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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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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/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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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 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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Permanent Deformation
Function of warm weather and traffic
Courtesy of FHWA