block 14 superpave mix design 13

HMA Superpave Mix Design 1

Superpave Mix Design

Senior/GraduateHMA Course


MIXTURE DESIGN


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


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


• Simulate field densification• traffic• climate

• Accommodate large aggregates• Measure compactability• Conducive to QC

Goals of Compaction Method


AASHTO T 312 Gyratory Compaction


• Basis• Texas equipment• French operational

characteristics• 150 mm <5.9”> diameter

• up to 37.5 mm nominal size• Height Recordation

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Superpave Gyratory Compactor


reactionframe

rotatingbase

loadingram

control and dataacquisition panel

mold

heightmeasurement

tilt bar


150 mm diameter mold

ram pressure600 kPa

1.25 degrees30 gyrationsper minute


Ndesign Table

Compaction Level

Traffic Level Ninitial Ndesign Nmaximum

Gyrations %Gmm

< 0.3 6 < 91.5 50 75

0.3 to < 3.0 7 < 90.5 75 115

3.0 to < 30.0 8 < 89.0 100 160

> 30.0 9 < 89.0 125 205


General Notes to Revised Table

• Traffic Level is Based Upon 20 Year Pavement Design Life

• Slow / Standing Traffic : Increase Ndesign

by 1 Level.


Superpave Gyratory Compaction

• Select mixing and compaction temperature based on asphalt binder properties

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


4 Steps of Superpave Mix Design

1. Materials Selection 2. Design Aggregate Structure

3. Design Binder Content 4. Moisture Sensitivity

TSR


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

The grading system is based on Climate

PG 64 - 22

Performance Grade

Average 7-day max pavement temperature

Min pavement temperature


Aggregate Consensus Properties

Coarse Aggregate Angularity

Fine Aggregate Angularity

Traffic Level

< 100 mm > 100 mm < 100 mm > 100 mm

< 0.3 75 / --- 50 / --- 40 40

0.3 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


Aggregate Properties

1 3 5

Traffic LevelSand

Equivalent, %Flat and

Elongated, %

< 0.3 40 ---

0.3 to < 3.0 45 10

3.0 to < 10.0 45 10

10 to < 30.0 45 10

> 30.0 50 10


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


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


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


Establish trial asphalt binder content

• Superpave Method

• Engineering judgement method


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


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)


Next steps

• Sample preparation

• Select mixing and compaction temperatures

• Preheat aggregates and asphalt

• Mix components

• Compact specimens

• Extrude and determine volumetrics


Place pre-heated aggregate in bowl and add hot asphalt

Mixing


Mixing

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


Short Term Aging


Short Term Aging Important

• 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


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 height of 150 mm

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)


d 2 hx

4 * 0.001 cm3/mm3Sample Volume = Vmx =

d 2

4 * 0.001 cm3/mm3 = 3.1416 * 150* 150 * 0.0001

4

= 17.671

Sample Mass Example Calc.


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 Mold into Gyratory Compactor


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

Compaction


Place mix in mold

Compaction


Place another paper disc on top

of the mix

Compaction


Slide mold into the compactor

Compaction


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


Start compactor

Compaction


Extrude sample and remove paper

from both sides while still warm

Compaction


% Gmm

Log Gyrations

10 100 1000

Nini

Ndes

Nmax

Three Points on SGC Curve


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





TSR


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.


% Gmm

Log GyrationsLog Gyrations

10 100 1000

increasingincreasingbinderbinder

Design Asphalt Binder Content


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


Mix VMA Requirements Voids in the Mineral Aggregate

Table 334-9

9.5 mm 15.0 12.5 mm 14.0 19.0 mm 13.0

Mix typeMinimum VMA, %

% asphalt binder

VMA


Mix VFA RequirementsVoids Filled with Asphalt

VFA

A 70 – 80B 65 – 78C 65 – 75D 65 – 75E 65 - 75

Traffic Level Design VFA, %

% asphalt binder


Mix Requirement for Dust Proportion

1001009283654836221594

% weight of - 0.075 material

% weight of effective asphalt binder

0.6 < < 1.6


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


Classroom 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


Classroom Example

Pba = 0.4 % & the percent of minus 200 is 6%

% AC Va VMA

4.5 5.5 15.1

5.0 4.5 15.0

5.5 3.3 14.9

6.0 2.4 15.0


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


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


63

67

72

76

81

85

4.0 4.5 5.0 5.5 6.0 6.5

% Asphalt Binder

% V

FA





TSR


DEFINITION

Stripping is the breaking of the adhesive bond between the aggregate surface and the

asphalt binder


Stripping potential is controlled by

• Asphalt binder properties

• Aggregate properties

• Mixture characteristics

• Climate

• Traffic

• Construction practices


Surface Chemistry

• Hydrophilic - “water loving”

• Those with high silica content

• Granites

• Hydrophobic - “water hating”

• Those with high carbon content

• Limestones

• But - it depends


ANTISTRIP ADDITIVESSurface Active Agents

• Generally they are chemical compounds containing amines

• Amines are basic compounds derived from ammonia

• Heat stability can be a problem• Dosage rate is generally 0.5 % (but it depends)• Can change the properties of the asphalt cement

- generally soften


ANTISTRIP ADDITIVESLime

• Hydrated lime - Ca(OH)

• AASHTO Specification -

• The result is a bonding of the calcium with the silicates in the aggregate

• Or an interaction or modification of the acidic portions of the asphalt

• Dosage rate is generally 1 to 1.5%


• 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 of for all six of the specimens

• Determine the percent retained strength

T-283 Procedure


Treatment with admixtures

• Liquid antistrip

• Asphalt binder is heated to 325 F

• Add liquid antistrip

• Stir for 2 minutes

• Lime

• Dry mixed to the hot aggregate or damp aggregate immediately before the asphalt binder is added and mixed (the process used should match that being used in the field).


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.


Vacuum saturation

• After 30 minutes determine the percent saturation

% Saturation = {(100) (D-A)}

{(C-B)(E)}

A: Dry wt

B: Wt in water before saturation

C: SSD wt. Before vacuum

D: SSD wt. After vacuum

E: Percent air voids in specimen


Vacuum Saturation


Heating Pills in Hot Water Bath


Specimens placed in chamber at 25 C


Applying Load


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


QUESTIONS ?

block 14 superpave mix design 13

Technology

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mixhmasuperpave mix

qchma superpave mix

hma mix design objective

design binder content

ps gshmasuperpave mix

steps of superpave mix

hma superpave mix design17