Notice

This document is disseminated under the sponsorship of the Department of Transportation in

the interest of information exchange. The United States Government assumes no liability for its

contents or use thereof. This report does not constitute a standard, specification, or regulation.

The United States Government does not endorse products or manufacturers. Trade and manufacturer's names appear in this report only because they are considered essential to the object of

the document.

WesTrack was the Federal Highway Administration's (FHWA) test facility in Nevada for

developing performance-related specifications for hot-mix asphalt pavement construction. It also provided some of the earliest data on the performance of Superpave asphalt

mixture designs under high rates of heavy truck loading. When Superpave-designed test

sections placed at the track in June 1997 had very rapid rutting failures, the highway community was concerned that the mixture design and construction procedures might be

missing important, but unknown, constraints. A forensic team composed of academicians,

asphalt industry representatives, and State highway agency engineers was assembled to

study the early failures and, if appropriate, to make recommendations for revising the

Superpave procedures. Their examination of the failures resulted in Report No. FHWA-RD99-134, Performance of Coarse-Graded Mixes at WesTrack—Premature Rutting, which is available from FHWA or on the Internet at the Turner-Fairbank Highway Research Center

homepage at www.tfhrc.gov.

During the team's investigation, its members concluded that the asphalt paving community needed a good guide on the design of Superpave mixtures. Such a guide would supplement existing specifications and supporting literature and would incorporate the experience of engineers across the country, including the WesTrack designers, in the initial

years of Superpave mixture design and placement. It would be a useful companion to the

National Asphalt Pavement Association's Superpave Construction Guidelines. This publication, Superpave Mixture Design Guide, was prepared by the forensic team. Its contents are

the views of the team and do not necessarily reflect the views of the U.S. Department of

Transportation.

Note that this version of the guide is not expected to be the final word on Superpave mixture design. Both current research studies and additional field experience are likely to

yield refinements in the future.

T. Paul Teng, P.E.

Director, Office of Infrastructure

Research and Development
WesTrack Forensic Team Consensus Report

February 2001

Washington, DC

SUPERPAVE MIXTURE
SUPERPAVE MIXTURE DESIGN GUIDE 1

Introduction

Superpave design methods and tools are

being implemented by many State agencies

to replace the Marshall and Hveem design

methods. In 1999, 2,515 projects, specifying

some 73 million metric tons of Superpave,

were let.[1] The majority of the projects in

1999 and in previous years were constructed

with little or no difficulty. On several projects, there were some problems during this

initial implementation. For the most part, the

causes of the problems have been identified

and have been solved. In 2000, estimates

were that more than 3,900 projects, specifying some 134 million metric tons of

Superpave, would be let; this would represent 62 percent of the total hot-mix asphalt

(HMA) tonnage expected to be contracted for

by State agencies during 2000 in the United

States.

[1] Superpave has become the mixture

design method of choice by most State transportation departments across the country.

This document, intended as a companion to

the National Asphalt Pavement Association's

(NAPA) Superpave Construction Guidelines,

[2]

is a guide for the HMA designer to maximize

the benefits of Superpave while avoiding

potential problems. The Superpave design

process is part of a total pavement design

system. Superpave is a system of components that work together to provide a

SUperior PERforming asphalt PAVEment. As

traffic levels and loading conditions increase

above 1 million 80-kN (18,000-lb) equivalent

single-axle loads (ESALs) during a pavement's design life, some design areas are not

adequately addressed by the current

Superpave specifications.

This guide discusses several issues that

should be considered during the mixture

design process to maximize the benefits of

this method. The Superpave design process

is documented in publications from the

Federal Highway Administration (FHWA),

the Strategic Highway Research Program

(SHRP), the Asphalt Institute (AI), and the

American Association of State Highway and

Transportation Officials (AASHTO).[3-8] Those

publications should be used for detailed

design information. This guide is intended to

serve as a bridge between existing knowledge and additional tools being developed to

measure and predict Superpave mixture performance under traffic.

A Superpave mix design includes several

processes and decision points. First, design

compaction levels are established and materials are selected and characterized. Then,

mixture samples are prepared and laboratory test results are compared to design criteria. However, the existing Superpave design

system does not properly address performance prediction testing on mixture samples

or decision-making during the design

process. This guide will address both of

these areas. 

Superpave Mixture Design

Compaction Level Determination

Prior to 1999, the design ESALs shown in the

Gyratory Compaction Criteria table of PP-28

(in AASHTO Provisional Standards[8]) did not

clearly indicate that they represent the pavement's cumulative ESALs for a 20-year

design life, rather than the cumulative

ESALs for a shorter or longer design life. The

WesTrack Forensics Team and the Lead

States Team both recently reminded users

that, regardless of the actual design life of

the pavement, the user should determine

the expected ESALs for 20 years and select

the design level for that traffic and loading.

For example, a project with a 5-year intended life may have a 5-year cumulative ESAL

count of 2.9 million. This corresponds to a

This document is a guide for the HMA designer

to maximize the benefits of Superpave while

avoiding potential problems. 
2 SUPERPAVE MIXTURE DESIGN GUIDE

20-year cumulative ESAL count of 11.6 million (neglecting traffic growth compounding), and this latter ESAL count should be

used in the design. The 1999 version of PP28 includes a footnote to the Gyratory

Compaction Criteria table with the appropriate guidance.

[8]

Experience has shown that rutting damage

often occurs in the first few years of a pavement's life; therefore, the design should be

based on the rate of loading. To properly

account for this in the mix design, the mix

designer should always use 20-year design

ESALs, essentially converting total loads to a

rate of loading. Estimating ESALs over a 20-

year life, instead of the actual design life,

may affect the mixture design compaction

level, the performance-graded (PG) binder

selection, and the aggregate consensus properties specified for the project. Compaction

criteria, aggregate properties, and volumetric properties are all more stringent at higher ESAL levels.

Superpave Performance-Graded

Binder Selection

The Superpave Performance-Graded Binder

Specification (AASHTO MP-1) is based on

providing a binder that is resistant to rutting,

fatigue cracking, and low-temperature cracking at specific pavement temperatures. The

binder temperature ranges in the specification are based on the high and low temperatures at which a binder reaches critical values of distress-predicting properties.

Reliability factors included in the design

method account for normal pavement temperature variations and allow the designer to

make a rational decision regarding the range

of temperature extremes for which to design.

Binder grade is selected based on design

high and low pavement temperatures

expected at the construction site and on

desired reliability.

The most common method of selecting a

binder grade is to determine the design air

temperature range for the specific project and

then to establish the corresponding design

pavement temperatures. Before selecting the

grade to be used, the designer must also consider traffic volume and traffic speed.

The owner should consider factors such as

cost and traffic levels in establishing reliability, and hence, the final binder grade selection, for a specific project. For example, if a

PG 64 binder provides 94 percent reliability

for high temperatures, it may not be costeffective to specify a PG 70 binder to obtain

98 percent reliability. However, if a PG 64

binder only provides 52 percent reliability, it

would probably be reasonable to specify a

PG 70 binder to obtain 98 percent reliability.

With respect to traffic volume, when the

design traffic exceeds 10 million ESALs,

Superpave suggests that an increase in the

high-temperature binder grade be considered. When design traffic is more than 30

million ESALs, Superpave requires a onegrade increase in the high-temperature

binder grade. With respect to traffic speed,

Superpave recommends increasing the hightemperature binder grade by one grade for

slow transient traffic (20 to 70 km/h) and by

two grades if standing traffic conditions

(<20 km/h) exist. The binder specifier

should increase the high-temperature grade

for traffic volume or traffic speed, but not for

both. If the system is used correctly, a pavement with high design ESALs with stopped

traffic conditions will require an asphalt

binder that is two high-temperature grades

higher than that required by the pavement

temperature alone.

It should be realized that when the hightemperature grade is increased by one grade,

the stiffness of the binder will approximately double. In other words, a PG 70 binder will

be twice as stiff as a PG 64 binder at a temperature of 64°C. Furthermore, a PG 76

binder will be four times stiffer than a PG 64

binder at a temperature of 64°C. Traffic

speed will also have an effect on binder stiffness in the pavement. At 50 km/h, a binder
SUPERPAVE MIXTURE DESIGN GUIDE 3

will have a lower apparent stiffness than it

does when carrying traffic at 100 km/h. In

other words, a mixture containing PG 70

binder in a pavement with traffic moving at

50 km/h will have roughly the same stiffness as a mixture containing PG 64 binder in

a pavement with traffic moving at 100 km/h;

thus, the increased high-temperature grade

of the binder effectively offsets the effect of

slower traffic speeds.

Consideration should be given to the impact

that increasing the binder grade will have on

the construction process. Depending on the

grade, such an increase could require mixing

and compaction temperatures beyond reasonable construction temperatures.

Only strong aggregate skeletons can experience significant performance increases with

increased asphalt binder stiffness. The

stiffer binder locks the aggregate particles in

place to prevent rutting. The binder cannot

carry the load alone and cannot overcome a

poor aggregate skeleton by itself.

The final step before selecting the binder

grade to be specified is to compare the grade

being considered with grades historically

used in the area. If the binder seems unreasonably soft for preventing rutting based on

past history, or unreasonably stiff for construction purposes, the selected grade

should be reconsidered.

Superpave Aggregate Selection

Aggregates are the largest component of

HMA, making up 80 to 85 percent of the

mixture by volume and roughly 95 percent

of the mixture by weight. Aggregate characteristics and quality are major factors in the

performance of HMA. As part of its focus on

binder and mixture properties, in the early

1990's SHRP convened an expert panel to

determine which aggregate properties were

most important for pavement performance.

The properties selected included coarse

aggregate angularity, fine aggregate angularity, flat and elongated particles, clay content, and gradation. Aggregate source properties, such as soundness, toughness, and

deleterious materials, were also found to be

important. However, the criteria applied to

the source properties were found to reflect

regional differences in aggregate quality,

and were usually based on aggregate availability. The panel determined that the

source properties were best left for each

State or local agency to establish. The following discussion addresses various aggregate properties (consensus and source) and

their effect on the Superpave design process.

Coarse Aggregate Angularity Mixtures

with crushed coarse aggregate with sharp,

angular shapes will usually have the greatest

shear resistance and, hence, the highest

resistance to rutting. These materials create

HMA mixtures with the highest voids in the

mineral aggregate (VMA). Coarse aggregate

angularity is defined as the percentage by

weight of the aggregate with one or more

fractured faces according to American

Society for Testing and Materials (ASTM)

D5821. Superpave requires increased percentages of crushed faces as the design ESAL

AGGREGATE PROPERTIES

CONSENSUS PROPERTIES

(required)

• coarse aggregate angularity (CAA)

• fine aggregate angularity (FAA)

• flat and elongated particles

• clay content

SOURCE PROPERTIES

(agency option)

• toughness

• soundness

• deleterious materials
4 SUPERPAVE MIXTURE DESIGN GUIDE

level increases. VMA increases somewhat as

coarse aggregate angularity increases.

Uncompacted Void Content of Fine

Aggregate (Fine Aggregate Angularity)

Similar to coarse aggregate, crushed angular

fine aggregate will usually have the greatest

shear resistance. The use of crushed angular

fine aggregate typically increases the mixture VMA. Fine aggregate angularity is established by AASHTO T304, Method A, which

measures the percentage of air voids present

in loosely compacted aggregate that passes

the 2.36-mm sieve. More fractured faces

generally result in higher uncompacted void

contents in this test. Superpave specifies

uncompacted void contents of at least 45

percent on high-volume roads (>3 x 106

ESALs). Crushed manufactured fine aggregates generally have uncompacted void contents of at least 44.5 percent, while rounded

natural sands typically are less than that

value. When a fine aggregate known to be

angular has test results lower than expected,

the aggregate's bulk specific gravity should

be verified since the test result is sensitive to

this property; a significant change in the

bulk specific gravity should trigger a

redesign of the mixture.

Particle shape can also influence the uncompacted void content. Some very cubical manufactured fine aggregates, especially some

limestones, have had less than 45 percent

(but more than 40 percent) uncompacted

void contents, but still have provided good

performance in pavements. If the performance has been satisfactory, the cubical manufactured fine aggregate may be used (with

caution).

The fine aggregate's uncompacted void content significantly influences the VMA. The

use of cubical angular fine aggregate is recommended to increase the VMA. Care

should be taken when using aggregate with

uncompacted void contents higher than 47

percent; use of these aggregates may result

in mixtures with excess VMA, which leads,

in turn, to a very high binder content. 

Flat and Elongated Particles The percentage of flat and elongated particles (not

flat or elongated) in coarse aggregate is

another important aggregate parameter. Flat

and elongated particles can break during the

construction process, changing the mixture

gradation and the overall mixture properties.

Soft aggregate has a greater tendency to

break than hard aggregate. Flat, slivered

aggregate particles also have a tendency to

lie flat in the pavement, creating slippage

planes and reducing aggregate interlock. A

small percentage of flat and elongated particles in the mixture may increase the VMA in

the laboratory-designed mix. A further

increase may, however, decrease the VMA in

the plant-produced mixture because of

aggregate breakage during mixing.

The critical measurement for a flat and elongated particle is the ratio of its maximum

and minimum dimensions. Current

Superpave standards allow no more than 10

percent of the coarse aggregate particles to

be flat and elongated (i.e., a ratio greater

than 5:1). Testing is performed according to

ASTM D4791, "Flat Particles, Elongated

Particles, or Flat and Elongated Particles in

Coarse Aggregate." Superpave establishes

that testing be done on material retained on

the 4.75-mm sieve, instead of on the 9.5-mm

sieve as specified in the ASTM method.

CONTRASTING STONE SKELETONS

Angular Aggregate

Rounded Aggregate
SUPERPAVE MIXTURE DESIGN GUIDE 5

Testing aggregate particles passing the 9.5-

mm sieve and retained on the 4.75-mm sieve

will be more difficult and results may be

more variable.

Sand Equivalent Sand equivalent, as

measured by AASHTO T176, "Plastic Fines

in Graded Aggregates and Soils by Use of the

Sand Equivalent Test," identifies the presence of clay in the fine aggregate. Clay can

make the mixture moisture sensitive and/or

combine with moisture to cause the mixture

to act "tender" (i.e., to lose density with continued compaction in the field). Clay content must be controlled by satisfying the

minimum sand equivalents specified in the

Superpave standards.

Aggregate Toughness Typically, mixtures

containing very hard aggregate (i.e., a Mohs

hardness of 7 or greater) do not have a problem meeting VMA criteria. A very hard

aggregate, such as basalt, does not easily

crush or degrade during laboratory compaction or during mix production in an HMA

plant. These aggregates can produce mixtures that have an adequate VMA.

Soft aggregates, such as some types of limestone having a Mohs hardness of about 5, are

often abraded during the gyratory compaction process; this can make it difficult to

meet VMA criteria during the design phase.

During production, the aggregates are often

abraded in the hot-mix plant to an even

greater degree than in the laboratory design

using a gyratory compactor. When plantproduced material is compacted in a gyratory compactor, the aggregate is abraded further and even more fines are generated in

the mixture; this further reduces the VMA.

Mixtures designed with soft aggregates often

have a problem meeting VMA criteria in the

design stage and, particularly, during production. It is extremely important that the

plant-produced mixture satisfy the minimum VMA requirement.

Mixtures designed with a blend of hard and

soft aggregate could have difficulty meeting

VMA specifications. The addition of hard,

coarse, or fine aggregate to these types of

aggregate blends will usually increase the

VMA. 

Superpave Mixture Design

Considerations

Superpave mixture design criteria include

air voids, VMA, and voids filled with asphalt

(VFA). Meeting the VMA minimum criterion

is usually difficult to achieve during mix

design and typically even more difficult to

achieve in the plant-produced material. This

document will only discuss VMA. 

Voids in the Mineral Aggregate In many

cases, achieving minimum VMA requirements during the design phase can be difficult. Many factors affect VMA. The most critical of these are aggregate characteristics

such as gradation, surface texture, and

shape. If the design VMA is close to (i.e., no

more than 0.6 percent above) the minimum,

aggregate properties may change during production and cause the VMA to drop below

the minimum during mixture production.

Differences between as-designed and plantproduced properties and other field problems are documented in the NAPA publication, Field Management of Hot-Mix Asphalt.[9]

As noted above, VMA is affected by both the

aggregate gradation (relationship to the

aggregate maximum density line) and the

aggregate's characteristics and properties.

For all designs, VMA should be plotted as a

function of binder content and the resulting

graph should be evaluated to check the VMA.

Typically, VMA will decrease with increasing

binder content to some minimum, then

increase as binder content continues to

increase. The design binder content, selected

at 4 percent air voids, should be near the

minimum of the plotted curve or preferably

on the lean binder content side of the curve.

If the VMA at the design binder content is on

the rich side of the VMA curve, adjustments

to the gradation should be considered; these
6 SUPERPAVE MIXTURE DESIGN GUIDE

are discussed later in this section. In

Superpave mixtures, however, VMA is sometimes insensitive to binder content and

shows little change. If the VMA at the design

binder content is close to the minimum

allowable VMA value and the curve is relatively flat, the mixture should be redesigned.

There are two competing demands during

the mix design process: sufficient inter-particle space must be available for a minimum

amount of binder, but, at the same time, the

aggregate must have a sufficiently strong

skeleton to carry the traffic loads. Superpave

mixture design specifications require that

adequate VMA be obtained without weakening the aggregate skeleton. 

Having representative aggregate bulk specific gravity values is necessary in order to

accurately calculate a mixture's VMA during

design and production. For this reason,

aggregate bulk specific gravity should be

determined at a frequency appropriate for

the variability of the source. 

Mixtures with high VMA need to be

reviewed for possible performance problems. The WesTrack Forensic Team recommended that the VMA of coarse-graded

Superpave mixtures be no more than 2.0

percent above the minimum required

value.

[10] Furthermore, the Team recommended running a draindown test (AASHTO

T305-97) on these mixtures if the VMA is 1.5

percent or more above the minimum value.

If a gradation yields a mixture with too high

of a VMA and, consequently, too high of a

binder content, the mixture design should

be repeated with a new gradation with lower

VMA.

• Gradation Effect Problem mixes typically will have a low VMA and may not be

responsive to changes in gradation (when

aggregate sources are not changed). Usually,

however, changing the gradation of a mixture

will influence the amount of void space in the

aggregate skeleton. The effect of gradation is

separate from the shape and surface texture

effects if all particles have the same shape

and texture. If the stockpiles in the blend are

of dissimilar materials, changing the stockpile

percentages will change the gradation, but it

will also influence the shape and texture of

the aggregate blend. Thus, VMA will change

not only because of gradation changes, but

also because of shape and texture changes. 

Research papers published by Nijboer in the

1940's, Goode and Lufsey in the 1960's, and

the Asphalt Institute in the 1980's provide a

basis for the 0.45 power chart. Nijboer investigated aggregate gradations plotted as the

log percent passing versus log particle size.

He showed a maximum packing density for

both gravel and crushed aggregates when

the slope was 0.45. Goode and Lufsey confirmed Nijboer's results on gravel aggregates.

Work by the Asphalt Institute evaluated the

maximum density line on a 0.45 power chart

for both gravel and crushed limestone mixtures and reconfirmed the previous results. 

Moving the gradation away from the 0.45

power maximum density line generally

increases the VMA for a fine gradation, i.e.,

when the gradation is above the maximum

density line. For a coarse gradation, VMA

may decrease slightly and then increase as

the gradation moves away from the maximum density line. For hard aggregates, the

Job Mix Formula (JMF) should be parallel to

the maximum density line until after passing the restricted zone, i.e., for aggregate

retained on the 4.75-mm sieve (for 25-mm or

larger mixes) or on the 2.36-mm sieve (for

19-mm or smaller mixes). Then the gradation line should be taken to the desired -

0.075-mm content. Therefore, with fine gradations, the JMF should be above and parallel to the maximum density line. For a

coarse gradation, it should be below and parallel to the maximum density line.

Many coarse Superpave mixes have an "S"

shape, starting on the fine side of the maximum density line and finishing the S on the

coarse side. If the same particle shape and

texture are used (same aggregate source, dif-
ferent sizes), the highest VMA that can be

achieved is that for the gradation that is the

farthest from the maximum density line. To

minimize the chance of mix tenderness during construction, the JMF should be 3 to 4

percent above the lower control point for the

2.36-mm sieve. 

If the aggregate is obtained from a gravel

source, normally the fine aggregate must be

removed before the coarse aggregate enters

the crusher. The crushed material should be

divided into three or more stockpiles that

can then be blended into a combination that

meets the minimum VMA requirements. All

of the aggregate processed may not be

usable; it may be necessary to waste some of

the material in order to meet the requirements of the mixture design.

If the aggregate is obtained from a quarried

source, the crushed material should not be

placed into a single stockpile, but should be

divided into at least three separate size

ranges, depending on the nominal maximum size of the aggregate required in the

mix. The use of multiple stockpiles allows

more flexibility to change gradation and,

thus, VMA. In addition, it may still be necessary to incorporate another size of aggregate

from the quarry or from a different source.

The VMA of coarse-graded mixes can generally be increased by reducing the amount of

material passing the 4.75- and 2.36-mm

sieves. The reason has to do with packing—

smaller particles fill the spaces between larger ones. By reducing the amount of material

passing the 4.75- and 2.36-mm sieves, intermediate material is removed and more space

is created between the coarse aggregate particles. Hence, the mixture cannot compact

as tightly, i.e., VMA is increased.

In fine-graded mixes, VMA is created by fine

aggregate—the material passing the 2.36-mm

sieve. To increase the VMA in fine-graded

mixes, the percentage of material passing

the 2.36-mm sieve should be increased. Care

should be taken not to create a hump in the

gradation on the 0.6- to 0.3-mm sieves using

an aggregate that has a low fine-aggregate

angularity value.

The dust content (i.e., the amount of material finer than 0.075 mm) in a mixture has a

significant effect on the VMA. Lowering the

dust content will increase the VMA. This

effect may not be entirely due to the gradation, but may also be due to characteristics

of the dust, such as shape and size. In general, reducing dust content to the extent that

the dust-to-binder ratio will allow will maximize the amount of VMA that can be

obtained for the specific gradation. 

If the dust content is coming from the addition of mineral filler, adjusting the dust conSUPERPAVE MIXTURE DESIGN GUIDE 7

SUPERPAVE AGGREGATE GRADATION

100

 0

.075 .3 2.36 12.5 19.0

Sieve Size (mm) Raised to 0.45 Power

Percent Passing

Design Aggregate Structure

FHWA 0.45 POWER GRADING CHART

100

80

60

40

20

0

.075 .3 .6 1.18 2.36 4.75 9.5 12.5 19.0

Sieve Size (mm) Raised to 0.45 Power

Percent Passing

Maximum

Size

Maximum Density Line
8 SUPERPAVE MIXTURE DESIGN GUIDE

tent can be simply a matter of reducing the

amount of filler being used. If the dust is predominantly from one of the aggregate stockpiles, e.g., screenings, reducing the percentage of that stockpile used in the blend should

be tried. If the screenings are the only manufactured fines coming into the mix, using

washed screenings or blending with a

washed screening may be necessary.

If baghouse fines will be introduced back into

the mix during production, some of the same

fines should be added during the mix design.

During the design, adding half of the quantity of baghouse fines expected to be added

during production is an appropriate procedure. These fines should be obtained from

the actual plant that will be used for production; otherwise, mineral filler or an alternate

source of baghouse fines could be used.

These fines will reduce the VMA of the mixture. If the aggregate in the mix contains friable particles, a greater quantity of dust

should be used in the laboratory mix design

since the friable particles tend to create more

dust during mix production. A mix design

that includes baghouse fines will be more

representative of the mix as produced. The

addition of baghouse fines during the mix

design will better simulate the reduction in

VMA that typically occurs during production.

• Surface Texture Effect The way in which

aggregate particles pack together for any

given gradation is influenced by the surface

texture of the particles. Rougher texture generates more friction between aggregate particles and the mixture therefore resists compaction. Hence, for a given number of

design gyrations, the mixture will not compact as much and the VMA will be higher.

Smooth texture, by contrast, does not generate as much friction between aggregate particles. For a given number of design gyrations, the mixture containing smoother particles will compact more easily and the VMA

will be lower.

Typically, crushed faces have more texture

than uncrushed faces. In the case of gravel

aggregate, the uncrushed portion of the

particles tends to have a smooth texture.

The greater the percentage of each individual particle surface area that is fractured,

the more surface texture that will be present. Usually, the more a gravel is crushed,

the more surface texture it will have.

Particles with two crushed faces tend to

have a greater percentage of surface area

with rough texture than will particles with

only one crushed face. However, crushing

will not always increase texture, because

some aggregates fracture with very smooth

faces.

If manufactured sand and natural sand are

being used together in a mix design, the percentage of manufactured sand can be

increased to increase surface texture.

Substituting 20 percent washed manufactured sand (with good "bite") for an equivalent amount of natural sand can increase the

VMA substantially. (What is good bite?

Squeeze a handful of angular manufactured

sand, then a handful of rounded natural

sand, and feel the difference in the way the

particles bite into one another.) If the manufactured sand contains more dust than the

natural sand, gains in VMA from the surface

texture may be decreased by the increase in

dust content. For example, if the natural

sand is relatively clean and the manufactured sand has a high minus 0.075-mm dust

content, the benefit of increased surface texture may be partially or completely offset by

the increased dust content.

• Shape Effect For any given gradation,

the density to which aggregate particles will

pack is influenced by the shape of the particles. Angular particles (i.e., those with

sharp, defined edges) tend to produce mixtures with a higher VMA than mixtures containing rounded particles. Cubical particles

that retain a sharp, angular edge tend to create a higher VMA than particles with rounded edges.

The effect of flat and elongated particles

depends on the laboratory compaction
SUPERPAVE MIXTURE DESIGN GUIDE 9

method. Under Marshall compaction, the

particles were not as free to rotate as they

are in a Superpave gyratory compactor. In

fact, flat particles tend to bridge in a

Marshall mold and give a high VMA.

Therefore, flat and elongated particles tend

to increase asphalt content in a Marshall mix

design. In the Superpave gyratory compactor, where the particles are kneaded into

a more stable condition, flat and elongated

particles tend to lie horizontally; this

reduces the VMA and the optimum binder

content. 

During construction, rollers tend to orient

flat and elongated particles horizontally. A

Marshall mix design containing excess flat

and elongated particles could compact very

easily or be compacted to a lower air void

content than desired during the roadway

compaction process. A Superpave mix

design will have a more appropriate binder

content since gyratory compaction better

simulates compaction during construction

than does Marshall compaction. Therefore,

the influence of particle shape must be considered when comparing the VMA of

Marshall specimens to that of Superpave

specimens. 

If a mix design has a low VMA, the amount of

flat and elongated particles must be determined. Superpave specifications limit the

percentage of particles with a maximum-tominimum dimension ratio of greater than 5.

If flat and elongated particles are contributing

to a low VMA in a mixture, the percentage of

particles that exceed a 3:1 ratio should be

determined. If the percentage of particles

exceeding the 3:1 ratio is high (i.e., greater

than 40 percent), material from a coarse

aggregate stockpile that has a lower percentage should be added. It may be possible to

change one of the coarse aggregate stockpiles

for another that contains more cubical and

angular aggregate particles. Adding an intermediate-size coarse aggregate with cubical

and angular shapes will prevent the larger

particles from lying flat. Thus, VMA will

increase. 

Crushing operations influence the amount

of flat and elongated particles produced. If

excess flat and elongated particles are being

produced, the crushing operation should be

evaluated. In some instances, the amount of

flat and elongated particles produced can be

reduced by changing the aggregate feed rate,

or by changing the opening of the cone or

jaw crushers. In some cases, it might be necessary to modify the crushing operation by

adding to or changing the equipment used.

Vertical-shaft impact crushers, for example,

tend to produce more cubical particles than

do some cone crushers (especially older

models). 

In summary, VMA depends on the gradation, surface texture, and particle shape of

the aggregate. In designing a mix, all of these

characteristics must be considered. When

there is difficulty in meeting the minimum

VMA requirements, some or all of the above

characteristics must be adjusted. It should be

remembered that the VMA of a plant-produced mixture is typically lower than the

VMA of the laboratory trial mix formula.

Allowances should be made for the reduction in VMA that will occur between the laboratory-designed and the plant-produced

mixtures.

[9]

Mixing and Compaction Temperatures 

For unmodified binders, the mixing and

compaction temperatures used during the

design process should be established with a

rotational viscometer. If the binder is modified, the binder supplier must provide recommended mixing and compaction temperatures. If the binder content determined in

the mix design process seems unrealistic,

the supplier should be consulted to determine whether the mixing and compaction

temperatures being used are still appropriate

for the material being delivered. The compaction temperature used during design

should also be used in plant production quality control and quality assurance testing. 

The laboratory mixing and compaction temperatures may not be appropriate for use in
1 0 SUPERPAVE MIXTURE DESIGN GUIDE

actual plant production and laydown.

Environmental conditions at the time of construction and other factors, such as haul

length and lift thickness, need to be considered in establishing the actual mixing temperature used at the plant. These temperatures should not be any greater than those

necessary to ensure complete mixing of the

HMA while minimizing premature aging of

the binder and providing for adequate compaction in the field.

If the quality control and/or quality assurance sample is taken behind the paver and

the sample requires reheating before compaction, a comparison should be made

between the properties of the reheated samples and samples that are compacted before

cooling. Detailed procedures for quality

assurance sampling and testing should be

established before construction begins.

Dust-to-Binder Ratio Superpave calculates the dust-to-binder ratio using the effective binder content. Using the effective

binder content rather than the total binder

content will normally result in a higher dustto-binder ratio because of binder absorption

into the aggregate. To account for absorption, the limit for the dust-to-binder ratio

should be increased. In the original

Superpave specification, the dust-to-binder

ratio was 0.6 to 1.2 by weight. FHWA's

Asphalt Mixture Expert Task Group and the

AASHTO Lead States recommended changing the limit to 0.6 to 1.6; AASHTO subsequently added a note to the mix design specification suggesting that agencies consider

changing the limit for coarse-graded mixes to

0.8 to 1.6.

During design, mixtures that are above the

maximum density line at the 2.36-mm (for

19-mm or smaller mixtures) or 4.75-mm (for

25-mm or larger mixtures) critical sieve

should have a dust-to-binder ratio of no more

than 1.4. For mixtures that pass below the

maximum density line at the critical sieve

size, the ratio should not exceed 1.6.

Characteristics of the fines will control the

amount that can be added to a mixture.

Changing the fines source or production

process will change how the fines affect the

mixture characteristics. For fine-graded mixtures (above the maximum density line at the

critical sieve), a ratio of about 1.0 has provided satisfactory performance. For coarse-graded mixtures (below the maximum density

line at the 4.75-mm sieve), as the VMA

increases the dust-to-binder ratio should

increase toward 1.6. If the mixture VMA is

high (more than 2.0 percent above the minimum), the ratio should approach 1.6. 

High dust-to-binder ratios will typically stiffen the mixture and improve permanent

deformation resistance. However, if the

VMA is more than 1.5 percent above the

minimum, it is preferable to adjust the

aggregate properties to reduce the VMA

instead of increasing the dust content.

Performance Indicator Tests No test is

currently available that is satisfactory, by

itself, as a performance predictor for mixtures generated by Superpave volumetric

procedures. Appendix A contains a discussion of various tests that may be used to indicate the relative performance of different

mixtures. The designer should have experience with one of the tests before assuming

that the test results will actually predict field

performance. Criteria developed elsewhere

may not apply to a particular combination of

materials, environmental conditions, pavement structure, and traffic.

Completing the Design After the Superpave design is completed, the designer needs

to ask two final questions:

• Is this HMA design reasonable

and logical?

• Is the binder content reasonable

for the type of aggregate, the nominal maximum aggregate size, the

VMA, and the gradation used in

the mixture? 
SUPERPAVE MIXTURE DESIGN GUIDE 1 1

If the answer to either question is "No," the

design should be re-evaluated and/or

redone. Once the answers to both questions

are "Yes" and mixing plant operation has

begun, the mixture volumetrics of the plantproduced mixture must be checked and

those volumetrics must meet the minimums

required in the design. In addition, performance tests should be repeated on the plantproduced mixture if these tests were performed during the original design phase.

During plant verification, enough mixture

should be produced to ensure that the plant

is operating uniformly. The designer should

be prepared to make mixture adjustments to

account for changes caused by plant production. If changes are made, the mixture

should be re-verified. A key to good mixture

performance is to verify the HMA plant-produced mixture properties. 

DESIGN CHECK LIST

Use a performance-graded (PG) binder and an N-design value

appropriate for the weather, traffic level, and traffic speed for the

project under consideration. Heavy, slow traffic will require a

stiffer PG binder than may have been used in the past.

Check that a complete mix design has been done in accordance

with specifications and that it meets all of the aggregate consensus property requirements and specified volumetric criteria.

Check that the submitted design contains a reasonable binder

content for the materials used and the design level specified.

Generally, more dust (material passing the 0.075-mm sieve) is

needed for coarse-graded mixtures. The character of the dust

will control how much can be added to the mixture. Laboratory

samples should contain the expected plant-produced amount of

material finer than 0.075 mm.

In coarse-graded mixtures, if the VMA is more than 1.5 percent

above the specified minimum, check for binder draindown.

Excessive draindown is an indication that the binder content is

too high for the binder grade, aggregate type, and/or gradation

being used.

Evaluate the mixture with a performance indicator test that has

worked satisfactorily based on local experience (until a universally acceptable test is included in Superpave). Does the mixture

perform as expected?

Verify the properties of the plant-produced mixture to check volumetric properties. Repeat the performance test on the plantproduced mixture if the test was run during the mixture design.

➊

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❸

❹

➎

❻

❼
1 2 SUPERPAVE MIXTURE DESIGN GUIDE

Introduction

Many different types of performance tests

are currently available for assessing a mixture's ability to resist permanent deformation (commonly referred to as "rutting").

These tests, which include Marshall

flow/stability, Hveem stability, the gyratory

testing machine, wheel-track testers, the

Superpave Shear Test device, and triaxial

testers, generally attempt to quantify mixture strength and/or stiffness. The individual tests have shown varying levels of success in capturing a mixture's ability to resist

rutting. Therefore, the designer must know

the limitations of each test and how to incorporate test results into mixture design selection. This appendix describes each test and

examines how suitable each is for assisting

engineers in designing rut-resistant mixtures. At the same time, mixture designers

are reminded that a mixture that is resistant

to rutting will not necessarily resist thermal

or fatigue cracking, moisture damage, or

durability problems such as raveling.

Marshall

The Marshall mixture design process seeks

to optimize a mixture's performance with

regard to fatigue cracking, rutting, and durability by determining the optimum binder

content for the gradation selected. Once the

optimum binder content is selected, the

mixture must meet minimum stability values and maximum flow values. A number of

European countries have modified the specification criteria to use a stability quotient

(stability/flow) criterion in lieu of the minimum stability and maximum flow values.

Many mixtures have stability values that are

two or three times the minimum, but also

exceed the maximum flow value. The

European approach appears more logical

because it normalizes the stability/flow values. Marshall flow does provide an indication when a mixture is over-asphalted—high

flow values indicate excess binder content.

The Marshall test conditions may significantly affect the test's value in predicting

rutting performance. First among these is

the ratio of the test specimen's size to the

nominal maximum aggregate size. A 100-

mm- (4-in.-) diameter specimen that

includes a large nominal maximum aggregate size (37.5 mm) or a more open-graded

mixture (one containing little intermediatesize material) does not provide good-quality

test data. The effects of the specimen edges

are amplified and the assumption that the

Marshall breaking head is applying a uniform load across the specimen is no longer

valid. The effective load on the specimen

(load divided by the contact area) is higher

for larger nominal maximum aggregate size

mixtures. Another shortcoming of the procedure is the 60°C (140°F) temperature at

which the Marshall test is conducted. The

mixture may encounter temperatures 5 to

10°C (9 to 18°F) higher in place in some

parts of the country.

Hveem

The Hveem Stabilometer is a mixture design

tool used primarily in the western United

States. The concept behind the Hveem

Stabilometer is an empirical measurement

of the internal friction within a mixture,

PERFORMANCE TESTS
SUPERPAVE MIXTURE DESIGN GUIDE 1 3

resulting from application of a vertical axial

load. Like the Marshall Method, Hveem testing is conducted on 100-mm- (4-in.-) diameter specimens at 60°C (140°F). As noted

above, this temperature does not always represent the highest temperature a mixture

will experience in the field. Furthermore,

stabilometer values are measurements of

internal friction, which is more a reflection

of the properties of the aggregate than of the

binder. As with Marshall flow values, Hveem

stability does provide an indication when a

mixture is over-asphalted—low stability values indicate excess binder content.

Gyratory Testing Machine

The gyratory testing machine (GTM), developed by the U.S. Army Corps of Engineers,

measures the increase in the angle of gyration during compaction. The gyratory shear

index, a measure of a mixture's stability, is

the initial angle of gyration divided by the

maximum angle. Shear indices above 1.1

usually indicate poor mixture stability, while

values nearer to 1.0 are more stable.

Wheel-Track Testers

Currently, three wheel-track testers are

available commercially—the French LCPC

[Laboratoire Central des Ponts et Chaussees]

Rutting Tester, the Georgia Loaded-Wheel

Tester (marketed as the Asphalt Pavement

Analyzer), and the Hamburg Wheel-Tracking

Device. Conceptually, the three devices are

the same (a rolling load is applied to laboratory-scale specimens), but they differ significantly in design, load configuration, and

test conditions. To complicate the comparison, each device has a different recommended pass/fail criterion for mixtures. The

machine design for each of the devices significantly affects how well its results can be

correlated with field performance.

[11]

The French LCPC Rutting Tester uses a 90-

mm-wide pneumatic tire to test specimens

that are 180-mm wide. This specimen width

and the closeness of the confining rigid specimen holder to the location of repeated loading distorts the development of the mixture's

shear plane, especially for mixtures containing larger aggregate. As a result, poor mixtures tend to perform better than expected

in the French device, and discriminating

between good- and poor-performing mixtures becomes difficult. The device should

not be used to test mixtures that have aggregate larger than 20-mm.

The Georgia Loaded-Wheel Tester (GLWT)

runs a concave steel wheel over a pressurized 29-mm-wide hose to apply loads on

specimens. Testing can be conducted on dry

specimens or underwater. For mixtures containing a larger size of aggregate, aggregate

bridging becomes a problem. The applied

footprint from the pressurized hose is much

narrower than the footprint of a vehicle tire

that the mixture will be subjected to under

field conditions. As a result, the GLWT test

criteria may allow for some poor mixtures to

be placed.

The Hamburg Wheel-Tracking Device

(HWTD) applies a sinusoidal load on specimens using a steel wheel underwater at an

elevated temperature. The HWTD measures

a mixture's ability to resist rutting and strip-
1 4 SUPERPAVE MIXTURE DESIGN GUIDE

ping. The probability that these same test

conditions will coincide in the field is unlikely. The use of a steel wheel further increases the severity of the test. Because a steel

wheel does not deform under the test conditions like a pneumatic tire, the effective load

per unit area is much higher than that occurring during actual field loading. A mixture

that survives the HWTD test should be rutresistant in the field; however, mixtures that

do not survive the test may also perform

well in the field. Use of this device in mixture pass/fail situations can result in the

rejection of acceptable mixtures.

FHWA's Asphalt Mixture Expert Task Group

recommends the following cautionary note

for wheel-track testers:

Rut testers, properly calibrated,

have been utilized by some agencies

as effective proof testers. However,

they should not be used to predict

actual pavement performance

because of differences in in-service

temperature and loading conditions.

The devices use empirical evaluation of some measured response to a

loaded wheel as an indicator of performance. Local criteria from one

region are not applicable in another.

As such, each potential user needs

to develop his/her own evaluation

of wheel test results using local conditions.

Superpave Shear Tester

The Superpave Shear Tester (SST) can be

operated in any of six different modes: volumetric, uniaxial strain, repeated shear at

constant stress ratio, repeated shear at constant height, simple shear at constant height,

and frequency sweep at constant height. All

but the repeated shear at constant height

test were included in the original Superpave

performance testing program. The report,

Background of SUPERPAVE Asphalt Mixture

Design and Analysis,

[4] describes the test

modes in detail. Problems have been

encountered in interpreting data from the

repeated shear at constant stress ratio test,

the simple shear at constant height test, and

the frequency sweep at constant height

tests.

[12-14] As a result of these problems, no

attempt was made to link the predicted performance from the laboratory tests to the

field performance.

Romero and Mogawer presented additional

SST results and compared the results of

repeated shear at constant height tests with

those from full-scale accelerated tests.

[15]

They stated that the repeated shear at constant height test mode is able to rank mixtures with different binders, but with high

variability in mixture stiffness. This variability often makes it impossible to place each

mixture into statistically different groups.

SST results have shown significant variability between laboratories for the simple shear

at constant height test mode. Until this variability can be reduced, it will not be possible

to adopt universally acceptable criteria. In

summary, the SST is still being studied to

determine the usefulness of the results from

each of its six test modes; work with the

device has not reached a point where its

results can be used in any standard mode to

predict rutting performance.

Creep Tests

Triaxial testing equipment has been used for

many years in soil mechanics and on

asphalt materials. The creep test and, to a
SUPERPAVE MIXTURE DESIGN GUIDE 1 5

lesser extent, the creep-creep recovery

(CCR) test have been used for HMA under

various triaxial stress states. The creep and

CCR tests are used to estimate rutting

potential. Most commonly, a uniaxial static

test is used in either a confined or an

unconfined mode. The unconfined test does

not simulate field conditions. The applied

pressure cannot exceed 207 kPa (30 psi)

without specimens failing, and the test temperature is kept at 40°C (104°F) in the

unconfined test, well below actual field

loading conditions that often reach 830 kPa

(120 psi) and 60°C (140°F). The confined

creep test can be run at higher pressures

and temperatures, with a confining pressure

of 138 kPa (20 psi). Research has shown that

confined creep testing has a higher correlation to permanent deformation than unconfined testing.[16] A viscoelastic layered pavement performance system that uses creep

and CCR testing to estimate the permanent

deformation in asphalt mixtures subjected

to repeated haversine loading of in-service

loading frequencies already exists.

[17]

However, this CCR testing does not exist as

input directly into constitutive models for

asphalt pavements. Research is underway to

examine the ability of this equipment to

measure "fundamental" material properties

and to include these measurements in constitutive modeling. Currently, the equipment and procedures to help engineers

make rational mixture design decisions are

not available in the context of measuring

engineering properties as input to constitutive models.

One test that is being recommended for performance evaluation of HMA by the

researchers on NCHRP Project 9-19 is the

Static Creep/Flow-Time test. In this test, a

cylindrical sample of bituminous paving

mixture is subjected to a static axial load.

The test can be performed without confinement or with a confining pressure applied to

better simulate in situ stress conditions. The

flow time is defined as the time after initial

load application when shear deformation,

under constant volume, starts. The applied

stress and the resulting permanent and/or

axial strain response of the specimen are

measured and used to calculate the flow

time. Using this test, the selection of the

design binder content and aggregate structure can be fundamentally enhanced by the

evaluation of the mix's resistance to shear

flow (flow time). This fundamental engineering property can be used as a performance criteria indicator for permanent deformation resistance of the asphalt concrete

mixture, or can simply be used to compare

the shear resistance properties of various

bituminous paving mixtures.

Conclusions

Currently, no single test is suitable as a

national standard for predicting rutting. The

development of such a procedure is urgently

needed, but a satisfactory procedure may be

years away. In the meantime, if an agency

has extensive experience with a particular

test over a range of materials typical of its

geographic area, it should consider using the

test to predict rutting performance. Each of

the devices outlined here has difficulty in

predicting the true performance of an

asphalt mixture and should be used with

great caution. 

Currently, no single test is suitable as a

national standard for predicting rutting.
1 6 SUPERPAVE MIXTURE DESIGN GUIDE

1. Superpave 1999-2000 National

Implementation, Report to the

AASHTO Task Force on SHRP

Implementation, May 2000.

2. Superpave Construction

Guidelines, Special Report 180,

National Asphalt Pavement

Association, Lanham,

Maryland, 1997.

3. McGennis, R.B.; Shuler, S.; and

Bahia, H.U. Background of

SUPERPAVE Asphalt Binder Test

Methods, Report No. FHWA-SA94-069, Federal Highway

Administration, Washington,

D.C., 1994.

4. McGennis, R.B.; Anderson,

R.M.; Kennedy, T.W.; and

Solaimanian, M. Background of

SUPERPAVE Asphalt Mixture

Design and Analysis, Report No.

FHWA-SA-95-003, Federal

Highway Administration,

Washington, D.C., 1995.

5. The SUPERPAVE Mix Design

System Manual of Specifications,

Test Methods, and Practices,

Report No. SHRP-A-379,

National Research Council

(Strategic Highway Research

Program), Washington, D.C.,

1994.

6. Superpave Performance-Graded

Asphalt Binder Specification and

Testing, Superpave Series No. 1

(SP-1), The Asphalt Institute,

Lexington, Kentucky, 1995.

7. Superpave Mix Design,

Superpave Series No. 2 (SP-2),

The Asphalt Institute,

Lexington, Kentucky, 1996.

8. AASHTO Provisional Standards,

Interim Edition, American

Association of State Highway

and Transportation Officials,

Washington, D.C., May 1999.

9. Field Management of Hot Mix

Asphalt, Report No. IS-124,

National Asphalt Pavement

Association, Lanham,

Maryland, 1997.

10. WesTrack Forensic Team.

Performance of Coarse-Graded

Mixes at WesTrack—Premature

Rutting, Report No. FHWA-RD99-134, Federal Highway

Administration, Washington,

D.C., June 1998. 
11. Williams, R.C., and Stuart, K.D.

"Evaluation of Laboratory

Accelerated Wheel Test

Devices," Proceedings, The 9th

Road Engineering Association

of Asia and Australasia

Conference, Wellington, New

Zealand, May 1998.

12. Hicks, R.G., and Finn, F.N. Stage

1 Validation of the Relationship

Between Asphalt Properties and

Asphalt-Aggregate Mix

Performance, Report No. SHRPA-398, National Research

Council (Strategic Highway

Research Program), Washington,

D.C., 1994.

13. Zhang, X. "Evaluating

Superpave Performance

Prediction Models Using a

Controlled Laboratory

Experiment," Journal of the

Association of Asphalt Paving

Technologists, Volume 66, 1997.

14. Romero, P., and Mogawer, W.S.

"Evaluation of the Superpave

Shear Tester's Ability to

Differentiate Between Mixtures

With Different Aggregate Size,"

Transportation Research Record

1630, National Research

Council, Washington, D.C.,

1998, pp. 69-76.

15. Romero, P., and Mogawer, W.S.

"Evaluation of the Superpave

Shear Tester Using 19-mm

Mixtures From the Federal

Highway Administration's

Accelerated Load Facility,"

Journal of the Association of

Asphalt Paving Technologists,

Volume 67, 1998, pp. 573-601.

16. Roberts, F.L.; Kandhal, P.S.;

Brown, E.R.; Lee, D.Y.; and

Kennedy, T.W. Hot Mix Asphalt

Materials, Mixture Design, and

Construction, National Asphalt

Pavement Association

Education Foundation, Lanham,

Maryland, 1991.

17. VESYS 3am User's Manual,

Office of Research and

Development, Federal Highway

Administration, McLean,

Virginia, 1996.

SUPERPAVE MIXTURE DESIGN GUIDE 1 7
1 8 SUPERPAVE MIXTURE DESIGN GUIDE

Team Members

Ray Brown, Director

National Center for Asphalt Technology

211 Ramsay Hall

Auburn University, AL 36849

Tel.: (334) 844-6228

Fax: (334) 844-6248

Larry Michael, Regional Engineer

Maryland Department of Transportation

528 East Main Street

Hancock, MD 21750

Tel.: (301) 678-6134

Fax: (301) 678-5190

Erv Dukatz, V.P., Materials & Research

Mathy Construction Company

P.O. Box 563

915 Commercial Court

Onalaska, WI 54650

Tel.: (608) 779-6392

Fax: (608) 781-4694

Gerald Huber

Heritage Research Group

7901 W. Morris Street

Indianapolis, IN 46231

Tel.: (317) 243-0811

Fax: (317) 486-5095

Ron Sines, Director, QC/QA Operations

P.J. Keating Co.

P.O. Box 367

Fitchburg, MA 01420

Tel.: (978) 582-5200

Fax: (978) 582-7130

Jim Scherocman, Consulting Engineer

11205 Brookbridge Drive

Cincinnati, OH 45249

Tel.: (513) 489-3338

Fax: (513) 489-3349

Liaison Members

John D'Angelo, Sr. Pavement Mtls. Engineer 

Office of Pavement Technology (HIPT-10)

Federal Highway Administration

Room 3118, Nassif Building

Washington, D.C. 20590

Tel.: (202) 366-0121

Fax: (202) 366-7909

E-mail: john.d'angelo@fhwa.dot.gov

Chris Williams (formerly Research Highway

Engineer, Federal Highway Administration),

Asst. Prof., Dept. of Civil & Env. Eng.

Michigan Technological University

870 Dow Env. Sci. & Eng. Building

Houghton, MI 49931

Tel.: (906) 487-1630

Fax: (906) 487-2943

Editor

Terry Mitchell, Research Mtls. Engineer

Office of Infrastructure Res. & Dev. (HRDI-11)

Federal Highway Administration

6300 Georgetown Pike

McLean, VA 22101

Tel.: (202) 493-3147

Fax: (202) 493-3161

E-mail: terry.mitchell@fhwa.dot.gov

FHWA-RD-01-052

VISIT US ON THE WEB A T : 

www.tfhrc.gov