Behavior of Asphalt Binder and Asphalt Concrete

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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 _at_
  reasonable cost
• VOLUMETRIC process
• Vair gt 3 to preclude bleeding, instability
• Vair lt 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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Factors Influencing the Behavior

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

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Permanent Deformation
Courtesy of FHWA
Function of warm weather and traffic
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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 (gt 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
• ? binder

?Flexibility
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Fatigue Resistance

• resistance to fracture caused by repetitive
  loading (bending)
• fatigue (alligator) cracking

• dense-graded aggregate
• ? binder
• ? degree of compaction

?Fatigue Resistance
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Tensile (Fracture) Strength

• resistance to thermal cracking
• important _at_ 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
• ? binder

?Tensile 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 compaction

?Durability
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Mix Design

• select proportion component materials to obtain
  desired properties _at_ 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
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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
Binder
Aggregate

• wettability
• viscosity (temp)
• composition (oxygen)
• durability

• surface chemistry (mineral composition)
• surface texture
• porosity
• surface condition (cleanliness, moisture)

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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 _at_ 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
• gt 90 by weight aggregate

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Composite Material
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Permanent Deformation
Courtesy of FHWA
Function of warm weather and traffic
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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
Vp volume of particulate Vm volume of matrix
The particulate and matrix carry the same strain.
Used to describe soft particles in a hard matrix
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Series Model
The particulate and matrix carry the same stress.
Used to describe hard particles in a soft matrix
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Hirschs Model
X represents the degree of bonding
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Viscoelastic Behavior of Asphalt Concrete
Viscoelastic response Immediate elastic
Time dependent viscous
Strain
Elastic
Stress
to
tr
time
Strain
to
tr
time
Viscous
to
tr
time
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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
Strain
Elastic
to
tr
time
Stress
Strain
Viscous
to
tr
to
tr
time
time
Strain
Viscous
time
to
tr
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Maxwell Model
Constant Stress (Creep)
Constant Strain (Relaxation)
Strain
Stress
time
time
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Kelvin Model
Constant Strain (Relaxation)
Constant Stress (Creep)
Strain
Stress
time
time
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Burger Model
Constant Stress (Creep)
Strain
time
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Asphalt Binder Behavior
Temperature scale
Elastic part is negligible Viscous behavior
Temperature Value depends on asphalt type
Viscoelastic behavior
fluid
Semi solid or solid
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Viscous Behavior of Fluids
Shear Stress
Shear Stress
Slope ? (Viscosity)
?yield Yield stress
Shear Rate
Shear Rate
Non Newtonian Bingham behavior
Newtonian
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Viscous Behavior of Fluids
Shear Stress
Shear Stress
Shear Rate
Shear Rate
Non Newtonian Shear Thinning
Non Newtonian Shear Thickening
Decrease in viscosity with increase in strain
rate
Increase 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
Courtesy of FHWA
Function of warm weather and traffic