5  HMA - Superpave Method

One of the principal results from the Strategic Highway Research Program (SHRP) was the Superpave mix design method.   The Superpave mix design method was designed to replace the Hveem and Marshall methods.  The volumetric analysis common to the Hveem and Marshall methods provides the basis for the Superpave mix design method.  The Superpave system ties asphalt binder and aggregate selection into the mix design process, and considers traffic and climate as well.  The compaction devices from the Hveem and Marshall procedures have been replaced by a gyratory compactor and the compaction effort in mix design is tied to expected traffic.

This section consists of a brief history of the Superpave mix design method followed by a general outline of the actual method.  This outline emphasizes general concepts and rationale over specific procedures.  Typical procedures are available in the following documents:

• Roberts, F.L.; Kandhal, P.S.; Brown, E.R.; Lee, D.Y. and Kennedy, T.W.  (1996).  Hot Mix Asphalt Materials, Mixture Design, and Construction.  National Asphalt Pavement Association Education Foundation.  Lanham, MD.   
• Asphalt Institute.  (2001).  Superpave Mix Design.  Superpave Series No. 2 (SP-02).  Asphalt Institute.  Lexington, KY.
• American Association of State Highway and Transportation Officials (AASHTO).  (2000 and 2001).  AASHTO Provisional Standards.  American Association of State Highway and Transportation Officials.  Washington, D.C. 

5.1  History

Under the Strategic Highway Research Program (SHRP), an initiative was undertaken to improve materials selection and mixture design by developing:

1. A new mix design method that accounts for traffic loading and environmental conditions.
2. A new method of asphalt binder evaluation.
3. New methods of mixture analysis.

When SHRP was completed in 1993 it introduced these three developments and called them the Superior Performing Asphalt Pavement System (Superpave).  Although the new methods of mixture performance testing have not yet been established, the mix design method is well-established.

5.2  Procedure

The Superpave mix design method consists of 7 basic steps:

1. Aggregate selection.
2. Asphalt binder selection.
3. Sample preparation (including compaction).
4. Performance Tests.
5. Density and voids calculations.
6. Optimum asphalt binder content selection.
7. Moisture susceptibility evaluation.

5.2.1  Aggregate Selection

Superpave specifies aggregate in two ways.  First, it places restrictions on aggregate gradation by means of broad control points.  Second, it places "consensus requirements" on coarse and fine aggregate angularity, flat and elongated particles, and clay content.  Other aggregate criteria, which the Asphalt Institute (2001) calls "source properties" (because they are considered to be source specific) such as L.A. abrasion, soundness and water absorption are used in Superpave but since they were not modified by Superpave they are not discussed here.

5.2.1.1  Gradation and Size

Aggregate gradation influences such key HMA parameters as stiffness, stability, durability, permeability, workability, fatigue resistance, frictional resistance and resistance to moisture damage (Roberts et al., 1996).  Additionally, the maximum aggregate size can be influential in compaction and lift thickness determination. 

Gradation Specifications

Superpave mix design specifies aggregate gradation control points, through which aggregate gradations must pass.  These control points are very general and are a starting point for a job mix formula.

Aggregate Blending

It is rare to obtain a desired aggregate gradation from a single aggregate stockpile.  Therefore, Superpave mix designs usually draw upon several different aggregate stockpiles and blend them together in a ratio that will produce an acceptable final blended gradation.  It is quite common to find a Superpave mix design that uses 3 or 4 different aggregate stockpiles (see Figure 5.11).    

Figure 5.11: Screen Shot from HMA View Showing a Typical Aggregate Blend from 4 Stockpiles

Typically, several aggregate blends are evaluated prior to performing a complete mix design.  Evaluations are done by preparing an HMA sample of each blend at the estimated optimum asphalt binder content then compacting it.  Results from this evaluation can show whether or not a particular blend will meet minimum VMA requirements and N_initial or N_max requirements.

Dust- to-Binder Ratio
In order to ensure the proper amount of material passing the 0.075 mm (No. 200) sieve (called "silt-clay" by AASHTO definition and "dust" by Superpave) in the mix, Superpave specifies a range of dust-to-binder ratio by mass.  The equation is:

Dust-to-binder ratio specifications are normally 0.6 - 1.2, but a ratio of up to 1.6 may be used at an agency's discretion (AASHTO, 2001).

5.2.1.2  Consensus Requirements

"Consensus requirements" came about because SHRP did not specifically address aggregate properties and it was thought that there needed to be some guidance associated with the Superpave mix design method.  Therefore, an expert group was convened and they arrived at a consensus on several aggregate property requirements - the "consensus requirements".  This group recommended minimum angularity, flat or elongated particle and clay content requirements based on:

• The anticipated traffic loading.  Desired aggregate properties are different depending upon the amount of traffic loading.  Traffic loading numbers are based on the anticipated traffic level on the design lane over a 20-year period regardless of actual roadway design life (AASHTO, 2000b).
• Depth below the surface.  Desired aggregate properties vary depending upon their intended use as it relates to depth below the pavement surface.

These requirements are imposed on the final aggregate blend and not the individual aggregate sources.  

Coarse Aggregate Angularity

Coarse aggregate angularity is important to mix design because smooth, rounded aggregate particles do not interlock with one another nearly as well as angular particles.  This lack of  interlock makes the resultant HMA more susceptible to rutting.   Coarse aggregate angularity can be determined by any number of test procedures that are designed to determine the percentage of fractured faces.  Table 5.5 lists Superpave requirements.

Table 5.5: Coarse Aggregate Angularity Requirements (from AASHTO, 2000b)

Fine Aggregate Angularity

Fine aggregate angularity is important to mix design for the same reasons as coarse aggregate angularity - rut prevention.  Fine aggregate angularity is quantified by an indirect method often called the National Aggregate Association (NAA) flow test.  This test consists of pouring the fine aggregate into the top end of a cylinder and determining the amount of voids.  The more voids, the more angular the aggregate.  Voids are determined by the following equation:

Table 5.6 shows the Superpave recommended fine aggregate angularity.

Table 5.6: Fine Aggregate Angularity Requirements (from AASHTO, 2000b)

The standard test for fine aggregate angularity is:

• AASHTO T 304: Uncompacted Void Content of Fine Aggregate

Flat or Elongated Particles

An excessive amount of flat or elongated aggregate particles can be detrimental to HMA.  Flat/elongated particles tend to breakdown during compaction (giving a different gradation than determined in mix design), decrease workability, and lie flat after compaction (resulting in a mixture with low VMA) (Roberts et al., 1996).  Flat or elongated particles are typically identified using ASTM D 4791, Flat or Elongated Particles in Coarse Aggregate.  Table 5.7 shows the Superpave recommended flat or elongated particle requirements. 

Figure 5.7: Flat or Elongated Particle Requirements (from AASHTO, 2000b)

Clay Content

The sand equivalent test measures the amount of clay content in an aggregate sample.  If clay content is too high, clay could preferentially adhere to the aggregate over the asphalt binder.  This leads to a poor aggregate-asphalt binder bonding and possible stripping.  To prevent excessive clay content, Superpave uses the sand equivalent test requirements of Table 5.8.

Table 5.8: Sand Equivalent Requirements (from AASHTO, 2000b)

5.2.2  Asphalt Binder Evaluation

Superpave uses its own asphalt binder selection process, which is, of course, tied to the Superpave asphalt binder performance grading (PG) system and its associated specifications.  Superpave PG asphalt binders are selected based on the expected pavement temperature extremes in the area of their intended use.  Superpave software (or a stand-alone program such as LTPPBind) is used to calculate these extremes and select the appropriate PG asphalt binder using one of the following three alternate methods (Roberts et al., 1996):

1. Pavement temperature.  The designer inputs the design pavement temperatures directly.
2. Air temperature.  The designer inputs the local air temperatures, then the software converts them to pavement temperatures.
3. Geographic area.  The designer simply inputs the project location (i.e. state, county and city).  From this, the software retrieves climate conditions from a weather database and then converts air temperatures into pavement temperatures.

Once the design pavement temperatures are determined they can be matched to an appropriate PG asphalt binder. 

5.2.2.1  Design Pavement Temperature

The Superpave mix design method determines both a high and a low design pavement temperature.  These temperatures are determined as follows:

• High pavement temperature - based on the  7-day average high air temperature of the surrounding area.
• Low pavement temperature - based on the 1-day low air temperature of the surrounding area. 

Using these temperatures as a starting point, Superpave then applies a reliability concept to determine the appropriate PG asphalt binder.  PG asphalt binders are specified in 6°C increments.   

5.2.2.2  Design Pavement Temperature Adjustments

Design pavement temperature calculations are based on HMA pavements subjected to fast moving traffic (Roberts et al., 1996).  Specifically, the Dynamic Shear Rheometer (DSR) test is conducted at a rate of 10 radians per second, which corresponds to a traffic speed of about 90 km/hr (55 mph) (Roberts et al., 1996).  Pavements subject to significantly slower (or stopped) traffic such as intersections, toll booth lines and bus stops should contain a stiffer asphalt binder than that which would be used for fast-moving traffic.  Superpave allows the high temperature grade to be increased by one grade for slow transient loads and by two grades for stationary loads.  Additionally, the high temperature grade should be increased by one grade for anticipated 20-year loading in excess of 30 million ESALs.  For pavements with multiple conditions that require grade increases only the largest grade increase should be used.  Therefore, for a pavement intended to experience slow loads (a potential one grade increase) and greater than 30 million ESALs (a potential one grade increase), the asphalt binder high temperature grade should be increased by only one grade.  Table 5.9 shows two examples of design high temperature adjustments - often called "binder bumping".

Table 5.9: Examples of Design Pavement Temperature Adjustments
for Slow and Stationary Loads

5.2.3  Sample Preparation

The Superpave method, like other mix design methods, creates several trial aggregate-asphalt binder blends, each with a different asphalt binder content.  Then, by evaluating each trial blend's performance, an optimum asphalt binder content can be selected.  In order for this concept to work, the trial blends must contain a range of asphalt contents both above and below the optimum asphalt content.  Therefore, the first step in sample preparation is to estimate an optimum asphalt content.  Trial blend asphalt contents are then determined from this estimate. 

The Superpave gyratory compactor (Figure 5.12) was developed to improve mix design's ability to simulate actual field compaction particle orientation with laboratory equipment (Roberts, 1996).   

Each sample is heated to the anticipated mixing temperature, aged for a short time (up to 4 hours) and compacted with the gyratory compactor, a device that applies pressure to a sample through a hydraulically or mechanically operated load.  Mixing and compaction temperatures are chosen according to asphalt binder properties so that compaction occurs at the same viscosity level for different mixes.  Key parameters of the gyratory compactor are:

• Sample size = 150 mm (6-inch) diameter cylinder approximately 115 mm (4.5 inches) in height (corrections can be made for different sample heights).  Nnote that this sample size is larger than those used for the Hveem and Marshall methods (see Figure 5.13).
• Load = Flat and circular with a diameter of 149.5 mm (5.89 inches) corresponding to an area of 175.5 cm² (27.24 in²)
• Compaction pressure = Typically 600 kPa (87 psi)
• Number of blows = varies
• Simulation method = The load is applied to the sample top and covers almost the entire sample top area.  The sample is inclined at 1.25° and rotates at 30 revolutions per minute as the load is continuously applied.  This helps achieve a sample particle orientation that is somewhat like that achieved in the field after roller compaction.

Figure 5.12 (left): Gyratory Compactor Figure 5.13 (below): Superpave Gyratory Compactor Sample (left) vs. Hveem/Marshall Compactor Sample (right)

The Superpave gyratory compactor establishes three different gyration numbers:

1. N_initial.  The number of gyrations used as a measure of mixture compactability during construction.  Mixes that compact too quickly (air voids at N_initial are too low) may be tender during construction and unstable when subjected to traffic.  Often, this is a good indication of aggregate quality - HMA with excess natural sand will frequently fail the N_initial requirement.  A mixture designed for greater than or equal to 3 million ESALs with 4 percent air voids at N_design should have at least 11 percent air voids at N_initial.
2. N_design.  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 N_design is desired in mix design.
3. N_max.  The number of gyrations required to produce a laboratory density that should never be exceeded in the field.  If the air voids at N_max 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 N_max should never be below 2 percent air voids.

Typically, samples are compacted to N_design to establish the optimum asphalt binder content and then additional samples are compacted to N_max as a check.  Previously, samples were compacted to N_max and then N_initial and N_design were back calculated.  Table 5.10 lists the specified number of gyrations for N_initial, N_design and N_max while Table 5.11 shows the required densities as a percentage of theoretical maximum density (TMD) for N_initial, N_design and N_max.  Note that traffic loading numbers are based on the anticipated traffic level on the design lane over a 20-year period regardless of actual roadway design life (AASHTO, 2001).

Table 5.10: Number of Gyrations for N_initial, N_design and N_max (from AASHTO, 2001)

Table 5.11: Required Densities for N_initial, N_design and N_max (from AASHTO, 2001)

The standard gyratory compactor sample preparation procedure is:

• AASHTO TP4: Preparing and Determining the Density of Hot-Mix Asphalt (HMA) Specimens by Means of the Superpave Gyratory Compactor

5.2.4  Performance Tests

The original intent of the Superpave mix design method was to subject the various trial mix designs to a battery of performance tests akin to what the Hveem method does with the stabilometer and cohesiometer, or the Marshall method does with the stability and flow test.  Currently, these performance tests, which constitute the mixture analysis portion of Superpave, are still under development and review and have not yet been implemented.  The most likely performance test, called the Simple Performance Test (SPT) is a Confined Dynamic Modulus Test.

5.2.5  Density and Voids Analysis

All mix design methods use density and voids to determine basic HMA physical characteristics.  Two different measures of densities are typically taken:

1. Bulk specific gravity (G_mb) - often called "bulk density"
2. Theoretical maximum density (TMD, G_mm)

These densities are then used to calculate the volumetric parameters of the HMA.  Measured void expressions are usually:

• Air voids (V_a), sometimes called voids in the total mix (VTM)
• Voids in the mineral aggregate (VMA)
• Voids filled with asphalt (VFA)

Generally, these values must meet local or State criteria. 

VMA and VFA must meet the values specified in Table 5.12.  Note that traffic loading numbers are based on the anticipated traffic level on the design lane over a 20-year period regardless of actual roadway design life (AASHTO, 2000b).

  Table 5.12: Minimum VMA Requirements and VFA Range Requirements (from AASHTO, 2001)

5.2.6  Selection of Optimum Asphalt Binder Content

The optimum asphalt binder content is selected as that asphalt binder content that results in 4 percent air voids at N_design.  This asphalt content then must meet several other requirements:

1. Air voids at N_initial > 11 percent (for design ESALs ³ 3 million).  See Table 5.11 for specifics.
2. Air voids at N_max > 2 percent.  See Table 5.11 for specifics.
3. VMA above the minimum listed in Table 5.8.
4. VFA within the range listed in Table 5.8.

If requirements 1,2 or 3 are not met the mixture needs to be redesigned.  If requirement 4 is not met but close, then asphalt binder content can be slightly adjusted such that the air void content remains near 4 percent but VFA is within limits.  This is because VFA is a somewhat redundant term since it is a function of air voids and VMA (Roberts et al., 1996).  The process is illustrated in Figure 5.14 (numbers are chosen based on 20-year traffic loading of ³ 3 million ESALs).

Figure 5.14: Selection of Optimum Asphalt Binder Content Example
(from Roberts et al., 1996)

5.2.7  Moisture Susceptibility Evaluation

Moisture susceptibility testing is the only performance testing incorporated in the Superpave mix design procedure as of early 2002.  The modified Lottman test is used for this purpose.

The typical moisture susceptibility test is:

• AASHTO T 283: Resistance of Compacted Bituminous Mixture to Moisture-Induced Damage.

5.3  Summary

The Superpave mix design method was developed to address specific mix design issues with the Hveem and Marshall methods.  Superpave mix design is a rational method that accounts for traffic loading and environmental conditions.  Although not yet fully complete (the performance tests have not been implemented), Superpave mix design produces quality HMA mixtures.  As of 2000, 39 states have adopted, or are planning to adopt, Superpave as their mix design system (NHI, 2000).

The biggest differentiating aspects of the Superpave method are:

1. The use of formal aggregate evaluation procedures (consensus requirements).
2. The use of the PG asphalt binder grading system and its associated asphalt binder selection system.
3. The use of the gyratory compactor to simulate field compaction.
4. Traffic loading and environmental considerations.
5. Its volumetric approach to mix design.

Even given its many differences when compared to the Hveem or Marshall methods, Superpave still uses the same basic mix design steps and still strives for an optimum asphalt binder content that results in 4 percent design air voids.  Thus, the method is quite different but the ultimate goals remain fairly consistent.