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

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21 The third candidate procedure was developed during this temperature. The minimum temperature suitable for mixing project. This technique considers the non-Newtonian visco- should be based on how well the binder coats the aggregate in elastic behavior of binders as measured using standard asphalt simulated plant mixing conditions. Two types of laboratory DSR equipment. In dynamic testing, the phase angle is the mixers were used to evaluate coating of binders on a base aggre- measure of the time lag between the applied stress and the re- gate material at four temperatures. Coating percentages were sulting measured strain. For dynamic shear rheology, the phase evaluated by the Ross count method (ASTM D 2489). angle identifies the relative elastic and viscous response to Mix workability tests were included in the test plan to char- shear and can be easily measured over a range of temperatures acterize how binder consistency affects the reaction of HMA and frequencies. Therefore, it was logical to explore the phase mixtures to manipulation by equipment and manual tools. angle as an alternate consistency parameter that could be used Workability is considered to be an intermediate state between to establish mixing and compaction temperatures. Although mixing and compaction. The simple workability test was phase angle is a fundamental material property, its relation- anticipated to show how different binders change how easy ship to coating and lubricating aggregate particles is empiri- or how difficult a mixture is to handle. cal. However, this is also the case for viscosity at any shear rate. Mix compaction tests were conducted with a SGC. Since Binders also were analyzed with regard to their potential many studies have shown that the densities of mixtures com- for emissions and thermal degradation over a range of tem- pacted in an SGC to a high number of gyrations are insensi- peratures normally used for storage and mixing. The results tive to binder consistency, compaction tests in this study used from these were anticipated to aid in establishing upper lim- 25 gyrations. It was believed that differences due to binder its for mixing temperatures in the field and the lab. consistency may be more apparent in the early part of the Mixture tests were performed to analyze the effects of tem- compaction process before aggregates lock up and dominate perature and binder consistency on aggregate coating, mix mixture shear resistance. workability, lubrication and shear resistance during laboratory A diagram illustrating the experimental plan is shown in Fig- compaction, and the mechanical properties creep compliance ure 8. Part 1 included the series of binder tests that included and indirect tensile strength. The results of these tests were used testing of the 14 binders described in the following section to identify temperatures at which each binder provides similar using the candidate methods for determining mixing and mixture responses or where properties change significantly with compaction temperatures. Also included in Part 1 were the 14 Asphalt Binders Part 1: Binder Testing Part 2: Mix Testing Steady Shear Predict Mix & Correlations Determine Minimum Mix Coating Flow Tests Compaction Temps Mixing Temps Tests Rotational Predict Mix & Determine Intermediate Mix Workability Viscosity Tests Compaction Temps Mix Handling Temps Tests Dynamic Shear Predict Mix & Determine Compaction Mix Compaction Rheology Compaction Temps Temps Range Tests & Reasonableness Smoke & Emissions Establish Max Temp Potential Test to Avoid Emissions Check for Determine Effect of IDT Creep & Excessive Temps on Mix Props Strength Test Grade Binders Before Establish Max Temp Temps & After SEP to Avoid Degradation Select Best Method Mix Coating Tests 4 Asphalt Binders Validation of Method Mix Workability Tests Mix Compaction Tests Draft New Test Method for Establishing Mixing & Compaction Temperatures Figure 8. Diagram illustrating experimental test plan.