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Superior Performing Asphalt Pavements

Performance based design system testing and design procedures simulate closer to the

actual loading and climatic conditions performance graded asphalt binders aggregate specifications hot mix asphalt design and analysis system

Level 1 material selection and volumetric designLevel 2 Level 1 design + performance testingLevel 3 Level 1 design + more performance testing

Enhanced mix performancePermanent deformation Fatigue Cracking and Low temperature cracking

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Superpave

SHRP established 1987 - 5 yrs. Binder specs in use by 100% of US, Ont., Quebec

in Canada Level 1 Mix design - partial use Still to come

simple performance tests performance prediction models

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Superpave Mixture Design

Key features are laboratory compaction and performance testing

Laboratory compactionSuperpave gyratory compactor (SGC)compacts test specimens to simulate actual

pavement and loading conditionsprovides information about the compactability of a

particular mixturedesign mixtures that do not exhibit tender mix

behavior and do not densify to low air voids under traffic

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Superpave Mixture Design

Performance testing and performance prediction models detailed predictions of actual pavement performance in

terms of ESAL’s Superpave Shear tester (been replaced by triaxial) Indirect Tensile tester

Short term aging protocolperformance of HMA immediately after construction

is influenced by mix properties from hot mixing and compaction

oven aged mix at 135 C 4 hours to simulate delays that can occur during construction

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Superpave Mixture Design

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Level 1 Mix Design

Volumetric mix design Select asphalt and aggregate materials develop several aggregate trial blends (3) that

meet Superpave aggregate specs blend asphalt, compact, analyze use best blend with several asphalt contents to

determine the design asphalt content at 4% air voids and compare to other criteria

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Gyratory Compactor

Better simulates field compaction than hammerCompacts specimen but also measures density

during compactionMeasures compactibility so tender mixes are

identifies Larger size aggregate blends can be used (50 mm

max size) because of larger mold sizes

Amount of compaction depends on traffic and climate Ndes is the design number of gyrations used for volumetric

properties Nmax < 98%, Nini < 89% of max. density - used to indicate

compactibility Log Nmax = 1.1 Log Ndes

Log Nini = .45 Log Ndes

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Trial Blends

The available aggregate stockpiles are combined in varying percentages, 3 blends are usually considered

Aggregate properties are determined (either on blends or estimated from individual aggregates)

Compact each blend and estimate volumetric properties

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Trial Blends

For each aggregate trial blend: Asphalt binder grade selected Mixing temperature (non modified)

viscosity .15 - .19 Pa-s (150 – 190 mm2/s) Compaction temperature

viscosity = 0.25 - .31 Pa-s (250 – 310 mm2/s) Prepare aggregates

2 specimens for each trial blend : 2 x 4700g for compaction 1 approx. 2000g for max density (uncompacted mix) 1 x 3700 g for moisture sensitivity

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Trial Blends

Heat, mix with asphalt and short term age for 4 hours at 135°C representing possible delays in the field

Compact specimens for Nmax gyrations, recording height

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Data Analysis

Measure density at Nmax, using volume of cylinder calculate estimated density at Nmax, Nini, Ndes

Errors in estimated densities due to surface irregularities of briquette are corrected using ratio of measured to estimated density at Nmax

Measure Max. theoretical density of the uncompacted mix

Calculate air voids, VMA, VFA at Ndes

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Data Analysis

When you calculate your air voids you want to be at 4%………..

Estimate asphalt content at 4% using

Pb,est = Pbi - (.4x(4-AV%))

Calculate corrected values of design VMA and VFA for 4% air voids using VMAest = VMAinitial+C x (4-AV%) where C

= .1 if AV% < 4% or .2 if AV% > 4% VFAest = ( VMAest- 4) / VMAest

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Blend Selection Check blend properties against design criteria

VMA VFA meets table 6-14,6-15 in text Nini < 89%, Nmax < 98% of max density

passes moisture sensitivity (T283)80 % minimum indirect tensile strength

dust content (% passing 0.075 mm sieve divided by effective asphalt content) between .6 and 1.2

Choose best blend - new samples ( 2 compaction 2 max density) are made and checked Pb @ 4% AV

one at 0.5% less 2 more at 0.5% and 1% higher

volumetric properties calculated at each a/c contenta/c content at 4 % chosen other criteria checked

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Level 2 & 3 Design

Performance tests for permanent deformation fatigue cracking and low temperature cracking Superpave Shear Tester (SST) (been replaced by triaxial) Indirect Tensile Tester (IDT)

Lab testing has not yet been validated with field results and is still in experimental stage

Current expectations are that Level 2 & 3 will be combined into one and Level 1 will include an additional simplified stability test

At present research - triaxial testing for rutting and fatigue and indirect tension test for thermal cracking - presently being evaluated

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Example - Level 1

Interstate 43 in Milwaukie, Wisconsin Nominal max aggregate size = 19mm Design ESALs = 18,000,000

this would actually require a level 3 design

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Select a PG 58-34 mix temp = 165 to 172°C compaction temp = 151 to 157°C

5 stockpiles - measure bulk and apparent specific gravities

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Measure 4 consensus properties can measure blends but will use individual in this case to

estimate blend properties which has an advantage in choosing blends

final design blends must meet criteria not individual stockpiles

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3 blends are produced 1 - Intermediate, 2 - Coarse, 3 - Fine

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Blends are checked against gradation requirements and for consensus properties - all 3 pass

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Trial asphalt content absorption water calculated from bulk and apparent specific

gravities of aggregate blends Absorption water % = 100 (1/Gsb-1/Gsa) Blend 1 = .9% water absorption Blend 2 = 1.0%, Blend 3 = .9%

From table 6-12 in text, initial binder content is 4.4% for all 3 blends

Two samples of each blend are compacted using N ini = 8, Ndes = 109, Nmax = 174 (from design table table 6-13 text)

Maximum and bulk specific gravities (RD) (Gmm, Gmb) are measured and compactor data obtained

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Trial Blend 1

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Trial Blend 1

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Trial Blend 2

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Trial Blend 2

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Trial Blend 3

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Trial Blend 3

Calculated air voids and VMA at Ndes

AV = 100 - %Gmm

1 = 100-96.2 = 3.8%, 2 = 4.3%, 3=4.8% %VMA = 100 - (%Gmm x Gmm x (1 - Pbi))/Gsb

1 = 100-(96.2x2.563x.956)/2.699 = 12.7%2 = 13.0%3 = 13.5%

• The % max density at Nini , Ndes, Nmax is calculated from an average of the two samples of each blend

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Estimate properties @ 4% air voids Pb,est = Pbi - (.4 x(4-AV))

1 = 4.4-(.4x(4-3.8)) = 4.3% 2 = 4.5% 3 = 4.7%

VMAest = VMAinitial + C x (4 - AV)

1 = 12.7 + (.1 x (4.0 - 3.8)) = 12.7% 2 = 13.0 + (.2 x (4.0 - 4.3)) = 13.0% 3 = 13.5 + (.2 x (4.0 - 4.8)) = 13.4%

%VFAest = 100x (VMAest - 4)/VMAest

1 = 100 x (12.7 - 4.0)/12.7 = 68.5% 2 = 69.2% 3 = 70.1%

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Estimate properties at 4% Air Voids %Gmm,est @ Nini = % Gmm,trial - (4-Av,trial)

1 = 87.1-(4-3.8) = 86.9%2 = 85.9%3 = 87.1%

Similarly for Nmax1 = 97.4%2 = 97.7%3 = 97.3%

Results for 4% air voids for 3 blends:

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Dust ration = P0.75/Pbe,est1 = 3.1/3.6 = .862 = 2.9/3.7 = .783 = 3.5/4.0 = .88All pass with values between .6 and 1.2

Blend 1 fails VMA, blend 2 just meets VMA, therefore select blend 3

If no blends were acceptable start over with new stockpile ratios

Evaluate blend 3 as before by mixing 2 specimens with 4.2%, 4.7%, 5.2% and 5.7% AC contents

Note: this time aggregate consensus properties must be measured on the blends and not calculated from stockpile values

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Calculating mix volumetric properties (not estimating) for the final blend use:

Calculated air voids, VMA, VFA at Ndes

%AV = 100 - %Gmm

%VMA = 100 - (%Gmm x Gmm x (1 - Pbi))/Gsb

%VFA = 100 x ((VMA - AV)/VMA)

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Blend is tested for moisture sensitivity and has tensile strength ratio of 82.6% which exceed the 80% minimum requirement

This blend meets all required design criteria and is approved for use!