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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


Warning: You could spend a week discussing mix design approaches and best practices. We only have 1 hour!

http://classes.engr.oregonstate.edu


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% +)




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% +)



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


"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


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


Review and Gather Needed Information

Materials Selection

Binder

Aggregate

Recycle

Aggregate Blending


Performance Testing

Utah DOT uses AASHTO M323 and R35 for Superpave Mix Design with some exceptions which will be noted.

General Mix Design Steps


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.


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


• 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


Proper preparation makes the mix design process go smoother and helps reduce surprises!

Materials Selection

Asphalt Binder


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


PG Grading System


Required Binder Determination Using the LTPP Bind Software


LTPP Bind – A View of Utah


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


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


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



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


• 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


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


Impact of Aggregate Specific Gravity


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


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)


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)


LA Abrasion

Indicates an aggregate’s ability to withstand breakdown during handling, mixing and placement (compaction)

AASHTO T96

Coarse Aggregate Angularity (aka Fractured Faces)


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


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


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


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)


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?


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.)


Recommended RAP Sampling and Testing Guidelines

NCAT recommendations on RAP sampling and testing. (From: NCHRP 9-46)


Utah DOT RAP Guidelines


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


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


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


SMA

Superpave

OGFC

Dense Graded Asphalt Mixes


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


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


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


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


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

53

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



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!)


Compaction


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


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


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



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


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


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.


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


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


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


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


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


Impact of Mix Characteristics on Performance


Utah requires the Hamburg Wheel Tracking testing of designed mixes to evaluate rutting and moisture susceptibility.

Hamburg Wheel Tracker


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


Shane Buchanan

Asphalt Performance Manager

Oldcastle Materials Company

[email protected]

205-873-3316

Questions?


http://www.pennyauctionwatch.com/

mailto:[email protected]

mailto:[email protected]

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