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1 SUMMARY Mixing and Compaction Temperatures of Asphalt Binder in Hot-Mix Asphalt Background The use of modified asphalt binders in hot-mix asphalt has steadily increased over the past several decades. Modified asphalt binders currently make up over 20% of paving grade asphalt sales in the United States, and the percentage continues to grow. However, selecting appro- priate temperatures for handling these binders has been an issue with the use of modified asphalt binders. The traditional method of determining appropriate mixing and compaction temperatures for an asphalt binder is based on relatively simple viscosity measurements of the asphalt. However, this method often yields excessively high temperatures for many mod- ified binders that have caused concerns with degradation of the binder's properties and emis- sion problems in laboratories during preparation of samples and during production and placement of asphalt mixtures in the field. In the absence of a reliable method for selecting mixing and compaction temperatures for modified asphalt binders, many agencies have writ- ten their specifications to simply allow the binder supplier to set an appropriate mixing tem- perature for each modified binder. In some cases, agencies or binder suppliers have based mixing and compaction temperature ranges on the Superpave® performance grades (PGs), and in other cases the mixing and compaction temperatures have been established based on field experience. Clearly, a standardized method or more formal process was needed for selecting mixing and compaction temperatures for asphalt mixtures. Research Approach The goal of this research was to identify or develop a simple, reliable, and accurate proce- dure for determining the mixing and compaction temperatures that is applicable for both modified and unmodified asphalt binders in hot-mix asphalt (HMA). An examination of available literature on the subject identified several possible methods for determining mix- ing and compaction temperatures. With the input from the project panel and the desire for this research to ultimately recommend a practical method for possible implementation, three candidate methods were selected for the laboratory evaluation. Each of the candidate methods was based only on measurements of binder properties using common equipment used in asphalt binder laboratories. Candidate Method A, developed by Yildirim Soaimanian, and Kennedy in Texas, was based on proof that most modified binders exhibit shear thinning behavior. This method, referred to as High Shear Rate Viscosity, used measurements from a rotational viscometer at shear rates ranging from 0.1 1/s to 93 1/s at 135 and 165°C. The Cross-Williams model was used to extrapolate the measured shear rate versus viscosity data to the higher shear rate of 500 1/s. The extrapolated viscosity data were plotted on a conventional log viscosity versus
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2 log temperature chart to obtain temperatures corresponding to the traditional viscosity cri- teria of 0.17 ± 0.02 Pa s for mixing, and 0.28 ±0.03 Pa s for compaction. Candidate Method B, developed by Reinke, is called the Steady Shear Flow test. This method uses a Dynamic Shear Rheometer (DSR) with a 25-mm-diameter parallel plate geometry and a 500-micron gap. The viscosities of binders were measured in a constant shear mode over a range of shear stresses at temperatures ranging from 76 to 94°C. At higher shear stresses, around 500 Pa, viscosities of most modified binders approach a steady state. Using a log vis- cosity versus log temperature chart, viscosity results at 500 Pa shear were extrapolated to 180°C. The mixing temperature was selected at a viscosity of 0.17 ± 0.02 Pa s, which is con- sistent with conventional practice. However, as recommended by Reinke, compaction tem- perature from the Steady Shear Flow method was selected at a viscosity of 0.35 ± 0.03 Pa s. Candidate Method C, developed by Casola, is referred to as the Phase Angle method, which is based on the observation that the Phase Angle from dynamic shear rheology is a binder consistency property that takes into account the visco-elastic nature of asphalt binders. This method uses a DSR with 25 mm parallel plate geometries and a 1-mm gap set up in oscillatory shear at angular frequencies from of 0.001 to 100 rad/s. The frequency sweep data is used to construct a Phase Angle versus frequency master curve at a reference temperature of 80°C. The binder's transition point from viscous behavior to viscoelastic behavior was represented by the Phase Angle of 86°. The frequency corresponding to this transition point was correlated to the temperatures where binders provide good aggregate coating during mixing and lubrication during compaction. The research approach included two series of experiments. The first series involved tests with the candidate methods and other methods to assess emissions potential and binder degradation due to exposure of binders to high temperatures. Modified and unmodified binders used in the experiments were obtained from several binder suppliers across the United States to include a range of crude sources, refining processes, and modification sys- tems. The second series of experiments were laboratory mixture tests to assess how tempera- tures affect the coating of aggregates during mixing, workability, compactability, and low temperature properties of asphalt mixtures with the different binders. The primary purpose of the mixture tests was to provide validation of the predicted mixing and compaction tem- peratures from the candidate methods. A few additional mixture test experiments also were conducted to evaluate how other factors affect coating and compactability results. Findings The High Shear Rate Viscosity method resulted in mixing temperatures for all of the mod- ified binders well above 177°C, which are widely considered in the asphalt paving industry to be too high and likely to result in emissions problems. Results from the high shear rate viscosity method were very similar to the traditional equiviscous method and therefore pro- vided no improvement to the current procedure. The Steady Shear Flow method yielded mixing and compaction temperatures lower than those from the traditional equiviscous method for modified and unmodified binders. Tem- perature differences between the two methods were greater for modified binders, which con- firmed shear thinning behavior for the modified binders. The Steady Shear Flow method also yielded lower mixing and compaction temperatures for the unmodified binders; in most cases, the mixing temperatures are more than 6°C lower than the equiviscous mixing tem- peratures. Correlation of the mixing temperatures from the Steady Shear Flow method with the binder producers' recommended mixing temperatures was reasonable, yielding a coefficient of determination (R2) of 70.1%.
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3 The Phase Angle method also resulted in lower mixing and compaction temperatures than did the equiviscous method for modified binders. However, for unmodified binders, some of mixing and compaction temperatures from the Phase Angle method were lower and some were higher than from the equiviscous method. The coefficient of determination for cor- relation of the Phase Angle results with the binder producers' recommended temperatures was 58.2%, which is lower than for the Steady Shear Flow method. However, the regression equation between the Phase Angle method and producers' recommendations was closer to the line of equality than for the Steady Shear Flow method. Results from the smoke and emissions test (SEP) showed that emissions increased with higher temperatures for some binders much more than for others. However, with the lim- ited set of binders from different crudes, different refineries, different modification types, and different grades, it was not possible to identify what factors may cause some binders to smoke more than others. Test results for various binder properties before and after the SEP test did show that binders increased in stiffness with exposure to higher temperatures, but there was no strong evidence of polymer degradation for the modified binders. Although the results of the mixture coating tests at different temperatures generally fol- lowed expected trends for each binder, using the data to estimate mixing temperatures to achieve specific coating percentages did not always provide reasonable results. Therefore, the validation of the candidate methods with mixture coating test results by direct correla- tion yielded poor goodness-of-fit statistics, even when a few outlier coating test results were removed from the dataset. The same was true for the results of the workability tests and their correlations with the results from the Steady Shear Flow and Phase Angle methods. Com- paction tests with the binders over a range of four temperatures did indicate that both the binder and temperature had significant effects on specimen density at 25 gyrations. A sepa- rate experiment showed that a mixture's aggregate components have a greater affect on com- paction behavior than do the binder characteristics. Maximum shear ratio was not a useful indicator of compactability. Reasonable correlation statistics were obtained for regressions between the compaction temperatures from the Steady Shear Flow and the Phase Angle method and the predicted compaction temperatures to reach a set level of density from the lab experiments. Compaction temperatures also significantly affected the low temperature properties of mix- tures. Higher mixing temperatures stiffened mixtures (reduced the creep compliance), which will reduce the ability of the pavement to dissipate thermal stresses. The stiffening effect was more evident for binders with lower PG grades. Conclusions Both the Steady Shear Flow method and the Phase Angle method provide lower mixing and compaction temperatures for modified asphalt binders compared with the traditional equiviscous method. Both methods also appear to provide reasonable temperatures for mix- ing and compaction temperatures for a variety of modified and unmodified asphalt binders being used across the United States. An advantage of both methods is that they can be set up and performed using existing standard DSR equipment used in most asphalt binder labs in the United States. Limitations of both methods include the restrictions normally applicable to parallel plate DSR testing, such as the binder test sample, must be homogenous and free of particulate matter (e.g., ground rubber particles) that may interfere with or distort the rheological response of the instrument. Correlations of the mixing and compaction temperatures with laboratory coat- ing, workability, and compactability were similar for both methods. Although the Steady Shear
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4 Flow and Phase Angle methods are based on different binder properties, both methods use a standard DSR and common parallel plate geometries for testing of the binder. Therefore, they have some practical limitations including the test temperatures at which the properties are measured and particulate matter begins to have an effect. Results from the two methods are well correlated. Differences in results exist primarily for unmodified binders where the Steady Shear Flow method generally yields lower mixing and compaction temperatures. No links were evident between opacity (i.e., smoking) of a binder and its grade, crude source, or whether the binder was modified. Performance grading of the binders after SEP tests showed that the critical high and critical low temperatures increased slightly with higher SEP temperatures. However, there was no evidence of degradation of the binders due to ex- posure at elevated temperatures in the SEP test. The lack of evidence of degradation does not mean that it is not possible; rather, it may mean that the conditions in the SEP test are not sufficiently severe to cause breakdown of polymer modifiers. Although the SEP test may have value in identifying binders with opacity problems, it does not appear that it is suitable for establishing maximum mixing temperatures. Recommendations Both the Phase Angle method and the Steady Shear Flow method are recommended for further evaluation. Draft AASHTO format procedures for both methods are provided. Mix- ing and compaction temperatures determined by these methods are only applicable to the laboratory setting for mix design work, quality assurance testing of HMA, and fabricating HMA samples for laboratory performance tests. The mixing and compaction temperatures determined by these methods should not be used to control plant production or pavement construction temperatures. Greater latitude in mixing temperatures is necessary in the field to allow for different ambient conditions, haul distances, and other mix characteristics that affect coating and compactability. Four additional steps are recommended to validate, refine, and assist the implementation of the Steady Shear Flow and/or Phase Angle methods: 1. Independent validation of the methods by asphalt suppliers with a much broader range of binders; 2. Refinement of the methods by developing standard DSR control and computer analysis routines, possibly eliminating steps to reduce testing times, and evaluation of alternate instrument geometries to avoid issues with temperatures and some filled binders; 3. Interlaboratory studies to establish precision information for the procedures; and 4. Training on the method(s) for full implementation by the asphalt paving industry.