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24 Description of Tests by Reinke (45). Steady shear flow tests with a DSR utilize a one directional turning action of the top plate with a constant Binder Tests stress. Viscosities were measured at three temperatures (76°C, Equiviscous Tests. Rotational viscosity tests were con- 82°C, and 88°C) over a series of stress levels from 0.33 Pa to ducted to establish the traditional equiviscous temperatures for 500 Pa to evaluate shear dependency of the binders. Figure 12 mixing and compaction. These tests were performed at 135°C shows example data obtained using Binder Y. The viscosities and 165°C with a Brookfield model DV-II+ Rotational Visco- at 500 Pa measured for each temperature are then plotted meter using a shear rate of 6.8 1/s. using a log viscosity versus log temperature chart as seen in High Shear Rate Viscosity. Following the method devel- Figure 13 and extrapolated to obtain temperatures correspon- oped by Yildirim, viscosity measurements were also made with ding to 0.17 ± 0.02 Pa s and 0.35 ± 0.03 Pa s for mixing and the Brookfield rotational viscometer at temperatures ranging compaction, respectively. from 120°C to 180°C (248°F to 356°F) in 15°C increments Phase Angle Method. This method, conceived and de- using as much of the range of shear rates possible with the in- veloped by John Casola as part of the study, is based on the strument as each individual binder and temperature combina- observation that the phase angle from dynamic shear rheol- tion would allow. Example data are shown in Figure 9 and ogy is a consistency measure that takes into account the visco- Figure 10. For binders that exhibit non-Newtonian behavior, elastic nature of asphalt binders. The development and test- a Cross-Williams model was fit to the viscosity-shear rate data ing of the Phase Angle method for this project was conducted to estimate viscosity of each binder corresponding at a shear at the Malvern Instruments facility in Southborough, MA. rate of 500 1/s. High shear rate viscosities were plotted on a log The procedure consists of performing a frequency sweep on viscosity versus log temperature chart, as shown in Figure 11, unaged binder using a DSR meeting Superpave PG binder test- to determine mixing and compaction temperatures corre- ing requirements and employing the same geometries and tem- sponding to 1.7 ± 0.02 Pa s and 2.8 ± 0.03 Pa s, respectively. peratures used in routine PG binder grading. Tests were con- Steady Shear Flow. Steady shear flow viscosity measure- ducted at four temperatures, typically 50°C, 60°C, 70°C, and ments were made with a TA Model CSA DSR using parallel 80°C, and at frequencies from 0.001 to 100 rad/s with 10 points/ plate geometry and following the procedure recommended decade. Strain was maintained at 12%. Data collected included Rotational Viscosity 165°C 0.600 Bnder X Test 1 Binder X Test 2 Binder Y Test 1 Binder Y Test 2 Binder Z Test 1 Binder Z Test 2 0.500 0.400 Viscosity, PaS 0.300 0.200 0.100 0.000 0 10 20 30 40 50 60 70 80 90 100 Shear Rate, 1/sec Figure 9. Example data from rotational viscosity test, viscosity at 165C versus shear rate for three binders.
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25 Rotational Viscosity 180°C 0.35 Binder X Test 1 Binder X Test 2 Binder Y Test 1 Binder Y Test 2 0.3 Binder Z Test 1 Binder Z Test 2 0.25 Viscosity, PaS 0.2 0.15 0.1 0.05 0 0 10 20 30 40 50 60 70 80 90 100 Shear Rate, 1/sec Figure 10. Example data from rotational viscosity test, viscosity at 180C versus shear rate for three binders. 500 100 10 Viscosity, Pa-s 1 0.1 52 58 64 70 76 82 88 100 120 135 150 165 180 200 Temperature, C Figure 11. Temperature viscosity plot showing mixing and compaction temperatures for high shear viscosity method.
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26 Binder Y Steady Shear Flow #1 2000 1800 1600 1400 76C 1200 Viscosity, PaS 82C 88C 1000 800 600 400 200 0 0 100 200 300 400 500 600 Shear Stress, Pa Figure 12. Example data of steady shear flow method, viscosity versus shear stress at three test temperatures. Steady Shear Flow Viscosity at 500 Pa 500 100 10 Viscosity, Pa-s 1 Compaction Range Mixing Range 0.1 52 58 64 7 0 76 82 88 100 12 0 1 35 150 165 180 200 Temperature, C Figure 13. Example results of steady shear flow method, viscosity data from Figure 12 at 500 Pa shear stress extrapolated to determine mixing and compaction temperatures.
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27 standard values of shear moduli, temperature, frequency, and atures that were considered lower than what are typically used phase angle. Phase angle master curves were developed from in practice, particularly for modified binders. The frequency- the data using a reference temperature of 80°C. This tempera- mixing temperature regression equation was adjusted to bal- ture range provides reliable phase angle resolution. Rheologi- ance the desire to increase the mixing temperatures, particularly cal testing of a wide variety of asphalt binders at temperatures for modified binders, with the aim to minimize differences with above 135°C generally yields Newtonian behavior (phase equiviscous mixing temperatures for unmodified binders. angles at or very near 90°). The selection of 80°C as the refer- The resulting adjusted equation for mixing temperature using ence temperature was also to avoid confusion with standard- the Phase Angle method was grade high temperatures (e.g., 76°C, 82°C). Figure 14 shows phase angle and shear modulus master curves for a typical Mixing Temperature ( ° F ) = 325 -0.0135 (3) modified binder. The shear modulus is shown simply to verify the shifting of the data to construct the phase angle master Figure 15 shows a plot of the EC 101 plant mixing tempera- curve. A smooth and straight shear modulus master curve ture ranges and midpoints with the Phase Angle method results illustrates a good data shift. The region of the phase angle based on the preliminary and the adjusted frequency-mixing master curve between = 90° and 85° represents the transi- temperature relationships for the eight binders that have PG tion from purely viscous to visco-elastic behavior. Therefore, that are provided in the EC 101 guide. It can be seen that the this is a region that can easily differentiate rheological behav- preliminary equation yielded results closer to the EC 101 iors among binders. The frequency corresponding to = 86° midpoint and the adjusted equation yielded results closer to was selected as a reasonable reference point for this technique. the maximum of the EC 101 range. A correlation of the mixing Casola established an initial relationship between this fre- temperatures using the adjusted Phase Angle method and the quency and mixing temperature using the recommended plant corresponding EC 101 maximum mixing temperature yielded mixing temperatures for each binder grade from the EC 101. the following regression: A curve-fitting program was used to establish a preliminary Phase Angle method power-law regression to fit the data: TM = 1.1 × ( EC 101 Tmax ) - 33, R 2 = 0.92 Mixing Temperature ( ° F ) = 310 -0.01 (2) Establishing a relationship between frequency and com- paction temperature began with the observation that com- where the frequency, , is in radians/sec. Since the midpoints paction temperatures for unmodified binders were typically of recommended plant mixing temperatures in EC 101 are 20°F to 25°F lower than the mixing temperature based on the conservative toward lower temperatures, the preliminary equiviscous method. Using this simple offset, a similar power frequency-temperature relationship yielded mixing temper- function to the one for mixing temperature was developed Phase Angle Master Curve G* Figure 14. Phase Angle master curve of a typical modified asphalt.
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28 360 Max 340 Midpoint EC 101 Plant Mixing Temperatures (F) Min 320 Preliminary Adjusted 300 280 260 240 220 PG 46-34 PG 46-28 PG 52-46 PG 52-28 PG 58-34 PG 58-28 PG 58-22 PG 64-34 PG 64-28 PG 64-22 PG 67-22 PG 70-28 PG 70-22 PG 76-28 PG 76-22 PG 82-22 Figure 15. Plot showing results of the preliminary and adjusted Phase Angle mixing temperatures to the recommended mixing range from EC 101. for compaction temperature. This relationship is shown as meter was added to the oven's flue to continuously record the Equation 4: opacity. Asphalt samples were placed in thin film oven pans and loaded in a rack placed on the balance in the oven. The Compaction Temperature ( °F ) = 300 -0.012 (4) rack held five pans, each filled with 50 grams of asphalt binder. Tests were run for 2 hours at 130°C, 150°C, 170°C, and 190°C Example results for two binders are used to illustrate the (266°F, 302°F, 338°F, and 374°F). Opacity, mass loss, and Phase Angle method for selecting mixing and compaction temperature were recorded versus time and temperature for temperatures. Figure 16 shows the results of frequency sweep each binder. Figure 17 illustrates opacity data for a binder testing for an unmodified PG 52-34 and an SBS-modified PG tested at four temperatures. 64-40. The figure shows that the PG 52-34 binder reached a Following the SEP tests, the binders were re-graded in accor- phase angle of 86 at a frequency of 158.5 rad/sec, whereas the dance with AASHTO M 320 and also tested with the MSCR PG 64-40 binder crossed the reference phase angle at a fre- test following AASHTO TP 70-07. The binder properties of quency of 1.1 rad/sec. Using the temperature-frequency rela- the original binders and binders recovered from the SEP tests tionships in Equations 3 and 4, the mixing and compaction were used to evaluate changes due to thermal degradation. The temperatures for the two binders are MSCR tests on the original unaged binders were conducted by the Turner-Fairbank Highway Research Center. Post SEP test · PG 52-34: binders were graded and tested at NCAT. Mixing temperature = 325(158.5)0.0135 = 303.5°F The MSCR test consists of a series of 1-second creep load- Compaction temperature = 300(158.5)0.012 = 282.3°F ing (constant stress) times followed by 9-second rest periods. · PG 64-40: The samples were loaded for 10 cycles each at creep loads of Mixing temperature = 325(1.1)0.0135 = 324.6°F 25, 50, 100, 200, 400, 800, 1600, 3200, 6400, and 12,800 Pa. Compaction temperature = 300(1.1)0.012 = 299.7°F Test temperatures were 58°C, 64°C, 70°C, and 76oC. An unmodified binder typically has low elasticity and, con- SEP Test. All binders were tested using the SEP test devel- sequently, does not recover most of the deformation caused oped by Stroup-Gardiner and Lange (30). All of the SEP tests by the loading stress during the 9-second rest period. This re- except for those on the validation binders were performed by sults in a plot that resembles stair steps, with vertical rises dur- Stroup-Gardiner at Auburn University. This test utilized a ing the 1-second loading period, and horizontal plateaus dur- Thermolyne moisture content oven equipped with an inter- ing the 9-second rest time. Unmodified binders accumulate nal scale for measuring mass loss during heating. To quantify unrecovered strain during the MSCR test. Binders that have the amount of smoke emitted from the binders, an opacity been modified with elastomeric modifiers, on the other hand,
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29 Phase Angle Master Curve G* (a) Phase Angle Master Curve G* (b) Figure 16. (a) Comparison of a PG 52-34 and (b) a PG 64-40 (bottom graph) at a Threshold Phase Angle of 86. typically recover nearly all of the shear strain under the same where 10 is strain at end of 10th cycle and 0 is strain at begin- conditions during the rest periods. ning of first cycle. The output of the MSCR test is the nonrecovered compli- For each stress level, Jnr is calculated using Equation 6: ance, Jnr, of the binder at each level of applied stress. For each stress level, the total amount of unrecovered strain is calcu- u J nr = (6) lated using Equation 5: u = 10 - 0 (5) where u is unrecovered strain and = applied stress, Pa.