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Sup erior Per forming Asphalt Pave ments. 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 - PowerPoint PPT Presentation
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ENCI 579 8 1
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terms of ESAL’s Superpave Shear tester (been replaced by triaxial) Indirect Tensile tester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
ENCI 579 8 14
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
ENCI 579 8 16
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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
ENCI 579 8 18
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
ENCI 579 8 24
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
ENCI 579 8 31
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%
ENCI 579 8 32
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)
ENCI 579 8 43
ENCI 579 8 44
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!