Superpave Mix Design

S. M DAVEReader

Civil Engg DepttThe M.S.University of Baroda

1

INTRODUCTION• SHRP (Strategic Highway Research Program

was established by USA congress in 1987 as a five year $150million research program to improve the performance and durability of roads.

• $50 million of SHRP research funds were used for development of performance based asphalt specifications to directly relate laboratory analysis with field performance.

• Superpave (SUperior PERforming Ashalt PAVEment) is a product of SHRP asphalt research

2

INTRODUCTION

• Superpave asphalt binder tests measure physical properties that can be directly related to field performance through engineering principles.

• Superpave binder tests are conducted at in-service pavement temperatures.

• Superpave gyratory compactor is used for preparing HMA specimens

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MIXTURE DESIGN

Asphalt Binder Behaviour• Temperature Susceptibility Asphalt is stiffer at colder temp and softer at

high temp.• Viscoelasticity Asphalt act as viscous fluid at high temp(>100

ºC) and as elastic solid at low temp.(<0 ºC ).At intermediate temp it act like viscous fluid.

• Aging Oxidation of asphalt causes age hardening

5

Mineral Aggregate Behaviour• Types: Natural, Processed (Quarried) and

Synthetic(Blast furnace slag),Recycled• Must provide enough shear strength to resist

repeated load application.• Shear strength depends on Internal friction

provided by aggregates• Cubical, Rough textured offer more

resistance.• Rounded aggregates not preferred (Natural

sand not desirable )6

Asphalt Mixture Behaviour

• Primary pavement Distress Types Permanent Deformation( Rutting) Fatigue Cracking Low temperature cracking

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Permanent Deformation

Courtesy of FHWA

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HMA Mix Design• Objective:

• Develop an economical blend of aggregates and asphalt binder that meet design and functional requirements

• Historical mix design methods• Marshall • Hveem

• New • Superpave gyratory Mix design

Marshall Mix DesignAdvantages:• Density & Void properties of asphalt mixture

relatively easy to measure• Required equipment is relatively inexpensive

and portableDisadvantages:• Impact compaction does not simulate mixture

densification in real pavement• Marshall stability does not estimate shear

strength of HMA12

Hveem Design MethodAdvantages: Mixture’s resistance to swell in presence of

water is also determined Kneading laboratory compaction simulate

densification of real pavement. Hveem stability is a direct measurement of

internal friction component of shear strengthDisadvantage: Testing equipment is expensive and not

portable Selecting binder content is too subjective 13

Superpave Mix Design• Individual steps used to select asphalt and

aggregate materials• Method integrates material selection and mix

design into procedures based on project’s climate and design traffic

• Laboratory compaction is accomplished using a Superpave gyratory compactor.

• SGC can be used to design mixtures that do not exhibit classic tender mix behaviour and do not densify to dangerously low air void content under traffic action

• Perfomance based tests and models developed 14

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Requirements in Common

• Sufficient asphalt binder to ensure a durable pavement

• Sufficient stability under traffic loads• Sufficient air voids

• Upper limit to prevent excessive environmental damage

• Lower limit to allow room for initial densification due to traffic

• Sufficient workability

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4 Steps of Superpave Mix Design

1. Materials Selection 2. Design Aggregate Structure

3. Design Binder Content 4. Moisture Sensitivity

TSR

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Step 1: Materials Selection

• Materials Selection consists of:• Choosing the correct asphalt binder• Choosing the aggregates that meet the

quality requirements for the mix

Superpave Asphalt Binder Specification

• Three critical stages during binder’s life:• First stage: Transport, storage &

handling• Second Stage: Mix production and

construction• Third Stage: Aging over a long period

as part of the hot mix asphalt

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Superpave Binder Test EquipmentEquipment

• Rolling thin film oven(RTFO)• Pressure Aging Vessel

• Dynamic Shear Rheometer(DSR)

• Rotational Viscometer(RV)

• Bending beam Rheometer

Purpose

• Simulate binder aging characteristics

• Measure binder properties at high and intermediate temp

• Measure binder properties at high temp

• Measure binder propertis at low temp

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Rolling Thin Film Oven Test

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Bottles in RTFO test

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Pressure Aging

Vessel(PAV)

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PanPan HolderPAV

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Table 1: Performance Graded Asphalt Binder RTFO Specification

Material Value SpecificationProperty of

Concern

Unaged binder Mass loss1 ≤ 1.0% None

Temperature Simulation194°F (90°C) cold climate

212°F (100°C) moderate climate230°F (110°C) hot climate

PAV Test Temp. Pressure:300Pa Time 20hours

Table 2

Dynamic Shear

Rheometer

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DSREquipments

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Upper and Lower plate of DSR Equipment

Rotational Viscometer

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Table 3: Performance Graded Asphalt Binder RV Specification

Material Value Specification Property of Concern

Unaged binder Dynamic viscosity ≤ 3 Pa•sPumping, mixing and

workability

Table 4: Performance Graded Asphalt Binder DSR specifications

Material Value SpecificationHMA Distress of

Concern

Unaged binder G*/sinδ ≥ 1.0 kPa (0.145 psi) Rutting

RTFO residue G*/sinδ ≥ 2.2 kPa (0.319 psi) Rutting

PAV residue G*sinδ ≤ 5000 kPa (725 psi) Fatigue cracking

Bending Beam Rheometer

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Bending Beam Rheometer

BBR Set up BBR Mould

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Bending Beam Rheometer

Air Bearing

Load Cell

Deflection Transducer

Fluid Bath

Computer

BBR Output

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Summary

FatigueCrackingRutting

RTFOShort Term AgingNo aging

Construction

[RV] [DSR]

Low TempCracking

[BBR]

[DTT][DTT]

PAVLong Term Aging

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Superpave Asphalt Binder Specification

The grading system is based on Climate

PG 64 - 22

Performance Grade

Average 7-day max pavement temperature

Min pavement temperature

Pavement Temperatures are Calculated

• Calculated by Superpave software AASHTO Suerpave program OR LTPP Bind (long term pav performance program

• High temperature – 20 mm below the surface of mixture

• Low temperature– at surface of pavement

Pave temp = f (air temp, depth, latitude)

Pavement Temperature

• T20mm =(Tair-0.00618Lat2+0.2289Lat+42.2) (0.9545)-17.78

Where T20mm= High pave. Design temp at depth

of 20mm Tair= Seven –day high air temp C

Lat= The geographical latitude of the project in degrees.

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Reliability concept

• Reliability is the percent probability in a single year that the actual temperature (one-day low or seven day high) will not exceed the design temperature.

• Higher reliability means lower risk.

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Binder selection on basis of traffic speed and traffic level

Design ESALs(million)

Adjustment to Binder PG gradeTraffic load Rate

Standing Slow Standard

<0.3 - - -

0.3to<3 2 1 -

3 to<10 2 1 --

10 to<30 2 1 --

>30 2 1 1

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Standing: Average traffic speed <20 kmphSlow: Average traffic speed 20—70kmphStandard :Average traffic speed >70kmph

Mineral Aggregate Selection

Consensus Properties• Coarse agg Angularity

• Fine agg Angularity• Flat & Elongated particles

• Clay Content

Source Properties

• Toughness

• Soundness

• Deleterious Materials

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Coarse Aggregate Angularity

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Fine Aggregate Angularity

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Flat & Elongated Particles

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Coarse Aggregate Angularity Criteria

Traffic Depth from SurfaceMillions of ESALs < 100 mm > 100mm

< 0.3< 1< 3< 10< 30< 100> 100

55/--65/--75/--85/8095/90

100/100100/100

--/----/--50/--60/--80/7595/90

100/100

First number denotes % with one or more fractured facesSecond number denotes % with two or more fractured faces

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Aggregate Consensus Properties

Coarse Aggregate Angularity

Fine Aggregate Angularity

Traffic Level< 100 mm > 100 mm < 100 mm > 100 mm

< 0.3 75 / --- 50 / --- 40 400.3 to < 3.0 85 / 80 60 / --- 45 40

3.0 to < 30.0 95 / 90 80 / 75 45 40> 30.0 100 / 100 100 / 100 45 45

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Aggregate Properties

1 3 5

Traffic Level Sand Equivalent, %

Flat and Elongated, %

< 0.3 40 ---0.3 to < 3.0 45 10

3.0 to < 10.0 45 1010 to < 30.0 45 10

> 30.0 50 10

Aggregate Size Definitions

• Nominal Maximum Aggregate Size• one size larger than the first sieve to

retain more than 10%

• Maximum Aggregate Size• one size larger than nominal

maximum size

10010010010090907272656548483636222215159944

100100999989897272656548483636222215159944

Superpave Mix Size Designations

SuperpaveSuperpave Nom Max SizeNom Max Size Max SizeMax SizeDesignationDesignation (mm) (mm) (mm) (mm)

37.5 mm37.5 mm 37.5 37.5 50 50 25 mm25 mm 25 25 37.5 37.5 19 mm19 mm 19 19 25 25 12.5 mm12.5 mm 12.5 12.5 19 19 9.5 mm9.5 mm 9.5 9.5 12.5 12.5

100100

00 .075.075 .3.3 2.36 2.36 4.75 4.75 9.59.5 12.5 19.012.5 19.0

Percent PassingPercent Passing

control pointcontrol point

restrictedrestricted zonezone

max density linemax density line

maxmaxsizesize

nomnommaxmaxsizesize

Sieve Size (mm) Raised to 0.45 PowerSieve Size (mm) Raised to 0.45 Power

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Table 1. 37.5 mm (1.5 inch) Nominal SizeSieve Size Control Points Restricted Zone

(mm) (U.S.) Lower Upper Lower Upper50 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 34.72.36 No. 8 15 41 23.3 27.31.18 No. 16 - - 15.5 21.50.60 No. 30 - - 11.7 15.70.30 No. 50 - - 10.0 10.00.15 No. 100 - - - -

0.075 No. 200 0 6 - -

Superpave Aggregate Gradation

100100

00 .075.075.3.3 2.36 2.36 12.5 12.5 19.019.0

Percent PassingPercent Passing

Design Aggregate StructureDesign Aggregate Structure

Sieve Size (mm) Raised to 0.45 PowerSieve Size (mm) Raised to 0.45 Power

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Step 2: Aggregate Gradation

• Establish trial aggregate blends• 3 suggested• evaluate combined aggregate properties

• Estimate optimum asphalt binder content• Manufacture and compact trial blends• Evaluate the trial blends• Select the most promising blend

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Steps of Superpave HMA Mix Design

1. Materials Selection1. Materials Selection 2. Design Aggregate Structure2. Design Aggregate Structure

3. Design Binder Content3. Design Binder Content 4. Moisture Sensitivity4. Moisture Sensitivity

TSRTSR

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• Simulate field densification• traffic• climate

• Accommodate large aggregates• Measure compactability• Conducive to QC

Goals of Compaction Method

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AASHTO T 312 Gyratory Compaction

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• Basis• Texas equipment• French operational

characteristics• 150 mm diameter

• up to 37.5 mm nominal size• Height Recordation

?

???

Superpave Gyratory Compactor

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reactionframe

rotatingbase

loadingram

control and dataacquisition panel

mould

heightmeasurement

tilt bar

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150 mm diameter mould

ram pressure600 kPa

1.25 degrees30 gyrationsper minute

Compaction• Gyratory compactor

• Axial and shearing action• 150 mm diameter molds

• Aggregate size up to 37.5 mm• Height measurement during compaction

– Allows densification during compaction to be evaluated

1.25o

Ram pressure600 kPa

% G% Gmmmm

Log GyrationsLog Gyrations

1010 100100 10001000

NNiniini

NNdesdes

NNmaxmax

Three Points on SGC Curve

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• Ninitial. The number of gyrations used as a measure of mixture compactability during construction. Mixes that compact too quickly (air voids at Ninitial are too low) may be tender during construction and unstable when subjected to traffic. Often, this is a good indication of aggregate quality – HMA with excess natural sand will frequently fail the Ninitial requirement. A mixture designed for greater than or equal to 3 million ESALs with 4 percent air voids at Ndesign should have at least 11 percent air voids at Ninitial.

• 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.

• 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.

SGC Critical Point Comparison%Gmm= Gmb / Gmm

Gmb = Bulk Mix Specific Gravity from compaction at N cycles

Gmm = Max. Theoretical Specific Gravity

Compare to allowable values at:NINI : %Gmm < 89%

NDES: %Gmm < 96%

NMAX: %Gmm < 98%

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Ndesign Table

Compaction LevelTraffic

Level Ninitial Ndesign Nmaximum

Gyrations< 0.3 6

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50 750.3 to < 3.0 75 1153.0 to < 30.0 100 160

> 30.0 9 125 205

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General Notes to Revised Table

• Traffic Level is Based Upon 20 Year Pavement Design Life

• Slow / Standing Traffic : Increase Ndesign

by 1 Level.

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Superpave Gyratory Compaction

• Select mixing and compaction temperature based on asphalt binder properties

• Select number of gyrations to use based on design traffic level

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Steps of Superpave HMA Mix Design

1. Materials Selection1. Materials Selection 2. Design Aggregate Structure2. Design Aggregate Structure

3. Design Binder Content3. Design Binder Content 4. Moisture Sensitivity4. Moisture Sensitivity

TSRTSR

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Establish Trial Blends

• Develop three gradations based on• Stockpile gradation information• Gradation specification

• Optimize use of materials in the most economical blends

• Estimate properties of combined stockpiles

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Trial Asphalt Binder Content

• Use known or estimated values for• Effective aggregate specific gravity, Gse

• Asphalt binder absorbed, Vba

• Calculate the effective binder content, Vbe

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Trial Asphalt Binder Content

• Calculate the initial asphalt binder content:

• Where:

Pbi = 100 Gb (Vbe + Vba)

(Gb (Vbe + Vba)) + Ws

Ws = Ps (1 – Va)

(Pb / Gb) + (Ps /Gs)

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Next steps

• Sample preparation• Select mixing and compaction

temperatures• Preheat aggregates and asphalt• Mix components• Compact specimens

• Extrude and determine volumetrics

72Temperature, C Temperature, C

Vis

cosi

ty, P

a-s

Vis

cosi

ty, P

a-s

0.050.05

0.10.1

11

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100100 110110 120120 130130 140140 150150 160160 170170 180180 190190 200200

Mixing Range

Compaction Range

Temp-Vis RelationshipTemp-Vis Relationship

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Determine the sample mass

• Estimate an asphalt binder content• Mix up a sample & determine Gmm

• Calculate the bulk gravity needed to achieve 4 % air voids (Va)

• Calculate the weight for a pill with a Diameter of 150 mm and height 115mm.

• Superpave mix specimen requires approximately 4700gms of aggregate.

Mixing & Compaction Temperatures

• Select the mixing and compaction temperature corresponding with following binder viscosity values:

• Mixing Temp:0.17 ± 0.02Pa* s• Compaction Temp: 0.28 ± 0.03 Pa* s• Practically Mixing temp should not exceed

165º C • Compaction temp. should not be lower than

115 º C 74

Sample Mass

h d

d 2 hx

4 * 0.001 cm3/mm3Sample Volume = Vmx =

Where: Vmx = volume of specimen in mold)d = diameter of mold (150 mm)hx = height of specimen in mold

Sample mass = (Est. Gmb) (Sample Volume)

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Place pre-heated aggregate in bowl and add hot asphalt

Mixing

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Mixing

Place bowl on mixer and mix until aggregate is well-coated

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Short Term Aging

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Short Term Aging Importance

• Allows time for aggregate to absorb asphalt binder

• Helps minimize variability in volumetric calculations• Most terms dependent upon volumes which

change with changes in the amount (volume) of absorbed asphalt binder

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After aging, take mix and preheated mold from oven. Place paper in bottom of mold.

Compaction

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Place mix in mold

Compaction

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Overview of Compaction Procedure

• Initialize Compactor• verify/set ram pressure at 600 kPa• verify/set number of gyrations for Ndes

• Fill Gyratory Mold With HMA• paper disk on bottom• one lift of HMA• slightly round top of HMA• paper disk on top

• Load Mould into Gyratory Compactor

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Place another paper disc on top

of the mix

Compaction

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Slide mold into the compactor

Compaction

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Overview of Compaction Procedure (cont.)

• Start Test (the following occurs automatically):• ram lowers• angle is applied• compaction occurs• ram raises

• Extrude Specimen• Allow Specimen to Cool• Determine Bulk Specific Gravity

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Start compactor

Compaction

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Extrude sample and remove paper

from both sides while still warm

Compaction

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% Gmm

Log Gyrations

10 100 1000

Nini

Ndes

Nmax

Three Points on SGC Curve

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Estimate Aggregate Blend Properties(Example)

Property Criteria Trial Blend 1 2 3

Ninitial, % < 89.0 87.1 85.6 86.3Ndesign, % 96.0 97.6 97.4 96.5Nmax, % < 98.0 96.2 95.7 95.2Air Voids, % 4 4.4 4.4 4.4VMA, % 13 12.7 13.0 13.5

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4 Steps of Superpave Mix Design

1. Materials Selection 2. Design Aggregate Structure

3. Design Binder Content 4. Moisture Sensitivity

TSR

91

General Guidance

• Compact the trial mixtures in accordance with AASHTO T 312 which now requires specimens be compacted to the design number of gyrations

• When doing a mix design when you compact a pair of samples to Nmaximum and check them to see if the Nmaximum value of 98% is exceeded.

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% Gmm

Log GyrationsLog Gyrations10 100 1000

increasingincreasingbinderbinder

Design Asphalt Binder Content

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Superpave Mixture Requirements

• Mixture Volumetrics• Air Voids (Va)• Mixture Density Characteristics• Voids in the Mineral Aggregate (VMA)• Voids Filled with Asphalt (VFA)

• Dust Proportion• Moisture Sensitivity

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Mix VMA Requirements Voids in the Mineral Aggregate

9.5 mm 15.0 12.5 mm 14.0 19.0 mm 13.0 25.0mm 12.0 37.5mm 11.0

Nominal sizeOf Aggregate Minimum VMA, %

% asphalt binder

VMA

95

Mix VFA RequirementsVoids Filled with Asphalt

VFA

A(<0.3) 70 – 80B (0.3to <3) 65 – 78C(3 to <10) 65 – 75D(10 to<30) 65 – 75E (≥ 30) 65 - 75

Traffic Level Design VFA, %

% asphalt binder

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Mix Requirement for Dust Proportion

1001009283654836221594

% weight of 0.075 mm sieve material

% weight of effective asphalt binder

0.6 < < 1.2

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DP

VMA

% asphalt binder

VFA

%Gmm at Nini

%Gmm at NmaxVa

Selection of Design Asphalt Binder ContentSelection of Design Asphalt Binder Content

% asphalt binder

% asphalt binder

% asphalt binder

% asphalt binder% asphalt binder

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Example

• Using the data on the next sheet, determine:• The design asphalt binder content• The VMA at the design asphalt binder• The VFA at the design asphalt binder• The dust to asphalt ratio

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Example

% AC Va VMA4.5 5.5 15.1

5.0 4.5 15.0

5.5 3.3 14.9

6.0 2.4 15.0

100

94.0

94.5

95.0

95.5

96.0

96.5

97.0

97.5

98.0

4.0 4.5 5.0 5.5 6.0 6.5

% Asphalt Binder

%G

mm @

Nde

s

101

14.7

14.8

14.9

15.1

15.2

15.3

4.0 4.5 5.0 5.5 6.0 6.5

% Asphalt Binder

% V

MA

102

63

67

72

76

81

85

4.0 4.5 5.0 5.5 6.0 6.5

% Asphalt Binder

% V

FA

103

4 Steps of Superpave Mix Design

1. Materials Selection 2. Design Aggregate Structure

3. Design Binder Content 4. Moisture Sensitivity

TSR

104

• Six specimens are made at optimum asphalt binder content

• VTM is 7.0 + 0.5 % for all other mixes• Three specimens are vacuum saturated

• 90 % saturation minimum• One freeze-thaw cycle • Determine the indirect tensile strength for all six

of the specimens• Determine the percent retained strength

T-283 Procedure

105

Vacuum Saturation

• Place the specimen in vacuum chamber covering with at least one-inch of water

• Drop the pressure by 26 inches of mercury for 30 minutes

• Tap the chamber to dislodge trapped bubbles• Release the vacuum and leave in water for 30

minutes.

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Vacuum saturation

• After 30 minutes determine the percent saturation

% Saturation = {(100) (D-A)} {(C-B)(E)}

A: Dry wtB: Wt in water before saturationC: SSD wt. Before vacuumD: SSD wt. After vacuumE: Percent air voids in specimen

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Vacuum Saturation

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Heating Pills in Hot Water Bath

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Specimens placed in chamber at 25 C

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Applying Load

111

INDIRECT TENSILE STRENGTH

S = 2p/ h DS – strength

P = load

H = width of specimen

D = the diameter

Dry Tensile Strength(average)

Wet Tensile Strength(average)

TSR = x 100 80 %Wet

Dry

Deformation Rate: 51 mm / min @ 25 oC

Moisture SensitivityAASHTO T 283 Test Procedure

112

Procedural Outline

I Selection of MaterialsA Selection of Asphalt Binder

(i)Determine project weather condition (ii) Select reliability(iii)Determine design temperature(iv) Verify Asphalt grade(v)Temperature-viscosity relationship for lab

mixing and compaction

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B Selection of Aggregates 1 Consensus properties (a)Combined gradation (b)Coarse aggregate angularity (C)Fine aggregate angularity (d) Flat & elongated particles 2 Source properties (a)Toughness (b)Soundness (c)Deleterious materials

II Selection of Design Aggregate Structure

A Establish Trial Blends (a)Develop three blends (b)Evaluate combined aggr.properties

B Compact Trial Blend Specimens (a) Establish trial asphalt binder content (b) Establish trial blend specimen size (c) Determine Ninitial and Ndesign

(d) Batch trial blend specimens (e) Compact specimen & generate

densification tables (f) Determine mix properties Gmm & Gmb

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C Evaluate Trial Blends(1) Determine % Gmm @ Ninitial & Ndesign

(2)Determine %Air voids and % VMA(3)Estimate asphalt binder content to get 4% Va(4)Estimate mix properties@ estimated asphalt

binder content(5)Determine dust asphalt ratio(6)Compare mixture properties to criteria

D Select Most Promising Design Aggregate Strucure for Further Analysis

III Selection of Design Asphalt Binder content A Compact Design Aggregate structure

Specimens at Multiple Binder Content(1)Batch design aggregate structure specimens(2)Compact specimens & generate densification

tableB Determine Mix Properties V/s Asphalt Binder(1)Determine %Gmm @ Ninitial & Ndesign

(2)Determine volumetric properties(3)Determine dust-asphalt ratio(4) Graph mixture properties V/s Asphalt Binder 117

118

C Select Design Asphalt Binder Content(1)Determine asphalt binder content at 4% Va(2)Determine mixture properties at selected

asphalt binder content (VMA, VFA,Dust proportion, %Gmm @N initial)

(3)Compare mixture properties to criteria

IV Evaluation of Moisture sensitivity of Design Asphalt Mixture Using AASHTO T283