1

Behavior of Asphalt Binder and Asphalt

Concrete

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Mixture Classification

type of binder asphalt cement liquid asphalt

aggregate gradation dense-graded (well-graded) open-graded

production method hot-mix (hot-laid)** cold-mix (cold-laid)

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AC Mix Design Asphalt Concrete = binder + aggregate

select & proportion components that provide adequate performance over design life @ reasonable cost

VOLUMETRIC process Vair > 3% to preclude bleeding, instability

Vair < 8% for durability

Vasp to coat, bind, & satisfy (absorption) agg

WEIGH components in production

4

AC Mix Design

adequate performance assessed based on MIXTURE PROPERTIES

stiffness stability durability flexibility fatigue resistance

fracture (tensile) strength thermal characteristics skid resistance permeability workability

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ASPHALT CONCRETECONCRETE MIXTURES

Asphalt Concrete = binder + aggregate 3 stages of Life

mixing (fluid asphalt cement) curing (viscoelastic solid) aging (environmental effects & loading)

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Behavior depends on: Temperature Time of loading (Traffic Speed) Aging (properties change with time)

Factors Influencing the Behavior

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Permanent Deformation

Function of warm weather and traffic

Courtesy of FHWA

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Stability

resistance to permanent deformation under repetitive loading

rutting, shoving Marshall Stability

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Stability

mechanical / frictional interlock between aggregate particles

same factors that influence creep

rough, angular, dense-graded aggregate

binder (w/ voids filled) Sac

degree of compaction (> 3% air)

Stability

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Stability

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Flexibility

ability to conform to long-term variations in underlying layer elevations

settlement (clay), heave (frost, moisture)

open-graded aggregate

binderFlexibility

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Fatigue Resistance

resistance to fracture caused by repetitive loading (bending)

fatigue (alligator) cracking

dense-graded aggregate binder degree of compactionFatigue Resistance

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Tensile (Fracture) Strength resistance to thermal cracking

important @ low temps large induced stresses (restrained contraction) weak subgrade

transverse cracking primarily controlled by binder limiting tensile strength (4-10 MPa) ~ limiting stiffness

dense graded aggregate degree of compaction binderTensile Strength

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Low Temperature Behavior

Low Temperature Cold Climates Winter

Rapid Loads Fast moving trucks

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Thermal Cracking

Courtesy of FHWA

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Aging

Asphalt reacts with oxygen “oxidative” or “age hardening”

Short term Volatilization of specific components During construction process

Long term Over life of pavement (in-service)

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Permeability

ease w/ which air & water can pass through or into AC moisture damage, accelerated aging inversely proportional to durability

dense graded aggregate degree of compaction binder

Permeability

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Durability

resistance to weathering & abrasive action of traffic exposure to air (aging), water, & traffic moisture damage (stripping, loss of stiffness),

accelerated aging

Sac

binder

strong, hard, clean, dry aggregate resistant to polishing, crushing, freeze-thaw effects; not water sensitive

dense graded aggregate degree of compactionDurability

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Mix Design

select & proportion component materials to obtain desired properties @ reasonable cost properties of component materials properties of composite material economic factors & availability of materials construction methods

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Mix Design

select aggregate blend determine optimum

binder content balance desired

properties

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Mix Design

AsphaltType

AggregateGradation

BinderContent

Property Hard Soft Dense Open High LowDegree of

Compaction

Stability X X X High

Durability ---- ---- X X High

FatigueResistance

X(thick)

X X High

TensileStrength

X X X High

SkidResistance

---- ----X

(surface)X ----

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Mix Design

selection of aggregate blend aggregate properties (primarily gradation) compactibility

selection of binder content surface area of aggregates volumetrics of mixture (air voids, voids between

aggregates) mechanical properties of mixture from laboratory

testing

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Thermal Cracking

Courtesy of FHWA

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Binder-Aggregate Bonding

wettability viscosity (temp) composition (oxygen) durability

surface chemistry (mineral composition)

surface texture porosity surface condition

(cleanliness, moisture)

Binder Aggregate

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Binder-Aggregate Bonding

ac wetting the aggregate surface low surface energy need dry aggregates polar nature of ac / electrostatic interaction

mechanical bonding failure

flaws @ interface stripping

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Binder-Aggregate Bonding

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Composite Material

2 components physically combined w/ some AIR VOIDS

1 continuous phase binder - viscous, viscoelastic aggregate** - solid

dense aggregate skeleton w/ sufficient binder to bind and provide durability

> 90% by weight aggregate

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Composite Material

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Permanent Deformation

Function of warm weather and traffic

Courtesy of FHWA

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Description of Asphalt Concrete

Particulate composite material that consists of: Aggregates. Asphalt. Air voids.

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Review of the Properties of Particulate Composites

The properties of the composite can be calculated from the properties of the constituents.

For simplicity, assume asphalt concrete to be represented by particulate (aggregates), and matrix (asphalt and air). Also, assume elastic behavior.

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Parallel Model

The particulate and matrix carry the same strain.

mmppc VEVEE

Vp = volume of particulate

Vm = volume of matrix

Used to describe soft particles in a hard matrix

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Series Model

The particulate and matrix carry the same stress.

mppm

mpc VEVE

EEE

Used to describe hard particles in a soft matrix

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Hirsch’s Model

a

a

p

p

aappc E

V

E

VX1

EVEV

1X

E

1

X: represents the degree of bonding

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to tr

Stress to trtime

Strain

to trtime

Strain

Elastic

Viscous

Viscoelastic Behavior of Asphalt Concrete

time

Viscoelastic response = Immediate elastic + Time dependent viscous

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Viscoelastic Models

Viscoelastic Model: Mathematical expression for the relationship between stress, strain, and strain rate.

Combinations of basic rheological models. The combinations mean that there are different

mechanisms due to different chemical and physical interactions that govern the response.

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Basic responses

G

Viscous

to tr

Stressto trtime

Strain

to trtime

Elastic

time

Viscous

to trtime

Strain

Strain

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Maxwell Model

total s d

total G

Constant Stress(Creep)

Constant Strain(Relaxation)

time

Strain

time

Stress

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Kelvin Model

dstotal

Constant Stress(Creep)

Constant Strain(Relaxation)

time

Strain

time

Stress

Gtotal

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Burger Model

Constant Stress(Creep)

time

Strain

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Asphalt Binder Behavior

Viscoelastic behavior

Temperature Value depends on asphalt type

Elastic partis negligible

Viscous behavior

Temperature scale

Semi solid or solid fluid

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Viscous Behavior of Fluids

yield

Shear Stress

Shear Rate

Slope = (Viscosity)

Shear Stress

Shear Rate

yield

Yield stress

NewtonianNon NewtonianBingham behavior

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1n

A n

Shear Stress

Shear Rate

Shear Stress

Shear Rate

1n

A n

Viscous Behavior of Fluids

Non NewtonianShear Thinning

Non NewtonianShear Thickening

Increase in viscosity with increase in strain rate

Decrease in viscosity with increase in strain rate

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Why do we need to model the response?

Conduct a creep or a relaxation test. Fit a model to the data. Determine the material parameters. Describe the material parameters based on design

conditions Use the model to predict performance under

different loads and applications.

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Permanent Deformation

Function of warm weather and traffic

Courtesy of FHWA