One of the Principal Results From the Strategic Highway Research Program

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One of the principal results from the Strategic Highway Research Program (SHRP) was theSuperpavemix designmethod. The Superpavemix designmethod was designed to replace the

HveemandMarshallmethods. The volumetricanalysiscommon to the Hveem and Marshall

methods provides the basis for the Superpavemix designmethod. The Superpave system tiesasphaltbinder andaggregateselection into themix designprocess, and considers traffic and

climate as well. Thecompactiondevices from the Hveem and Marshall procedures have beenreplaced by a gyratory compactor and thecompactioneffort inmix designis tied to expected

traffic.

This section consists of a brief history of the Superpavemix designmethod 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 [1]).HotMixAsphaltMaterials, Mixture Design, andConstruction. NationalAsphaltPavement

Association Education Foundation. Lanham, MD.

AsphaltInstitute. (2001

[2]

). SuperpaveMix Design. Superpave Series No. 2 (SP-02).AsphaltInstitute. Lexington, KY.

American Association of State Highway and Transportation Officials (AASHTO).(2000

[3]and 2001

[4]).AASHTO Provisional Standards. American Association of State

Highway and Transportation Officials. Washington, D.C.

Superpave History

Under the Strategic Highway Research Program (SHRP), an initiative was undertaken to

improvematerialsselection and mixture design by developing:

1. A newmix designmethod that accounts for traffic loading and environmental conditions.2. A new method ofasphaltbinder evaluation.3. New methods of mixtureanalysis.

When SHRP was completed in 1993 it introduced these three developments and called them theSuperior PerformingAsphaltPavement System (Superpave). Although the new methods of

mixture performance testing have not yet been established, themix designmethod is well-

established.

Superpave Procedure

The Superpavemix designmethod 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.

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6. Optimum asphalt binder content selection.7. Moisture susceptibility evaluation.

Aggregate Selection

Superpave specifiesaggregatein two ways. First, it places restrictions onaggregategradation bymeans of broad control points. Second, it places consensus requirements on coarse and fineaggregateangularity, flat and elongated particles, and clay content. Otheraggregatecriteria,

which theAsphaltInstitute (2001[2]

) calls source properties (because they are considered to besource specific) such asL.A. abrasion,soundnessand water absorption are used in Superpave

but since they were not modified by Superpave they are not discussed here.

Gradation and Size

Aggregate gradationinfluences such key HMA parameters as (read about these parameters here)

stiffness, stability, durability, permeability, workability, fatigue resistance, frictional resistance

and resistance to moisture damage (Roberts et al., 1996[1]

). Additionally, themaximumaggregate sizecan be influential incompactionand lift thickness determination.

GradationSpecifications

Superpavemix designspecifiesaggregategradation control points, through whichaggregategradations must pass. These control points are very general and are a starting point for ajob mix

formula.

AggregateBlending

It is rare to obtain a desiredaggregategradation from a singleaggregatestockpile. Therefore,Superpave mix designs usually draw upon several differentaggregatestockpiles and blend themtogether in a ratio that will produce an acceptable final blended gradation. It is quite common to

find a Superpavemix designthat uses 3 or 4 differentaggregatestockpiles (Figure 1).

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Figure 1. Screen shot from HMA View showing a typical aggregate blend from 4 stockpiles.

Typically, severalaggregateblends are evaluated prior to performing a completemix design.

Evaluations are done by preparing an HMA sample of each blend at the estimated optimum

asphaltbinder content then compacting it. Results from this evaluation can show whether or not

a particular blend will meet minimumVMArequirements and Ninitial or Nmax 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 arange of dust-to-binder ratio by mass. The equation is:

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Dust-to-binder ratiospecificationsare normally 0.61.2, but a ratio of up to 1.6 may be used at

an agencys discretion (AASHTO, 2001)[4]

.

Consensus Requirements

Consensus requirements came about because SHRP did not specifically addressaggregateproperties and it was thought that there needed to be some guidance associated with the

Superpavemix designmethod. Therefore, an expert group was convened and they arrived at aconsensus on severalaggregateproperty requirementsthe consensus requirements. This

group recommended minimum angularity, flat or elongated particle and clay content

requirements based on:

The anticipated traffic loading. Desiredaggregateproperties are different dependingupon 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

roadwaydesign life(AASHTO, 2000b[5]

).

Depth below the surface. Desiredaggregateproperties vary depending upon theirintended use as it relates to depth below the pavement surface.

These requirements are imposed on the finalaggregateblend and not the individualaggregate

sources.

CoarseAggregateAngularity

Coarseaggregateangularity is important tomix designbecause smooth, roundedaggregate

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. Coarseaggregateangularity can

be determined by any number of test procedures that are designed to determine thepercentage offractured faces. Table 1 lists Superpave requirements.

Table 1. CoarseAggregateAngularity Requirements (from AASHTO, 2000b )

20-yr Traffic Loading

(in millions of ESALs)

Depth from Surface

100 mm (4 inches) > 100 mm (4 inches)

< 0.3 55/- -/-

0.3 to < 3 75/- 50/-

3 to < 10 85/80 60/-

10 to < 30 95/90 80/75 30 100/100 100/100

Note: The first number is a minimum requirement for one or more fractured faces and the second

number is a minimum requirement for two or more fractured faces.

FineAggregateAngularity

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Fine aggregate angularityis important tomix designfor the same reasons as coarseaggregateangularityrut prevention. Fineaggregateangularity is quantified by an indirect method often

called the NationalAggregateAssociation (NAA) flow test. This test consists of pouring the

fineaggregateinto the top end of a cylinder and determining the amount of voids. The more

voids, the more angular theaggregate. Voids are determined by the following equation:

Table 2 shows the Superpave recommended fineaggregateangularity.

Table 2. FineAggregateAngularity Requirements (from AASHTO, 2000b )

20-yr Traffic Loading

(in millions of ESALs)

Depth from Surface

100 mm (4 inches) > 100 mm (4 inches)

< 0.3 - -

0.3 to < 3 40

403 to < 10

4510 to < 30

30 45

Numbers shown represent the minimum uncompacted void content as a percentage of the total

sample volume.

The standard test for fineaggregateangularity is:

AASHTO T 304: Uncompacted Void Content of FineAggregateFlat or Elongated Particles

An excessive amount offlat or elongated aggregate particlescan be detrimental to HMA.

Flat/elongated particles tend to breakdown duringcompaction(giving a different gradation thandetermined inmix design), decrease workability, and lie flat aftercompaction(resulting in a

mixture with low VMA) (Roberts et al., 1996 [1]). Flat or elongated particles are typically

identified using ASTM D 4791, Flat or Elongated Particles in Coarse Aggregate. Table 3 shows

the Superpave recommended flat or elongated particle requirements.

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Table 3. Flat or Elongated Particle Requirements (from AASHTO, 2000b )

20-yr Traffic Loading

(in millions of ESALs)

Maximum Percentage

of Particles with

h/Thickness > 5

< 0.3 -

0.3 to < 3

103 to < 10

10 to < 30

30

Clay Content

Thesand equivalent testmeasures the amount of clay content in anaggregatesample. If clay

content is too high, clay could preferentially adhere to theaggregateover theasphaltbinder.

This leads to a pooraggregate-asphaltbinder bonding and possiblestripping. To preventexcessive clay content, Superpave uses the sand equivalent test requirements of Table 4.

Table 4. Sand Equivalent Requirements (from AASHTO, 2000b )

20-yr Traffic Loading

(in millions of ESALs)Minimum Sand Equivalent (%)

< 0.340

0.3 to < 3

3 to < 1045

10 to < 30 30 50

Asphalt Binder Evaluation

Superpave uses its ownasphaltbinder selection process, which is, of course, tied to theSuperpaveasphaltbinderperformance grading (PG) systemand its associatedspecifications.

Superpave PGasphaltbinders are selected based on the expected pavement temperature

extremes in the area of their intended use. Superpave software (or a stand-alone program such asLTPPBind) is used to calculate these extremes and select the appropriate PGasphaltbinder using

one of the following three alternate methods (Roberts et al., 1996[1]):

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 andcity). From this, the software retrieves climate conditions from a weather database and

then converts air temperatures into pavement temperatures.

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Once the design pavement temperatures are determined they can be matched to an appropriate

PGasphaltbinder.

Design Pavement Temperature

The Superpavemix designmethod determines both a high and a low design pavementtemperature. These temperatures are determined as follows:

High pavement temperaturebased on the 7-day average high air temperature of thesurrounding area.

Low pavement temperaturebased on the 1-day low air temperature of the surroundingarea.

Using these temperatures as a starting point, Superpave then applies areliability conceptto

determine the appropriate PGasphaltbinder. PGasphaltbinders are specified in 6C

increments.

Design Pavement Temperature Adjustments

Design pavement temperature calculations are based on HMA pavements subjected to fast

moving traffic (Roberts et al., 1996[1]). Specifically, theDynamic Shear Rheometer (DSR) testis

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[1]

). Pavements subject tosignificantly slower (or stopped)trafficsuch as intersections, toll booth lines and bus stops should contain a stifferasphaltbinder

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 stationaryloads. 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 thatrequire 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 30million ESALs (a potential one grade increase), theasphaltbinder high temperature grade should

be increased by only one grade. Table 5 shows two examples of design high temperature

adjustmentsoften called binder bumping.

Table 5. Examples of Design Pavement Temperature Adjustments for Slow and Stationary Loads

Original GradeGrade for Slow Transient

Loads

(increase 1 grade)

Grade for

Stationary Loads

(increase 2 grades)

20-yr ESALs

> 30 million

(increase 1 grade)

PG 58-22 PG 64-22 PG 70-22 PG 64-22

PG 70-22* PG 76-22 PG 82-22 PG 76-22

*the highest possible pavement temperature in North America is about 70C but two more high

temperature grades were necessary to accommodate transient and stationary loads.

Sample Preparation

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The Superpave method, like othermix designmethods, creates several trialaggregate-asphaltbinder blends, each with a differentasphaltbinder content. Then, by evaluating each trial blendsperformance, an optimumasphaltbinder content can be selected. In order for this concept to

work, the trial blends must contain a range ofasphaltcontents both above and below theoptimumasphaltcontent. Therefore, the first step in sample preparation is to estimate an

optimumasphaltcontent. Trial blendasphaltcontents are then determined from this estimate.

The Superpave gyratory compactor (Figure 2) was developed to improvemix designs ability to

simulate actual fieldcompactionparticle orientation with laboratory equipment (Roberts,

1996[1]

).

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 samplethrough a hydraulically or mechanically operated load. Mixing andcompactiontemperatures are

chosen according toasphaltbinder properties so thatcompactionoccurs 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) inheight (corrections can be made for different sample heights). Note that this sample sizeis larger than those used for theHveemandMarshallmethods (Figure 3).

Load = Flat and circular with a diameter of 149.5 mm (5.89 inches) corresponding to anarea of 175.5 cm

2(27.24 in

2)

Compactionpressure = 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 minuteas the load is continuously applied. This helps achieve a sample particle orientation that

is somewhat like that achieved in the field after roller compaction.

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Figure 2. Gyratory compactor.

Figure 3. Superpave gyratory compactor sample (left) vs. Hveem/Marshall compactor sample

(right).

The Superpave gyratory compactor establishes three different gyration numbers:

1. Ninitial. The number of gyrations used as a measure of mixture compactability duringconstruction. Mixes that compact too quickly (air voids at N initial are too low) may be

tenderduringconstructionand unstable when subjected to traffic. Often, this is a good

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indication ofaggregatequalityHMA with excess natural sand will frequently fail theNinitial requirement. A mixture designed for greater than or equal to 3 million ESALs with

4 percent air voids at Ndesign should have at least 11 percent air voids at Ninitial.

2. Ndesign. This is the design number of gyrations required to produce a sample with thesame density as that expected in the field after the indicated amount of traffic. A mix with

4 percent air voids at Ndesign is desired inmix design.3. Nmax. The number of gyrations required to produce a laboratory density that should neverbe exceeded in the field. If the air voids at Nmax are too low, then the field mixture maycompact 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.

Typically, samples are compacted to Ndesign to establish the optimumasphaltbinder content andthen additional samples are compacted to Nmax as a check. Previously, samples were compacted

to Nmax and then Ninitial and Ndesign were back calculated. Table 6 lists the specified number of

gyrations for Ninitial, Ndesign and Nmax while Table 7 shows the required densities as a percentage

of theoretical maximum density (TMD) for Ninitial, Ndesign and Nmax. Note that traffic loading

numbers are based on the anticipated traffic level on the design lane over a 20-year periodregardless of actual roadwaydesign life(AASHTO, 2001[4]

).

Table 6. Number of Gyrations for Ninitial, Ndesign and Nmax (from AASHTO, 20014 )

20-yr Traffic Loading

(in millions of ESALs)

Number of Gyrations

Ninitial Ndesign Nmax

< 0.3 6 50 75

0.3 to < 3 7 75 115

3 to < 10* 8 (7) 100 (75) 160 (115)

10 to < 30 8 100 160 30 9 125 205

* When the estimated 20-year design traffic loading is between 3 and < 10million ESALs, the agency may, at its discretion, specify

Ninitial = 7, Ndesign = 75 and Nmax = 115.

Table 7. Required Densities for Ninitial, Ndesign and Nmax (from AASHTO, 20014 )

20-yr Traffic Loading

(in millions of ESALs)

Required Density (as a percentage ofTMD)

Ninitial Ndesi n Nmax

< 0.3 91.5

96.0 98.0

0.3 to < 3 90.5

3 to < 10

89.010 to < 30

30

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The standard gyratory compactor sample preparation procedure is:

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

Performance Tests

The original intent of the Superpavemix designmethod was to subject the various trial mix

designs to a battery of performance tests akin to what the Hveem method does with thestabilometer and cohesiometer, or the Marshall method does with the stability and flow test.

Currently, these performance tests, which constitute the mixtureanalysisportion of Superpave,

are still under development and review and have not yet been implemented. The most likelyperformance test, called the Simple Performance Test (SPT) is a Confined Dynamic Modulus

Test.

Density and Voids Analysis

Allmix designmethods use density and voids to determine basic HMA physical characteristics.

Two different measures of densities are typically taken:

1. Bulk specific gravity(Gmb).2. Theoretical maximum specific gravity(TMD, Gmm).

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

Air voids(Va), sometimes expressed as 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 8. Note that traffic loading numbers

are based on the anticipated traffic level on the design lane over a 20-year period

regardless of actual roadwaydesign life(AASHTO, 2000b[5]

).

Table 8. Minimum VMA Requirements and VFA Range Requirements (from AASHTO, 2001[4])

20-yr Traffic

Loading

(in millions of

ESALs)

Minimum VMA (percent)

VFA Range

(percent)9.5 mm

(0.375

inch)

12.5 mm

(0.5

inch)

19.0 mm

(0.75

inch)

25.0

mm

(1 inch)

37.5 mm

(1.5

inch)

< 0.3

15.0 14.0 13.0 12.0 11.0

7080

0.3 to < 3 6578

3 to < 10 6575

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10 to < 30

30

Selection of Optimum Asphalt Binder Content

The optimumasphaltbinder content is selected as thatasphaltbinder content that results in 4

percent air voids at Ndesign. Thisasphaltcontent then must meet several other requirements:

1. Air voidsat Ninitial> 11 percent (for design ESALs 3 million). See Table 5 forspecifics.

2. Air voids at Nmax > 2 percent. See Table 5 for specifics.3. VMAabove the minimum listed in Table 2.4. VFAwithin the range listed in Table 2.

If requirements 1,2 or 3 are not met the mixture needs to be redesigned. If requirement 4 is notmet but close, thenasphaltbinder 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[1]

). The process isillustrated in Figure 4 (numbers are chosen based on 20-year traffic loading of 3 million

ESALs).

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Figure 4. Selection of optimumasphaltbinder content example: 4 basic steps.

Moisture Susceptibility Evaluation

Moisture susceptibilitytesting is the only performance testing incorporated in the Superpavemix

designprocedure as of early 2002. Themodified Lottman testis used for this purpose.

The typicalmoisture susceptibilitytest is:

AASHTO T 283: Resistance of Compacted Bituminous Mixture to Moisture-InducedDamage.

Surveys

Superpave Dust to Binder Ratio Survey

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Questions

AASHTO M 323 (Superpave VolumetricMix Design) allows an agency to modify the required

dust to binder ratio from 0.6-1.2 to 0.8-1.6 if theaggregategradation passes beneath the PCS

Control Point. Does your agency allow (or require) a dust to binder ratio of 0.8 1.6? If so, when

is this allowed/required?

Results

Superpave Dust to Binder Ratio Survey by NJDOT

Footnotes ( returns to text)

1. Hot MixAsphaltMaterials, Mixture Design, andConstruction. NationalAsphaltPavement Association Education Foundation. Lanham, MD.

2. HMAConstruction. Manual Series No. 22 (MS-22). AsphaltInstitute. Lexington, KY.3. American Association of State Highway and Transportation Officials (AASHTO).

(2000b). AASHTO Provisional Standards, April 2000 Interim Edition. American

Association of State Highway and Transportation Officials. Washington, D.C.4. AASHTO Provisional Standards, April 2001 Interim Edition. American Association of

State Highway and Transportation Officials. Washington, D.C.

5. AASHTO Provisional Standards, April 2000 Interim Edition. American Association ofState Highway and Transportation Officials. Washington, D.C.