MIXING AND COMPACTION TEMPERATURES FOR HOT MIX ASPHALT CONCRETE

According to Superpave mixture design, gyratory specimens are mixed and compacted at equiviscous binder temperatures corresponding to viscosities of 0.17 and 0.28 Pa*s, respectively. These were the values previously used in the Marshal mix design method to determine the mixing and compaction temperatures. In order to estimate the appropriate mixing and compaction temperatures for Superpave mixture design, a temperature-viscosity relationship for the binder should be developed (ASTM D 2493, Calculation of Mixing and Compaction Temperatures). This approach is simple and provides reasonable temperatures for unmodified binders. However, some modified binders have exhibited unreasonably high temperatures for mixing and compaction using this technique. These high temperatures could result in construction problems, asphalt damage, and fume production. ASTM D 2493 was established for unmodified asphalt binders, which are Newtonian fluids at high temperatures. For these materials, viscosity does not depend on shear rate. However, most of the modified asphalt binders exhibit a phenomenon known as pseudoplasticity, in which viscosity depends on shear rate. Thus, at the high shear rates that occur during mixing and compaction, it is not necessary to use very high temperatures. A research study was undertaken to determine the shear rate during compaction so that the effect of this parameter could be included during viscosity measurements. The use of practical shear rate results in reasonable mixing and compaction temperatures for hot mix asphalt design and construction with modified asphalt binders. It was found that application of the shear rate concept rather than the traditional approach used for unmodified binders can reduce the mixing and compaction temperatures between roughly 14 to 38 degrees C for the former and 10 to 27 degrees C for the latter, depending on the type and the amount of modifier.