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Asphalt Pavement Technology
Bituminous Mixtures Laboratory (BML)
Equipment SUPERPAVE Gyratory Compactor
Simulates the kneading action of rollers used to compact asphalt concrete pavements by applying a vertical load to an asphalt mixture while gyrating a mold tilted at a specified angle. SGC is the new equipment used in volumetric pavement design. Specimens produced by this compactor can be used in the Superpave SST and IDT. (AASHTO T312)
The Superpave Gyratory Compactor (SGC) is used in the Superpave mixture design system to prepare asphalt concrete specimens for determining volumetric and mechanical properties. The Strategic Highway Research Program chose this compactor because it produces specimens that are similar to pavements in aggregate orientation and mechanical properties, and it can be used for quality control at hot-mix plants.
To compact a specimen, an asphalt mixture is placed in a steel mold that has an inside diameter of 150 mm and a steel base plate that serves as a lower platen. The assembly is placed inside the SGC where a load is applied through an upper ram and platen. The bottom of the mold is shifted horizontally along one diameter to provide the required angle of 0.0218 rad (1.25 degrees). The angle is then applied to the mold in a circular manner at a constant speed of 30 gyrations (revolutions) per minute. The platens remain parallel to each other during compaction, but are free to move with respect to the mold as the mixture densifies. Compaction occurs due to the pressure from the ram and the kneading action provided by the revolving angle.
The standard ram pressure is 600 kPa. As the specimen densifies and becomes shorter in height, a pressure gauge signals the loading system to adjust the position of the loading ram so that the 600-kPa pressure is maintained throughout the compaction process.
The SGC uses a linear variable differential transformer to record the position of the upper loading ram. The vertical change in ram position provides a measurement of the specimen height during compaction. The SGC methodology uses the change in height to determine the change in density with gyrations. Density is the mass of the specimen by its volume. Thus, the SGC provides a compaction curve, which is the relationship between density and the number of gyrations. See figure 1.
In the Superpave volumetric mixture design, the optimum binder content is chosen so that it provides a 4-percent air-void level at the design number of gyrations, called Ndesign. Ndesign for dense-graded mixtures ranges from 68 to 172. It depends on the traffic level and the climate where the mixture will be placed. Ndesign for stone matrix asphalt is 100. Two other gyration levels are also used to evaluate a mixture: the initial number of gyrations, called Ninitial, and the maximum number of gyrations, called Nmaximum. Ninitial is used to eliminate tender mixtures. Nmaximum is used to eliminate rutting if the air-void level in the pavement falls below the design level of 4 percent because of uncertainties such as increased traffic.
The FHWA Bituminous Mixtures Laboratory has SGC's from two manufacturers: a Troxler model 4140 from Troxler Electronic Laboratories and a Pine model AFGC125X from Pine Industries. See figures 2 and 3. The main difference between these two gyratories is the way they apply the angle. The Troxler SGC fixes the position of the top of the mold and directly induces the angle by offsetting the base of the mold. The Pine molds have an outer flange that is closer to the bottom of the mold than the top. Three sets of rollers contact the upper and lower sides of the flange to guide the movement. One set of rollers is raised to apply the angle. Both models cost $26,000.