Optimum Compaction Temperature for Modified Binders

With increasing use of modified asphalt binders, the correct compaction temperature range for asphalt mixtures containing these binders needs to be determined. The current procedure uses asphalt binder viscosities to determine the compaction temperature range. However, the current procedure often provides unacceptably high temperatures for modified binders. Past work showed that lower temperatures can be used and that the volumetric properties of Superpave gyratory-compacted specimens can be the same over a very wide range in compaction temperatures, even when the mixtures are short-term oven aged at the compaction temperature. However, that study did not examine the effect of temperature on the internal characteristics and mechanical properties of the mixtures. The present study addressed these issues for one of the mixtures used in the previous study. The shear properties of the mixture compacted at four temperatures were evaluated. These properties improved to some extent with an increase in temperature, except for strain-controlled fatigue damage, which did not change significantly with temperature. The effect of temperature on aggregate orientation and the effect of aggregate orientation on shear properties were also investigated. Based on these analyses, a narrower range of optimum compaction temperatures was found compared to the range based on volumetric properties alone. The changes in the shear stiffness of the asphalt binder, in the vertical air void distribution, and in the preferred orientation of the aggregates with compaction temperature were the responsible factors for the changes in mixture shear properties.