Mixing and Compaction Temperatures for Modified Asphalt Binders
Superpave mixture design requires that gyratory specimens be mixed and compacted at equiviscous binder temperatures corresponding to viscosities of 0.17 and 0.28 Pa.s, respectively. Those values were previously used in the Marshal mix design method to determine the mixing and compaction temperatures. Estimation of the proper mixing and compaction temperatures involves developing a temperature viscosity relationship for the binder (ASTM D2493, 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 mixing and compaction temperatures when using this technique. This is due to the fact that modified binders are very sensitive to shear rate. Currently, low shear rate values are used to attain viscosities of 0.17 and 0.28 Pa.s. It is common that modified binders tested at low shear values result in high temperatures, where binder temperatures in excess of 190º C have often been reported. Currently, mix designers address this issue by consulting with the supplier of the modified binder to obtain recommended temperatures. In many cases, however, the supplier has no solid information on which to base a recommendation. Furthermore, binder suppliers may even be dealing with a material with which they have little or no experience. Thus, there is a need to establish a more rigorous and fundamentally sound procedure for selecting reasonable mixing and compaction temperatures for use in Superpave mix design. In this study the objective is to develop a new protocol that can be used to establish sensible values for mixing and compaction temperatures.
It is important to emphasize that ASTM D2493 was established for unmodified asphalt binders, which are Newtonian fluids at high temperatures. For these materials, viscosity does not depend on shear rate. However, modified asphalt binders exhibit a phenomenon known as Pseudoplasticity, commonly referred as shear thinning at high temperatures where viscosity values depend on shear rate. Figure.1 shows the viscosity-shear rate relationship for a multigrade modified asphalt binder (MG 40-20 that grades as a PG 70-22). It can be seen that viscosity decreases with an increase in shear rate. Figure.2 displays the viscosity-shear rate relationship for an unmodified asphalt binder (PG 64-22). The viscosity of the unmodified asphalt binder does not change as the shear rate increases. The viscosity of the unmodified asphalt binders does not depend on shear rate.
The shear rate during the mixing and compaction process is greater than the shear rate that is used during viscosity measurements. Therefore, viscosity measurements with the current procedure for modified asphalt binders do not reflect the viscosity values of the binder during mixing and compaction. ASTM D2493 does not specify any shear rate value for viscosity measurements.
In this research study, the shear rate during mixing and compaction will be determined such that the effect of it will be included during viscosity measurements. The use of practical shear values will result in sensible mixing and compaction temperatures used for hot mix asphalt design with modified asphalt binder.
There are several factors that affect the bulk specific gravity (Gmb) of a bituminous specimen. They include gradation of the aggregate, aggregate type, asphalt content, viscosity of the asphalt binder, and the type of compactor. If all factors that have an effect on Gmb are kept the same, the Gmb for any two specimens will be similar. This idea can be used in the calculation of shear rate inside the SGC.
Specimens will be compacted at different temperatures and their Gmb values will be calculated. All variables other than temperature will be kept the same. Generally the Gmb values will increase with increasing temperature. The relationship between Gmb and temperature will be determined for compacted specimens made with a modified asphalt binder and an unmodified asphalt binder. By using this relationship for the modified and the unmodified asphalt binder, the temperatures that give the same Gmb values will be found. Thus it is believed that specimens compacted with modified and unmodified asphalt binders with the same Gmb will have the same viscosity during compaction. The shear rate viscosity relationship, similar to that as seen in Figures 1 and 2, will have determined for these two binders at the temperatures yielding the same Gmb. The Rotational Viscometer and Dynamic Shear Rheometer will be used to find these relationships. The shear rate experienced during compaction for these two binders will be calculated from the measured shear rate-viscosity relationship.
In summary, the significance of this shear rate is that it can be used in determining viscosity for mixing and compaction temperature calculations for modified asphalt binders. It is foreseen that this procedure will produce practical temperatures because it is based on laboratory compacted specimens, as well as advanced testing devices.