National Academies Press: OpenBook

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

Chapter: Chapter 4 - Conclusions and Recommendations

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Page 72
Suggested Citation:"Chapter 4 - Conclusions and Recommendations." 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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Page 72
Page 73
Suggested Citation:"Chapter 4 - Conclusions and Recommendations." 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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Page 73
Page 74
Suggested Citation:"Chapter 4 - Conclusions and Recommendations." 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.
×
Page 74
Page 75
Suggested Citation:"Chapter 4 - Conclusions and Recommendations." 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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Page 75

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72 Summary of Key Findings The results of the experiments conducted as part of this study have led to the following conclusions. 1. The high shear rate viscosity method does not provide an improvement in selecting mixing and compaction tem- peratures for modified binders compared with the equiv- iscous method. The mixing and compaction temperatures determined by the high shear viscosity method are very similar to the temperatures determined by the equiviscous method that are considered too high for modified binders based on general field experience of binder suppliers. 2. Mixing and compaction temperatures determined with the SSF method are substantially lower than the equivis- cous mixing and compaction temperatures. The differ- ences between the results are greater for the modified binders. This indicates that many of the modified asphalt binders exhibit shear thinning behavior. However, the SSF method also results in lower mixing and compaction tem- peratures for unmodified binders; in most cases, the SSF mixing temperatures are more than 10°F lower than the equiviscous mixing temperatures. 3. For modified asphalt binders, the mixing and compaction temperatures determined by the Phase Angle method are also substantially lower and more reasonable than by the equiviscous method. For unmodified binders, compar- isons of mixing and compaction temperatures between the Phase Angle method and the equiviscous method were mixed. In some cases, the Phase Angle method yielded tem- peratures within a few degrees of the equiviscous method results. However, some Phase Angle results were also more than 10°F above or 10°F below the results from the equi- viscous method. 4. The results of the SEP test showed that opacity and mass loss from asphalt binders increase with higher tempera- tures. However, the amount and rate of emissions increase differs among the binders. No links were evident between opacity and binder high grade number, binder low grade number, grade spread, whether the binder was modified, or crude source. Grading of the binders after SEP tests showed that the critical high temperatures and critical low temper- atures increased slightly with higher SEP temperatures. Also, the non-recoverable compliance values (Jnr) decrease (i.e., the binders became more elastic) with higher SEP temperatures. However, there was no evidence of degra- dation of the binders due to exposure at elevated temper- atures in the SEP test. The lack of evidence of degradation from the SEP test does not mean that binder degradation is not possible. It is entirely possible that the conditions in 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 is not rec- ommended as a test for establishing maximum tempera- tures for mixing with aggregates. 5. Coating experiment results showed that the binder, mix- ing temperature, and mixer type significantly affected the coating percentage. However, neither of the lab mixers consistently provided reasonable mixing temperatures for all of the binders. Coating test results with completely dry aggregates and wet aggregates were not significantly different. Foaming of the asphalt was not observed dur- ing the tests with the wet aggregates. Correlations between the mixing temperatures from the candidate methods and the coating test results were weak. The poor correla- tions were likely due to inconsistencies with the subjective coating test. 6. Poor repeatability was also a problem with the workability experiments. This test did not provide dependable results by which to evaluate relationships between temperature and workability of mixes with different binders. 7. Compaction experiments using the SGC indicate that a mixture’s aggregate components have a greater affect on compaction behavior than the binder characteristics. How- C H A P T E R 4 Conclusions and Recommendations

73 ever, when aggregate type and gradation were held con- stant, the binder ID and compaction temperature signifi- cantly affect the mix density at 25 gyrations. Maximum shear ratio was not a useful indicator of compactability. 8. Compaction temperature has a significant effect on low temperature properties of mixtures: lower compaction tem- peratures resulted in slightly higher creep compliance val- ues. Conversely, aging of the binders due to high mixing temperatures stiffened mixtures (lower creep compliance), which reduces the ability of the pavement to dissipate thermal stresses. The effect of aging was more evident for binders with lower PG grades. In other words, a PGXX-34 is affected more by increases in mixing and compaction temperatures than a PGXX-16. 9. Correlations of the mixing and compaction temperatures determined from the candidate methods with laboratory tests on mixtures for coating and workability were poor. The weak correlations between the binder tests and the mix tests are partially attributable to the poor precision of the coating and workability tests. On the other hand, com- paction temperatures from both of the candidate methods correlated well with the results of the laboratory com- paction tests. Results of the candidate methods also agree reasonably well with the binder suppliers’ recommended mixing temperatures. Recommendation of a New Method for Determining Mixing and Compaction Temperatures The objective of this research was to identify or develop a simple, reliable, and accurate procedure for determining mix- ing and compaction temperatures that is applicable to modified and unmodified binders in HMA. Two candidate procedures, the SSF method and the Phase Angle method, were thoroughly evaluated. • An advantage of both candidate methods is that they can be set up and performed using existing standard DSR equip- ment used in most asphalt binder labs in the United States. Limitations of both methods include the restrictions nor- mally applied 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. • Both methods also appear to provide reasonable tempera- tures for mixing and compaction temperatures for a variety of modified and unmodified asphalt binders being used across the United States. • Correlations of the mixing and compaction temperatures with laboratory coating, workability, and compactability were similar for both methods. Draft AASHTO format procedures for the SSF method and the Phase Angle method in are included in Appendix C. Mix- ing and compaction temperatures determined by these meth- ods are only applicable to the laboratory setting for mix de- sign work, quality assurance testing of HMA, and fabricating HMA samples for laboratory performance tests. The mixing and compaction temperatures determined by this method should not be used to control plant production or pavement construction temperatures. Greater latitude in mixing tem- peratures is necessary in the field to allow for different ambi- ent conditions, haul distances, and other mix characteristics that affect coating and compactability. Although excessive temperatures may cause emission problems for some binders, no evidence of this was found in the laboratory work nor did tests demonstrate degradation. At this time, the best guidance for plant mixing temperatures is the EC 101 guidance (38). Recommendations for Further Work There are several additional key steps that should be con- sidered in order to validate, refine, and eventually implement the use of the SSF and/or Phase Angle methods. Four addi- tional steps are outlined as follows: 1. Independent validation of the methods by asphalt suppliers; 2. Refinement of the SSF and Phase Angle methods; 3. Interlaboratory studies to determine precision informa- tion for the new procedures; and 4. Training on the new method(s) for full implementation by the asphalt paving industry. Independent Validation The first major step is to validate the SSF and Phase Angle methods with many more asphalt binders in other laboratories. This would logically begin with asphalt suppliers conducting both methods in parallel with grading tests on their current slate of paving grade asphalt products. Simple DSR control and analysis programs to run the tests and analyze results need to be developed and distributed for the variety of DSR’s being used in the asphalt industry. To begin this effort, preliminary training on the methods would be necessary. A single organization should be identified to collect the results of this broader field validation effort in an organized fashion. Ideally, an online data- base would be established to facilitate entry of results from users anywhere in the world. The following list shows basic in- formation that would be useful to gather in such a database: • Binder supplier. • Binder PG grade. • Modification type(s). • Crude source.

• Location(s) the binder is used (state, region, etc.). • Supplier’s recommended mixing and compaction tem- peratures. • Equiviscous mixing and compaction temperatures. • SSF mixing and compaction temperatures. • Phase Angle mixing and compaction temperatures. • Issues with the procedures and/or equipment. • Comments and observations regarding laboratory mixing and compaction. • Comments and observations regarding field mixing and compaction. This information would allow users and researchers to see how well the methods gain acceptance and if problems occur, where and with what sort of binders. A single organization should be responsible for initial training on the methods, es- tablishing the database, trouble-shooting problems, and re- porting of the field validation data. This initial field validation effort should take about 18 months. Refinement of the SSF and Phase Angle Methods Gerald Reinke, developer of the SSF method, has recently recommended using a higher shear stress of 1000 Pascals com- pared with the 500 Pascals used in this study. The recom- mended change is evidently to try to reach a better steady state viscosity for some highly modified binders. Since viscosities of such binders decrease at higher shear stresses (shear thinning behavior), the resulting mixing and compaction tempera- tures could be expected to be slightly lower for those binders. Reinke’s recommended refinement of the SSF method simply adds one additional stress level to the procedure, so it would be relatively easy to analyze the data using both stress levels as part of the independent validation work described earlier. Although the Phase Angle method developed in this study is simple, reasonable, and innovative, criticisms of the method are (1) that the relationships between phase angle and mixing and compaction temperatures are too empirical; (2) the selec- tion of frequencies at δ=86° seems arbitrary; and (3) the com- mon water-cooled DSRs may not be able to reach the desired phase angle transition region within the temperature range of the water bath. Further research is warranted to explore sev- eral possible refinements of the concept. One idea suggested by the AI is to simply perform the frequency sweep testing at only 80°C and find the frequency corresponding to the phase angle at 86°. This could substantially reduce the testing and analysis time. Another path of further study should evaluate the use of the phase angle measurements on aged binders (e.g., RTFO or some other aging protocol) since rheological behaviors of asphalts change at different rates during plant mixing or lab conditioning protocols. Another idea to evalu- ate would be to change from the parallel plate geometry to a cup and bob geometry in the DSR for the phase angle mea- surements. This would allow for testing at higher tempera- ture ranges (closer to mixing and compaction temperatures) and examination of the binder behavior at the more funda- mental phase angle point of 45° (the point where loss mod- uli and storage moduli are equal). This analysis would provide more points on the phase angle master curve for analysis of the frequency-temperature relationships for determining mix- ing and compaction temperatures. Interlaboratory Studies Interlaboratory studies are useful for improving test meth- ods and determining repeatability and reproducibility infor- mation of the results. A ruggedness study should be conducted on new methods to identify what procedural factors have the greatest influence on the results. The recommended method for conducting a ruggedness study is ASTM C1067, Standard Practice for Conducting A Ruggedness or Screening Program for Test Methods for Construction Materials. Steps for developing a ruggedness study include 1. Identify seven procedural factors for the test method; 2. Establish high and low levels for each factor; 3. Design the experiment: normally eight combinations of 14 factor levels (7 factors × 2 levels) with two replicates for each combination; 4. Identify at least three laboratories to participate in the study; and 5. Determine three to five materials that cover the range of ma- terials properties to which the test method is applicable. The selection of the procedural factors should be selected follow- ing the independent validation work described above. For many test methods used in pavement materials analysis, a second interlaboratory study is conducted to establish preci- sion statistics. The recommended procedure for this type of interlaboratory study is ASTM C802, Standard Practice for Conducting an Interlaboratory Test Program to Determine the Precision of Test Methods for Construction Materials. However, given that the purpose of either the SSF method or the Phase Angle method would be only to prepare asphalt mixture sam- ples and they would not be used in determining whether a ma- terial met a criteria for the purpose of payment, the repeatabil- ity and reproducibility statistics are not critical. Alternatively, a more economical way to obtain useful precision information for either of these procedures would be to include it in the AMRL proficiency testing program for asphalt binder testing. Training A key final step in implementation of a new method is training on the procedure and getting users to understand how the results should be used. Multiple avenues should be 74

75 used for delivery of training on using the Phase Angle method ranging from traditional instructor-led courses with hands on workshops to more contemporary methods like self-paced web-based training. Many technicians can be reached effectively within existing regional and national binder technician certifi- cation programs. To aid in these training venues, simple educa- tional materials, such as Powerpoint slides with examples, need to be developed and distributed. Online delivery of training ma- terials is likely to become a very important method for training in the near future. Interactive web-based training content is able to reach a broad audience at any time, with self-paced learn- ing using new media including animation and video.

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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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