Superpave and Texas Gyratory Compactors: How Do They Compare?
Texas Department of Transportation (TxDOT) has been using the Texas gyratory compactor (TGC) for years. There are about 300 units of this compactor available in different districts of the state. However, as of today, approximately 30 Superpave gyratory compactors (SGC) are available in the districts. TxDOT has been investigating how the results from these two machines compare with each other for Superpave mixes.
Three independent studies have been conducted for this purpose. The first study was conducted on laboratory prepared mixtures. The loose mixtures were sent to various districts for compaction. The specimens were compacted in both types of presses in the districts. In another study, TxDOT collected density data from various districts on plant mixes. These mixes were compacted with both TGC and SGC at different districts. The design number of gyrations for this comparison was 96. Later, a third study was conducted on plant mixes with the idea of having all the compaction taking place at a central laboratory. This way, the variability in operation and equipment is significantly reduced. Superpave plant mixes were shipped from different districts to SCSC where they were compacted with both SGC and TGC. The same mixing and compaction temperatures were used for both compactors. One operator was used for each compactor to reduce the variability.
The results of the three studies are presented in Figures 1 through 3, respectively. Figure 1, which is based on laboratory mixes, does not exhibit a good correlation. However, Figures 2 and 3 indicate that there is a relatively good correlation between the two presses. As shown in Figure 4, the largest difference in magnitude of air voids from the two compactors is about one percent. In most cases, the difference is less than 0.6 percent. The same conclusion can be drawn based on the results presented in Figure 3.
An important consideration in making comparison between SGC and TGC is the design number of gyrations (Ndes). Obviously, as the Ndes changes, the air voids from SGC will change. Figure 1 and 2 are based on an Ndes of 96 while Figure 3 is based on an Ndes of 86. If an Ndes of 96 had been used for this last study, there would have been a shift of data points to the left presenting results similar to what is shown in Figure 2.
Considering the above correlation as well as the fact that in most cases, the difference between air voids from the two presses is less than 0.6 percent (only for the Ndes used in this study), one could conclude that the results are comparable within reasonable limits. There may be the possibility of developing shift factors between the results from the machines for other Ndes values.