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

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5Introduction The use of modified asphalt binders in hot-mix asphalt (HMA) has steadily increased over the past several decades. The Association of Modified Asphalt Producers (AMAP) esti- mates that modified asphalt binders currently make up ap- proximately 20% of paving grade asphalt sales (1), and the quantity of modified binders used in HMA is increasing at an annual rate of about 4%. However, a suitable method for de- termining mixing and compaction temperatures for these binders has not been established. The traditional equiviscous principle used to determine mixing and compaction tempera- tures was developed using unmodified binders having New- tonian behavior. However, using this method with many modified asphalt binders often results in excessively high mix- ing temperatures that have caused concerns with emission problems and degradation of the binder’s properties. The objective of this research was to identify or develop a simple, reliable, and accurate procedure for determining the mixing and compaction temperatures applicable for modified and unmodified asphalt binders in HMA. The research project consisted of six tasks: • Task 1-Literature Search, • Task 2-Design of Experiments, • Task 3-Interim Report, • Task 4-Conduct Experiments and Analyze Results, • Task 5-Prepare Procedure in AASHTO Format and Field Validation Plan, and • Task 6-Final Report. Literature Search A review of available literature was conducted on the subject of mixing and compaction temperatures for asphalt mixtures. The purpose of the literature review was to gain an understand- ing of the historical development of the current method, iden- tify shortcomings of this method, and identify potential new methods that may be evaluated or explored. A survey of state highway agencies and materials suppliers also was conducted to evaluate the current practices for determining mixing and compaction in the United States and abroad. The literature review is organized into the following topics: • A note on the units of viscosity. • Background on the development of mixing and compaction temperature criteria. • Effect of temperature on degradation of asphalt binders. • Mixing and compaction temperatures for modified asphalts. • Survey on current practices for selecting mixing and com- paction temperatures. • Recent research on proposed new methods for determining mixing and compaction temperatures. • Shear rates during mixing and compaction. • Summary of key findings. A Note on Units of Viscosity Viscosity may be expressed in several different ways, and this often leads to some confusion. One of the early methods for measuring asphalt viscosity was the Saybolt Furol viscosity test, which measured the time for a given volume of asphalt to flow through an orifice of specific dimensions. Thus the results of this test were reported as seconds Saybolt Furol (SSF). Another expression of viscosity is kinematic viscosity, which is actually the viscosity divided by the material’s density. The most commonly used unit of kinematic viscosity is the centis- tokes, which is written as mm2/s. The original equiviscous cri- teria in AASHTO T 312 used ranges of kinematic viscosity to define mixing and compaction temperatures. These criteria were 170 ± 20 mm2/s and 280 ± 30 mm2/s for mixing and com- paction, respectively. The measurement of viscosity using a rotational viscometer is often recorded in units of centipoise (cP). However, the SI C H A P T E R 1 Background

unit of viscosity is the Pascal-second (Pa  s). The conversion from centipoises to Pa  s is made by multiplying by 0.001. In some references cited in this report, the viscosity units used by the original author(s) were retained for clarity. It is much less confusing to use a single unit of viscosity for the test measurement and the specification criteria. Since it is more convenient to use Pa  s and not have to worry about temperature-density corrections for asphalt binders, this unit will be used in this report for both measurements and crite- ria. This requires the conversion of the kinematic viscosity to absolute viscosity. If the density of an asphalt is assumed to be 1.000 g/mm3, the equiviscous criteria convert to 0.17 ± 0.02 Pa s and 0.28 ± 0.03 Pa s for mixing and compaction, respec- tively. Although most asphalts have densities between 0.93 and 0.98 g/mm3 at temperature ranges used for mixing and compaction, the small error due to the assumed density of 1.000 g/mm3 is not considered to be significant. The Asphalt Institute (AI) has used this practice in its Superpave mix de- sign manual, SP-2. In 2009, AASHTO balloted a revision for T 312 to include viscosity criteria for mixing and compaction as 0.17 ± 0.02 Pa  s and 0.28 ± 0.03 Pa  s. Background on the Development of Mixing and Compaction Temperature Criteria The origin of the equiviscous concept, which has been the standard method for determining appropriate temperatures for mixing and compacting asphalt mixtures for many decades, was a logical place to begin. However, there is scarce docu- mentation of the origin of this technique. It is clear that the AI guided its development and evolution through several decades in the mid 1900s. The first mention of equiviscous temperatures found in the literature was a 1951 paper titled “Viscosity Effects in the Marshall Stability Test” (2) by Fink and Lettier of the Shell Laboratories in Wood River, Illinois. They conducted re- search aimed at evaluating the influence of type and consis- tency of asphalt binders on Marshall stability values. Experi- mental factors included compaction temperature and the test temperature for measuring Marshall stability. A variety of types and grades of asphalt binders was incorporated in the mixes. They reported that compaction temperature had little effect on the density of the specimens. However, stability val- ues increased with increasing compaction temperature. Flow values were influenced almost entirely by asphalt content, but not affected by compaction temperature. They concluded that, “neither stability nor flow was influenced by the consistency or source of the asphaltic binder if compaction temperature is carried out at equiviscous temperatures above the soften- ing point of the binder.” However, different results were obtained by Parker. In his 1950 paper titled “Use of Steel-Tired Rollers,” (3) Parker de- scribes an experiment in which Marshall specimens were compacted at temperatures ranging from 100°F to 350°F. Eleven sets of samples were compacted over the range at 25°F increments. Results show that specimen densities were simi- lar for temperatures of 275°F and above, but decreased in a linear fashion for temperatures down to 150°F. Below 150°F, specimen density dropped significantly. Another reference dating to work in the mid 1950s alluded to specific ranges for Saybolt Furol viscosity as the basis for plant mixing temperature. In the description of constructing the Michigan Test Road in 1954, Serafin et al. (4) state that mixing temperatures were set so that the asphalts would have Saybolt Furol viscosities of 75 to 200 SSF. Six asphalts were used in the test road. At the midpoint of the viscosity range, mixing temperatures for the asphalts ranged from 290°F to about 312°F. Therefore, it is apparent that a recommended range for plant mixing viscosity was established prior to 1954. According to Vyt Puzinauskas, former AI chief chemist, the viscosity range cited above was essentially based on the field experience of AI’s field engineers. These studies may have provided the impetus for establish- ing the equiviscous criterion for laboratory compaction. The first edition of the AI’s Mix Design Methods for Hot-Mix Asphalt Paving (5) in 1956 does not include viscosity criteria for mixing and compaction temperatures. In the 1960 paper titled “The Effect of Compaction Temperature on the Prop- erties of Bituminous Concrete,” (6) Kiefer states that “a mixing viscosity of about 100 seconds Saybolt Furol (SSF) was cho- sen in accordance with the recommendations of the Asphalt Institute which advised a mixing viscosity of between 75 and 150 sec Saybolt Furol.” The reference cited in Kiefer’s work was an informal paper by John W. Griffith, “The Effects of Viscosity of Asphalt Cement at the Temperature of Mixing on the Properties of Bituminous Materials,” presented at the 38th annual meeting of the Highway Research Board in 1959. Kiefer’s work (6) with the Hveem mix design procedure also concluded that compaction temperature also should be stan- dardized. Samples were compacted in a Hveem kneading compactor at temperatures of 150, 190, 230, 270, 310, and 350°F. The range of temperatures was shown to affect sam- ple density, air voids, Hveem stabilometer value, and cohesio- meter value. The second edition of the AI’s Manual Series No. 2 Mix De- sign Methods for Asphalt Concrete (7) includes the equiviscous criteria for mixing and compaction of laboratory samples. The publication states: “The temperature to which the asphalt must be heated to produce 85 ± 10 seconds Saybolt Furol and 140 ± 15 seconds Saybolt Furol shall be established as the mix- ing temperature and compaction temperature respectively.” In 1965, Bahri and Rader reported on experiments with the recommended mixing and compaction viscosities for the Marshall method (8). The recommended ranges in ASTM D 6

1559 were the same as the MS-2 criteria. Effects of tempera- ture and mineral filler content were studied. The results showed that variations in mixing and compaction viscosities produced significant changes in Marshall stability, flow, spe- cific gravity, and voids. They concluded that the mixing and compaction viscosities recommended by ASTM were satis- factory for the mixture with 8.6% mineral filler (bitumen filler ratio 1.81). For the mixture with 11.3% mineral filler (bitumen filler ratio 2.38), an optimum mixing viscosity of 54 SSF and an optimum compaction viscosity of 250 SSF were recommended. In 1984, Kennedy et al. (9) conducted a study to analyze the effect of lower compaction temperatures on the engineering properties of HMA. The study was prompted by an investiga- tion of premature rutting of a recycled asphalt concrete over- lay that had met in-place density specifications even though unusually low compaction temperatures had been used. Field records showed that the average delivery temperature to the roadway was 93°C (200°F). Laboratory experiments involved compacting samples over the range of temperatures during construction and determining the tensile strengths of the sam- ples. They concluded that the low compaction temperature had an adverse effect on the properties of the HMA and thus contributed to the early pavement failure. In 1985, Crawley (10) conducted a field evaluation of lower compaction temperatures and reached a different conclusion. This research evaluated engineering properties of HMA placed on a field project in which about half the asphalt base and binder courses were produced at the normal temperature of 149°C (300°F) and half were produced at 107°C (225°F). After 3.5 years, cores were taken from the project, and it was found that there was no significant difference in mixture properties or performance, although mixtures were placed at the differ- ent temperatures. The study further quantified a reduction of 20.6% in energy consumption by using the lower production mixing temperatures. Other studies have demonstrated the effects of laboratory compaction temperature on the results of mechanical tests. Newcomb et al. (11) conducted a lab and field study to eval- uate asphalt mixtures with plastic and latex modifiers. Part of their work examined the effect of compaction temperature on air voids and resilient modulus. Compaction was accom- plished with a California kneading compactor at four tempera- tures: 79, 116, 154, and 191°C (175, 240, 310, and 375°F). The lowest air voids were achieved when compaction tempera- ture was 154°C (310°F). Resilient moduli increased linearly with increasing temperature. Aschenbrener and Far (12) eval- uated the influence of compaction temperature on results of the Hamburg Wheel Tracking Device. They reported that a higher compaction temperature improved the resistance to deformation in the Hamburg test. Azari et al. (13) compacted samples of an HMA mixture in a Superpave Gyratory Com- pactor (SGC) over a wide range of compaction temperatures and found that except for strain-controlled fatigue testing using a Superpave shear tester, the shear properties of the mixture improved with increasing compaction temperature. The strain-controlled fatigue results were not significantly affected by mixing temperature. A number of studies have shown that laboratory com- paction temperature has little to no influence on the volu- metric properties of samples compacted in a SGC. Findings of NCHRP Project 9-10 (14) indicated very minor changes in density of SGC samples when compaction temperature var- ied from 80° to 155°C (176° to 311°F). Over this same tem- perature range, the binder viscosity increased by three orders of magnitude. It is interesting to note, however, that when the mixtures were compacted with a U.S. Army Corps of Engi- neers gyratory testing machine and with a Marshall hammer, the air voids increased by 56% and 44%, respectively, over the same temperature range. Huner and Brown (15) investigated the effect of reheating HMA and the effect of varying com- paction temperature over a range of 28°C (50°F) on volumet- ric properties for mixtures compacted in the SGC. Their work included eight mixtures using two aggregate types, two grada- tion types, and two binders. None of the mixtures were affected by changing the compaction temperature. In a ruggedness study of the SGC compaction procedure, McGennis et al. (16) also found that compaction temperature was not a significant factor on the compacted density of mixtures for unmodified asphalts, but it was a significant factor for mixtures containing modified binders. De Sombre et al. (17) conducted research to determine the range of temperatures over which the compactive effort of HMA is maximized. In this study, an Intensive Compaction Tester (ICT) gyratory compactor produced in Finland was used to compact samples at different temperatures. Density data were recorded at several points during compaction. The density data, the change in height of the sample during com- paction, the size of the sample, and the pressure used to com- pact the sample were used to calculate the total work energy during compaction. By plotting the shear stress and power used during compaction against the number of gyrations for each temperature, the compactability of the mixtures was evaluated. By testing samples at several temperatures, it was possible to determine a desirable range of compaction tem- perature for a given mixture. Six laboratory mixes using three binder grades and two aggregate gradations, dense-graded and Stone Matrix Asphalt (SMA), as well as five field mixes were used in the study. For both the laboratory and field mixes, there was an attempt to establish a relationship be- tween temperature and shear stress during compaction at dif- ferent temperatures for each mix. A comparison of shear stress versus temperature showed that there was very little change in shear stress although the temperature changed 7

significantly. It appeared that aggregate type, angularity, and gradation had a greater effect on the energy required for com- paction than did the compaction temperature. A difference in the type of polymer used also was found to be important, even though the binders met the same PG specifications. In the field, however, temperature is considered one of the primary factors to affect the compactability of HMA. Leiva and West (18) found that field compactability of HMA was dominated by mat temperature and mat thickness relative to mixture characteristics such as gradation, aggregate type, and binder grade. Willoughby et al.(19), in a study of temperature segregation during placement of HMA, found significant dif- ferences for in-place density resulting from localized tempera- ture differentials of only 14°C (25°F). In the field validation of the Pavecool program for estimating cooling rates of HMA during compaction, Chadbourn et al. (20) showed that almost no increase in mat density changes with continued passes of rollers as the mat temperature dropped below 100°C. Many in- dustry references (20–23) cite 79°C (175°F) as the cessation temperature for compaction of HMA in the field. However, it is probable that the cessation temperature limit is a general rule of thumb and will vary from mix to mix depending to a large degree on the grade or consistency of the binder. Effect of Temperature on Degradation of Asphalt Binders A significant amount of research during the 1950s and 1960s also investigated asphalt aging. Some studies focused on the hardening of asphalts that occurs during manufacturing of asphalt mixtures. In the 1958 AAPT symposium on asphalt hardening, Clark (24) summarized that the mechanisms involved in age deterioration of asphalts are volatilization, oxi- dation, action of water, action of light, and chemical changes. He stated that volatilization was the primary cause of harden- ing and that oxidation assumes a secondary role in aging of asphalt. Fink’s paper (25) stated that “most oxidation reactions approximately double for each 10°C increase.” Serafin (26) dis- cussed ways of minimizing binder hardening during the man- ufacture of asphalt mixtures. The discussion touched on topics such as oven aging procedures, recovery procedures, tempera- ture control charts, and penalties. Of interest for this research was how Michigan dealt with temperature control at plants. The Michigan specification at the time required the “mixture be delivered at the temperature, between 275°F and 375°F, as directed by the Engineer, and shall not vary more than 20°F, plus or minus, from that temperature, except that no mixture shall exceed a temperature of 375°F.” The discussion says that normal mix temperature was in the range of 285°F to 325°F. Lottman et al. (27) investigated the effect of mixing tem- perature on viscosity changes (hardening) of asphalt binders. They found “a linear relationship between the dependent variable of final asphalt viscosity (after mixing) and the inde- pendent variables of aggregate temperatures and initial asphalt viscosity.” However, they concluded that mixing temperature was not a significant factor on asphalt hardening. The discus- sions of the paper drew several criticisms of the analysis and points of view disagreeing with the conclusions. Excessive temperatures during processing and storage of binders as well as during HMA production have several seri- ous consequences. Airey and Brown (28) reported that stor- age of binders at elevated temperatures can cause breakdown of long chain polymers in modified asphalts, thereby negat- ing the benefits of modified binders. Linde and Johansson also examined the effect of processing and storage tempera- ture on degradation of polymer modified binders (29). Tests were conducted with size exclusion chromatography (SEC) to detect changes in molecular sizes in the bitumen and the poly- mer phase of the binders. Binders were stored at 200°C (392°F), and aliquots taken periodically for SEC testing and analysis. After just a few hours, the polymer degradation occurred as evidenced by a decrease in molecular size. The bitumen phase showed increases in molecular size most likely the result of oxidation and polymerization reactions. However, in an inert atmosphere, the polymer phase did not show changes in molecular size. The changes in molecular size were correlated to changes in mechanical properties of the binders. Tensile properties dropped significantly for the binders showing polymer degradation. Stroup-Gardiner and Lange demonstrated environmental concerns associated with excessive HMA temperatures (30). They reported greater volatile loss, emissions, and concentra- tions of odor-causing compounds with increasing temperatures for a range of asphalt binders. They conducted a number of studies on fumes and odors that can be released from asphalts and asphalt additives at different temperatures (31, 32). They used gas chromatography (GC) analyses to identify specific compounds that can be linked to nuisance odors. They also de- veloped a practical method of quantifying smoke and emis- sions potential from asphalt binders, called the SEP test. They demonstrated that the release of volatile organic compounds (VOCs), SEP mass loss rates, and opacity for asphalt binders are strongly influenced by temperature and crude source. One of the most widely used references for asphalt around the world is The Shell Bitumen Handbook (33). This handbook provides useful guidance on handling of asphalt binders. Several specific recommendations are worth noting: Bitumen should always be stored and handled at the lowest temperature possible, consistent with efficient use . . . to prevent auto-ignition of the bitumen 230°C (446°F) must never be ex- ceeded. . . . During mixing the hot bitumen must be readily able to coat the dried and heated mineral aggregate, given the shear- ing conditions employed, in a relatively short period of time (typically 30 to 90 seconds); this determines the lowest mixing 8

temperature. Whilst the mixing temperature must be sufficiently high to allow rapid distribution of the bitumen on the aggregate, the use of the minimum mixing time at the lowest temperature possible should be advocated. The higher the mixing tempera- ture the greater will be the oxidation of the bitumen exposed in thin films on the aggregate surface. . . . There are, therefore, upper and lower limits to mixing temperature. . . . These different con- siderations combine to give an optimal bitumen viscosity of 0.2 Pa s (2 poise) at mixing temperatures. . . .When materials are being laid at low ambient temperatures, or if haulage over long distances is necessary, mixing temperatures are often increased to offset these two factors. However, increasing the mixing temper- ature will considerably accelerate the rate of bitumen oxidation which will increase the viscosity of the bitumen. Thus a significant proportion of the reduction in viscosity achieved by increasing the mixing temperature will be lost because of additional oxida- tion of the bitumen. . . . Once the mat has been spread it must still be sufficiently workable to enable the material to be satisfacto- rily compacted with the available 30 Pa  s (300 poise). At viscosi- ties lower than 5 Pa  s the material will probably be too mobile to compact and at viscosities greater than 30 Pa  s the material will be too stiff to allow any further compaction. Mixing and Compaction Temperatures for Modified Asphalt Binders The use of polymer-modified asphalt binders has become much more common over the past two decades. Many types of polymers have been used in paving asphalts to enhance the per- formance of asphalt pavements in a wide range of climates and loading conditions. The AMAP now estimates that modified asphalt binders make up about 20% of paving grade asphalt sales in the United States (1). Most modified binders require higher temperatures for mix- ing and compaction in the field and the laboratory to achieve the same workability as mixes with unmodified binders. In “Using Additives and Modifiers in Hot Mix Asphalt,” Terrel and Epps (34) include construction guidelines for a number of specific modifiers. The guidelines regarding temperatures for mixing and placing vary widely for the several specific HMA polymers. For example, mixing information for Butonal NS 175, a styrene/butadiene latex, states that “the temperature of the aggregate, when introduced into the mixture should not ex- ceed 182°C (360°F), and the temperature of the mixture when discharged from the hauling unit shall be 149°C (300°F) mini- mum.” Another polymer required much higher temperatures. Rosphalt 50®, a virgin polymeric additive, primarily used as an additive to HMA for bridge deck sealing, recommended the discharge temperature be at least 199°C (390°F). For Kraton®, a block copolymer, mix preparation instruc- tions simply state that “it may be necessary to adjust the mixing and compaction temperatures when conducting laboratory work.” For plant operations, asphalt mixtures containing Kraton® should be as workable as a typical asphalt mixture because of “the relatively high shear forces seen at the manu- facturing level compared to the lower shear forces used in lab- oratory viscosity measurements.” Shuler et al. (35) showed that viscosities increased with higher polymer contents for Kraton® and Styrelf® modified binders. Based on the viscos- ity tests, significantly higher mixing and compaction temper- atures would be required. However, experience with these binders on previous construction projects indicated that the extremely high mixing and compaction temperatures were not necessary. For environmental reasons, they considered the upper limit for field mixing to be 160°C (320°F). New- comb et al. (11) conducted laboratory and field studies of mixtures containing polyolefin and styrene-butadiene latex. In the field, normal construction procedures were used, and they noted that the only modified mixtures that created any handling difficulty were mixtures with 3% latex that tended to cause sticking problems in the trucks and screeds. Other- wise, modified mixtures behaved similar to conventional unmodified mixtures. Shenoy (36) studied the temperature-viscosity relationships of two polymer modified asphalts, Styrelf® and Novophalt®. Both of these binders have been studied extensively at the FHWA Turner Fairbank Research Center’s Accelerated Load- ing Facility and laboratories. The Novophalt® binder used in this study was an AC-10 modified with about 6.5% low-density polyethylene. The Styrelf® binder was an AC-20, which was air blown to AC-40 and then modified with styrene-butadiene. Sulfur was also added as a cross-linking agent. Testing utilized a Brookfield viscometer with three spindles to generate a wide range of shear rates. Arrhenius plots for the binders showed that the viscosity-temperature relationships are influenced by the melting points of the polymers. For the Novophalt®, the polyethylene has a melting point around 125°C (257°F). For the Styrelf®, the melting point of the polystyrene occurs in the range of 115°C to 150°C (239 to 302°F) and the polybutadi- ene melts between 150°C and 163°C (302°F and 325°F). Ev- idence of polymer degradation also was presented. Aging of the binders and breakdown of the polymers occurred in the same temperature range. Aging tended to cause the viscosity to increase whereas polymer degradation caused viscosity to decrease. In a field trial of various modified binders and one unmodi- fied control binder, Albritton et al. (37) used a rotational viscometer to determine viscosity of the different modified binders. At the lower temperature of 135°C (275°F), the vis- cosities of the binders range from a high viscosity value of 2.60 Pa  s for the multigrade binder to a lower value of 0.50 Pa  s for the unmodified binder control section. At the higher temperature of 190°C (374°F), the viscosities of the modified binders grouped closer together, ranging from a high value of 0.40 Pa  s for the Novophalt® modified binder to a lower value less than 0.10 Pa s for the control and multigrade 9

binders. The study showed that mixing and compaction temperatures for polymer modified mixes were higher than for conventional unmodified binders. Mixing temperature ranged from 160°C to 177°C (320°F to 351°F). They noted that the linear temperature-viscosity relationship assumed for unmodified binders may not be valid for modified asphalt binders. Recognizing the potential problems with using the equiv- iscous method with modified asphalts, a note was added to AASHTO T 312 in the section on preparation of specimens: Note 4 – Modified asphalts may not adhere to the equiviscosity requirements notes, and the manufacturer’s recommendation should be used to determine mixing and compaction temperatures. In 2000, the Asphalt Pavement Environmental Council pub- lished an industry guidance document titled Best Management Practices to Minimize Emissions During Construction also known as EC 101 (38). This document includes several important rec- ommendations regarding mixing and compaction tempera- tures. It states that the equiviscous method is meant to be used only for laboratory purposes and should not be used as a start- ing point for plant mixing and field compaction temperatures. For modified binders, it is advised to use the binder supplier’s recommendation for proper laboratory and plant mix produc- tion and field compaction temperatures. Table 1, reproduced from this publication, serves as a guide for the industry on suitable temperatures for field operations. These recommended HMA plant temperature ranges were considered as practical guidelines for mixing temperatures in this study. Survey of Current Practices for Determining Mixing and Compaction Temperatures A survey was conducted to examine procedures used by highway agencies to determine mixing and compacting tem- peratures for mix design and construction operations. Survey results were obtained from 65 agencies including the highway agencies for all 50 states and the U.S. Army Corps of Engi- neers. Survey responses also were received from highway agencies in Canada, Australia, China, Denmark, India, Japan, and Malaysia. A table listing each agency’s responses is in- cluded in Appendix A. In the survey summary, the survey responses add up to more than 100% for each of the questions because some agencies used more than one method for determining mixing and compaction temperatures. For example, an agency may gen- erally follow AASHTO T 245 and T 312 to determine mixing and compaction temperatures, but if the temperatures exceed a certain value then the supplier’s recommendation would be used. Likewise, for setting production and placement temper- atures, some agencies may follow AASHTO T 245 and T 312, but allow the contractor some flexibility to change those tem- peratures depending on ambient temperature, haul distance, and layer thickness. The survey questions, along with a summary of responses, are as follows: 1. What procedure does your agency/organization currently use to determine mixing and compaction temperature for asphalt binders used in unmodified hot-mix asphalt? As shown in Figure 1, the largest response (46%) indi- cated that the guidelines in AASHTO T 245 and T 312 were used to determine mixing and compaction tempera- tures for unmodified asphalt. Several agencies reported using the AASHTO procedures unless the temperature ex- ceeded a certain limit, such as 166°C (325°F), and if that limit were exceeded, they would use the binder supplier’s recommendation. A few agencies reported that they avoided the mixing and compaction temperature issue altogether 10 Question 1 Response 46% 30% 13% 10% 1% T245/T312 Supplier Own Other N/A Figure 1. Survey responses to Question 1. Asphalt Pavement Environmental Council Best Practices Typical Asphalt Binder Temperatures Binder Grade HMA Plant Asphalt Tank Storage Temperature (ºF) HMA Plant Mixing Temperature (ºF) Range Midpoint Range Midpoint PG 46-28 PG 46-34 PG 46-40 260 – 290 260 – 290 260 – 290 275 275 275 240 – 295 240 – 295 240 – 295 264 264 264 PG 52-28 PG 52-34 PG 52-40 PG 52-46 260 – 295 260 – 295 260 – 295 260 – 295 278 278 278 278 240 – 300 240 – 300 240 – 300 240 – 300 270 270 270 270 PG 58-22 PG 58-28 PG 58-34 280 – 305 280 – 305 280 - 305 292 292 292 260 – 310 260 – 310 260 – 310 285 285 285 PG 64-22 PG 64-28 PG 64-34 285 – 315 285 – 315 285 - 315 300 300 300 265 – 320 265 – 320 265 – 320 292 292 292 PG 67-22 295 – 320 320 275 – 325 300 PG 70-22 PG 70-28 300 – 325 295 – 320 312 308 280 – 330 275 – 325 305 300 PG 76-22 PG 76-28 315 – 330 310 – 325 322 318 285 – 335 280 – 330 310 305 PG 82-22 315 – 335 325 290 – 340 315 Table 1. Recommended plant temperatures for different binder grades (38).

by using the supplier’s recommendation for all binders. A few agencies also reported that they have established a standard mixing and compacting temperature based on their own experience and use those temperatures for all unmodified binders. Nevada indicated that they no longer use unmodified asphalt binders. Of those using other methods, ASTM D2493 was the pro- cedure most often used. The ASTM procedure is a method for determining the standard viscosity-temperature chart but does not specify mixing and compacting temperatures. The Japan Road Association uses a procedure that is similar to AASHTO T 245 and T 312, but allows a slightly higher temperature. The mixing temperature range is that which gives a viscosity of 180 ±20 mm2/s, and the com- paction temperature range is that which gives a viscosity of 300 ± 30 mm2/s. 2. What procedure does your agency/organization currently use to determine mixing and compaction temperatures for asphalt binders used in polymer-modified hot-mix asphalt? The overwhelming response to Question 2 (Figure 2) was that binder supplier recommendations are used for deter- mining mixing and compacting temperatures for polymer- modified asphalt mixtures. Fifty-six percent of the agencies responding stated they use supplier recommendations. A few of those indicated they use AASHTO T 245 and T 312 unless unusually high temperatures are needed, and in that case, they would use supplier recommendations. Even though supplier recommendations were used most often, some agencies also established a maximum mix- ing and compaction temperature. Four agencies (British Columbia, Canada; Hawaii; New York State Thruway Authority; and Puerto Rico) reported that they do not cur- rently use polymer-modified asphalt. Since supplier recommendations are typically used for polymer-modified asphalts, an obvious question is, “What procedures do suppliers use?” For answers to this ques- tion, 17 surveys were sent to representatives of the modi- fied asphalt industry. Unfortunately, no responses were received to the surveys. However, Gaylon Baumgardner of Paragon Technical Services, Inc., (a division of Ergon) and Frank Fee of NuStar Energy, L.P., were very helpful in pro- viding information by telephone and e-mail in regard to industry procedures. Suppliers most often use temperature-viscosity charts based on AASHTO T 245 and T 312 for determining mixing and compacting temperatures for unmodified asphalt. Some suppliers of modified asphalt use a rough adjustment of 10°F for each grade increase with polymer modification. Other suppliers set mixing and compaction temperatures solely based on experience with the partic- ular binder and modification system. It was stated that prior experience has shown that a mixing temperature in the range of 160°C to 171°C (320°F to 340°F) and a compaction temperature in the range of 143°C to 155°C (290°F to 310°F) are adequate for most mixes with polymer- modified binders. Suppliers also may allow some flexibil- ity in adjusting those temperatures depending on project conditions such as ambient temperature, haul distance, layer thickness, etc. For highly modified binders such as PG 82-22, the temperatures may need to be increased slightly. However, suppliers generally recommend that mixing temperatures never exceed 190°C (375°F), which is consistent with EC 101. 3. What procedure does your agency/organization currently use to determine mixing and compaction temperatures for asphalt binders used in crumb rubber-modified hot- mix asphalt? Sixty-four percent of agencies responded that they do not use crumb rubber modifiers (Figure 3). Of those that use crumb rubber, most rely on supplier recommendations that are generally based on experience. Some states supplement the supplier recommendations with a maximum mixing temperature of their own. From the responses, the highest mixing temperature allowed in the United States and Canada is 176°C (350°F). Asian countries generally allow slightly 11 Question 2 Response 16% 56% 18% 5% 5% T245/T312 Supplier Own Other N/A Question 3 Response 9% 15% 9% 3% 64% T245/T312 Supplier Own Other N/A Figure 2. Survey responses to Question 2. Figure 3. Survey responses to Question 3.

higher temperatures. For example, India uses a mixing temperature range of 165°C to 185°C (330°F to 365°F) and Japan has a maximum mixing temperature of 185°C (365°F) for all types of modified asphalts. 4. Does your agency/organization have experience with air- blown or chemically modified asphalt? If so, what proce- dure is currently used to determine mixing and compaction temperatures for asphalt binders used in hot-mix asphalt modified in this manner? Fifty-nine percent of the agencies responded that they do not have experience with or do not allow air-blown or chemically modified asphalt (Figure 4). Most of the agen- cies that allow these modifiers use supplier recommen- dations or a combination of AASHTO T 245 and T 312 and supplier recommendations. The Quebec Ministry of Transportation uses a combination of AASHTO T 245 and T 312 and specification limits. If the temperature that corresponds to 0.17 Pa  s exceeds 170°C (338°F), then 170°C (338°F) becomes the maximum temperature and a mixing temperature range of 156°C to 170°C (313°F to 338°F) is used. 5. Are the same procedures used to determine plant produc- tion and laydown temperatures? Most agencies (61%) responded that they use the same procedures for establishing mixing and compaction tem- peratures in the field as they do for laboratory work (Fig- ure 5). Fifteen percent of the responding agencies allow the contractor flexibility to set the temperatures for construc- tion (or to adjust temperatures to account for ambient temperature, haul distance, layer thickness, etc.) so long as a maximum temperature such as 177°C (350°F) is not ex- ceeded. Four agencies vary production temperature based on PG grade, and two others vary production temperatures depending on whether the binder is virgin or modified. In Denmark, the maximum mixing temperature is reg- ulated by legislation with the Danish Working Environment Authority. Based on those guidelines, contractors must not exceed 190°C (375°F) for polymer-modified mixtures. If the maximum mixing temperature is exceeded, the con- tractor is required to provide special safety precautions (such as “space suits”) for workers. In order to keep from being associated with hazardous materials, contractors avoid going over the maximum limit. For unmodified asphalt, the maximum mixing temperature is 180°C (355°F). Where polymer-modified asphalt is used in China, the agency has set temperatures for heating the aggregate and the modified binder, and if the combined mixture exceeds 195°C (383°F), the mixture is discarded. A minimum com- paction temperature is also established depending on sur- face temperature and layer thickness as indicated in Table 2. The Japanese method for determining mixing and com- paction temperatures for mixes with polymer-modified binders is interesting and rather straightforward. A 13-mm maximum aggregate size mix using the same aggregates as will be used in the project is mixed with straight-run un- modified asphalt and compacted using their standard temperature-viscosity curves. The density of the samples are determined and used as the standard density. Using the same mix with the modified asphalt specified for the proj- ect, mixture is compacted at three to five incremental tem- perature levels up to a compaction temperature of 185°C (365°F). The density of each set of samples is determined 12 Question 4 Response 12% 19% 6% 4% 59% T245/T312 Supplier Own Other N/A Figure 4. Survey responses to Question 4. Surface Temperature of Existing Layer (°C) Minimum Compaction Temperature Related to the Following Thickness (°C) Unmodified Modified or SMA <50mm 50-80 >80mm <50mm 50-80 >80mm <5 NA NA 140 NA NA NA 5 - 10 NA 140 135 NA NA NA 10 - 15 145 138 132 165 155 150 15 - 20 140 135 130 158 150 145 20 - 25 138 132 128 153 147 143 25 - 30 132 130 126 147 145 141 >30 130 125 124 145 140 139 Table 2. Minimum compaction temperatures of HMA in China. Question 5 Response 61% 11% 11% 14% 3% Yes Own Supplier Contractor Other Figure 5. Survey responses to Question 5.

13 and compared with the standard density. The temperature that results in the same density for modified samples as ob- tained in the standard unmodified samples becomes the minimum compaction temperature. An increment of 10°C (18°F) is added to the minimum compaction temperature to determine the maximum compaction temperature. An increment of 10°C (18°F) is added to the maximum com- paction temperature to determine the minimum mixing temperature. Another increment of 10°C (18°F) is added to the minimum mixing temperature to obtain the maxi- mum mixing temperature. However, the maximum mix- ing temperature cannot exceed 185°C (365°F). Responses from each agency that replied to the question- naire are provided in Appendix A. Although AASHTO T 245 (Marshall method) and T 312 (Superpave Gyratory Com- paction) are similar with regard to establishing mixing and compaction temperatures, some agencies specified one over the other. In those cases, the specified AASHTO procedure is underlined. Recent Research on Proposed New Methods for Determining Mixing and Compaction Temperatures Over the past decade, several studies have specifically ad- dressed the issue of mixing and compaction temperatures for modified binders. Several of these research projects proposed new approaches for determining mixing and compaction temperatures. Zero Shear Viscosity In NCHRP Report 459: Characterization of Modified Asphalt Binders in Superpave Mix Design, Bahia et al. (39) determined that the equiviscous requirements in AASHTO T 312 did not give practical results for 17 out of the 38 modified binders tested. For these modified binders, mixing temperatures of over 165°C (329°F) were calculated. The researchers hypoth- esized that this was due to the fact that most of the modified binders were sensitive to shear rate and therefore did not meet the current assumption that all binders are Newtonian fluids. They introduced the concept of Zero Shear Viscosity (ZSV) and recommended its use for determining mixing and compaction temperatures for modified asphalt binders. They reasoned that since the vertical compression rate during most of the SGC compaction process is very low, the measurement of the viscosity for the binder should be made at a very low shear rate. They also found that low shear viscosity data for the binders correlated better to air voids in SGC-compacted specimens compared with high shear (300 1/s) viscosity mea- surements. Initially, the viscosity corresponding to a shear rate of 0.001 1/s was estimated with a curve-fitting model. The criteria at this shear rate were set at 3.0 Pa s and 6.0 Pa s, for mixing and compaction, respectively. Laboratory compaction in an SGC at the ZSV compaction temperature for four mixtures and five binders yielded very similar air void contents for two of the mixes, but the other two mixtures had increased air voids of 0.7% to 1.1% for the modified binders compared with results with the unmodified binder. To simplify the approach so that the Brookfield rota- tional viscometer could be used, low shear rate viscosity crite- ria were set at 0.75 ± 0.05 Pa s and 1.4 ± 0.10 Pa s respectively, for mixing and compaction. The shear rate for the Brookfield rotational viscometer, as currently used in AASHTO T 316, is 6.8 1/s. This resulted in about a 40°C (72°F) reduction in mix- ing and compaction temperatures compared with the standard equiviscous temperatures. Laboratory mix coating tests at the low shear rate mixing temperature with four mixtures and five binders yielded good coating for most mix-binder combina- tions in a restaurant-type mixer and a bucket-type mixer. A couple of independent studies evaluated the ZSV method. Grover (40) presented research and discussion of mixing and compaction temperatures based on the ZSV method and the equiviscous temperature method. Included in the presentation were several different definitions of ZSV and a description of Bahia’s method of determining a value for Low Shear Viscos- ity. Data for both the equiviscous temperature method and the ZSV method were presented for several modified and unmod- ified asphalt binders. The data showed that the equiviscous temperature method required excessive heating of the modi- fied binders. The ZSV method yielded mixing and compaction temperatures that were 35°C to 40°C lower than those calcu- lated by the equiviscous method. Tang and Haddock (41) evaluated seven Superpave mix- tures and one SMA mixture obtained from Indiana DOT projects. Three projects utilized a PG 64-22, two projects utilized a PG 70-22, and three projects used a PG 76-22. Raw materials from the projects were obtained and used in the laboratory research. Since four of the binders were not shear rate dependent (Newtonian), their mixing and compaction temperatures were determined by the traditional equivis- cous technique. All binders also were tested to determine their ZSV mixing and compaction temperatures using the procedure described by Khatri et al. (42). Mix designs were performed in accordance with Superpave procedures. They found that the ZSV mixing and compaction temperatures yielded the same optimum asphalt content as was used on the project. The paper did not state how the mixing and com- paction temperatures were determined for the original mix designs. Field mixing and compaction temperature for the modified binders were based on “experience.” Correlations of mixing and compaction temperature from the equivis- cous technique and the ZSV principle showed that the ZSV

temperatures were about 40°C below the equiviscous tem- peratures. They recommended that the equiviscous method be used to determine mixing and compaction temperatures for Newtonian binders and the ZSV method be used for modified binders. Extrapolated High Shear Rate Viscosity Yildirim et al. (43) presented an approach that estimates high shear rate binder viscosity from rotational viscosity measurements for the determination of mixing and com- paction temperatures. The shear rate of 490 1/s was selected from experimental work with mixtures compacted in a SGC. The same mixtures were prepared with four different binders and then compacted in the SGC over a range of relatively low temperatures (between 50°C and 95°C) to amplify the effect of temperature on mix density. It was hypothesized that equivalent mix densities would occur when the viscosities of binders were the same. To find the point where binder vis- cosities were the same, the binders were tested in a Brookfield viscometer at shear rates ranging from 0.1 1/s to 93 1/s. These viscosity data were extrapolated to find the shear rate where the unmodified asphalt and the modified asphalts had the same viscosity. The hypothesis was that the shear rate at which the viscosities of binders intersect is equivalent to the shear rate that the mix experiences in the gyratory mold during compaction. The average equiviscous shear rate for the eight pairs of mixtures was 487 1/s, which was rounded to 490 1/s. The extrapolated viscosity of binders at 490 1/s was referred to as the high shear rate viscosity. Using a shear rate of 490 1/s, the traditional viscosity criteria of 0.17 ± 0.02 Pa  s for mix- ing, and 0.28 ± 0.03 Pa  s for compaction, they determined mixing and compaction temperatures for the four modified binders to be 10°C to 40°C (18°F to 72°F) below the respec- tive temperatures from AASHTO T 312. This method was criticized on several points (44). One issue with this approach was the use of unrealistically low compaction temperatures and then using viscosity measure- ments at those temperatures to extrapolate viscosity-shear rate data. The accuracy of the estimates for viscosities at the recommended shear rate of 490 1/s was questioned. At higher temperatures typical of the range normally used in the labo- ratory, the binders would have exhibited less shear thinning behavior and thus the extrapolations would be different. Steady Shear Flow Reinke (45) presented another concept, which he called the steady shear flow test, to determine mixing and compaction temperatures. This approach uses a DSR with a 500-micron gap and a 25-mm-diameter plate geometry. The viscosities of binders are tested over a range of shear stresses at tempera- tures ranging from 76°C to 94°C (169°F to 201°F). At high shear stresses, around 500 Pa, the viscosities of modified binders approach a steady state (i.e., very small change in vis- cosity with increasing shear stress). This is illustrated in Fig- ure 6. Using a log-log temperature-viscosity chart, the viscosi- ties from the 500-Pa shear flow tests are extrapolated out to 180°C (356°F). As with unmodified binder using the equivis- cous principle, the recommended mixing temperature is based on a viscosity of 0.17 ± 0.02 Pa  s. The recommended compaction temperature from the steady shear flow tech- nique is 0.35 ± 0.03 Pa s, which is higher than the equiviscous compaction range of 0.28 ± 0.03 Pa s. Reinke indicated that the mixing and compaction temperatures derived from the steady shear flow method for the few polymer-modified binders eval- uated matched well to the temperature ranges successfully used in practice. Shear Rate Dependency As noted previously, Shenoy (36) proposed a technique of selecting mixing temperatures based on shear rate dependency and other factors. The study included only two polymer- modified asphalts, Styrelf® and Novophalt®. The binders and a diabase filler were mixed at four temperatures ranging from 150 to 200°C (302 to 392°F) and tested in the Brookfield vis- cometer with three spindles to generate viscosities over a range of shear rates. The ratio of viscosities for the filled binder divided by the unfilled binder showed a pessimum point for the Novophalt® around 180°C (356°F). For the Styrelf®, the viscosity ratio was at an apparent minimum around 163°C. Shenoy selected the mixing temperature range for the binders based on a table of pass/fail criteria for shear-rate dependency, fluidity, Arrhenius plot smoothness, degradation, aging, and viscosity ratio. This yielded mixing temperatures of 180°C (356°F) for the Novophalt® binder and a range of 163°C to 180°C (325°F to 356°F) for the Styrelf® binder. 14 0 50 100 150 200 250 300 350 400 0 100 200 300 400 500 600 Shear Stress, Pa Vi sc os ity (P aS ) 76°C 82°C 88°C 92°C Figure 6. Steady shear viscosity over a range of shear stresses.

Extensional Viscosity Sudduth et al. (46) evaluated eight polymer additives used to modify an AC-20 asphalt binder using a technique referred to as “Pseudo Extensional Viscosity, ηext.” ηext is defined as the difference in the viscosity measured using the Rotational Vis- cometer for the smaller 27 spindle and the viscosity measured using the larger 21 spindle. The asphalts were evaluated at strain rates of 1 1/sec and 100 1/sec. It was concluded that a positive value for ηext was most desirable for roadway paving, and a negative value would be undesirable as it would indi- cate a softer asphalt pavement. Equivalent Mixture Properties Stuart (47) used an SGC to compact three mixtures with an unmodified binder over a range of compaction temper- atures, then substituted the unmodified binder with two dif- ferent modified binders (Novophalt® and Styrelf®) at the same asphalt contents and repeated the SGC compaction tests. The temperature range that yielded the same volumet- ric properties as the unmodified binder was determined for each modified binder. Rheological properties of the binders and mastics were measured to determine which property provided the same temperature ranges given by the com- paction process. The results of the compaction tests indi- cated that a compaction temperature of 145°C (293°F) could be used for each of the binders and achieve the same air void content. The allowable compaction temperature range was found to be 20°C to 40°C (36°F to 72°F), which indicates that compaction was insensitive to temperature. The tempera- ture at which smoking was observed for each of the binders during mixing or short-term oven aging was used to set the maximum compaction temperature. Stuart concluded that a single binder viscosity range could not be used to select the laboratory compaction range for all binders. He also commented that the viscosity range of 1.4 ± 0.10 Pa  s, as recommended by the NCHRP Project 9-10 study, was too low for the unmodified binder. Azari et al. (13) conducted a follow up study to the work by Stuart. They tested mechanical properties and physical characteristics of the limestone-Novophalt® mixture from Stuart’s study compacted at four different compaction tem- peratures. Mixture specimens were fabricated by mixing at 145°C (293°F) and then short-term oven-aged and com- pacted at the following temperatures: 119°C, 139°C, 159°C, and 179°C (246°F, 282°F, 318°F, and 354°F). The samples were analyzed using computer-aided tomography to study the distribution of air voids and aggregate orientation. The shear testing conducted was repeated load at constant height and frequency sweep at constant height at 25 and 50°C. The compaction temperature affected the total air void content and the distribution of the air voids within specimens. The vertical gradient of the air voids was higher than the lateral gradient in the gyratory-compacted specimens. The vertical air void gradients (top to bottom) in the SGC specimens were lowest for samples compacted at 139°C. Lateral air void gra- dients (side to side) were of a much smaller magnitude but were lowest at the highest compaction temperature of 179°C. Aggregate orientation, measured as vector magnitude, peaked at 159°C. A stepwise regression was performed to relate the physical properties (air voids, air void distribution, aggregate orientation, and binder shear stiffness) to the mix shear prop- erties sinδ/G, Gsinδ, and permanent shear strain. Binder stiffness and the distribution of the air voids were important factors that affected the mechanical properties. All shear prop- erties except Gsinδ improved with increasing compaction temperature. For this binder, they recommended an optimum compaction temperature in the range of 139°C to 159°C (282 to 318°F). Workability One of the earliest attempts to evaluate workability of HMA was performed in 1979. Marvillet and Bougalt (48) developed a prototype stirring device for loose mixtures to evaluate the factors that affect workability. They defined work- ability as the inverse of the torque required to rotate the stir- ring blade through a sample of mixture. The prototype device was used to test various combinations of materials and it was found that • Workability increased as viscosity decreased. • Workability was unaffected by changes in asphalt content. • Workability was reduced as the dust content (percent pass- ing the No. 200 sieve) was increased. • Mixes with angular aggregate particles were less workable than mixes with rounded particles. Their work showed that an increase in workability resulted in a corresponding increase in compactability. However, they cautioned that just because two different mixes have the same workability does not necessarily mean the mixes will have the same compactability. It was pointed out in the discussion of the paper that two mixes can start out with the same den- sity and reach the same final density but have different com- paction slopes. The steeper the compaction slope, which was referred to as coefficient of compactability, the easier the material would be to compact. Gudimettla et al. (49, 50) developed another prototype workability device similar to the French workability machine. The first study evaluated several operational parameters such as paddle configuration and speed of rotation and their effect on torque measurements with a few mix factors. This work 15

indicated that the prototype workability device could effec- tively differentiate the effects of mixture gradations and binder grades. This led to development of the current work- ability device by Instrotek, Inc. Further research with the Instrotek workability device studied additional materials factors including three aggregate types (granite, crushed gravel, limestone); two NMAS (19 mm, 12.5 mm); five gradations; and three binder types (PG 64-22, 70-22, 76-22). The research found that each mix has a range of workability as the mix temperature decreases. When the same binder grade was used with three different aggregate types, each mix had different workability levels. This indicates that binder properties alone may not be the best measure of the compactability of an asphalt mixture. When compared with the equiviscous method for determining mixing and compacting temperatures, the workability tests resulted in compaction temperatures 9°C to 28°C (16°F to 50°F) lower for modified asphalts and about the same temperature for unmodified asphalt. The data were examined to determine the temperatures at which mixtures with different binders had the same workability. The study also evaluated an ap- proach to define compaction temperatures for each mixture based on the workability versus temperature data. The re- search concluded that every mix has a unique relationship be- tween temperature and workability based on aggregate type, binder type, gradation, and NMAS. Table 3 provides a summary of methods in use or proposed by researchers for determining mixing and compaction tem- peratures of hot-mix asphalt. Shear Rates During Mixing and Compaction If viscosity is the binder parameter used to establish mix- ing and compaction temperatures, the shear rate(s) used to determine the viscosity should approximate the shear rates that occur during mixing and compaction. However, very lit- tle information was found in the literature regarding the shear rates that exist during mixing and compaction in the labora- tory or during plant production and construction. The max- imum shear rate that exists in a rotating shaft mixer can be approximated using Equation 1 and substituting tangential velocity of the mixing tip for the relative velocity, V: where γ ′ = the shear rate; V = the relative velocity of the solid elements shearing the fluid; and d = distance between solids. ′ =γ V d ( )1 vt = rω where vt = the tangential velocity; r = the radius; and ω = the angular velocity. The rotation speed for a popular model of continuous mix plant (Astec 400 ton/hr double barrel plant) is 7.68 rpm (0.8 rad/s). This plant has a drum radius of 1.69 m (5.56 ft). The tangential velocity at the point of mixing for this plant is 1,352 mm/s. In a popular size batch plant, the pugmill is driven at a rate of 33.6 rpm (3.52 rad/s). The length of the mixing arms from the center of the mixer shaft is 0.47 m (1.55 ft). Therefore, the maximum tangential velocity during mixing in this pugmill is 1,654 mm/s. If the nominal thickness of the asphalt coating on an aggregate particle during the initial mix- ing is approximated as 10 microns (0.01 mm), then the instan- taneous shear rate of the binder film on a particle in contact with the pugmill tip can be estimated with Equation 1. Using this approach, the pugmill will yield a maximum in- stantaneous shear rate of 165,400 1/s, and for the drum plant, the maximum instantaneous shear rate is estimated to be 135,200 1/s. These estimated shear rates for mixing represent the high end of a range of shearing that occurs during mixing. In reality, the mixing process in a plant or in laboratory mixers produce a turbulent mass mixing action with an extremely wide range in shear rates. For comparison, the shear rates for several laboratory mixers and binder test methods are shown in Table 4. These estimates also reveal how different the conditions may be in routine laboratory binder tests versus an HMA plant. For estimating shear rates during compaction, it is necessary at this point to only be concerned with laboratory compaction since that is the issue at hand. Although the conditions of lab- oratory compaction are much more controlled than in the field, there are still numerous complications in estimating shear rates during compaction. In NCHRP Report 459, Bahia et al. (39) reasoned that the shear rate during compaction in the SGC was very low. This logic was based on the observation that the change in specimen heights is very low during most of the compaction, especially after the first 10 or so gyrations. However, the one-dimensional vertical strain rate of the mixture and the shear rate of the binder films coating the aggregate particles during compaction are not the same thing. Since the aggregate particles are essen- tially nondeformable rigid bodies, the strain or deformation only occurs due to manipulation of aggregate particles around one another, thus shearing the asphalt films during those movements. One valid point is that the strain rate changes throughout the compaction process. The early part of com- paction is where the binder consistency plays a greater role. 16

17 Method Description Advantages Disadvantages Equiviscous Temperatures The rotational viscometer is used to determine the viscosity at 2 temperatures and 1 shear rate. The viscosities are plotted vs. temperature and a temperature range corresponding to 0.17±0.02 Pa·s is chosen for mixing and a temperature range corresponding to 0.28±0.03 Pa·s is chosen for compaction. • Simple to obtain and analyze results • Can be completed in less than 1 hour • Assumes linear relationship between viscosity and temperature • Assumes that all asphalt binders are Newtonian liquids; does not account for shear rate dependency • Can result in unnecessarily high mixing and compacting temperature for some modified asphalt binders High Shear Rate Viscosity Uses the rotational viscometer to determine the shear rate dependency of an asphalt binder at 2 temperatures (135°C, 165°C). For each • Takes into account the shear rate dependency of modified asphalt • Requires extrapolation of results to a high shear rate temperature, the data is fit to an inverse power curve and extrapolated to estimate the viscosity at a shear rate of 490 1/s. The high shear viscosities are plotted versus temperature, and mixing and compaction temperature ranges are determined at target values of 0.17±0.02 Pa·s and 0.28±0.03 Pa·s, respectively. binders • Testing is simple to perform • Does not require complicated modeling Steady Shear Flow Uses a DSR Steady State Flow test at 76°C, 82°C, 88°C, and 94°C. Measurements of steady state viscosity are made over a range of 0.16 to 500 Pa stress. The viscosity values at 500 Pa are plotted versus temperature and mixing and compaction temperature ranges are determined at target values of 0.17±0.02 Pa·s and 0.35±0.03 Pa·s, respectively. • Simple to perform, uses standard DSR equipment and testing procedures • Can be time consuming for modified asphalts • Not all modified asphalts reach a state of steady shear by 500 Pa • Requires extrapolation of viscosity to much higher temperatures Zero (Low) Shear Viscosity Uses the rotational viscometer to determine the shear rate dependency of an asphalt binder at 3 temperatures (120, 135, 165°C). The Cross- Williams model is used to fit a curve to the data at each temperature from which the viscosity at a shear rate of 0.001 1/s is estimated. The low shear viscosities are plotted versus temperature and mixing and compaction temperature ranges are determined at target values of 3.0 Pa·s and 6.0 Pa·s, respectively. • Takes into account the shear rate dependency of modified asphalt binders • Testing is simple to conduct • Results in lower mixing and compaction temperatures for modified asphalt binders • May not accurately represent the shear thinning behavior of modified asphalt binders • Requires extrapolation of results to a low shear rate • Cross-Williams regression model is complicated • No clear agreement on definition of zero shear viscosity • Results for some binders yield unrealistically low mixing and compaction temperatures Table 3. Summary of researched methods for determining mixing and compaction temperatures. (continued on next page)

As the aggregates compact more closely together, the com- paction resistance and mixture strain is dominated more by the aggregate texture, shapes, and gradation. In a typical gy- ratory compaction record, the height change during the first gyration is 2.8 mm, and by the tenth gyration the height change is down to about 0.4 mm. The speed of gyration for SGC compactors is specified to be 30 gyrations per minute or 0.5 gyrations/sec. Multiplying the height changes by the gyration rate gives an approximate instantaneous vertical velocity within the compacting mixture. For the first gyra- tion, it is 1.4 mm/s and for the tenth gyration, 0.2 mm/s. However, due to the fixed tilting angle, there is also a rota- tional shear. Using a 2-D approximation of this 3-D prob- lem, the horizontal displacement for a 120-mm tall speci- men gyrated at 1.16° is 2.43 mm. Figure 7 illustrates the shear movement during SGC compaction. As the specimen is compacted, the shear moves in the opposite direction for a total displacement of 4.86 mm within one-half of a gyration, which occurs in 1 second. Thus, at the top of the specimen, there is an approximate horizontal shear rate of 4.86 mm/s. Resolving the vertical and horizontal shear movements yields Dividing this velocity by a nominal film thickness of 10 mi- crons yields estimated shear rate of 506 1/s for the first gyra- tion, with a slight reduction in shear rates as the compaction process continues. Note that this is very similar to the shear rate estimated for gyratory compaction by Yildirim et al. (43, 44) based on experimental analyses. 1 4 4 86 5 06 2 2 . . . .( ) + ( ) = mm s 18 Method Description Advantages Disadvantages Mixture Workability Uses a large stirring device to measure the torque required to stir a mix as it cools. Torque is inversely proportional to workability. The relationship • Considers the effects of aggregate particle shapes and size on compactability of • New equipment • Time-consuming procedure • Not practical for between workability and temperature can be used to help establish temperature range where a mix is easiest to work. mixtures routine use • Aggregate characteristics and gradation may overwhelm binder effects Compaction Test A standard mix is compacted with an unmodified “control” binder to establish a baseline density. The modified binder is then added to the standard mix and samples are compacted at temperature intervals. The temperature that provides the same density as the control binder is the compaction temperature for the modified binder. • Easy to analyze based on density and volumetric properties • Time-consuming procedure • SGC is insensitive to binder consistency • Only provides results for compaction temperature • Results are dependent on the “standard” mixture. Other mixes may provide different results. Table 3. (Continued). Laboratory Device Model # RPM Radius (in.) Tangential velocity (mm/s) shear rate (1/s) Bucket mixer KOL M-60 65 5.6 961 96,100 Pugmill mixer 7590-H 128 3.9 1341 134,100 Workability device Instrotek 20 6.0 319 31,919 Bowl mixer Hobart A200 48 4.0 425 42,558 Rotational viscometer Brookfield DV-II+ 20 8.4 mm 17.5 6.8 Dynamic Shear Rheometer 10rad/s 25 mm 8 mm 125 20 Table 4. Summary of shear rates for some field and lab equipment.

These are only estimates of shear rates at specific moments and locations during mixing and compaction. However, they show that shear rates during mixing and compaction are likely to be very different in the lab and in the field. It is also apparent that high rates occur at short instances of time. In reality, it is not reasonable to select a single shear rate that is representative of the extreme range of shear and flow of binder films on aggregate particles during mixing and compaction operations. Summary of Key Findings from the Literature Review Key findings from the literature search and state of practice survey for mixing and compaction temperatures are listed here. 1. The equiviscous concept for selecting mixing and com- paction temperatures was established by the Asphalt Insti- tute between 1956 and 1962 (5, 7). However, the methods used to establish the viscosity criteria are not known. Sev- eral studies in the 1950s reference mixing temperatures based on asphalt viscosity ranges (2, 3, 4, 6). 2. Criteria for mixing and compaction temperatures were initially based on viscosity measurements made with a Saybolt Furol viscometer (4, 5, 6, 7, 8). 3. Numerous studies have demonstrated that compaction temperatures have a significant effect on the mechanical properties of the fabricated specimens (2, 3, 4, 6, 8, 13). 4. Volumetric properties of asphalt mixtures compacted with SGC, on the other hand, appear to be insensitive to compaction temperature (15, 16, 17). 5. The use of the equiviscous concept for many polymer- modified asphalts results in excessively high mixing and compaction temperatures (34, 35, 36). 6. High mixing temperatures cause asphalt binders to harden, primarily through mechanisms of volatilization and oxida- tion (24). Most oxidation reactions approximately double as temperature increases by 10°C (25). Polymer additives in asphalt can break down at excessively high temperatures. However, the temperatures where these changes become detrimental are not clearly established (28, 29, 36). 7. High temperatures also cause emission and odor prob- lems for some asphalt binders (30, 31, 32). 8. Most modified asphalt binders are shear rate dependent and exhibit shear thinning behavior (reduced viscosity at high shear rates). 9. In some cases, mixtures with polymer modified asphalts can be more difficult to work with in the field (34). How- ever, in other cases, some mixtures with modified binders have been reported to be as workable as mixes with un- modified binders (11). 10. Most agencies use the equiviscous mixing and com- paction criteria cited in AASHTO T 245 and T 312 (e.g., 0.17 ± 0.02 Pa s for mixing and 0.28±0.03 Pa s for com- paction) for unmodified asphalt binders. However, many agencies refer instead to the binder supplier for recom- mended mixing and compaction criteria even for un- modified binders. 11. Some countries use slightly higher viscosity ranges than given in AASHTO T 312 for setting mixing and com- paction temperatures. 12. Supplier recommendations are most often used for set- ting mixing and compaction temperatures for polymer modified binders. Some agencies set their own criteria for modified binders. 13. Field experience (i.e., trial and error) has generally been used to determine appropriate mixing and compaction temperatures for modified binders. 14. For many agencies and suppliers, mixing and compaction temperature ranges are based on the Superpave PG of the binder. 15. Most agencies use the same mixing and compaction tem- perature ranges for laboratory testing and field operations of HMA production and construction. Some agencies specify a temperature range for plant produced HMA and reject mix outside of that range. 16. The Zero (or Low) Shear Viscosity concept has been shown to yield unrealistically low mixing and compaction tem- peratures for some binders. 17. Yildirim’s method of estimating binder viscosities at high shear rates from rotational viscosity data attempts to take the shear rate dependency of modified binders into ac- count. Although this method requires extrapolation of viscosity-shear rate data, reasonable mixing and com- paction temperatures for a limited number of modified binders were demonstrated in the study. 18. Reinke’s method of using a DSR steady shear flow test uses viscosity measurements taken at shear stress levels where binders appear to have Newtonian behavior. How- ever, this method relies on extrapolating viscosity data to much higher temperature ranges. 19. Research has yet to clearly identify a reliable method of determining mixing and compaction temperatures for modified and unmodified asphalt binders. 19 2.43 mm 2.43 mm 1.16º Figure 7. Illustration of horizontal strain during SGC compaction.