ensayo superpave gyratory compactor

Superpave Gyratory CompactorOverview · Background · Test Description · Results

Overview

The Superpave Gyratory Compactor (SGC) (Slideshow 1) is used to prepare HMA samples for Superpave mix design testing. It prepares a HMA sample using a compaction effort intended to simulate the effects of field compaction. This way, relatively small, inexpensive samples intended to simulate actual HMA field conditions can be used to test potential mix designs and select the most appropriate one.

The basic SGC uses about 9.9 to 10 lbs (4500 to 5000 g) of HMA and compacts it into a mold to produce a sample 6 inches (150 mm) in diameter and about 4.5 inches (115 mm) high (Figure 1). Compaction is achieved by applying a continuous load to the top of a sample while the sample is inclined at 1.25° and rotates at 30 revolutions per minute. Ideally, this helps achieve a sample particle orientation that is somewhat like that achieved in the field after roller compaction.

The standard gyratory compaction procedure is:

AASHTO T 312: Preparing and Determining the Density of Hot-Mix Asphalt (HMA) Specimens by Means of the Superpave Gyratory Compactor

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Superpave Gyratory CompactorOverview · Background · Test Description · Results

Background

The Superpave Gyratory Compactor (SGC) provides a method of preparing laboratory HMA samples for testing that is meant to simulate field compaction with a roller. Essentially, a load is placed on top of a cylindrical mold (Figure 2) filled with HMA. The load is applied as the offset angle of 1.25° rotates around at 30 revolutions per minute for a given number of revolutions, referred to as "gyrations" (Figure 3). Key parameters of the gyratory compactor are:

Sample size = 6 inch (150 mm) diameter cylinder approximately 4.5 inches (115 mm) in height (corrections can be made for different sample heights). Other heights can be produced.

Load = Flat and circular with a diameter of 5.89 inches (149.5 mm) corresponding to an area of 27.24 in2 (175.5 cm2)

Compaction pressure = Typically 87 psi (600 kPa)

Number of GyrationsThe SGC establishes three different gyration numbers:

1. Ninitial. The number of gyrations used as a measure of mixture compactability during construction. Mixes that compact too quickly (air voids at Ninitial are too low) may be tender (prone to displace and shove rather than compact) during construction and unstable when subjected to traffic. Thus, Superpave mix design specifies a maximum relative density (minimum air void content) at Ninitial.

2. Ndesign. This is the design number of gyrations required to produce a sample with the same density as that expected in the field after the indicated amount of traffic. A mix with 4 percent air voids at Ndesign is usually the goal in mix design.

3. Nmax. The number of gyrations required to produce a laboratory density that should never be exceeded in the field. If the air voids at Nmax are too low, then the field mixture may compact too much under traffic resulting in excessively low air voids and potential rutting. The air void content at Nmax should never be below 2 percent air voids. Thus, Superpave mix design specifies a maximum relative density (minimum air void content) at Nmax.

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IssuesThere are two major issues with the SGC that should be known and understood before using it because they can significantly affect test results based on SGC compacted samples.

Non-Uniform Sample Density

Samples compacted with the SGC typically demonstrate non-uniform density with lower densities near the sample edges. Procedures used to core the middle of a full 6 inch (150 mm) diameter sample to obtain a 4 inch (100 mm) sample (Figure 4) for use in the SPT have shown that the inner core can have on the order of 1.5% fewer air voids than the full 6 inch (150 mm) sample. This issue is widely known but accepted.

Internal Angle of Gyration

Measurable differences in compacted specimen specific gravities have been attributed to differences in compaction equipment (Harman et al., 2001). One specimen compacted in one manufacturer's gyratory compactor will not give the same specific gravity as the same specimen compacted in another manufacturer's gyratory compactor. Differences in air voids between samples of up to 0.8% can be expected (Prowell et al., 2003).

This difference is likely the result of differences in the internal angle of gyration. AASHTO T 312 specifies a value (1.25°) and tolerance (± 0.02°) for the external angle of gyration but not the internal one. The external angle of gyration is defined as the angle between the mold wall and vertical. The internal angle of gyration (Figure 5) is defined as the angle between the mold walls and the upper and lower plates.

It was originally thought that the external angle of gyration, which was easier to measure, accurately reproduced the internal angle of gyration and thus was a good representation of what geometry the HMA sample was actually experiencing. However, if the upper and lower plates do not remain parallel and horizontal, there can be a significant different between the internal and external angles of gyration. This out of parallel condition usually results from the end plates flexing under load during compaction. In Alabama SGCs Prowell et al. (2003) found that while the external angles were within tolerance, the internal angles varied from 0.98° to 1.19° with the upper internal angle usually being less than the lower one.

It is thought that this variation in internal angle is causing differences in

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compacted specimen specific gravities. In response, the FHWA has developed specifications for angle validation kits (AVK) (Figure 6, Figure 7 and Figure 8) that can be used inside any standard mold and that are capable of measuring the internal angle of gyration during HMA sample compaction (Figure 9). This device was used by Prowell et al. (2003) to determine internal angles. The newest AVKs do not even require an HMA sample; they can be used alone in the SGC mold.

Mold Wear

Mold wear can also contribute to measurable differences in compacted specimen specific gravities due to compaction equipment. Specifically, Prowell et al. (2003) noticed that an NCAT mold (which is used extensively) showed visible signs of wear in the compaction area. Further analysis showed that this excessive wear led to greater clearances between the end plates and mold walls, which resulted in unusually high end plate movement. This end plate movement likely caused changes in the internal angle of gyration. Mold wear can have a dramatic effect on sample specific gravities causing changes of up to 0.028 in bulk specific gravity (Prowell et al., 2003).

AASHTO T 312 does not currently specify measurement of the compaction area of the molds nor does it specify what seems to be the critical parameter: clearance between the mold walls and end plates.

Currently, internal angle of gyration issues have not been fully resolved, yet the SGC is in use throughout the country. It is not uncommon to see differences in measured sample specific gravities attributable to differences in compaction equipment. Therefore, when comparing data from specimens produced with SGCs, it is important to determine what types of compactors made the specimens and if there is a known offset between the two compactors based on internal angle of gyration issues.

History (Harman et al., 2001)By the late 1950s HMA mix design was pretty well dominated by the Hveem and Marshall methods. Other methods did exist, such as the Hubbard-Field method, the Smith Triaxial Method, and the Texas gyratory method, each one using their own unique method of specimen compaction.

The original gyratory compaction concept is attributed to Philippi, Raines, and Love of the Texas Highway Department. The first actual unit was used experimentally from 1939 to 1946. Two offshoots of the Texas gyratory shear test concept were the U.S. Army Corps of Engineers' Gyratory Test

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Machine (GTM) and a French version developed at the Laboratoroire Central des Ponts et Chausées (LCPC) called the PGC.

On the basis of NCHRP Project 9-06, the gyratory method of compaction was identified as an acceptable way to compact HMA specimens based on a comparison between field cores and laboratory compacted samples. The GTM was identified as an effective machine with the Texas gyratory shear test machine as an acceptable alternative. Subsequent to this, the FHWA's Demonstration Project 90, Innovative Asphalt Mix Laboratory Techniques found the GTM impractical and the Texas gyratory shear test machine deficient.

A hybrid idea of the Texas gyratory shear test machine and the PGC was suggested and pursued. Eventually, the angle of gyration was set at 1.25°. This was a compromise between the Texas machine's 6° (which caused compaction to occur too quickly - generally in only 15 to 18 gyrations) and the French machine's 1° (which, in some cases could not achieve enough compaction).

Two manufacturers were contracted to build prototype gyratory compactors based on the hybrid design. Although both met design criteria, it was found that they did not compact mixes equally. After modifications, these two machines were brought to within acceptable tolerances, however significant issues (as discussed above) still exist with the SGC.



Superpave Gyratory Compactor

Overview · Background · Test Description · Results

Results

Parameters MeasuredRelative density at different gyration levels. Apart from these parameters, the Superpave Gyratory Compactor (SGC) is used to prepare HMA samples for further testing.

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SpecificationsSuperpave mix design specifies density (as a percentage of theoretical maximum density) to be achieved at each of the three compaction levels.

Table 2: Superpave Mix DesignRequired Relative Density Specification

20-yr ESALs1

(millions)

Required Relative Density(% of Gmm)

Ninitial Ndesign Nmax

< 0.3 ≤ 91.5%

96.0% ≤ 98.0%

0.3 to < 3 ≤ 90.5%

3 to < 10

≤ 89.0%10 to < 30

≥ 30

Note 1Requirements are based on the expected loading in the design lane for a 20-year period regardless of the anticipated design life.

Typical ValuesCompaction values depend on mix design goals but are most often 96% of theoretical maximum density (4% air voids) at Ndesign. If a mix is designed properly, it should also meet the Ninitial and Nmax requirements. Typical values for Ninitial are 84 to 91% of theoretical maximum density (fine-graded mixes are higher and coarse-graded mixes are lower), while Nmax is typically 97 to 98% of theoretical maximum density (fine-graded mixes are typically lower and coarse-graded mixes are typically higher).

CalculationsMany tests and associated calculations can be done on samples prepared with the SGC. There are, however, only two calculations specifically associated with the SGC: uncorrected and corrected relative density. They are used to determine sample bulk density at any point during the compaction process.

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If the bulk specific gravity (Gmb) and theoretical maximum specific gravity (Gmb) are known, then the relative density at any point during the compaction process can be calculated using the following equations:

Uncorrected Relative Density (Interactive Equation)

Where: %Gmmux = uncorrected relative density expressed as a percentage of Gmm

Wm = mass of specimen (g)

Gmm = theoretical maximum specific gravity

Gm = unit weight of water (1 g/cm3)

Vmx = specimen volume after "x" gyrations (cm3)

Relative Density (Interactive Equation)

Where:%Gmmx = corrected relative density expressed as a percentage of

Gmm

Gmb = bulk specific gravity

Gmm = theoretical maximum specific gravity

hm = specimen height (mm)

hx = specimen height after "x" gyrations (mm)




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Test Description

The following description is a brief summary of the test. It is not a complete procedure and should not be used to perform the test. The standard gyratory compaction procedure can be found in:


SummaryA sample of HMA is proportioned, mixed and short-term aged and then compacted with the Superpave Gyratory Compactor (SGC) (Figure 10). Compaction occurs in a cylindrical mold (Figure 11) that is 6 inches (150 mm) in diameter with a final sample height goal of 4.5 inches (115 mm). Corrections can be made for different sample heights. Compaction occurs when a 87 psi (600 kPa) load is applied to the top of the mold as the mold is rotated at 30 revolutions per minute at a 1.25° offset. The number of rotations, or "gyrations", is determined by Superpave mix design procedure and is dependant on anticipated 20-year traffic volume.

Approximate Test TimeAbout 5 minutes of actual test time in the SGC. Most of the time is associated with sample mixing and conditioning.

Basic Procedure

WARNING

The SGC has several known issues that should be understood before performing the compaction procedure or using any test results obtained from SGC compacted samples.

1. Prepare the HMA mixture to be compacted (Figure 12). This consists of heating the aggregate to mixing temperature and mixing it with a known quantity of asphalt binder (Video 1).

2. Condition the mixture.3. Heat the compaction mold, base and upper plates in an oven at the required mixing

temperature for 30 to 60 minutes.

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4. Add the HMA to the mold in one lift (Figure 13).5. Place the mold in the SGC (Figure 14).6. Apply 87 psi (600 kPa) of pressure and a 1.25° incline to the mold assembly. These

values are programmed into the SGC. 7. Compact the sample to the desired number of gyrations (Video 2). Table 1 shows

the standard specified number of gyrations for each of the three compaction levels. Figure 15 shows a typical progression of relative density vs. number of gyrations.

Table 1: Number of Gyrations vs. Design 20-Year ESALs

20-yr ESALs1

(millions)

Number of Gyrations


< 0.3 6 50 75

0.3 to < 3 7 75 115

3 to < 30 8 100 160

≥ 30 9 125 205


8. Remove the angle from the mold, retract the loading ram and extrude the sample from the mold (Video 3 and Figure 16).





Test Description

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The following description is a brief summary of the test. It is not a complete procedure and should not be used to perform the test. The standard gyratory compaction procedure can be found in:


SummaryA sample of HMA is proportioned, mixed and short-term aged and then compacted with the Superpave Gyratory Compactor (SGC) (Figure 10). Compaction occurs in a cylindrical mold (Figure 11) that is 6 inches (150 mm) in diameter with a final sample height goal of 4.5 inches (115 mm). Corrections can be made for different sample heights. Compaction occurs when a 87 psi (600 kPa) load is applied to the top of the mold as the mold is rotated at 30 revolutions per minute at a 1.25° offset. The number of rotations, or "gyrations", is determined by Superpave mix design procedure and is dependant on anticipated 20-year traffic volume.

Approximate Test TimeAbout 5 minutes of actual test time in the SGC. Most of the time is associated with sample mixing and conditioning.

Basic Procedure

WARNING

The SGC has several known issues that should be understood before performing the compaction procedure or using any test results obtained from SGC compacted samples.

1. Prepare the HMA mixture to be compacted (Figure 12). This consists of heating the aggregate to mixing temperature and mixing it with a known quantity of asphalt binder (Video 1).

2. Condition the mixture.3. Heat the compaction mold, base and upper plates in an oven at the required mixing

temperature for 30 to 60 minutes.4. Add the HMA to the mold in one lift (Figure 13).5. Place the mold in the SGC (Figure 14).6. Apply 87 psi (600 kPa) of pressure and a 1.25° incline to the mold assembly. These

values are programmed into the SGC.

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7. Compact the sample to the desired number of gyrations (Video 2). Table 1 shows the standard specified number of gyrations for each of the three compaction levels. Figure 15 shows a typical progression of relative density vs. number of gyrations.

Table 1: Number of Gyrations vs. Design 20-Year ESALs

20-yr ESALs1

(millions)

Number of Gyrations


< 0.3 6 50 75

0.3 to < 3 7 75 115

3 to < 30 8 100 160

≥ 30 9 125 205


8. Remove the angle from the mold, retract the loading ram and extrude the sample from the mold (Video 3 and Figure 16).



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ensayo superpave gyratory compactor

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