Asphalt Concrete and Asphalt Binder Behaviors

7/30/2019 Asphalt Concrete and Asphalt Binder Behaviors

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Behavior of Asphalt Binderand 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


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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 CONCRETE

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 optimumbinder 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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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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Composite Material

2 components physically combined w/someAIR 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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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 elasticbehavior.


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Parallel Model

The particulate and matrixcarry 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 matrixcarry the same stress.

mppm

mp

cVEVE

EEE

Used to describe hardparticlesin a soft matrix


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Hirschs Model

aa

p

p

aappc EV

E

VX1

EVEV1X

E1

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 andphysical interactions that govern the response.


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Basic responses

G

Viscous

to tr

Stress to trtime

Strain

to trtime

Elastic

time

Viscous

to trtime

Strain

Strain


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Maxwell Model

total s d

totalG

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

Viscoelasticbehavior

Temperature

Value depends

on asphalt type

Elastic part

is negligible

Viscous

behavior

Temperature scale

Semi solid or solidfluid


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Viscous Behavior of Fluids

yield

ShearStress

Shear

Rate

Slope = (Viscosity)

ShearStress

Shear

Rate

yieldYieldstress

NewtonianNon NewtonianBingham behavior


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1n

An

ShearStress

ShearRate

ShearStress

ShearRate

1n

An

Viscous Behavior of Fluids

Non Newtonian

Shear Thinning

Non Newtonian

Shear Thickening

Increase in viscosity withincrease in strain rate

Decrease in viscosity withincrease 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 designconditions

Use the model to predict performance under

different loads and applications.


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Courtesy of FHWA

Date post:	14-Apr-2018
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Asphalt Concrete and Asphalt Binder Behaviors

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