74. Mix Design Superpave

Major Topics on this Page (History- Procedure - Summary)

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 The compaction devices from the Hveem and Marshall procedures have been replaced by a gyratory compactor and the compaction effort in mix design is tied to expected traffic. (See appendix 01). 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.

•

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.

f/d:bk/Yas/Mix Design (MRT)

5-1


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. (See appendix 02).

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


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 5.1).


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Figure 5.1: 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 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:

where:

P0.075

=

mass of particles passing the 0.075 mm (No. 200) sieve

Pbe

=

effective binder content = the total asphalt binder content of a paving mixture less the portion of asphalt binder that is lost by absorption into the aggregate particles.

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


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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. rutting.

This lack of

interlock makes the resultant HMA more susceptible to

Coarse aggregate angularity can be determined by any number of test


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procedures that are designed to determine the percentage of fractured faces. Table 5.5 lists Superpave requirements. Table 5.1: Coarse Aggregate Angularity 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. 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:

where:

V

=

volume of cylinder (mL)

W

=

weight of loose fine aggregate to fill the cylinder (g)

Gsb

=

bulk specific gravity of the fine aggregate

Table 5.2 shows the Superpave recommended fine aggregate angularity. Table 5.2: Fine Aggregate Angularity Requirements (from AASHTO, 2000b) Depth from Surface

20-yr Traffic Loading (in millions of ESALs)

≤ 100 mm (4 inches)

> 100 mm (4 inches)

< 0.3

-

-

0.3 to < 3

40

40

3 to < 10

45

10 to < 30


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45

≥ 30

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). Flat or elongated particles are typically identified using ASTM D 4791, Flat or Elongated Particles in Coarse Aggregate. Table 5.3 shows the Superpave recommended flat or elongated particle requirements. Figure 5.3: Flat or Elongated Particle Requirements (from AASHTO, 2000b)


Maximum Percentage of Particles with Length/Thickness > 5

< 0.3

-

0.3 to < 3 3 to < 10 10 to < 30

10

≥ 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 5.4. Table 5.4: Sand Equivalent Requirements (from AASHTO, 2000b) 20-yr Traffic Loading (in millions of ESALs)

Minimum Sand Equivalent (%)

< 0.3

40


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0.3 to < 3 3 to < 10

45

10 to < 30

50

≥ 30

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:


5-8


•

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. (See appendix 04). 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 5.5 shows two examples of design high

temperature adjustments - often called "binder bumping". Table 5.5: 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 70 °C but two more high temperature grades were necessary to accommodate transient and stationary loads.


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5.2.3 Sample Preparation The

Superpave

trial

aggregate-asphalt

content.

Then,

method, by

like

binder

evaluating

other

mix

blends, each

design

each trial

with

blend's

methods, a

different

creates

several

asphalt

performance,

an

binder

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 5.2) 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 5.3).

•

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


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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 5.2 : Gyratory Compactor

Figure 5.3 : Superpave Gyratory Compactor Sample (left) vs. Hveem/Marshall Compactor Sample (right) The Superpave gyratory compactor establishes three different gyration numbers:


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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 N design 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 N max 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 5.5 lists the specified number of gyrations for Ninitial, Ndesign and Nmax while Table 5.6 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). Table 5.5: Number of Gyrations for Ninitial, Ndesign and Nmax (from AASHTO, 2001)

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


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*

When the estimated 20-year design traffic loading is between 3 and < 10 million ESALs, the agency may, at its discretion, specify Ninitial = 7, Ndesign = 75 and Nmax = 115.

Table 5.6: Required Densities for Ninitial, Ndesign and Nmax (from AASHTO, 2001) 20-yr Traffic Loading (in millions of ESALs)

Required Density (as a percentage of TMD) Ninitial

< 0.3

≤ 91.5

0.3 to < 3

≤ 90.5

3 to < 10 10 to < 30 ≥

Ndesign

Nmax

96.0

≤ 98.0

≤ 89.0

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

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


5-13


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 (Gmb) - often called "bulk density"

2.

Theoretical maximum density (TMD, Gmm)

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

Air voids (Va), 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 5.7.

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 5.7: Minimum VMA Requirements and VFA Range Requirements (from AASHTO, 2001) Minimum VMA (percent)

20-yr Traffic Loading

9.5 mm 12.5 mm (in millions of ESALs) (0.375 inch) (0.5 inch)

19.0 mm (0.75 inch)

25.0 mm (1 inch)

37.5 mm (1.5 inch)

VFA Range (percent)

< 0.3

70 - 80

0.3 to < 3

65 - 78

3 to < 10 10 to < 30

15.0

14.0

13.0

12.0

11.0 65 - 75

≥ 30 See appendix 05


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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 Ndesign. This asphalt content then must meet several other requirements:

1.

Air voids at Ninitial > 11 percent (for design ESALs

≥ 3 million). See Table 5.6 for

specifics.

2.

Air voids at Nmax > 2 percent. See Table 5.6 for specifics.

3.

VMA above the minimum listed in Table 5.7.

4.

VFA within the range listed in Table 5.7.

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 5.4

(numbers are chosen based on 20-year traffic loading of


≥ 3 million ESALs).

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Figure 5.4: 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 MoistureInduced 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.


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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.

APPENDIX 1 WSDOT Superpave Mix Design Process


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WSDOT Superpave Mix Design Process The WSDOT process is quite similar to the Hveem method described in the Asphalt Institute's Superpave Mix Design (SP-02), 3rd edition, however, there are some differences. Some major points to note about the WSDOT method are: •

WSDOT is responsible for determining the optimum asphalt content and antistrip percentage needed for the contractor-submitted mix design.

The

contractor produces several trial blends then sends in his/her proposed aggregate blends (three blends total - the proposed blend plus a coarse and a fine blend on either side of the proposal) along with the proposed asphalt binder type and asphalt content. WSDOT then runs the proposal through the mix design process. In many states the individual contractors are responsible for their own mix design. •

When using the AASHTO tables for the various Superpave requirements (Tables 5.5, 5.6, 5.7, 5.8, 5.10, 5.11), WSDOT uses a 15-year traffic loading instead of the listed 20-year period because WSDOT typically designs overlays for a 15year design life. This difference does result in fewer design ESALs (because WSDOT uses 5 fewer years) but usually does not result in a selected category being different than if a 20-year traffic loading were used.

•

An approved aggregate stockpile need not be tested for aggregate properties on subsequent mix designs.


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•

WSDOT tests asphalt binder contents at the contractor's desired level and typically at ± 0.5 percent.

•

WSDOT makes separate Hveem samples and tests them for stability. This test is not required in Superpave mix design and is only done for informational purposes.

•

WSDOT uses the modified Lottman test to determine the optimum amount of antistripping asphalt binder modifier.

The following is a brief description of the WSDOT Superpave mix design process. 1. Aggregate Selection The contractor who will be doing the paving sends WSDOT three trial aggregate blends (typically a coarse, fine, and middle ground gradation) along with laboratory data for each of these blends.

The contractor indicates which of the gradations

he/she would like to use and designates a design asphalt content. The aggregate requirements for Superpave are checked by the contractor during his/her trial blend process, then again by WSDOT during the confirmation of the contractor's proposal. The coarse aggregate angularity requirements are determined by the number of ESALs to which the roadway will be subjected. Additionally: •

Unless the aggregate comes from a previously WSDOT-approved stockpile, testing is done to confirm the aggregates meet WSDOT specifications. As of 2002 aggregate sources are approved for 5 years, although some sources have not been switched over from the previous 10-year approval interval.

•

Each of these trial blends must be within the Superpave gradation requirements and preferably not pass through the restricted zone (although WSDOT does accept a mix design that goes through the restricted zone if it meets all other requirements).

The bulk specific gravity (Gsb) of the coarse and fine aggregate is determined for each stockpile. In this case, material retained on the 4.75 mm (No. 4) is considered "coarse", while the rest is considered "fine".


5-19


Figure 1: Aggregate Samples

Figure 2: Preparing the Graded Sample 2. Binder Selection The contractor who will be doing the paving sends WSDOT the brand and type of binder and antistrip modifier to be used. Actual asphalt binder samples are sent from the asphalt producers whenever needed throughout the year.

Producers

typically send anywhere from 10 to 40 cans (see Figure 3) at a time (depending on the binder type - typically they send fewer of the modified binders). The asphalt binder shall conform to AASHTO MP 1 requirements (Superpave PG binder system). WSDOT only allows one asphalt binder type submission for Superpave jobs. WSDOT determines the asphalt binder's specific gravity for use in the mix design process.


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Figure 3 : Asphalt Binder Sample 3. Sample Preparation Typically, six initial samples are made: two at the design asphalt content, two at 0.5 percent below the design asphalt content and two at 0.5 percent above the design asphalt content. These six samples are then cured and conditioned according to AASHTO PP 2 and compacted in the Superpave gyratory compactor in accordance with AASHTO TP 4. Additionally, three samples (one at each of the above asphalt contents) are made and compacted in the California kneading compactor for use in stability tests. •

AASHTO PP 2: Mixture Conditioning of Hot Mix Asphalt

•

AASHTO TP 4: Method for Preparing and Determining the Density of Hot Mix Asphalt (HMA) Specimens by Means of the Superpave Gyratory Compactor

4. Stabilometer The three Hveem-compacted samples are tested for stability. Because Superpave mix designs do not have to pass stability requirements, these tests are done for informational purposes only. 5. Density and Voids Analysis


5-21


First, bulk specific gravity (Gmb) is determined for each sample and the two results for each asphalt content are averaged.

Second, one sample from each asphalt

content is broken down for density and volumetric determinations to include theoretical maximum density (abbreviated TMD or called "Rice" density after its originator, and often designated Gmm), air voids, VMA and VFA. At this time the Gmm at Ninitial , Ndesign and Nmaximum are checked, as well as the dust to asphalt ratio. The effective asphalt content (Pbe) and percent absorbed asphalt content (Pba) are also checked. 6. Selection of Optimum Asphalt Binder Content Using the data from the three asphalt contents, the optimum binder content is selected as that which corresponds to 4.0 percent air voids (4.5 percent air voids for Superpave designs that will be paid for based on volumetric properties). Usually, this asphalt content must be interpolated between two of the sample asphalt contents. For example, a 5.0 percent asphalt sample may have 4.8 percent air voids and a 5.5 percent asphalt sample may have 3.8 percent air voids. In this case the design asphalt content would be interpolated as 5.4 percent. This selected asphalt content must also meet VMA, VFA, density and dust-to-asphalt requirements. 7. Determine the Amount of Antistripping Modifier Six samples are mixed per binder type at the determined asphalt content (from step 6 above). Note that these samples are 100 mm (4 inch) diameter cylinders instead of the usual Superpave 150 mm (6 inch) diameter cylinders. Two samples are kept as controls and the other four samples contain varied amounts of an antistripping modifier.

These samples are then cured and compacted the same way as in the

Hveem mix design process. Next, all of the samples except one of the two that does not contain any antistripping modifier are tested for bulk specific gravity (Gmb). The one that is not tested is kept as an unconditioned sample. The remaining 5 samples are

then

subjected

to

the

modified

Lottman

test

to

determine

moisture

susceptibility. The minimum TSR is 0.80. 8. Ignition Furnace Calibration Samples are then mixed at the design asphalt content and antistripping modifier amount for use in determining an ignition furnace calibration factor. The ignition


5-22


furnace

is

used

to

determine

field

sample

asphalt

content

during

manufacturing/construction. 9. Mix Design Report Finally, the recommended mix design is reported on a standard form that includes the manufacturer's recommended mix and compaction temperatures. These reports are quite valuable because they include the contractor's proposed JMF, the laboratory analysis information from WSDOT and the recommendations for asphalt content and antistrip amount for the particular JMF and aggregate source submitted.

APPENDIX 2

Superpave Overview


5-23


Superpave Overview (Strategic Highway Research Program (SHRP) - Superpave) "Superpave" is an overarching term for the results of the asphalt research portion of the 1987 - 1993 Strategic Highway Research Program (SHRP). Superpave consists of (1) an asphalt binder specification, (2) an HMA mix design method and (3) HMA tests and performance prediction models. Each one of these components is referred to by the term "Superpave". This section provides a brief overview and background of Superpave. Strategic Highway Research Program (SHRP) In 1987 the U.S. Congress established a 5-year, $150 million applied research program aimed at improving the performance, durability, safety, and efficiency of the Nation’s highway system.

Called the Strategic Highway Research Program

(SHRP), this program was officially authorized by the Surface Transportation and Uniform Relocation Act of 1987 and consisted of research concentrated in four key areas (FHWA, 1998): •

Asphalt. This area consists of research to develop a completely new approach to HMA mix design.


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•

Concrete and structures. This area consists of research in the areas of mix design and assessing, protecting and rehabilitating concrete pavements and structures.

•

Highway operations. This area consists of pavement preservation, work zone safety and snow and ice control research.

•

Pavement performance.

This area consists of the Long Term Pavement

Performance Program (LTPP), a 20-year study of over 2,000 test sections of inservice U.S. and Canadian pavements to improve guidelines for building and maintaining pavements. SHRP research activities were completed in 1992 and SHRP was closed down in 1993.

To date, SHRP has produced more than 100 new devices, tests and

specifications and, perhaps more importantly, has spawned a full-scale on-going implementation drive by such organizations as the FHWA, AASHTO and TRB. Now that this first SHRP effort has reached the implementation stage, Congress has requested that the Transportation Research Board initiate a new process of setting priorities and designing a program for another focused research and development effort. This new study was initiated in 1999 and was completed in 2001 (TRB, 2001). Superpave The SHRP asphalt research program, the largest SHRP program at $53 million (FHWA, 1998), had three primary objectives (NECEPT, 2001): •

Investigate why some pavements perform well, while others do not.

•

Develop tests and specifications for materials that will out-perform and outlast the pavements being constructed today.

•

Work with highway agencies and industry to have the new specifications put to use.

The final product of this research program is a new system referred to as "Superpave", which stands for SUperior PERforming Asphalt PAVEments. Superpave, in its final form consists of three basic components:

1.

An asphalt binder specification. This is the PG asphalt binder specification.


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2.

A design and analysis system based on the volumetric properties of the asphalt mix. This is the Superpave mix design method.

3.

Mix analysis tests and performance prediction models. This area is not yet complete. Test development and evaluation is on-going as of 2001.

Each one of these components required new specifications and performance standards as well as new testing methods and devices. As of late 2001, most states (48) have adopted or will adopt the Superpave PG asphalt binder specification and 39 states either have adopted or will adopt the Superpave mix design method (NHI, 2000).

APPENDIX 3

Superpave Gradation Requirements


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Superpave Gradation Requirements These tables (data taken from AASHTO MP 2, Standard Specification for Superpave Volumetric Mix Design) show typical Superpave aggregate specifications for 37.5 mm (1.5 inch) down to 9.5 mm (0.375 inch) nominal aggregate sizes. Significant figures are the same as those in AASHTO MP 2. Table 1: 37.5 mm (1.5 inch) Nominal Size Sieve Size Control Points (mm)

(U.S.)

Lower

Restricted Zone

Upper

Lower

Upper

50

2 inch

100

-

-

-

37.5

1.5 inch

90

100

-

-

25

1 inch

-

90

-

-

19

3/4 inch

-

-

-

-

12.5

1/2 inch

-

-

-

-

9.5

3/8 inch

-

-

-

-

4.75

No. 4

-

-

34.7

34.7

2.36

No. 8

15

41

23.3

27.3

1.18

No. 16

-

-

15.5

21.5

0.60

No. 30

-

-

11.7

15.7

0.30

No. 50

-

-

10.0

10.0

0.15

No. 100

-

-

-

-

0.075

No. 200

0

6

-

-

Table 2: 25 mm (1 inch) Nominal Size


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Sieve (mm) 37.5 25 19 12.5 9.5 4.75 2.36 1.18 0.60 0.30 0.15 0.075

Size (U.S.) 1.5 inch 1 inch 3/4 inch 1/2 inch 3/8 inch No. 4 No. 8 No. 16 No. 30 No. 50 No. 100 No. 200

Control Points Lower Upper 100 90 100 90 19 45 1 7

Restricted Zone Lower Upper 39.5 39.5 26.8 30.8 18.1 24.1 13.6 17.6 11.4 11.4 -

Table 3: 19 mm (3/4 inch) Nominal Size Sieve Size

Control Points

Restricted Zone

(mm)

(U.S.)

Lower

Upper

Lower

Upper

25

1 inch

100

-

-

-

19

3/4 inch

90

100

-

-

12.5

1/2 inch

-

90

-

-

9.5

3/8 inch

-

-

-

-

4.75

No. 4

-

-

-

-

2.36

No. 8

23

49

34.6

34.6

1.18

No. 16

-

-

22.3

28.3

0.60

No. 30

-

-

16.7

20.7

0.30

No. 50

-

-

13.7

13.7

0.15

No. 100

-

-

-

-

0.075

No. 200

2

8

-

-

Table 4: 12.5 mm (1/2 inch) Nominal Size Sieve Size

Control Points

(mm)

(U.S.)

Lower

Restricted Zone

Upper

Lower

Upper

19

3/4 inch

100

-

-

-

12.5

1/2 inch

90

100

-

-

9.5

3/8 inch

-

90

-

-

4.75

No. 4

-

-

-

-

2.36

No. 8

28

58

39.1

39.1

1.18

No. 16

-

-

25.6

31.6

0.60

No. 30

-

-

19.1

23.1

0.30

No. 50

-

-

15.5

15.5

0.15

No. 100

-

-

-

-


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0.075

No. 200

2

10

-

Table 5: 9.5 mm (3/8 inch) Nominal Size Sieve Size Control Points (mm)

(U.S.)

Lower

12.5

1/2 inch

100

9.5

3/8 inch

90

4.75

No. 4

2.36 1.18 0.60

Upper

-

Restricted Zone Lower

Upper

-

-

100

-

-

-

90

-

-

No. 8

32

67

47.2

47.2

No. 16

-

-

31.6

37.6

No. 30

-

-

23.5

27.5

0.30

No. 50

-

-

18.7

18.7

0.15

No. 100

-

-

-

-

0.075

No. 200

2

10

-

-

APPENDIX 4

Reliability Concept in PG Asphalt Binder Selection


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Reliability Concept in PG Asphalt Binder Selection Reliability is defined as the percent probability in a single year that the actual temperature (seven-day high or one-day low) will not exceed the corresponding design temperatures. The animation below describes the basic process for selecting the pavement temperature extremes for a PG asphalt binder. Note that pavement temperatures are more extreme than air temperatures.


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APPENDIX 5

HMA Weight-Volume Terms and Relationships


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HMA Weight-Volume Terms and Relationships Major Topics on this Page : Specific Gravities - Voids (Air, VMA, VFA) - Other Definitions Effective Asphalt Content Volume Filled with Asphalt.

Basic HMA weight-volume relationships are important to understand for both mix design and construction purposes. Fundamentally, mix design is meant to determine the volume of asphalt binder and aggregates necessary to produce a mixture with the desired properties (Roberts et al., 1996). However, since weight measurements are typically much easier, they are typically taken then converted to volume by using specific gravities.

The following is a brief discussion of the more important volume

properties of HMA. In general, weight and volume terms are abbreviated as, Gxy, where:

x: b = binder s = stone (i.e., aggregate) m = mixture y: b = bulk e = effective a = apparent m = maximum


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For example, Gmm = gravity, mixture, maximum = the maximum gravity of the mixture. Other common abbreviations are:

VT

=

Total volume of the compacted specimen

WT

=

Total weight of the compacted specimen

Va

=

Volume of air voids

WD

=

Dry weight

Vb

=

Volume of asphalt binder

WSSD

=

Saturated surface dry (SSD) weight

Vbe

=

Volume of effective asphalt binder

Wsub

=

Weight submerged in water

Vba

=

Volume of absorbed asphalt binder

Wb

=

Weight of the asphalt binder

Vagg

=

Volume of aggregate

Wbe

=

Weight of effective asphalt binder

Veff

=

Effective volume of aggregate = (VT - VAC)

Wba

=

Weight of absorbed asphalt binder

Wagg

=

Weight of aggregate

Gsa

=

Apparent specific gravity of the aggregate

Gb

=

Asphalt binder specific gravity

Pb

=

Asphalt content by weight of mix (percent)

Gsb

=

Bulk specific gravity of the aggregate

Ps

=

Aggregate content by weight of mix (percent)

Gse

=

Effective specific gravity of the aggregate

Pa

=

Percent air voids

Gmb

=

Bulk specific gravity of the compacted mixture

Gmm

=

Maximum theoretical specific gravity of the mixture

γW

=

Unit weight of water

Specific Gravities Bulk Specific Gravity of the Compacted Asphalt Mixture (Gmb). The ratio of the mass in air of a unit volume of a permeable material (including both permeable and impermeable voids normal to the material) at a stated temperature to the mass in air (of equal density) of an equal volume of gas-free distilled water at a stated temperature.

This value is used to determine weight per unit volume of the

compacted mixture. It is very important to measure Gmb as accurately as possible. Since it is used to convert weight measurements to volumes, any small errors in G mb will be reflected in significant volume errors, which may go undetected. The standard bulk specific gravity test is: •

AASHTO T 166: Bulk Specific Gravity of Compacted Bituminous Mixtures Using Saturated Surface-Dry Specimens


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Theoretical Maximum Specific Gravity of Bituminous Paving Mixtures (Gmm). The ratio of the mass of a given volume of voidless (Va = 0) HMA at a stated temperature (usually 25°C) to a mass of an equal volume of gas-free distilled water at the same temperature. It is also called Rice Specific Gravity (after James Rice who developed the test procedure). Multiplying Gmm by the unit weight of water gives Theoretical Maximum Density (TMD). The standard TMD test is: •

AASHTO T 209 and ASTM D 2041: Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures

Voids (expressed as percentages) Air Voids (Va). The total volume of the small pockets of air between the coated aggregate particles throughout a compacted paving mixture, expressed as a percent of the bulk volume of the compacted paving mixture. The amount of air voids in a mixture is extremely important and closely related to stability and durability.

For

typical dense-graded mixes with 12.5 mm (0.5 inch) nominal maximum aggregate sizes air voids below about 3 percent result in an unstable mixture while air voids above about 8 percent result in a water-permeable mixture.

Voids in the Mineral Aggregate (VMA) The volume of intergranular void space between the aggregate particles of a compacted paving mixture that includes the air voids and the effective asphalt content, expressed as a percent of the total volume of the specimen. When VMA is too low, there is not enough room in the mixture to add sufficient asphalt binder to adequately coat the individual aggregate particles. Also, mixes with a low VMA are


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more sensitive to small changes in asphalt binder content. Excessive VMA will cause an unacceptably low mixture stability (Roberts et al., 1996). Generally, a minimum VMA is specified and a maximum VMA may or may not be specified.

Voids Filled with Asphalt (VFA). The portion of the voids in the mineral aggregate that contain asphalt binder. This represents the volume of the effective asphalt content. It can also be described as the percent of the volume of the VMA that is filled with asphalt cement. VFA is inversely related to air voids: as air voids decrease, the VFA increases.

Other Definitions Effective Asphalt Content (Pbe). The total asphalt binder content of the HMA less the portion of asphalt binder that is lost by absorption into the aggregate. Volume of Absorbed Asphalt (Vba). The volume of asphalt binder in the HMA that has been absorbed into the pore structure of the aggregate. It is the volume of the asphalt binder in the HMA that is not accounted for by the effective asphalt content.


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74. Mix Design Superpave

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