Asphalt

Superpave Volumetric Mix Design

Dec. 28, 2000

Superpave is an improved system for specifying hot-mix asphalt (HMA) component materials, HMA mix design and analysis, and pavement performance prediction. Implementation of Superpave, when fully developed, should minimize or eliminate rutting, fatigue cracking, and low temperature cracking.

As states are beginning to experiment and/or implement Superpave, the need for refining certain aspects of the method are becoming apparent. Two are given here.

A Superpave gyratory compactor (SGC) is used in the volumetric mix design method to compact the specimen to Nmaximum gyrations.

By: Prithvi "Ken" Kandhal, P.E.

As states are beginning to experiment and/or implement Superpave, the need for refining certain aspects of the method are becoming apparent. Two are given here.

A Superpave gyratory compactor (SGC) is used in the volumetric mix design method to compact the specimen to Nmaximum gyrations. The bulk specific gravity (Gmb) of the compacted specimens can be estimated by the computer program used in SGC. However, the actual Gmb is different than the estimated Gmb and has to be measured. A correction factor is then obtained from the ratio of measured and estimated Gmb. This correction factor is then used to obtain the corrected Gmb at Ndesign and Ninitial gyrations, which are then used to calculate the air voids content (or percentage of theoretical maximum density) at Ndesign and Ninitial. Recent experience has shown that the correction factor obtained at Nmaximum is not applicable at Ndesign and, therefore, can give an erroneous air void content at Ndesign. This means that the correction factor must be obtained at Ndesign gyrations to ensure that 4% air voids are actually obtained at the optimum asphalt content.

The Superpave volumetric mix design method has a table for selecting the gyratory compaction effort corresponding to seven categories of design ESALs and four categories of high air temperatures. Therefore, the table gives 7 x 4 = 28 different compactive efforts depending on traffic and climate. This table needs to be shortened especially in view of inadequate supporting data. Recent experience has also shown that the gyratory compaction effort in this table appears to be excessive.

The excessive compaction effort results in low voids in the mineral aggregate (VMA), which may not meet the minimum VMA requirements. This forces the mix designer to coarsen the mix gradation excessively. Such mixes have been observed to be permeable in some instances. It should be noted that the minimum VMA requirements were established on 75-blow Marshall compactive effort. In view of the preceding discussion, there is a need to refine the table on gyratory compactive effort.