Superpave Mix Design _ Pavement Interactive

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Design Mix Design Flexible Pavement Mix Design Superpave Mix Design

Publish date: January 26, 2011 | Author: Pavement Interactive

Superpave Mix Design

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Hveem and Marshall procedures have been replaced by a gyratory compactor and the compaction effort in mix design is tied to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[1]). Hot Mix Asphalt Materials, Mixture Design,

and Construction. National Asphalt Pavement Association Education Foundation. Lanham, MD.

Asphalt Institute. (2001[2]). Superpave Mix Design. Superpave Series No. 2 (SP-02). Asphalt Institute. 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 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.

Superpave 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.

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[2]) 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.

Gradation and Size

Aggregate gradation influences 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, 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 (Figure 1).

Figure 1. Screen shot from HMA View show ing 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 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 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 agencys discretion (AASHTO,

2001)[4].

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[5]).

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 1 lists Superpave requirements.

Table 1. Coarse Aggregate Angularity Requirements (from AASHTO, 2000b[5])

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 minimumrequirement for two or more fractured faces.

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 2 shows the Superpave recommended fine aggregate angularity.

Table 2. Fine Aggregate Angularity Requirements (from AASHTO, 2000b[5])



Depth from Surface

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

< 0.3 - -

0.3 to < 3 40 40

3 to < 10 45

10 to < 30

30 45

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

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[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.

Table 3. Flat or Elongated Particle Requirements(from AASHTO, 2000b[5])



Maximum Percentage

of Particles with

h/Thickness > 5

< 0.3 -

0.3 to < 3 10

3 to < 10

10 to < 30

30

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 4.

Table 4. Sand Equivalent Requirements (from AASHTO,2000b[5])



Minimum Sand Equivalent (%)

< 0.3 40

0.3 to < 3

3 to < 10 45

10 to < 30

30 50

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]):

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.

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 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, 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[1]). 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 shows two examples of design high temperature adjustments often called binder bumping.

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

Original Grade Grade 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 werenecessary to accommodate transient and stationary loads.

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 blends 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 2) was developed to improve mix designs ability to simulate actual field compaction

particle 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 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). Note that this sample size is larger than those used for the Hveem and Marshall methods (Figure 3).

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 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 during construction. Mixes that compact too

quickly (air voids at Ninitial 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 Ninitial 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 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 desired 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.

Typically, samples are compacted to Ndesign to establish the optimum asphalt binder content and then 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 period regardless of actual roadway design life (AASHTO,

2001[4]).

Table 6. Number of Gyrations for Ninitial, Ndesign and Nmax (from AASHTO,2001[4])

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

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Table 7. Required Densities for Ninitial, Ndesign and Nmax (from AASHTO,2001[4])


Required Density (as a percentage of TMD)

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

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

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.

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 (Gmb).

2. Theoretical maximum specific gravity (TMD, Gmm).

These densities are then used to calculate the volumetric parameters of the HMA. Measured void expressions 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 roadway design life (AASHTO, 2000b[5]).

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


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 70 80

0.3 to < 3 65 78

3 to < 10 65 75

10 to < 30

30

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 Ndesign. This

asphalt content then must meet several other requirements:

1. Air voids at Ninitial > 11 percent (for design ESALs 3 million). See Table 5 for specifics.

2. Air voids at Nmax > 2 percent. See Table 5 for specifics.

3. VMA above the minimum listed in Table 2.

4. VFA within the range listed in Table 2.



Copyright 2012 Pavia Systems, Inc.

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[1]). The process is illustrated in

Figure 4 (numbers are chosen based on 20-year traffic loading of 3 million ESALs).

Figure 4. Selection of optimum asphalt binder content example: 4 basic steps.

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.

Surveys

Superpave Dust to Binder Ratio Survey

Questions

AASHTO M 323 (Superpave Volumetric Mix Design) allows an agency to modify the required dust to binder ratio from 0.6-1.2 to

0.8-1.6 if the aggregate gradation 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 Mix Asphalt Materials, Mixture Design, and Construction. National Asphalt Pavement Association Education Foundation. Lanham, MD.

2. HMA Construction. Manual Series No. 22 (MS-22). Asphalt Institute. Lexington, KY.

3. American Association of State Highw ay and Transportation Officials (AASHTO). (2000b). AASHTO Provisional Standards, April 2000 Interim Edition. American Association of

State Highw ay and Transportation Officials. Washington, D.C.

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

5. AASHTO Provisional Standards, April 2000 Interim Edition. American Association of State Highw ay and Transportation Officials. Washington, D.C.

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