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Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt (2010)

Chapter: Chapter 2 - Research Approach

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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14367.
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14367.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14367.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14367.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14367.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14367.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14367.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14367.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14367.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14367.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14367.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14367.
×
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Suggested Citation:"Chapter 2 - Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14367.
×
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20 Experimental Plan Approach In developing the research approach for this study, several principles helped guide the plan for how and what should be in- vestigated. As stated in the objective for this study, the desired outcome was the development of a simple and effective method for determining mixing and compaction temperatures for asphalt binders. The researchers have taken the approach that for the procedure to be simple, it should be based only on binder properties and not involve aggregates or mineral filler. It is recognized that this is a significant practical limitation, but to include interactions of asphalt and aggregate would complicate the matter greatly. The researchers also constrained the evaluation to candidate tests that utilize existing equip- ment found in typical asphalt binder labs. Several candidate methods for determining mixing and compaction temperatures were examined in the experimen- tal plan. An expectation was that some measure of binder con- sistency would be a good indicator of how well binders coat and lubricate aggregate particles during mixing and compact- ing. One concern was that there also may need to be a maxi- mum mixing temperature to protect against overheating and damaging the binder during mixing. Therefore, some addi- tional experimentation was conducted to evaluate binder degradation due to exposure to elevated temperatures. However, in order to validate any simple binder procedure, a variety of mixture tests was considered necessary to char- acterize when the consistency of the binders is suitable for mixing with aggregates and subsequent handling and com- pacting of the mixtures. Since volumetric properties of asphalt mixtures compacted in SGC are insensitive to changes in binder consistency, other mix characteristics were examined to help identify temperatures that cause significant changes in the mixtures. Since experience is an excellent guide for what has worked and what has caused problems with regard to mixing and com- paction temperatures, consideration was given to such infor- mation. Accordingly, the equiviscous principle has worked well for unmodified binders. For modified binders, producers and users have found reasonable mixing and compaction tempera- tures for a wide variety of asphalt binders. These practical tem- perature ranges developed through field experience were used to check the reasonableness of the laboratory results. Overview of the Experimental Research Plan Based on the literature review and the guidance from the research panel, three candidate methods for selecting mixing and compacting temperatures were explored: 1. High shear rate viscosity, 2. Steady shear rate viscosity, and 3. A new approach based on the phase angle of binders. The high shear rate viscosity approach is based on the hy- pothesis that modified binders are shear thinning and that the reason SGC compaction is insensitive to binder viscosity or moduli is because the compaction process is a high shear domain for asphalt coatings on aggregates. The viscosity data generated with this method are easy to obtain and indicate the shear rate dependency of binders through typical mixing and compaction temperature ranges. The steady shear flow approach also takes into account the shear rate dependency of binders. Flow profiles for shear de- pendent binders show that viscosities tend to stabilize with increasing shear. The target shear stress of 490 Pascals was chosen by Reinke as a value where the viscosity measurements appeared more stable and within the capabilities of stan- dard asphalt DSR. Although this approach requires viscosity- temperature data to be extrapolated to high temperatures outside of the range of most DSRs, the method is fairly sim- ple and, based on a limited set of binders, appears to yield rea- sonable results for mixing and compaction temperatures. C H A P T E R 2 Research Approach

21 The third candidate procedure was developed during this project. This technique considers the non-Newtonian visco- elastic behavior of binders as measured using standard asphalt DSR equipment. In dynamic testing, the phase angle is the measure of the time lag between the applied stress and the re- sulting measured strain. For dynamic shear rheology, the phase angle identifies the relative elastic and viscous response to shear and can be easily measured over a range of temperatures and frequencies. Therefore, it was logical to explore the phase angle as an alternate consistency parameter that could be used to establish mixing and compaction temperatures. Although phase angle is a fundamental material property, its relation- ship to coating and lubricating aggregate particles is empiri- cal. However, this is also the case for viscosity at any shear rate. Binders also were analyzed with regard to their potential for emissions and thermal degradation over a range of tem- peratures normally used for storage and mixing. The results from these were anticipated to aid in establishing upper lim- its for mixing temperatures in the field and the lab. Mixture tests were performed to analyze the effects of tem- perature and binder consistency on aggregate coating, mix workability, lubrication and shear resistance during laboratory compaction, and the mechanical properties creep compliance and indirect tensile strength. The results of these tests were used to identify temperatures at which each binder provides similar mixture responses or where properties change significantly with temperature. The minimum temperature suitable for mixing should be based on how well the binder coats the aggregate in simulated plant mixing conditions. Two types of laboratory mixers were used to evaluate coating of binders on a base aggre- gate material at four temperatures. Coating percentages were evaluated by the Ross count method (ASTM D 2489). Mix workability tests were included in the test plan to char- acterize how binder consistency affects the reaction of HMA mixtures to manipulation by equipment and manual tools. Workability is considered to be an intermediate state between mixing and compaction. The simple workability test was anticipated to show how different binders change how easy or how difficult a mixture is to handle. Mix compaction tests were conducted with a SGC. Since many studies have shown that the densities of mixtures com- pacted in an SGC to a high number of gyrations are insensi- tive to binder consistency, compaction tests in this study used 25 gyrations. It was believed that differences due to binder consistency may be more apparent in the early part of the compaction process before aggregates lock up and dominate mixture shear resistance. A diagram illustrating the experimental plan is shown in Fig- ure 8. Part 1 included the series of binder tests that included testing of the 14 binders described in the following section using the candidate methods for determining mixing and compaction temperatures. Also included in Part 1 were the Dynamic Shear Rheology Rotational Viscosity Tests Smoke & Emissions Potential Test Grade Binders Before & After SEP Mix Workability Tests IDT Creep & Strength Test Mix Compaction Tests Mix Coating Tests Steady Shear Flow Tests Predict Mix & Compaction Temps Predict Mix & Compaction Temps Predict Mix & Compaction Temps Establish Max Temp to Avoid Emissions Establish Max Temp to Avoid Degradation Determine Minimum Mixing Temps Determine Intermediate Mix Handling Temps Determine Compaction Temps Range Correlations Determine Effect of Temps on Mix Props Check for Excessive Temps & Reasonableness 14 Asphalt Binders Part 1: Binder Testing Part 2: Mix Testing Select Best Method Validation of Method4 Asphalt Binders Mix Coating Tests Mix Workability Tests Mix Compaction Tests Draft New Test Method for Establishing Mixing & Compaction Temperatures Figure 8. Diagram illustrating experimental test plan.

22 analyses of emissions and degradation potential of the 14 binders using the SEP test. Part 2 of the testing plan included conducting the series of mixture tests with the 14 binders. Selection of the best method for determining mixing and compaction temperatures would be based on how well the temperatures predicted using the candidate methods from Part 1 correlated with the temperatures needed for good coat- ing, workability, and compaction as determined with the mixture tests in Part 2. Analysis of the SEP results and the change in binder properties before and after the SEP test were expected to provide information to be used for establishing maximum mixing temperatures so that emissions and/or binder degradation would not be a problem. Indirect tensile mixture tests also would be used to assess the need for upper limits for mixing temperatures. A small validation experiment was planned using a set of four new binders with a range of grades and modification types. The validation testing of the binders would include testing of the binders with the selected candidate method and performing mix coating tests, workability tests, and com- paction tests to see whether the predicted mixing and com- paction temperatures provided reasonable results. Materials The experimental plan included 14 different binders that represent a range of commonly used U.S. binder grades, crude sources, and modifiers as well as a few more unique modifi- cations systems. The list of binders with their reported grades, crude source, and modification systems is shown in Table 5. Throughout this report, data for modified binders are iden- tified with shaded rows in tables. Also shown in Table 5 are the recommended mixing and compaction temperatures that the producers have provided to their customers. Inconsisten- cies in the recommended temperatures are apparent. Some suppliers recommend a single temperature, whereas others give a range of temperatures as wide as 18°F (10°C). Also, Binders B and C are different PG, but have the same exact rec- ommended mixing and compaction temperatures. Binders G and H, which are both PG 76-22, have recommended mix- ing temperatures that differ by at least 10°F (6°C). A set of four different binders were selected for valida- tion testing of the method selected for establishing mixing and compaction temperatures. The validation binders set is shown in Table 6. Organization of the Test Plan As previously noted, the overall testing plan was divided into two parts. The first part dealt with testing binders and the sec- ond part involved testing those binders in aggregate mixtures. The mixture tests included in the experimental plan were not candidate procedures for establishing mixing and compaction temperatures. They were used in the experimental plan to eval- Binder I.D. Producer’s Reported Binder Grade Producer’s Recommended Mixing Temperatures Producer’s Recommended Compaction Temperatures Modification Type Crude Source B PG 64-40 302-320 284-311 SBS Canada C PG 70-34 302-320 284-311 SBS Canada D PG 58-28 295-309 275-284 None Alaskan Slope /Canada E PG 58-34 293-308 273-284 Air Blown Canada F PG 64-22 315 295 None North Sea G PG 76-22 335 315 SBS + PPA North Sea H PG 76-22 315-325 305-315 SBS Venezuela I PG 70-28 322-336 302-313 Air Blown Alaskan Slope /Canada J PG 64-16 307-313 267-273 None West Texas K PG 64-16 285-305 265-285 None California Valley L PG 76-22 320-330 290-300 18% Crumb Rubber California Valley M PG 82-22 285-300 250-260 SBS + Sasobit® Venezuela N PG 82-22 325-340 300-320 SBS Mexico O PG 64-28 313-324 291-300 None Venezuela Table 5. Binders included in the research.

uate how the consistency of the binders affects mixing and com- paction behavior. The mixture tests were a means to an end, the end being an independent set of measurements on the coatabil- ity and compactability to assess the predicted mixing and compaction temperatures from the candidate binder test meth- ods. Unless noted otherwise, all testing was conducted at the National Center for Asphalt Technology (NCAT) laboratory. Part 1: Binder Tests For each binder, a series of tests or experiments were used to characterize properties of the binder and evaluate changes in high temperature binder properties. These tests were per- formed at temperatures that span typical mixing and com- paction temperatures used for asphalt mixtures. The binder tests include the candidate procedures and additional tests to assess binder degradation. The binder tests were • Viscosity measurements using AASHTO T 316 to estimate traditional equiviscous temperatures. • Steady shear flow viscosity from a DSR extrapolated to higher temperatures. • Viscosity at various shear rates from a rotational visco- meter extrapolated to high shear rates. • Phase angle master curve from oscillation frequency sweeps using a DSR. • Opacity (smoke) and mass loss measurements from the SEP test. • Binder grading in accordance with AASHTO M 320 and Multiple Stress Creep Recovery (MSCR) tests following AASHTO TP 70 on binder samples before and after the SEP test to evaluate degradation. Part 2: Mixture Tests Mixture tests were included in the experimental plan to validate the predicted mixing and compaction temperatures from the candidate binder tests. For each of the study binders, mixture tests were used to establish relationships among tem- perature and coating, workability, and compactability. The mixture experiments included • Mix coating tests using a laboratory bucket mixer and a laboratory pugmill mixer. • Mix workability tests with the Instrotek workability device. • Mix compaction tests using a specially equipped Pine Instruments SGC model AFG1A (i.e., baby Pine). • Indirect tensile creep compliance and strength tests in accor- dance with AASHTO T 322. Most of the mixture tests were based on a fine-graded Super- pave mix design containing a blend of NCAT’s lab standard granite aggregate. The mix design for this baseline mix is in- cluded in Appendix B. To further evaluate the effects of aggre- gate type, aggregate gradation, and RAP content, an additional compaction experiment was conducted with a subset of four binders. The subset of four binders selected for this mixture ef- fects experiment were based on binders that exhibited dis- tinctly different behaviors. A description of the mixtures used in this subset of tests is shown in Table 7. Fine and coarse gra- dations were to assess how aggregate particle size distributions affect mixing and compaction. The two aggregate types (gran- ite and gravel) are distinctively different with respect to texture, shape, and absorption. Recycled asphalt pavement (RAP) also was included to evaluate the interaction of new asphalt and aged RAP binder. Binder I.D. Producer’s Reported Binder Grade Producer’s Recommended Mixing Temperature Producer’s Recommended Compaction Temperature Modification Type Crude Source W PG 82-22 345 325 SBS Venezuela X PG 70-28 308-315 279-285 Elvaloy North Sea Y PG 64-22 314-327 293-302 None Venezuela Z PG 76-22 347-353 317-322 SBS + Air Blown West Texas Table 6. Binders for validation of proposed new method for establishing mixing and compaction temperatures. Table 7. Mixture variations used in Compaction Experiment B. Aggregate Type Gradation Type RAP Content Granite Fine none Granite Fine 15% Granite Coarse 15% Gravel Fine 15% Gravel Coarse 15% 23

Description of Tests Binder Tests Equiviscous Tests. Rotational viscosity tests were con- ducted to establish the traditional equiviscous temperatures for mixing and compaction. These tests were performed at 135°C and 165°C with a Brookfield model DV-II+ Rotational Visco- meter using a shear rate of 6.8 1/s. High Shear Rate Viscosity. Following the method devel- oped by Yildirim, viscosity measurements were also made with the Brookfield rotational viscometer at temperatures ranging from 120°C to 180°C (248°F to 356°F) in 15°C increments using as much of the range of shear rates possible with the in- strument as each individual binder and temperature combina- tion would allow. Example data are shown in Figure 9 and Figure 10. For binders that exhibit non-Newtonian behavior, a Cross-Williams model was fit to the viscosity-shear rate data to estimate viscosity of each binder corresponding at a shear rate of 500 1/s. High shear rate viscosities were plotted on a log viscosity versus log temperature chart, as shown in Figure 11, to determine mixing and compaction temperatures corre- sponding to 1.7 ± 0.02 Pa  s and 2.8 ± 0.03 Pa  s, respectively. Steady Shear Flow. Steady shear flow viscosity measure- ments were made with a TA Model CSA DSR using parallel plate geometry and following the procedure recommended by Reinke (45). Steady shear flow tests with a DSR utilize a one directional turning action of the top plate with a constant stress. Viscosities were measured at three temperatures (76°C, 82°C, and 88°C) over a series of stress levels from 0.33 Pa to 500 Pa to evaluate shear dependency of the binders. Figure 12 shows example data obtained using Binder Y. The viscosities at 500 Pa measured for each temperature are then plotted using a log viscosity versus log temperature chart as seen in Figure 13 and extrapolated to obtain temperatures correspon- ding to 0.17 ± 0.02 Pa  s and 0.35 ± 0.03 Pa  s for mixing and compaction, respectively. Phase Angle Method. This method, conceived and de- veloped by John Casola as part of the study, is based on the observation that the phase angle from dynamic shear rheol- ogy is a consistency measure that takes into account the visco- elastic nature of asphalt binders. The development and test- ing of the Phase Angle method for this project was conducted at the Malvern Instruments facility in Southborough, MA. The procedure consists of performing a frequency sweep on unaged binder using a DSR meeting Superpave PG binder test- ing requirements and employing the same geometries and tem- peratures used in routine PG binder grading. Tests were con- ducted at four temperatures, typically 50°C, 60°C, 70°C, and 80°C, and at frequencies from 0.001 to 100 rad/s with 10 points/ decade. Strain was maintained at 12%. Data collected included 24 Rotational Viscosity 165°C 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0 10 20 30 40 50 60 70 80 90 100 Shear Rate, 1/sec Vi sc os ity , Pa S Bnder X Test 1 Binder X Test 2 Binder Y Test 1 Binder Y Test 2 Binder Z Test 1 Binder Z Test 2 Figure 9. Example data from rotational viscosity test, viscosity at 165C versus shear rate for three binders.

25 Rotational Viscosity 180°C 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0 10 20 30 40 50 60 70 80 90 100 Shear Rate, 1/sec V isc o sit y, Pa S Binder X Test 1 Binder X Test 2 Binder Y Test 1 Binder Y Test 2 Binder Z Test 1 Binder Z Test 2 Figure 10. Example data from rotational viscosity test, viscosity at 180C versus shear rate for three binders. Temperature, C V isc o sit y, Pa - s 0.1 1 10 52 58 64 70 76 82 88 100 150 165 180 200135120 100 500 Figure 11. Temperature viscosity plot showing mixing and compaction temperatures for high shear viscosity method.

26 Binder Y Steady Shear Flow #1 0 200 400 600 800 1000 1200 1400 1600 1800 2000 0 100 200 300 400 500 600 Shear Stress, Pa Vi sc o si ty , Pa S 76C 82C 88C Temperature, C Steady Shear Flow Viscosity at 500 Pa Vi sc os ity , P a- s 0.1 1 10 52 58 88 100 150 165 180 200 Mixing Range Compaction Range 64 76 82 53102107 100 500 Figure 12. Example data of steady shear flow method, viscosity versus shear stress at three test temperatures. 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.

standard values of shear moduli, temperature, frequency, and phase angle. Phase angle master curves were developed from the data using a reference temperature of 80°C. This tempera- ture range provides reliable phase angle resolution. Rheologi- cal testing of a wide variety of asphalt binders at temperatures above 135°C generally yields Newtonian behavior (phase angles at or very near 90°). The selection of 80°C as the refer- ence temperature was also to avoid confusion with standard- grade high temperatures (e.g., 76°C, 82°C). Figure 14 shows phase angle and shear modulus master curves for a typical modified binder. The shear modulus is shown simply to verify the shifting of the data to construct the phase angle master curve. A smooth and straight shear modulus master curve illustrates a good data shift. The region of the phase angle master curve between δ = 90° and 85° represents the transi- tion from purely viscous to visco-elastic behavior. Therefore, this is a region that can easily differentiate rheological behav- iors among binders. The frequency corresponding to δ = 86° was selected as a reasonable reference point for this technique. Casola established an initial relationship between this fre- quency and mixing temperature using the recommended plant mixing temperatures for each binder grade from the EC 101. A curve-fitting program was used to establish a preliminary power-law regression to fit the data: where the frequency, ω, is in radians/sec. Since the midpoints of recommended plant mixing temperatures in EC 101 are conservative toward lower temperatures, the preliminary frequency-temperature relationship yielded mixing temper- Mixing Temperature F°( ) = −310 20 01ω . ( ) atures that were considered lower than what are typically used in practice, particularly for modified binders. The frequency- mixing temperature regression equation was adjusted to bal- ance the desire to increase the mixing temperatures, particularly for modified binders, with the aim to minimize differences with equiviscous mixing temperatures for unmodified binders. The resulting adjusted equation for mixing temperature using the Phase Angle method was Figure 15 shows a plot of the EC 101 plant mixing tempera- ture ranges and midpoints with the Phase Angle method results based on the preliminary and the adjusted frequency-mixing temperature relationships for the eight binders that have PG that are provided in the EC 101 guide. It can be seen that the preliminary equation yielded results closer to the EC 101 midpoint and the adjusted equation yielded results closer to the maximum of the EC 101 range. A correlation of the mixing temperatures using the adjusted Phase Angle method and the corresponding EC 101 maximum mixing temperature yielded the following regression: Establishing a relationship between frequency and com- paction temperature began with the observation that com- paction temperatures for unmodified binders were typically 20°F to 25°F lower than the mixing temperature based on the equiviscous method. Using this simple offset, a similar power function to the one for mixing temperature was developed Phase Angle method T EC TM = ×( )−1 1 101 33. ,max R2 0 92= . Mixing Temperature F°( ) = −325 30 0135ω . ( ) 27 Phase Angle Master Curve G* Figure 14. Phase Angle master curve of a typical modified asphalt.

for compaction temperature. This relationship is shown as Equation 4: Example results for two binders are used to illustrate the Phase Angle method for selecting mixing and compaction temperatures. Figure 16 shows the results of frequency sweep testing for an unmodified PG 52-34 and an SBS-modified PG 64-40. The figure shows that the PG 52-34 binder reached a phase angle of 86 at a frequency of 158.5 rad/sec, whereas the PG 64-40 binder crossed the reference phase angle at a fre- quency of 1.1 rad/sec. Using the temperature-frequency rela- tionships in Equations 3 and 4, the mixing and compaction temperatures for the two binders are • PG 52-34: – Mixing temperature = 325(158.5)–0.0135 = 303.5°F – Compaction temperature = 300(158.5)–0.012 = 282.3°F • PG 64-40: – Mixing temperature = 325(1.1)–0.0135 = 324.6°F – Compaction temperature = 300(1.1)–0.012 = 299.7°F SEP Test. All binders were tested using the SEP test devel- oped by Stroup-Gardiner and Lange (30). All of the SEP tests except for those on the validation binders were performed by Stroup-Gardiner at Auburn University. This test utilized a Thermolyne moisture content oven equipped with an inter- nal scale for measuring mass loss during heating. To quantify the amount of smoke emitted from the binders, an opacity Compaction Temperature °F( ) = −300 40 012ω . ( ) meter was added to the oven’s flue to continuously record the opacity. Asphalt samples were placed in thin film oven pans and loaded in a rack placed on the balance in the oven. The 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 (266°F, 302°F, 338°F, and 374°F). Opacity, mass loss, and temperature were recorded versus time and temperature for each binder. Figure 17 illustrates opacity data for a binder tested at four temperatures. Following the SEP tests, the binders were re-graded in accor- dance with AASHTO M 320 and also tested with the MSCR test following AASHTO TP 70-07. The binder properties of the original binders and binders recovered from the SEP tests were used to evaluate changes due to thermal degradation. The MSCR tests on the original unaged binders were conducted by the Turner-Fairbank Highway Research Center. Post SEP test binders were graded and tested at NCAT. The MSCR test consists of a series of 1-second creep load- ing (constant stress) times followed by 9-second rest periods. The samples were loaded for 10 cycles each at creep loads of 25, 50, 100, 200, 400, 800, 1600, 3200, 6400, and 12,800 Pa. Test temperatures were 58°C, 64°C, 70°C, and 76oC. An unmodified binder typically has low elasticity and, con- sequently, does not recover most of the deformation caused by the loading stress during the 9-second rest period. This re- sults in a plot that resembles stair steps, with vertical rises dur- ing the 1-second loading period, and horizontal plateaus dur- ing the 9-second rest time. Unmodified binders accumulate unrecovered strain during the MSCR test. Binders that have been modified with elastomeric modifiers, on the other hand, 28 220 240 260 280 300 320 340 360 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 EC 10 1 Pl a n t M ix in g Te m pe ra tu re s (F ) Max Midpoint Min Preliminary Adjusted Figure 15. Plot showing results of the preliminary and adjusted Phase Angle mixing temperatures to the recommended mixing range from EC 101.

typically recover nearly all of the shear strain under the same conditions during the rest periods. The output of the MSCR test is the nonrecovered compli- ance, Jnr, of the binder at each level of applied stress. For each stress level, the total amount of unrecovered strain is calcu- lated using Equation 5: γ ε εu = −10 0 5( ) where ε10 is strain at end of 10th cycle and ε0 is strain at begin- ning of first cycle. For each stress level, Jnr is calculated using Equation 6: where γu is unrecovered strain and τ = applied stress, Pa. Jnr u = γ τ ( )6 29 (a) (b) Phase Angle Master Curve G* G* Phase Angle Master Curve Figure 16. (a) Comparison of a PG 52-34 and (b) a PG 64-40 (bottom graph) at a Threshold Phase Angle of 86.

Mixture Tests Mix Coating Tests. Each of the binders was mixed with the baseline mix using a common bucket-type mixer and a laboratory pugmill mixer at four temperatures to evaluate coating of the binder on the aggregates. The two mixer types (bucket and pugmill) were selected because they provide rea- sonable simulations of the mixing action occurring in drum mix plants and batch plants. Ratings of mix coating were de- termined using the Ross count method, ASTM D 2489. Re- sults of the coating tests for both mixer types were analyzed by developing regression equations based on a Sigmoid func- tion of the form: where C is the percentage coating at any temperature T, and a and b are regression constants. The regressions were used to estimate the coating percentage of the binders at any tempera- ture. Using the equiviscous mixing temperatures of the un- modified binders, baseline coating percentages for the bucket mixer and the pugmill mixer were determined to be 98% and 89%, respectively. The temperatures to achieve these coating percentages for the modified binders were then estimated with the regression equations. Results of the coating percent- ages for both mixer types were correlated to mixing tempera- tures from the candidate methods. An additional coating experiment was conducted to assess the effect of residual aggregate moisture on how well asphalt binders coat the aggregate. The motivation for this experi- C ae b T = + − 1 1 7( ) ment was that in the field, aggregate stockpiles are often wet and the plant’s drier may not completely dry the moisture from the coarse aggregate before mixing with the asphalt binder. As residual moisture escapes the aggregate and expands to steam, the asphalt binder can be foamed, which greatly increases it volume and enhances the coating process. The procedure used for this experiment was as follows: 1. The baseline mix with granite aggregate was batched to 4,600 grams and then split into coarse (+4.75 mm) and fine fractions. 2. The coarse aggregate fraction was placed in a deep pan, covered with water, then placed in an oven overnight at 210°F. The fine aggregate fraction was heated in a separate oven at about 20°F above the target mixing temperature, either 248°F, 284°F, 320°F, or 356°F. 3. The heated coarse and fine aggregate were combined in the pugmill mixer, and the combined blend was heated with a propane blowtorch until the blend reached the tar- get mixing temperature. For higher mixing temperatures, the blowtorch heating took as much as 20 minutes. 4. The asphalt was added (161.2 grams) to the aggregate in the mixer and mixing continued for 1 minute. 5. The mixture was discharged from the pugmill and imme- diately sieved on a 2.36-mm sieve, and the coarse aggregate particles were retained for later assessment of percentage coating in accordance with the Ross count method. This experiment included four binders selected following the main coating experiment as having a wide range of coat- ing results. 30 0 1 2 3 4 5 6 7 8 9 10 0 20 40 60 80 100 120 Time, minutes O p ac it y, % 130C 150C 170C 190C Figure 17. Example data from a SEP test, opacity measurements versus time for four temperatures.

Mixture Workability Test. Initially, the workability tests were to be performed with each of the binders and the base- line mixture containing granite aggregate. After several early workability test trials, a decision was made to change from the granite mix to a standard 0.6-mm silica sand to reduce raw data noise that could possibly have been due to binding of ag- gregate particles between the paddle tips and the workability bowl. An asphalt content of 5.1% was used for the sand mix- tures used in the workability tests. The sand-binder mixtures were short-term oven aged at 180°C (356°F), then transferred to the NCAT workability de- vice to measure the resistance to shearing in the loose state as it cools to approximately 120°C (248°F). The workability de- vice is a large fixed bucket with a two-pronged paddle that is rotated at a constant speed. Twenty-kilogram mix samples were prepared and then dumped into the heated workability bucket. The torque required to turn the paddle through the binder- sand mix as it cooled in the bucket was automatically recorded every second along with the binder-sand temperature. The raw torque data was filtered to remove extraneous in- strument noise in a three-step process using Microsoft Excel. A 60-second running average and standard deviation of the raw torque values were calculated in the first step. Then, at each second, a moving range was calculated as the 60-second average torque plus and minus the 60-second standard devia- tion. Finally, raw torque data outside of this 1-sigma range were removed. The filtered torque versus temperature data were plotted, and a second-order polynomial equation was fitted to the data with a least-squares regression. An example of the workability test data is shown in Figure 18. Mix Compaction Tests. Two compaction experiments were conducted. The main compaction experiment consisted of testing the 13 binders mixed with the baseline mixture. This experiment examined the effect of the main experimen- tal factors, binder type and compaction temperature, on changes in the lab compacted density and shear resistance of a single mixture. The second, smaller compaction experiment consisted of testing four unique binders mixed with four other mix designs. Selection of the four unique binders was based on materials that provided a range of binder properties. The additional mix designs include different aggregate types, particle shapes, gradations, and RAP contents. The results from this experiment provide an indication of the range of compaction temperatures necessary to provide good com- pactability for different mix types. All mix compaction sam- ples were mixed at 150°C (302°F), then short-term oven aged and compacted in an SGC at four temperatures: 110°C, 130°C, 150°C, and 170°C (230°F, 266°F, 302°F, and 338°F). Indirect Tensile Creep Compliance/Strength Tests. Since indirect tensile creep compliance and strength are highly dependent on binder properties, this test was used to assess how compaction temperatures affect tensile properties of two mixtures. Specimens were prepared with each of the binders mixed in the baseline mix design and tested in accordance with AASHTO T 322 and the recommendations from NCHRP Report 530 (51). The effect of compaction temperature on compliance and strength was analyzed. Summary of Research Plan The experimental plan involved testing and analysis of a variety of binders to characterize their visco-elastic responses over the range of temperatures for typical field operations. The testing utilized DSR and rotational viscosity equipment 31 y = 0.0004x3 - 0.1377x2 + 14.446x - 131.38 R2 = 0.8169 100 150 200 250 300 350 100 110 120 130 140 150 160 170 180 190 Temperature, °C To rq ue , in - lb Figure 18. Example results from a workability test, torque versus temperature.

common to Superpave binder laboratories. Binders were also analyzed with regard to their potential for emissions and ther- mal degradation. Mixture tests were performed to analyze how temperature and different binders affect aggregate coating, mix workabil- ity, shear resistance during laboratory compaction, and me- chanical properties. The results from these mix tests were used to identify appropriate mixing and compaction temperatures for each binder. These temperatures were then correlated to predicted mixing and compaction temperatures from the can- didate binder tests. The study recommends a new method for establishing mixing and compaction temperatures based on the simplest method that provides the best correlation statis- tics. A small validation experiment with an independent set of binders from a range of crude sources and with a range of properties and modification types was used to verify the selected method as a viable new procedure for determining mixing and compaction temperatures of asphalt binders. 32

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 648: Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt explores enhanced test methods for determining laboratory mixing and compaction temperatures of modified and unmodified asphalt binders.

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