ...
Average difference in design VBE (WMA-HMA) for the mix design study (error bars are ± 95-percent one-sided confidence intervals)
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From page 37... ...
The paired difference statistical analysis presented in Section E7 of Appendix E found that the compactability of WMA mixtures (as measured by the increase in the gyrations to 92-percent relative density when the compaction temperature is decreased 54°F [30°C]
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From page 38... ...
Effect of WMA process on the average difference in increase in gyrations to 92-percent relative density for a 54°F decrease in compaction temperature (WMA-HMA) for the mix design study (error bars are ± 95-percent one-sided confidence intervals)
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From page 39... ...
Table 24 summarizes the tensile strength ratios for all of the mixtures included in the study. Most of the WMA mixtures had tensile strength ratios below the AASHTO M 323 minimum of 80 percent.
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From page 40... ...
3.3.1.4 Rutting Resistance Rutting resistance in the revised preliminary mixture design procedure is evaluated using the flow number, AASHTO TP 79, Determining the Dynamic Modulus and Flow Number for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT)
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From page 41... ...
Analysis of the data in Table 25 suggests that it will be difficult for WMA mixtures designed for 10 MESAL or greater to meet the flow number rutting resistance criteria developed in NCHRP Project 09-33. 3.3.2 Field Validation Study The field validation study addressed several parts of the revised preliminary mixture design procedure including (1)
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From page 42... ...
Project Process Production Temperature (°F) High Intermediate Low Specified NA 58.0 19.0 28.0 Control 280 59.3 14.2 30.6 Advera 250 60.0 13.7 31.6 Evotherm 250 61.3 14.1 31.1 Colorado I-70 Sasobit 250 63.9 15.1 29.9 Specified NA 58.0 16.0 34.0 Control 325 60.0 11.1 34.1 Advera 275 56.3 8.9 36.2 Yellowstone National Park Sasobit 275 60.7 10.1 35.6 Specified NA 64.0 22.0 28.0New York Route 11 LEA 210 60.5 14.0 31.1 Specified NA 64.0 25.0 22.0 Control 320 67.7 22.0 24.6PennsylvaniaSR2007 Evotherm 250 67.2 22.0 24.9 Specified NA 64.0 25.0 22.0 Control 310 66.6 24.1 22.5 Advera 250 67.0 22.9 24.1 Gencor 250 67.5 21.7 25.7 LEA 210 63.2 21.6 25.4 Pennsylvania SR2006 Sasobit 250 72.9 23.3 22.5 Specified NA 70.0 28.0 22.0Monroe, North Carolina Astec 275 71.5 23.7 23.9 Average Difference in Continuous Grade Temperature (°C)
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From page 43... ...
. The mixture dynamic modulus master curve is the required material property for this analysis.
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From page 44... ...
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.1 1 10 100 Traffic, Million ESALs Pr ed ic te d Ru t D ep th , i n Control Sasobit Advera 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.1 10 1001 Pr ed ic te d Ru t D ep th , i n Traffic, Million ESALs PG 64-28 PG 64-28 LEA PG 70-22 LEA
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From page 45... ...
3.3.2.3 Short-Term Oven Conditioning For WMA and HMA, short-term oven conditioning of 2 h at the compaction temperature was determined by comparing properties of field-mixed, laboratory-compacted specimens with properties of laboratory-mixed, laboratory-compacted specimens for the mixtures from the Colorado I-70 project. The properties that were compared were maximum specific 45 Table 28.
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From page 46... ...
Figure 34 shows differences in indirect tensile strength for the field mixtures minus the laboratory mixtures. The error bars for the average difference in this figure are 95-percent con46 Figure 33.
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From page 47... ...
3.3.2.4 Specimen Fabrication In the validation study, the WMA specimen-fabrication procedures were used to fabricate specimens for several WMA processes including Advera, Astec Double Barrel Green, Evotherm DAT, Gencor Ultrafoam, LEA, and Sasobit. Figure 35 shows the difference in air voids at Ndesign between the WMA mixture and either the HMA job mix formula or the corresponding HMA control mixture.
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From page 48... ...
After mixing, the foamed mix is short-term aged at the compaction temperature for 2 h and compacted. The operation of the foaming machine presents some practical concerns for WMA laboratory mixture design.
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From page 49... ...
3.3.2.5 Coating and Compactability As required by the revised preliminary WMA mixture design procedure, coating and compactability were measured on all of the WMA mixtures in the field validation study. Coating was evaluated using AASHTO T 194, which counts the number of fully coated coarse aggregate particles in the mixture.
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From page 50... ...
Per the preliminary WMA mixture design procedure, the mixture was conditioned 2 h at the compaction temperature. The flow number test was conducted at the 50-percent reliability high pavement temperature from LTPPBind 3.1 for the project location.
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From page 51... ...
Although it is generally accepted that this conditioning represents the binder stiffening that occurs during construction, it appears from the shortterm conditioning study that this level of conditioning is more representative of the stiffness of the binder after some short period in service. The findings from the mix design study and the field validation study show that the rutting resistance of WMA mixtures that are conditioned 2 h at the compaction temperature, which represents the stiffness of WMA mixtures at the time of construction, generally fail criteria that are based on 4 h of conditioning at 275°F (135°C)
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From page 52... ...
3.3.4 Fatigue Study One of the potential benefits of WMA mixtures is improved fatigue characteristics in comparison to HMA mixtures due to the reduced aging that occurs during plant mixing at the lower WMA process temperatures. The fatigue study was designed to evaluate the fatigue resistance of WMA in comparison to HMA.
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From page 53... ...
. 4 6 75 100 1.6 1.3 No No 9.5 mm 9.5 mm Coarse PA Gravel VA Diabase Fine PA Limestone PA Gravel VA Diabase Natural Sand RAP None None Sieve Size, mm 12.5 100 100 9.5 98 98 4.75 63 53 2.36 44 40 1.18 32 31 0.6 22 22 0.3 12 12 0.15 5 7 0.075 3.0 4.8 FAA 43.5 48.3 CAA 98/95 100/100 Flat & Elongated 7.4 7.6 Sand Equivalent 80.2 76.7 6.3 5.7 5.3 4.7 4.3 3.7 16.3 15.1 12.0 11.4 73.6 75.5 0.6 1.0 Effective Binder Content, vol % Voids Filled With Asphalt, % Dust to Effective Asphalt Ratio Gradation Binder Content, wt % Effective Binder Content, wt % Air Voids, vol % Voids in Mineral Aggregate, vol % Aggregate Sources Aggregate Properties Mix Number Design Gyrations Aggregate Water Absorption, % RAP NMAS Table 33.
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From page 54... ...
The reduced cycles damage curves are very similar for the WMA processes and the HMA controls, providing further evidence that the fatigue performance of WMA and HMA mixtures produced from the same aggregates and binders will essentially be the same. 3.4 Draft AASHTO Standards Table 35 summarizes the major findings of the studies conducted during NCHRP Project 09-43 and the final disposition of each finding in the draft AASHTO standards that are the primary products of NCHRP Project 09-43.
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From page 55... ...
Summary of continuum damage fatigue parameters.
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From page 56... ...
0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 D am ag e Ra tio Reduced Cycles Control Advera Evotherm Sasobit Figure 42. Comparison of continuum damage fatigue curves for the 100-gyration mix.
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From page 57... ...
RAP RAP and new binders do mix at WMA process te mp eratures when conditioned 2 h at the com paction te mp erature. The Draft Appendix to AASHTO R 35 recommends limiting the high-temperature grade of the recovered RAP binder to the planned field compaction temperature of the WMA to ensure adequate mixing of the RAP and new binders.
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From page 58... ...
One of the major findings of the mixture design study conducted in Phase II of NCHRP Project 09-43 was that the volumetric properties of HMA and WMA mixtures having 1 percent or less binder absorption were very similar. It is, therefore, not necessary to have a separate design procedure for WMA because the major differences in the way WMA and HMA mixtures are designed are the specimenfabrication procedures and the evaluation of coating and compactability.
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From page 59... ...
The only additional requirement for WMA is that the high temperature of the "as recovered" RAP binder should be less than the planned field compaction temperature of the WMA mixture in order to ensure adequate mixing of the RAP and new binders. A table for low-temperature grade improvement of the virgin binder for RAP blending chart analysis is provided.
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