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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compaction devices from the Hveem and Marshall procedures have been replaced by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.
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Major Topics on this Page (History- Procedure - Summary)
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.
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Major Topics on this Page (History- Procedure - Summary)
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
Major Topics on this Page (History- Procedure - Summary)
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
convened and they arrived at a consensus on several aggregate property
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Major Topics on this Page (History- Procedure - Summary)
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.
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Major Topics on this Page (History- Procedure - Summary)
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)
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 < 1045
10 to < 30
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Major Topics on this Page (History- Procedure - Summary)
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). 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)
20-yr Traffic Loading(in millions of ESALs)
Maximum Percentage of Particles with Length/Thickness > 5
< 0.3 -
0.3 to < 3
103 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 5.4.
Table 5.4: Sand Equivalent Requirements (from AASHTO, 2000b)
20-yr Traffic Loading(in millions of ESALs) Minimum Sand Equivalent (%)
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Major Topics on this Page (History- Procedure - Summary)
< 0.340
0.3 to < 3
3 to < 10 45
10 to < 30
30 50
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:
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Major Topics on this Page (History- Procedure - Summary)
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. (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 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.
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Major Topics on this Page (History- Procedure - Summary)
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 5.2) 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
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
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Major Topics on this Page (History- Procedure - Summary)
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.
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 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 Ninitial requirement. A mixture designed for greater
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Figure 5.2 : Gyratory Compactor
Figure 5.3 : Superpave Gyratory Compactor Sample (left) vs. Hveem/Marshall Compactor Sample (right)
Major Topics on this Page (History- Procedure - Summary)
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 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)
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 < 10 million ESALs, the agency may, at its discretion, specify N initial = 7, Ndesign = 75 and Nmax = 115.
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Major Topics on this Page (History- Procedure - Summary)
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 Ndesign Nmax
< 0.3 91.5
96.0 98.0
0.3 to < 3 90.5
3 to < 10
89.010 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
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:
f/d:bk/Yas/Mix Design (MRT) 5-13
Major Topics on this Page (History- Procedure - Summary)
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)
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
70 - 80
0.3 to < 3 65 - 78
3 to < 10
65 - 7510 to < 30
30
See appendix 05
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.
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Major Topics on this Page (History- Procedure - Summary)
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)
Major Topics on this Page (History- Procedure - Summary)
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.
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Major Topics on this Page (History- Procedure - Summary)
APPENDIX 1WSDOT Superpave Mix Design Process
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Major Topics on this Page (History- Procedure - Summary)
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 15-
year 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.
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
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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".
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Figure 1: Aggregate Samples
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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 2: Preparing the Graded Sample
Figure 3 : Asphalt Binder Sample
Major Topics on this Page (History- Procedure - Summary)
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
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
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Major Topics on this Page (History- Procedure - Summary)
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
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.
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APPENDIX 2Superpave Overview
Superpave Overview
(Strategic Highway Research Program (SHRP) - Superpave)
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"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.
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 in-
service 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).
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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.
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).
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APPENDIX 3 Superpave Gradation Requirements
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
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Sieve Size Control Points Restricted Zone
(mm) (U.S.) Lower 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 SizeSieve Size Control Points Restricted Zone
(mm) (U.S.) Lower Upper Lower Upper
37.5 1.5 inch 100 - - -
25 1 inch 90 100 - -
19 3/4 inch - 90 - -
12.5 1/2 inch - - - -
9.5 3/8 inch - - - -
4.75 No. 4 - - 39.5 39.5
2.36 No. 8 19 45 26.8 30.8
1.18 No. 16 - - 18.1 24.1
0.60 No. 30 - - 13.6 17.6
0.30 No. 50 - - 11.4 11.4
0.15 No. 100 - - - -
0.075 No. 200 1 7 - -
Table 3: 19 mm (3/4 inch) Nominal SizeSieve 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
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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 SizeSieve Size Control Points Restricted Zone
(mm) (U.S.) Lower 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 - - - -
0.075 No. 200 2 10 - -
Table 5: 9.5 mm (3/8 inch) Nominal SizeSieve Size Control Points Restricted Zone
(mm) (U.S.) Lower Upper Lower Upper
12.5 1/2 inch 100 - -
9.5 3/8 inch 90 100 - -
4.75 No. 4 - 90 - -
2.36 No. 8 32 67 47.2 47.2
1.18 No. 16 - - 31.6 37.6
0.60 No. 30 - - 23.5 27.5
0.30 No. 50 - - 18.7 18.7
0.15 No. 100 - - - -
0.075 No. 200 2 10 - -
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APPENDIX 4 Reliability Concept in PG Asphalt Binder Selection
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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Major Topics on this Page (History- Procedure - Summary)
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Major Topics on this Page (History- Procedure - Summary)
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APPENDIX 5HMA Weight-Volume Terms and Relationships
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
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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
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
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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 Gmb
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
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
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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
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.
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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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