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