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Asphalt Mixes and Design
U TA H A S P H A LT C O N F E R E N C E
F E B R UA RY 2 5 – 2 6 , 2 0 1 5
S H A N E B U C H A N A N , O L D C A S T L E M AT E R I A L S
Utah Asphalt Mixes
Dense Graded (focus)
Stone Matrix Asphalt
Open Graded Friction Course
Asphalt Mix Design
Materials Selection
Aggregate Blending
Compaction and Volumetrics
Performance Testing
Asphalt Mixes and Design Discussion Topics
Utah Asphalt Conference | 2015
http://www.lib.utexas.edu/maps/us_2001/utah_ref_2001.jpg
Hot Mix Asphalt (HMA) – Focus of this presentation
UTAH Standard Specification SECTION 02741
Stone Matrix Asphalt (SMA)
Open Graded Friction Course (OGFC)
Utah Asphalt Mixes
Utah Asphalt Conference | 2015
Warning: You could spend a week discussing mix design approaches and best practices. We only have 1 hour!
http://classes.engr.oregonstate.edu
Stone Matrix Asphalt (SMA)
Premium, rut resistance mix
Typically placed on higher traffic routes
Characteristics
Increased amount of coarse aggregate
Increased amount of minus No. 200 (filler)
Stabilizing fiber to prevent draindown
High asphalt binder content (6% +)
Stone Matrix Asphalt (SMA)
Utah Asphalt Conference | 2015
Open Graded Friction Course
Similar to SMA, but without the increased filler content
Used to reduce surface water spray and to reduce noise
Typically placed on higher traffic routes
Characteristics
Increased amount of coarse aggregate
Stabilizing fiber to prevent draindown
High asphalt binder content (6% +)
Open Graded Friction Course
Utah Asphalt Conference | 2015
http://www.highwaysmaintenance.com/bitdiags.htm
Mix Design Thoughts
Combining aggregates, asphalt binder, and recycled materials to meet project requirements while being cost effective.
Remember both Performance and Economics
What are the specifications?
What aggregates are available?
What asphalt binder is available?
What does past experience provide?
Did some mixes work better than others?
Mix Design Thoughts
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"Boonsborough Turnpike Road" between Hagerstown and Boonsboro, Maryland in 1823.
Mix Design is a Balance (3 legged stool)
Materials Selection
Producability
Constructability
You must not design a mix in a vacuum!
Get input from production and construction personnel
Know the cost of mix components
Conduct mix audits to understand what worked and what didn’t
Decision
Redesign
Accept
Tweak
The Mix Design Cycle
Utah Asphalt Conference | 2015
1890 •Barber Asphalt Paving Company
•Asphalt cement 12 to 15% / Sand 70 to 83% / Pulverized carbonite of lime 5 to 15%
1905
•Clifford Richardson, New York Testing Company
•Surface sand mix: 100% passing No. 10, 15% passing No. 200, 9 to 14% asphalt
•Asphaltic concrete for lower layers, VMA terminology used, 2.2% more VMA than current day mixes or ~0.9% higher binder content
1920s
•Hubbard Field Method (Charles Hubbard and Frederick Field)
•Sand asphalt design
•30 blow, 6” diameter with compression test (performance) asphaltic concrete design (Modified HF Method)
1927
•Francis Hveem (Caltrans)
•Surface area factors used to determine binder content; Hveem stabilometer and cohesionmeter used
•Air voids not used initially, mixes generally drier relative to others, fatigue cracking an issue
1943
•Bruce Marshall, Mississippi Highway Department
•Refined Hubbard Field method, standard compaction energy with drop hammer
•Initially, only used air voids and VFA, VMA added in 1962
1993
• Superpave
• Level 1 (volumetric)
• Level 2 and 3 (performance based, but never implemented)
Evolution of Mix Design
http://asphaltmagazine.com/history-of-asphalt-mix-design-in-north-america-part-2/
ANPC 2015
The Path to Mix Design
AASHTO M323 Superpave Volumetric Mix Design
AASHTO R35 Standard Practice for Superpave Volumetric Design for Hot Mix Asphalt (HMA)
UDOT Part 8 – Materials Manual: Section 960: Guidelines for Superpave Mix Design and Verification
Utah DOT Specific Guidance Documents
Utah Asphalt Conference | 2015
Review and Gather Needed Information
Materials Selection
Binder
Aggregate
Recycle
Aggregate Blending
Compaction and Volumetrics
Performance Testing
Utah DOT uses AASHTO M323 and R35 for Superpave Mix Design with some exceptions which will be noted.
General Mix Design Steps
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Perform Critical Review of Historical Mixes
Review the design, production and construction of historical mixes
Thoughts:
Identify successful mixes (maximized profitability, minimized cost, good producability and constructability).
Tweak existing mixes to increase success.
Eliminate the mixes with significant issues.
Design new mixes based on past successful performing mixes
Don’t re-invent the wheel (they are always round!)
Don’t stop trying to advance the ball forward!
Opportunity that exists must be acted upon.
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General Information
Mix specifications and special provisions
Agency or Private / Commercial
Nominal maximum size (NMS)
Allowable recycle
WMA
Lift thickness
Project location relative to plant location Materials
Aggregates
Asphalt binders
Recycle materials
RAP, RAS, ground tire rubber, etc.
Gather Needed Information
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• Aggregate information (Internal or External)
• Sources available
• Products
• Average grading and standard deviation
• Physical properties (chemical, if needed)
• Specific gravities and absorption
• Cost (total) • Capped Products
• Asphalt binder information
• PG continuous grade (obtain from supplier)
• Sources of crude stock
• Polymer type and loading (if modified)
• Alternate binders available for grade bumping
• Cost
Recycled Products
RAP and RAS
Stockpile average grading and standard deviation
Asphalt content
Continuous PG grading
Physical properties
Specific gravities and absorption
Cost (total)
Other materials
Rejuvenators
Fiber
Mineral filler
Gather Needed Information
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Proper preparation makes the mix design process go smoother and helps reduce surprises!
Materials Selection
Asphalt Binder
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Today’s binders are classified using the Superpave Binder Classification System
Greater the absolute difference between the high and low, the wider the range of expected performance.
Generally abs. differences above “92” indicate the need for polymer modifiers
Bumping a grade would be going from “64” to a “70” or from a “-22” to a “-28”
What grades do you use?
Asphalt Binder
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PG Grading System
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Required Binder Determination Using the LTPP Bind Software
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LTPP Bind – A View of Utah
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Aggregates are selected based on local availability, price, and adherence to specification requirements.
Utah has many aggregate types
Sand and gravel, limestone, granite, etc.
What is the best aggregate?
Aggregate
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Specific Gravity and Absorption
Fractured Face
Fine Aggregate Angularity
Los Angeles Abrasion (Wear)
Sand Equivalent
Soundness (sodium sulfate)
Clay Lumps and Friable Particles
Plasticity Index
Typical Aggregate Required Testing
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Specific Gravity
Specific Gravity or “Relative Density”
Bridge between mass and volume
Ratio of the solid unit weight of a substance relative to the weight of de-aired, water at room temperature displaced by that object
One of the most important properties
Utah Asphalt Conference | 2015
Utah Asphalt Conference | 2015
FHWA RAP Expert Task Group
Gse is an aggregate property
For a given mix design, the relationship between Gse to Gsa and Gsb should not change (within test variability)
Relationship may change based on mix temperature (binder absorption)
If mix temperature is constant and a change is seen, something has changed in the aggregate blend (e.g., “heavy vs. light” aggregate).
Aggregate Control Chart
Develop control chart showing the Gsa and Gsb of the aggregate blend and the Gse determined values.
Identify trends before problems show up.
Monitoring Gse During Production
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• Evaluates up to 30 test results during production
• User inputs of Gsa, Gsb, Gse
• Allows trends in Gse and binder absorption to be identified
Aggregate Specific Gravity Control Chart
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Bulk specific gravity (Gsb) in a saturated surface dry (SSD)
Similar to Gsb (Dry), but the SSD mass is used with the bulk volume.
Utah DOT Also Uses of Gsb SSD
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Impact of Aggregate Specific Gravity
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Understand the impact of changing specific gravity
Variable gravities will impact volumetrics
Changing aggregate gravities can impact binder volume in the mix
Heavy aggregate relative to design = higher binder volume (and vice versa)
0.1 Gsb ∆ = 0.2% AC ∆
Do your gravities change around your quarry/pit? Probably!
How much?
0.01 Gsb ∆ = 0.3% VMA ∆
Impact of Aggregate Specific Gravity
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Example:
100 ft3 of
Aggregate A w/ specific gravity = 2.90
Mass = 2.90 (100 ft3) (62.4 lb / ft3) = 18,096 lb
Aggregate B w/ specific gravity = 2.70
Mass = 2.70 (100 ft3) (62.4 lb / ft3) = 16,848 lb
Impacts outside of binder content
Inventory
Yield (lay down thickness)
Coarse Aggregate Angularity
“Fractured Faces”
Identified as 2nd most important predicator of pavement performance
Aggregate “interlock” and internal friction
AASHTO T335
Highly subjective visual examination
Gravel sources can not meet highest ESAL criteria
Fails to distinguish aggregate 100%> 2 fractured faces
Coarse Aggregate Angularity (aka Fractured Faces)
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Fine Aggregate Angularity Uncompacted Voids
Particle shape, angularity and texture help define rut resistant fine aggregate
AASHTO T304, Method A
Fine Aggregate Angularity (aka Uncompacted Voids)
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LA Abrasion
Indicates an aggregate’s ability to withstand breakdown during handling, mixing and placement (compaction)
AASHTO T96
Coarse Aggregate Angularity (aka Fractured Faces)
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Aggregates in HMA must be able to withstand freezing and thawing conditions. Unsound material will result in breakdown and potential pavement failure
Sulfate solution simulates salt solution
Material is soaked / dried for 5 to 10 cycles and loss measured
ASTM C88
Sodium Sulfate Soundness
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Goal is to determine the amount of claylike materials present in the aggregate blend
Clay coating may degrade the bond between binder and aggregate
AASHTO T176
Sand Equivalent
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SE = Sand/Clay *100
Determining the “flat and elongated” shape of coarse aggregate (+3/8”).
Used in lieu of, or sometimes along with, the flat and elongated test.
UDOT MOI 933
“The Flakiness Index of an aggregate is the mass of particles in that aggregate which will pass the appropriate slot or slots for the individual size fraction expressed as a percentage of the total mass of that aggregate. The width of the slots is determined by multiplying the mean of upper and lower sieve size for the slot by 0.6.”
Flakiness Index
Utah Asphalt Conference | 2015
Recycled Asphalt Pavement (RAP) is utilized in most all asphalt mixes.
Utilized percentage is based on specification, quality, and overall economics.
Typical specifications allow a certain threshold addition of RAP below which no binder grade adjustment is required.
Recycled Asphalt Pavement (RAP)
Utah Asphalt Conference | 2015
RAP Bulk Specific Gravity (Gsb)
Gsb
Substantial VMA errors can occur if the incorrect RAP Gsb is utilized
Especially critical with high RAP mixes
What is the best way to determine the RAP Gsb?
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Acceptable RAP Property Tolerances (i.e., Engineering Limits)?
NCAT recommendations on RAP AC and grading.
Can you make RAP with higher variability work?
Yes, but it becomes more difficult!
Increases risk of potential pay factor deductions.
Must evaluate the use of any product (virgin or recycle) against down stream results (e.g., volumetrics, constructability, in-place density, re-work history, etc.)
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Recommended RAP Sampling and Testing Guidelines
NCAT recommendations on RAP sampling and testing. (From: NCHRP 9-46)
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Utah DOT RAP Guidelines
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Aggregate Blending
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
Mix Aggregate Blend Gradings
Utah Asphalt Conference | 2015
First step in blending is to identify the target grading bands for the mix being designed.
Master control bands are typically established by agencies.
Max size
NMS
Intermediate or “break point” sieve
No. 200
Mix Aggregate Blend Gradings
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Uniformly Graded
Few points or contact
Poor Interlock (shape dependent)
High permeability
Well Graded
Good interlock
Low permeability
Gap Graded
Only limited sizes
Good interlock
Low permeability
Types of Gradings
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SMA
Superpave
OGFC
Dense Graded Asphalt Mixes
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Blends are normally plotted on a “45 Power” chart
Each sieve in mm is raised to the 0.45 power and that value plotted.
12.5 mm = 12.50.45 = 3.12
• 45 power grading should yield the tightest packing of aggregate possible
Typical Aggregate Blend Grading
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Many blending spreadsheet can be used to determine a blend that will meet specification.
However, not the vast majority of determined blends are optimized.
Must consider the total picture when evaluating blends
Total picture includes aggregate grading, variability, moisture, asphalt absorption, etc.
Must move beyond a “deterministic” approach into a “probabilistic” approach, which considers variability and offers the ability to evaluate risk.
Optimize the Aggregate Blend
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The standard deviation is an indication in the “spread” of the data
Data falls within the normal distribution 68, 95, 99.7%
Use statistics to optimize your operations (i.e. maximize benefit + minimizing risk)
Evaluate Aggregate Product Variability
Utah Asphalt Conference | 2015
You MUST know and understand the gradation variability of available aggregates (including recycle)
Inconsistent supply WILL impact mix grading and volumetrics
Accurate and precise aggregate grading
Accurate means the average remains near where the mix design was developed
Precise means the standard deviation (variability) is not too great
You can work with a grading that is at the high or low side of a target size grading
Example: Coarse 78’s vs fine 78’s
BUT, don’t give me coarse 78’s one day and fine 78’s the next!
Aggregate Grading Variability
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Accuracy (Avg.) and Precision (Std. Dev.)
Utah Asphalt Conference | 2015 Which is Best/Worst?
Aggregate Grading Variability and PWL
Arkansas APA Quality Asphalt Conference | January 2015
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Excessive aggregate grading variability will is highly detrimental within a PWL specification.
Impacts go beyond grading to volumetrics, in-place density and performance
What are Acceptable Levels of Variability?
Do you know your typical variability for key properties?
Property Acceptable Standard Deviation ?
Asphalt Content < 0.15 to 0.20 %
Coarse aggregate (+No. 4 blend) < 2 %
Fine aggregate (- No. 4 blend) < 1 to 1.5 %
P200 (blend) < 0.5 %
Air voids and VMA < 0.3 to 0.4 %
Field density < 1.0 to 1.5 %
Film thickness < 0.50 microns
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Compaction and Volumetrics
NCHRP Report 673
Design and optimum are often used interchangeably
However, they mean two different things
There can be many design binder contents for a mix, but only one truly optimum
Optimum indicates the best binder content based on specification adherence, performance, and ultimately economics
Goal is to get as close as possible to the true optimum for the mix
Binder Content – Design vs. Optimum (There is a difference!)
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Compaction
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Generally, lab compaction is accomplished using the Superpave Gyratory Compactor.
Ninitial, Ndesign, Nmax
Marshall hammer is also utilized
Blows
Regardless of the compaction device, the compaction level is normally selected based on the anticipated traffic volume
High volume = Interstate
Low volume = County road
AASHTO M323 Standard Specification for Superpave Volumetric Mix Design
http://www.udot.utah.gov/main/uconowner.gf?n=13200916496713292
Volumetrics Basics
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Mass/Volume relationship
Asphalt mixture produced by weight
Asphalt mixture attributes are reported by volume
Asphalt mixture consists of three Phases
Aggregate, Binder, Air
Typical mixture contains 95% aggregate and 5% binder, by weight
Typical mixture contains 85% aggregate, 10-11% binder and 4% air, by volume
Volumetrics Terms - Definition
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Air Voids (Va)
Volume of air in a compacted specimen
Maximum Theoretical Specific Gravity (Gmm)
Specific gravity with zero air present
Bulk Specific Gravity of the Compacted Mix (Gmb)
Specific gravity of the specimen with air present
Voids in the Mineral Aggregate (VMA)
Volume of effective asphalt + air voids / total specimen volume
Voids Filled with Asphalt (VFA)
How much of the VMA is effective asphalt
Dust to Asphalt Ratio (D/Aceff)
Minus No. 200 sieve materials / effective binder content (mass)
Volumetrics Requirements - UDOT
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Utah Asphalt Conference | 2015
Gmb and Gmm
Mass of agg. and AC
Vol. agg., AC, air voids Gmb =
Mass agg. and AC
Vol. agg. and AC Gmm =
• Compacted Mix • AASHTO T166 • Air/water/ssd weights • Wt / Vt
• Loose Mix • AASHTO T209 • Max density w/o air • Wt / (Vt – Va)
Mix is air, asphalt, and aggregate
Everything during mix design comes back to the very basics of this diagram
Critical to understand what mix volumetrics mean, not just how to calculate using a formula
Mix designers should work a phase diagram from start to finish at least once
Don’t be too proud to review
Mixture Phase Diagram
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VMA: A Key Volumetric Property
Understand what VMA means
VMA is the volume of air voids and effective binder (binder not absorbed into the aggregate)
Excludes the portion of binder absorbed into the aggregate
Volume air = 0.076 cm3
Volume effective AC = 0.106 cm3
Volume total = 1 cm3
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VMA = ((0.076 + 0.106) / 1) x 100 = 18.2%
Plotting VMA in Mix Design
The optimum binder content of a mix is normally located near the bottom of the VMA vs binder content curve.
Why does the curve have this shape?
Binder initially acts as a lubricant which aids in aggregate packing to a maximum state. Afterwards, excess binder acts to push the aggregate apart resulting in a lower density.
Utah Asphalt Conference | 2015
Factor Effect on VMA
Aggregate Gradation Dense aggregate blend grading decrease VMA
Aggregate Shape Rounded aggregates decrease VMA
Aggregate Texture Smooth/polished aggregates decrease VMA
Asphalt Absorption Increased asphalt absorption results in lower effective asphalt content and lower VMA (for same compaction level)
Dust Content Higher dust contents increase surface area, decrease film thickness and tend to lower VMA
Baghouse Fines/Dust Generation Same as above
Plant Production Temperature Higher production temperatures decrease asphalt binder viscosity, resulting in more asphalt absorption, lower effective asphalt content and lower VMA
HMA Temperature During Paving Higher temperatures during paving increases mix viscosity, resulting in lower air voids and lower VMA
Haul Time Longer haul times allow for more asphalt absorption, lower effective asphalt content and lower VMA
Aggregate Handling More steps in handling increases potential for degradation, resulting in more fines and lower VMA.
Factors Influencing VMA
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Shape
Angular particles produce mixtures with higher VMA than rounded
Cubical particles create higher VMA than those with rounded edges
Texture
Rougher texture particles pack less tightly
Smooth particles compact more easily and result in lower VMA
Fractured aggregate generally rougher texture and results in higher VMA
“ Bite”
Manufactured sand vs. Concrete sand
Aggregate Specific Impacts on VMA
Utah Asphalt Conference | 2015
Grading
Move away from Maximum Density line (in either direction) improves VMA
Reducing #200
Coarse graded mixtures require reducing passing 4.75 to reduce packing
Fine graded mixture increase #2.36 (although this might increase #200)
Reducing #200 to the extent D/Be will allow improves VMA
Grading Specific Impacts on VMA
Utah Asphalt Conference | 2015
Depends on many factors including
VMA minimum specification, VMA control during production, VMA as pay item, historical VMA production “collapse”, aggregate Gsb variability impacts, VMA production variability
The decision should be made locally, BUT it should be well thought out and documented.
Generally 0.3 to 0.7% VMA is lost during production.
Very dependent on breakdown potential of aggregates
If you are paid on VMA, use historical VMA variability to minimize the amount of extra VMA targeted for during design
Strongly suggest reviewing VMA requirements through statistical measures.
Mix Nominal Max Size Minimum VMA, %
4.75 16.0
9.5 15.0
12.5 14.0
19.0 13.0
25.0 12.0
37.5 11.0
AASHTO M323
REMEMBER: Every 0.25% of unnecessary VMA results in $0.50 of unnecessary mix cost!
Determine How Much VMA is Needed
Utah Asphalt Conference | 2015
Performance Testing
A mix needs appropriate binder to have good stability (resist rutting) and durability (resist cracking) performance
Additionally, the mix should have resistance to moisture damage (i.e., stripping).
Performance Testing Importance
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Impact of Mix Characteristics on Performance
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Utah requires the Hamburg Wheel Tracking testing of designed mixes to evaluate rutting and moisture susceptibility.
Hamburg Wheel Tracker
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Many tests are currently being evaluated to predict asphalt mix cracking potential.
Bending beam fatigue
Semi circular bend
Disk shaped compact tension
Indirect tensile strength / fracture energy
No one test seems to be able to accurately predict performance.
Search continues…..
Durability Performance Testing
Utah Asphalt Conference | 2015
Shane Buchanan
Asphalt Performance Manager
Oldcastle Materials Company
205-873-3316
Questions?
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