An Introduction to SuperPave

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APPENDI X

D

A n In t rodu c t ion to Sup erpav e

by Kamyar C . Mahboub, Ph

.D

., P

.E

Superpave is a product of the Strategic Highway Research Program (SHRP)

. This re

search effort led to a new system for design of hot mix asphalt based upon mechanisti

c

concepts. The Superpave

T M

has been fully implemented by most of the state highwa

y

agencies

. Superpave is an acronym for Superior Performing Asphalt Pavements

. Th

e

Superpave system accounts for materials characteristics in light of climatic and traffi

c

considerations (AI, 2001, 1996, 1997) . Perhaps the most significant component of Su

perpave is its new asphalt binder grading system, which is designed to link with pave

ment performance . The Superpave methodology is believed to be the best available at

this time. However, it is an evolving methodology, and as such there are various asphal t

characterization routines that are under consideration as future additions to th e

Superpave (Witczak, et al

2002)

D 1

SPH LT BINDER GR DING SYSTE

M

The asphalt binder grading system in Superpave is called the performance gradin

g

(PG) system

. This system is a radical departure from the previous viscosity or penetra

-

tion based systems

. All PG binders are characterized based upon fundamental engi-

neering parameters . Additionally, Superpave accounts for the impact of climatic factor s

on binder characteristics at both hot and cold temperature regimes

. This is a major im

-

provement over previous systems of asphalt binder grading

. In addition to climati

c

conditions, traffic and aging control the performance of the asphalt pavement . To sim -

ulate climate conditions, testing is conducted at three pavement temperatures : hot, in-

termediate, and cold pavement temperatures

. These temperatures are derived fro

m

weather data for various geographical locations . The climatic data is further trans-

formed to represent the pavement temperature

. To simulate traffic conditions, an aver-

age rate of loading was assumed for normal highway traffic speeds

. Heavy traffi

c

conditions may be addressed by selecting a binder corresponding to higher tempera-

ture regimes. To simulate binder aging, a new rolling thin-film oven procedure wa

s

682



D .1 Asphalt

Binder Grading System 68

3

C rite ria for The rm al C rite rion for

Criteria for

Criterion for

Cracking

Fatigue Cracking

Rutting

Workability

S (60s) <300MPa G*sins G*/sin6

Viscosity at 2

0

m (60s)>0

.300

<5

.OMPa

unaged>1

.OkPa

rpm < 3

.0Pa-se

c

Failure strain>0 .01

RTFO >2 .2kP

a

Pressure Ag ing Vesse l

Direct Tension

Thin Film O ve n

Residu

e

Intermediate

H ig h

Temperature

Temperatur e

D ynamic Shear

D ynamic Shea r

Rheometer Rheometer

Bending

Beam

Rheometer

-2

0

FIGURE D

A summary of mechanical tests related to asphalt binder PG grading

developed under Superpave, which allows for rapid aging/oxidation of an asphal

t

binder under simulated conditions

The Superpave binder grading tests are based upon engineering properties that

control three major modes of distress in asphalt pavements

: rutting, fatigue cracking

and thermal cracking. The contribution of the asphalt binder to these modes of distres

s

is characterized through a battery of rheological tests which are outlined in Figure D1

The test data are analyzed in light of climatic conditions for determining the asphal

t

binder grade . For example, a PG 64-28 is suitable for an environment where the maxi

mum pavement temperature will not exceed 64°C, and the minimum pavement tem-

perature will not drop below -28°C

.

Figure D

.1 presents a summary of mechanical tests related to asphalt binder PG grad

ing

. The direct tension (DT) test is intended to determine the resistance of asphalt to ther

mal cracking

. Similarly, the Bending Beam Rheometer (BBR) is designed to measure th

e

critical stiffness (S) at which the asphalt becomes brittle and susceptible to thermal crack

ing (m=slope of stiffness courve)

. In order to simulate the most severe case, the thermal

cracking analysis is conducted using the asphalt which has gone through the accelerate d

aging process using the pressurized aging vessel (PAV)

. The Dynamic Shear Rheomete r

(DSR) is the device that is used for fatigue and rutting characterization . The rheometer

protocols are designed to measure elastic and damping properties of the asphalt binde

r

via the complex shear modulus parameter (G*)

. The rutting parameter is G*/sin 8

Brookfiel d

Viscosity

Pavement Tem perature, °C



68 Appendix D ntroduction to Superpav

where S is the phase angle and is related to damping . On the other hand, the G*sin S i

s

used to characterize the fatigue-cracking potential of asphalt

. The fatigue characteriza

tion is conducted on asphalt, which is aged via the PAV process, while the rutting char

acterization is conducted on the asphalt binder that is aged using a Rolling Thin-Fil

m

Oven (RTFO) test

. Superpave binder grading protocols are outlined in AASHT O

MP1 specifications (AASHTO, 1999a)

. Tables D

.1 through D .4 present Superpave Per

formance grade CPG binder specifications

D 2 AGGREGATES IN HM

A

Experience has shown that aggregates play a key role in HMA performance, and thi

s

was realized by SHRP researchers, which led to the refining of existing procedures t

o

fit within the Superpave system

. SHRP researchers produced an aggregate gradatio

n

specification without the benefit of experimentation to support or verify its formula-

tion . Thus, in lieu of experimentally verifiable protocols, a panel of SHRP experts de

-

veloped a set of recommendations for Superpave aggregate specifications (NCHR

P

Project 9-14,1997)

. This led to a number of controversial issues including flat and elon

-

gated aggregates, and the restricted zone

D 2

1 Flat and Elongated Aggregate s

The recommendation for flat and elongated aggregate content was that, for high traffi

c

(greater than 10

6

equivalent single axle loads—ESALs), no more than 10% of th

e

aggregate particles retained on the 4

.75 mm sieve should have a ratio of maximum-

to-minimum dimension greater than 5

:1 (Cominsky

et al

.

1994)

.

Vavrik et al

(1999) recommended performance based testing as a requirement t

o

establish if the use and breakdown of F&E particles had a detrimental effect on mix-

ture performance . Brown

et al

(1997) evaluated the effect of flat and elongated parti

cles on SMA mixes

. Stephens and Sinha (1978) studied the significance of flat an d

elongated particles on the characteristics of bituminous mixtures

. In a mix design, the

y

recommended 40-70 % cubical aggregates, 5-45 % flat aggregates and 5-45 To elon-

gated aggregates. Oduroh et al (2000) reported that coarse aggregates of 3

:1 size rati

o

at 40% and higher had the highest tendency to lie flat (horizontally) during HM

A

compaction . Overall, Superpave laboratory mixture performance tests did not sho

w

any significant changes in mixture properties due to the presence of up to 40% of 3

:

1

flat and elongated aggregates

D 2 2 Coarse Aggregate Angularit y

The aggregate interlock and internal friction is responsible for the HMA rutting resis

-

tance

. Aggregate angularity is quantified as the percent by weight of aggregates large

r

than 4

.75 mm with one or more fractured faces

. The standard test for measuring coarse

aggregate angularity is ASTM D 5821

:

Standard Test Method for Determining the Per-

centage of Fractured Particles in Coarse Aggregate

Superpave specifies a higher degre e

of aggregate angularity for higher traffic

. This is illustrated in Table D

.S



Table D

.1 Superpave S pecifications for Ruttin g

Superpave Binder Specifications

Average 7-day Maximum Pavemen

t

Design Temperature, °C

Minimum Pavemen

t

Design Temperature, ° C

Flash Point Temp, T48

: minimum, ° C

Viscosity, ASTM D 4402 :

Maximum, 3-Pa-s (3000 cP)

Test Temp, °

C

Dynamic Shear, TP

5

G /sin S, Minimum, 1

.00 kPa

Test Temperature@ 10 rad/s,°C

olling Thin Film Oven (T240

)

Mass Loss, Maximum,

Dynamic Shear, TP5 :

G*/sin 8, Minimum, 2 .20 kP

a

Test Temp @ 10 rad/sec,°C

Table D

.2 Superpave Sp ecifications for Fatigue Cracking

Superpave Binder Specification

PAV Aging Temp,° C

Dynamic Shear, TP5

:

G*sin

8, Maximum, 5000 kP

Test Temperature@ 10 rad/s,°

C

Physical Hardening

Creep Stiffness, TP1 :

S, Maximum, 300 MP

a

m-value, Minimum, 0

.300

Test Temperature@ 60sec,° C

Direct Tension, TP3

:

Failure Strain, Minimum, 1 .0

Test Temperature@ 1 .0mm/min, °C

Superpav e

Specification Limit

s

for Rutting

Superpav

e

Specification Limi t

for Fatig u e

Cracking

68



T able D

.3 Supe rpave Spe cifications for Thermal Crackin

g

Superpave Binder Specifications

PAV Aging Temp,°

C

Dynamic Shear, TP5

:

G*sin S, Maximum, 5000 kPa

Test Temperature@ 10 rad/s,°

C

Physical Hardening

Creep Stiffness, TP1

:

S, Maximum, 300 MPa

m-value, Minimum, 0

.300

Test Temperature@ 60sec,°

C

Direct Tension, TP3 :

Failure Strain, Minimum, 1

.0

Test Temperature@ 1

.0mm/min, °C

Table D .4 Superpave A sphalt Binder Grade

s

High Temperature Grades (°C)

Low Temp erature Grades (°C )

46

-34, -40, -4

6

5 2

-10, -16, -22, -28, -34, -40, -4

6

5 8

-16,-22,-28,-34,-4

0

64

-10,-16,-22,-28,-34,-40

70

-10, -16, -22, -28, -34, -4

0

76

-10,-16,-22,-28,-34

80

-10,-16,-22,-28,-3

4

Performanc

e

Grad

e

Designation :

Superpav

e

Specification Limit

s

for Therma l

Crackin

g

PG Hot Tem

p

Example : PG

64-22

Cold T em

p

686



Aggregates in HMA 68 7

TABLE D

.5

Superpave Coarse Aggregate Angularit

y

Requirement

s

Traffic,

Minimum Fracture d

Surface Requirements (%

)

million ESALs D <

100

mm D

100 m

m

<

.3 55/

65 1

—

< 3

75/—

50/

<10

85/80

60/

<3

95/90

80/75

<100

100/100 95/90

100

100/100 100/100

Note. 85/80

means that 8 of the coarse agg regate has one frac-

tured face, and 80 has two fractured face s D

= depth from surfac

e

D 2

3 Fine Aggregate Angularit

y

Fine aggregate contribution to the internal friction of HMA is quantified as the per

-

cent of air voids present in loosely compacted fine aggregates (smaller than 2

.36 mm)

Higher void content in this case reflects a more textured fine aggregate . The standar d

test for measuring this property is AASHTO T 304

: Uncompacted V oid Content–Method

A

. Superpave specifies a higher degree of fine aggregate angularity for higher traffic

This is illustrated in Tables D

.6

D 2

4 Aggregate Clay Conten

Clay is a highly undesirable material in HMA

. The clay content is characterized via a

suspension in the water test

: AASHTO T 176 :

Plastic Fines in Graded A ggregates an

d

Soils by Use of the Sand Equivalent Test

The clay content is controlled using a mini

mum sand equivalent criteria

. Superpave requires higher sand equivalent (i .e . lowe r

clay content) for higher traffic

. This is illustrated in Table D.7

TABLE D .6 Superpave Fine Aggregate Angularit

y

Requirements

Traffic,

Minimum Uncompacted Fine

Aggregat e

Air Voids Requirements (%

million ESALs

D < 1 0 0 mm D >

100

m m

<0

3

—

<1

40 —

<3

40 40

<10 45 40

<30

45

40

<100

45 45

1 0 0

45 45

Note Air voids criteria are presente d as perce nt air voids i n

loosely compacted fine agg reg ate D = depth from surfac e



688 ppendix D n Introduction to Superpav e

TABLE

D .7 Superpave Fine Aggregat

e

Angularity Requirement

s

Traffic, million

ESALs

Sand Equivalent, minimu

m

<0

.3 4

0

<1

4 0

<3

4 0

<10

4 5

<30

4 5

<100 5

0

100

5 0

D 2 5 Aggregate Toughnes s

Toughness is characterized using the Los Angeles Abrasion test

. The procedure i

s

described in AASHTO T 96

:

Resistance to Abrasion of Small Size Coarse Aggregate b

y

Use of the Los Angeles Machine . This test simulates the resistance of coarse aggregat e

to abrasion and mechanical impact during handling, construction, and in service . The

test is based upon comparing the coarse aggregate gradation before and after subject-

ing the aggregate to a mechanical degradation test

. The test measures the percent los

s

in the coarse aggregate

. The percent loss should be less than 35-45%

D 2 6 Aggregate Soundness

Soundness is the percent loss of materials from an aggregate blend during the sodiu

m

or magnesium sulfate soundness test

. The procedure is stated in AASHTO T 104

:

Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate

This test esti

-

mates the resistance of aggregate to weathering while in service . It can be performe

d

on both coarse and fine aggregate

. The test is performed alternately by exposing an ag-

gregate sample to repeated immersions in saturated solutions of sodium or magnesiu

m

sulfate each followed by oven drying. One immersion and drying process is considere

d

one soundness cycle

. During the drying phase, salts precipitate in the permeable voi

d

space of the aggregate

. Upon re-immersion, the salt rehydrates and exerts internal ex-

pansive forces that simulate the expansive forces of freezing water

. The test result i

s

the total percent loss over various sieve intervals for a required number of cycles

. Max -

imum-loss values range from approximately 10—20% for five cycles

D 2 7 Aggregate Gradatio n

Superpave aggregate gradation requirements posed several controversial issues . For ex-

ample, the initial versions of Superpave included a restricted zone in the gradation . Thi

s

zone was identified on a 0

.45-power gradation chart to define a permissible gradation . Th e

0

.45-power chart (Figure D2) is a common format for plotting aggregate gradation, be

cause it can easily illustrate the maximum density line as depicted in Figure D2 . It was ini -

tially hypothesized that gradations that violate the restricted zone possess weak aggregat

e

skeletons which may exhibit tenderness during construction and poor performance

. How

ever, recent research (TRB, 2002) suggests that Independent results from the literatur

e



3 Asphalt Mix Design 68

9

0 .075 .3

6 1

18 2.3 6 4 .7 5 9 .5 12 .5 19 0

Sieve Size, mm Raised to 0

.45 Powe r

FIGURE D

2

0

.45-Power curve

clearly indicated that no relationship exists between the Superpave restricted zone an

d

HMA rutting or fatigue performance . This report further recommends that perhap s

all references to the restricted zone should be deleted from AASHTO MP2 (1999b )

and AASHTO PP28 (1999c)

The maximum density gradation represents a tight aggregate packing

. This typ

e

of gradation does not necessarily produce the best performing HMA

. Superpav

e

gradations are recommended to have a strong aggregate interlock, which is common i n

more open mixes

D 3 ASPHALT MIX DESIG N

The Superpave mix design is based upon mixture volumetric properties at a specifie d

level of compaction . These volumetric properties are assumed to produce well performin

g

mixtures (AASHTO MP2 : Superpave Volumetric Mix Design)

Advanced Superpav

e

protocols are available for mixture performance analysis

:

Standard Test Method fo

Determining the Permanent Deformation and Fatigue Cracking Characteristics of Hot Mi

x

Asphalt HMA) Using the Simple Shear Test SST) Device, AASHTO TP7 Provisional

Standards, 1998

The mixture compaction is accomplished via the Superpave gyratory compacto

r

(SGC), and the resulting volumetric properties are used to select the optimum asphal

t

content

. The Superpave volumetric terms are defined in Table D B

D 3 1 Superpave Laboratory Compactio n

The Superpave gyratory compactor (SGC) is designed to produce specimens in th

e

laboratory which exhibit particle orientation similar to the field compacted mixtures



690 Appendix D

A n Introduction t o Superpav e

T A B L E D

Superpave Volumetric Terminology

Constituent Abbreviated Term

Paramete r

Aggregate P—mass percen

t

G—specific gravit y

s—stone (aggregate

)

b—bul

k

a—apparent

e — effective

Asphalt binder P—mass percent

G—specific gravit y

b—binder (asphalt

a— absorbed

e— effect iv e

Mixture

G—specific gravit

y

b—bul k

m — ma x imu m

m — m ixtur e

V—volume percent

Ps

—percent of mixture which is ston

e

Gsb

—bulk specific gravity of stone

G

s a

—apparent specific gravity of stone

G

S e

—effective specific gravity of ston

e

Pb

—percent of mixture which is binde

r

P e

—

P

ercent effective binde r

Pba — P

ercent binder absorbe d

Gb —specific gravity of binder

Gmb —bulk gravity of mi

x

G

mm —maximum theoretical gravity of mi

x

V

a

—volume of air in compacted mi

x

VMA—voids in mineral aggregat

e

VFA—voids filled with asphalt

D :B ratio—dust to binder ratio

This is achieved with a mold gyrating 30 revolutions per minute at 1

.25-degree pivo t

angle at the compaction pressure of 600 kPa

. The number of revolutions are adjuste d

to produce a target density.

There are a number of issues surrounding the Superpave laboratory com-

paction. The main concern is the relationship between laboratory and field com-

paction (Blankenship, et al

., 1994) . For example, Peterson et al

(2003) hav

e

suggested a number of modifications in order to improve upon the existin

g

Superpave compaction protocols

. These modifications include revisiting the angl e

of gyration and compaction pressure

. Additionally, there are standardization tool s

and techniques which are becoming available for the calibration of the angle o f

gyration

D 3

2 Mix Design Criteri

a

Once the proper aggregate and grade of asphalt binder have been selected, the nex

t

step is to produce a mixture that meets the Superpave criteria

. The goal of the mi

x

design process is to determine the optimum asphalt content corresponding to a set o

f

Superpave volumetric criteria . The most critical criterion is a 4% air content in the lab

-

oratory compacted specimens

. Additionally, voids in the mineral aggregate (VMA

)

and voids filled with asphalt (VFA) are checked

. Superpave volumetric characteristic

s

are determined at various compaction levels, which correspond to various traffic level

s

in the field

. AASHTO reports the required compactive effort for various traffi

c

(AASHTO, 1999b and 1999c)



D 3 Asphalt Mix Design 69

The mixtures are compacted to N

i , N

and N

f

in accordance with the following :

N

;—Number of initial gyrations

: This parameter indicates a potential for tender-

ness in the mix during construction

N

—Design number of gyrations : The number of gyrations for which the mix i

s

designed to produce 4% air content

Nf—Final number of gyrations

: The final number of gyrations is designed to sim-

ulate the post-compaction densification due to traffic

. A mixture demonstratin

g

less than 2 % air content at this point would be susceptible to rutting

N„ —Maximum number of gyrations that should never be exceeded

The volumetric properties of trial batches at 0

.5% asphalt content increments ar

e

measured, and the optimum asphalt content is selected based upon an air content of

4

% (AASHTO MP-2)

Finally, the moisture sensitivity of the mixture is determined in accordance wit

h

AASHTO T283

. This test is designed to measure the effect of moisture and cycles o

f

freeze—thaw on a mixture containing 7 % air

. The indirect tensile strength test is use

d

to quantify the laboratory-simulated moisture damage

. A strength ratio of 80% or

higher is normally required

. A strength ratio below 80% hints at a potential suscepti-

bility to stripping

.

Example

A Superpave mixture was prepared using for t rial ba tches at 0

.5%

asphalt content increments

All volumetric and com paction data are presented in the tab le below

:

B atch 1 Batch

2

Batch

3

Batch

4

1-AC(%)

4 .5 5 .0

5 .5 6

0

2-

Air (% ) a t N d

6

1

4

1

3 .0 2 0

3 - G m m

2 .467

2.444 2 .430 2.410

4-

VMA ( )

15.6

15 .1 15 .2 15 3

5- VFA

(% ) 62.1

72 .7 81 .5 8 9 1

6 - % G m m

at N

i 8 4.1

86

1

87 .0 8 8 1

7 - % G m m

at N

m 95 .4 97

1

98 . 6 99 .5

olut on

Using an interpolation routine or a graphical solution will reveal that

5

.1% would b e

the optimum asphalt content at which the

4% air content requirement is sat isf ied. As a cros

s

check, all other volum etric propert ies are reviewed at this optimum asphalt content to ensur e

their compliance with Superpave requirements

SUMMARY

Superpave has put the asphalt mix design and analysis on a rational platform

. There

are many who may argue that Superpave is not purely mechanistic

. However, mos

t



692 Appendix D

An Introduction to Superpav e

would agree that features such as the PG binder systems and mixture analysis proto-

cols, are major improvements over the empirical methods of the past

. The PG binde r

system incorporates the climatic information in the binder selection process

. The volu

metric properties of HMA are used for Superpave mix design

. Many of these proper

-

ties lend themselves to quality control measurements

Protocols related to Superpave aggregates were developed based upon literatur

e

review rather than sound experimentation

. This resulted in many early, less-than-per

fect aggregate specifications, which were later adjusted

. Additionally, there are ne

w

procedures being developed in order to do a better job with addressing modifie

d

binders, recycled asphalt, and waste materials in HMA

.

A mechanistic mix design methodology provides us with the opportunity to inte

grate asphalt mix design and flexible pavement structural design (Mahboub and Little

1990) . Superpave is hoped to link with the latest AASHTO pavement design guide in

a

mechanistic manner

P R O B L E M

S

D

What is Superpave? Why is it innovative ?

D 2

What is the PG system? Why is it considered an improvement over the viscosit

y

and penetration systems ?

What are the critical factors affecting asphalt performance in pavements

?

What are the engineering parameters used in rating the asphalt binders? Ho

w

are these parameters tested in Superpave

?

5

For an asphalt pavement, the maximum consecutive seven day pavement tem-

perature is 66°C and single event coldest temperature is -20°C . The desig

n

ESALs for this pavement is 10,000,000, and traffic speed is standard

. Select a P

G

grade for this pavement

.

6

Can Superpave be used for both base and surface mixes

?

Use the maximum gradation chart in Figure D

.3, and draw a typical Superpav e

surface mix gradation

D 8

List the steps in the Superpave mix design

.

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An Introduction to SuperPave

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