Development of the
Superpave Gyratory Compactor
By Gerald A. Huber
The first application of gyratory compaction which remains in use today was developed in Texas. Today there are three general types of gyratory compaction
- Texas four inch gyratory
- Texas 6 inch compaction
- Corps of Engineers Gyratory compactor
- LCPC (French) gyratory compaction
Texas Roots of Gyratory Compaction
Gyratory compaction has been used in asphalt mixture design since the 1930's when a procedure was developed y the Texas Highway Department (Department of Transportation today).The original gyrator compaction procedure was done manually. A mold, constructed from a section of 4 inch inside diameter pipe, was placed between two parallel plates. The plates were spaced one half inch further apart than the mold height which allowed the mold to be tilted approximately 6 degree until the diagonal corners contacted the upper and lower plate.
The compaction procedure used a hydraulic jack mounted upside down to apply a vertical load. In the procedure, the mold, loaded with hot mix was placed between the parallel plates. The hydraulic jack was pumped until a pressure of 50 psi was applied to the specimen. The mold was grasped using two long leveraging rods and tilted until opposite corners of the mold contacted the plate. Then the mold was gyrated three times such that opposite corners remained in contact with the parallel plates. During the gyrations vertical pressure on the specimen changes. Before applying the angle the pressure is 50 psi. When the angle is applied the pressure increases initially and then decreases as the three gyrations are applied.
The angle was removed from the mold by allowing it to sit on the lower plate and the pressure in the hydraulic jack was readjusted to 50 psi and the process was repeated. Sets of triplicate gyrations were continued until the endpoint was reached. The endpoint was defined as an increase in pressure to 150 psi with one full stroke of the hydraulic jack. When the endpoint was reached a "level up" load of 2500 psi was applied to complete the compaction.
In the late 1950's-early 1960's a mechanized compactor was developed to simulate the manual process. This compactor remains in use today in the Texas method of mix design.
A second gyratory compactor, 6 inches in diameter, was developed in Texas to design mixtures with large aggregates. The Texas 6 inch protocol is significantly different from the 4 inch method. Gyrations are applied continuously and the vertical pressure is held constant. Gyrations continue until the rate o height change per revolution decreases below a specified limit.
Corps of Engineers Develops Gyratory Testing Machine
During the post World War II era the Corps of Engineers was involved in adopting military technology to civilian use. The Marshall mix design method which had been initiated by the Mississippi Highway Department and developed for airfield construction was adopted to use on civilian highways.
At about the same time work began on a testing machine based on the gyratory compaction process. The Gyratory Testing Machine (GTM) was designed to measure forces during the compaction process. The method of applying the angle was changed from the Texas method. The angle was applied at two points on the diameter instead of the three points used in the Texas methods. Holding the angle constant across the diameter allows a degree of freedom for the mold to rotate or swivel about the line across the diameter. It was found that the change in angle during compaction could be related to permanent deformation performance.
LCPC (French) Gyratory Compaction
In the late 1950's during a technology exchange visit the idea of gyratory compaction was introduced to France. During the 1960's and early 1970's development o f the French gyratory compaction protocol occurred.. Extensive studies occurred to investigate the gyratory mechanism. Studies included the shape of the gyratory densification curves and the effects of aggregate gradation, mineral filler content, and asphalt properties on the position and slope of the curve. During the same time studies were done to investigate the compaction characteristics of mixture under rollers and relate the results to densification properties of the mixture in the compactor.
The LCPC gyratory compactor applies a fixed angle of one degree. A constant vertical pressure is maintained on the specimen and the gyrations are applied at a steady rate of 6 gyrations per minute throughout the compaction process.
The French method of mix design is based on the results of these studies. In the early 1970's LCPC replaced the Marshall method of mix design with a new LCPC method. The LCPC mix design standardizes the compaction effort on the road. The concept of standardizing compaction effort on the road is the reverse of the Marshall approach which standardizes compaction in the laboratory.
In the French system the standard compactive effort in the field is defined as a standard roller train. In the laboratory the number of gyrations used with the compactor is dependent on the lift thickness. As the lift thickness increases, the mixture becomes easier to compact on the road and the number of gyrations increases. For any given aggregate structure means the asphalt content will decrease and the mix will more difficult to compact. The increased difficulty of compaction is offset by increased lift thickness which reduces the compactive effort required to obtain density. In the mix design method the change in gyrations is designed to balance the two effect of the lift thickness. The end result is constant compactability on the road.
History of the Superpave Gyratory Compactor.
The decision to use gyratory compaction as the Superpave compaction is based on NCHRP Study 9-5. NCHRP 9-5, which was designed to be a lead-in to the Strategic Highway Research Program, focused on compaction methods and developed a preliminary mix design and analysis system using pre-SHRP performance related tests.
Midway through the Strategic Highway Research Program, as the Superpave method of mix design was being assembled, an evaluation of available gyratory compaction research was done.
An underlying premise of the gyratory protocol selection was that material property parameters were not expected to come from the compactor. The primary objective of SHRP was to develop and validate material properties, and test methods to measure the properties, which could be used to predict performance. Therefore, the need for fundamental or empirical engineering properties from a compactor did not exist. Hence, the material properties which can be measured with the Gyratory Testing Machine were not required.
The ability to evaluate the rate of densification was selected as a desirable characteristic. The constant angle and constant vertical pressure of the Texas 6 inch gyratory allowed the densification curves to be developed. Early testing showed that the high angle, five degrees, produced a very rapid rate of compaction and produced densification curves which were difficult to measure. An angle of one degree was then selected which matched the LCPC protocol. Subsequent work indicated that the rate of densification was not sufficient; hence, the final angle selected for Superpave was 1.25 degrees.
The decision was made to develop a gyratory protocol which would simulate the density achieved at the end of life, a concept similar to the approach used in Marshall compaction. In the end, the decisions which influenced the Superpave gyratory protocol led to the following issues:
- The angle was required to be constant during compaction.
- A constant vertical pressure was required.
- A constant rate of gyrations was required throughout the test.
- The design number of gyrations was expected to simulate pavement density at the end of life.
Studies conducted at the Asphalt Institute during SHRP investigated the effect of angle of gyration, speed of gyration and vertical pressure. Density was most influenced by the angle of gyration. Speed of gyration showed little effect on density. Vertical pressure had a small effect on the density achieved.
Determination of the number of design gyrations required a much larger experiment than was possible within the constraints of budget and time during SHRP. It was recognized that two main parameters would influence asphalt mixture density at the end of life. Densification under traffic would be controlled by the stresses applied to the mixture which would cause the densification and the mixture properties (stiffness) which would control the mixture reaction to the applied stresses.
Applied stresses come predominantly from truck traffic. Automobile traffic was assumed to apply negligible stresses to the pavement as compared to loaded trucks. The stream of trucks was assumed to be represented by the number of Equivalent Single Axle Loads (ESALs). Under any given loading, stresses induced by the pavement will be influenced by the rate of loading and depth from the pavement surface. It was recognized, therefore, that the N-design Experiment should consider the amount of truck traffic experienced by the pavement, speed of traffic and the depth from the surface.
In the limited experiment conducted during SHRP, the amount of traffic was considered as ESAL's. Loading speed was constrained by selecting sites with free flow highway speed conditions. Depth of mixture was constrained by selecting only mixtures which were within the top 100 mm of the pavement surface.
Densification of in-place mixtures is influenced by the high temperature environment, specifically the amount of time the pavement is at elevated temperature, and the amount of truck traffic, assuming that traffic distributions are similar and that the amount of truck loads applied to the pavement is proportional to the total amount of truck traffic.