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19 2.43 mm 2.43 mm 8. Most modified asphalt binders are shear rate dependent and exhibit shear thinning behavior (reduced viscosity at 1.16 high shear rates). 9. In some cases, mixtures with polymer modified asphalts Figure 7. Illustration of horizontal strain during can be more difficult to work with in the field (34). How- SGC compaction. ever, in other cases, some mixtures with modified binders have been reported to be as workable as mixes with un- modified binders (11). These are only estimates of shear rates at specific moments 10. Most agencies use the equiviscous mixing and com- and locations during mixing and compaction. However, they paction criteria cited in AASHTO T 245 and T 312 (e.g., show that shear rates during mixing and compaction are 0.17 0.02 Pa s for mixing and 0.280.03 Pa s for com- likely to be very different in the lab and in the field. It is also paction) for unmodified asphalt binders. However, many apparent that high rates occur at short instances of time. In agencies refer instead to the binder supplier for recom- reality, it is not reasonable to select a single shear rate that is mended mixing and compaction criteria even for un- representative of the extreme range of shear and flow of binder modified binders. films on aggregate particles during mixing and compaction 11. Some countries use slightly higher viscosity ranges than operations. given in AASHTO T 312 for setting mixing and com- paction temperatures. 12. Supplier recommendations are most often used for set- Summary of Key Findings from ting mixing and compaction temperatures for polymer the Literature Review modified binders. Some agencies set their own criteria Key findings from the literature search and state of practice for modified binders. survey for mixing and compaction temperatures are listed here. 13. Field experience (i.e., trial and error) has generally been used to determine appropriate mixing and compaction 1. The equiviscous concept for selecting mixing and com- temperatures for modified binders. paction temperatures was established by the Asphalt Insti- 14. For many agencies and suppliers, mixing and compaction tute between 1956 and 1962 (5, 7). However, the methods temperature ranges are based on the Superpave PG of the used to establish the viscosity criteria are not known. Sev- binder. eral studies in the 1950s reference mixing temperatures 15. Most agencies use the same mixing and compaction tem- based on asphalt viscosity ranges (2, 3, 4, 6). perature ranges for laboratory testing and field operations 2. Criteria for mixing and compaction temperatures were of HMA production and construction. Some agencies initially based on viscosity measurements made with a specify a temperature range for plant produced HMA and Saybolt Furol viscometer (4, 5, 6, 7, 8). reject mix outside of that range. 3. Numerous studies have demonstrated that compaction 16. The Zero (or Low) Shear Viscosity concept has been shown temperatures have a significant effect on the mechanical to yield unrealistically low mixing and compaction tem- properties of the fabricated specimens (2, 3, 4, 6, 8, 13). peratures for some binders. 4. Volumetric properties of asphalt mixtures compacted 17. Yildirim's method of estimating binder viscosities at high with SGC, on the other hand, appear to be insensitive to shear rates from rotational viscosity data attempts to take compaction temperature (15, 16, 17). the shear rate dependency of modified binders into ac- 5. The use of the equiviscous concept for many polymer- count. Although this method requires extrapolation of modified asphalts results in excessively high mixing and viscosity-shear rate data, reasonable mixing and com- compaction temperatures (34, 35, 36). paction temperatures for a limited number of modified 6. High mixing temperatures cause asphalt binders to harden, binders were demonstrated in the study. primarily through mechanisms of volatilization and oxida- 18. Reinke's method of using a DSR steady shear flow test tion (24). Most oxidation reactions approximately double uses viscosity measurements taken at shear stress levels as temperature increases by 10C (25). Polymer additives where binders appear to have Newtonian behavior. How- in asphalt can break down at excessively high temperatures. ever, this method relies on extrapolating viscosity data to However, the temperatures where these changes become much higher temperature ranges. detrimental are not clearly established (28, 29, 36). 19. Research has yet to clearly identify a reliable method of 7. High temperatures also cause emission and odor prob- determining mixing and compaction temperatures for lems for some asphalt binders (30, 31, 32). modified and unmodified asphalt binders.