Behavior of Asphalt Binder and Asphalt Concrete

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					Behavior of Asphalt Binder
  and Asphalt Concrete




                             1
             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)




                                      2
                     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


                                                                  3
                   AC Mix Design

   adequate performance assessed based on MIXTURE
    PROPERTIES

   stiffness               fracture (tensile) strength
   stability               thermal characteristics
   durability              skid resistance
   flexibility             permeability
   fatigue resistance      workability



                                                           4
          ASPHALT CONCRETE
              MIXTURES

   Asphalt Concrete = binder + aggregate
   3 stages of Life
       mixing (fluid asphalt cement)
       curing (viscoelastic solid)
       aging (environmental effects & loading)




                                                  5
Factors Influencing the Behavior


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




                                              6
  Permanent Deformation




                           Courtesy of FHWA



Function of warm weather and traffic
                                              7
Stability

   resistance to
    permanent
    deformation under
    repetitive loading
   rutting, shoving
   Marshall Stability




                         8
                     Stability
   mechanical / frictional interlock between
    aggregate particles
   same factors that influence creep

                                rough, angular, dense-
                                 graded aggregate
                                 binder (w/ voids filled)
                                  Sac
          Stability
                             

                                 degree of compaction
                                 (> 3% air)



                                                              9
Stability




            10
                     Flexibility

   ability to conform to long-term variations in
    underlying layer elevations
   settlement (clay), heave (frost, moisture)


                                    open-graded
                                     aggregate
           Flexibility              binder




                                                    11
             Fatigue Resistance

    resistance to fracture caused by repetitive loading
     (bending)
    fatigue (alligator) cracking
                               dense-graded aggregate
                                binder
Fatigue Resistance             degree of compaction




                                                           12
13
        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
Tensile Strength                   binder

                                                                   14
        Low Temperature Behavior

   Low Temperature
       Cold Climates
       Winter
   Rapid Loads
       Fast moving trucks




                                   15
Thermal Cracking




               Courtesy of FHWA   16
                       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)


                                                17
                   Permeability

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

                             dense graded aggregate
   Permeability              degree of compaction
                              binder




                                                       18
                     Durability

   resistance to weathering & abrasive action of traffic
   exposure to air (aging), water, & traffic
   moisture damage (stripping, loss of stiffness),
    accelerated aging
                         strong, hard, clean, dry aggregate
         Sac            resistant to polishing, crushing, freeze-
         binder         thaw effects; not water sensitive
                         dense graded aggregate
                           degree of compaction
    Durability       




                                                                 19
                      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




                                                       20
               Mix Design


   select aggregate blend
   determine optimum
    binder content
   balance desired
    properties




                             21
                        Mix Design
              Asphalt      Aggregate       Binder
               Type        Gradation       Content
                                                     Degree of
 Property    Hard Soft Dense    Open       High Low Compaction
 Stability    X            X                    X      High

Durability   ----   ----   X                X          High
 Fatigue      X
                           X                X          High
Resistance (thick)
 Tensile
              X            X                X          High
 Strength
   Skid                            X
             ----  ----                         X      ----
Resistance                     (surface)


                                                                 22
                     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

                                                           23
Thermal Cracking




               Courtesy of FHWA   24
     Binder-Aggregate Bonding

Binder                       Aggregate

   wettability             surface chemistry (mineral
   viscosity (temp)         composition)
   composition (oxygen)    surface texture
   durability              porosity
                            surface condition
                             (cleanliness, moisture)



                                                      25
        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




                                                         26
Binder-Aggregate Bonding




                           27
              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


                                                                28
Composite Material




                     29
  Permanent Deformation




                           Courtesy of FHWA



Function of warm weather and traffic
                                              30
Description of Asphalt Concrete

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




                                                       31
       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.

                                                 32
                        Parallel Model


Vp = volume of particulate

Vm = volume of matrix




                             The particulate and matrix
                             carry the same strain.


                        E c  E p Vp  E m Vm
                  Used to describe soft particles in a hard matrix
                                                                     33
        Series Model




         The particulate and matrix
         carry the same stress.

                      EpEm
      Ec 
               E m Vp  E p Vm
Used to describe hard particles in a soft matrix

                                                   34
         Hirsch’s Model




                             Vp Va 
1
    X
            1       1  X   
Ec    V E V E              E E 
       p p   a a             p   a 

         X: represents the degree of bonding

                                               35
         Viscoelastic Behavior of Asphalt
                     Concrete
 Viscoelastic response =
 Immediate elastic +             Strain

 Time dependent viscous    Elastic


Stress                                        to    time   tr


                                     Strain
          to   time   tr
                            Viscous


                                               to           tr
                                                    time         36
             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.

                                                  37
                          Basic responses
                            Strain

                            Elastic
                                                                G
                                       to   time    tr
Stress
                           Strain

                          Viscous


         to   time   tr               to    time   tr
                                                                
                                                                   
                          Strain
                          Viscous



                                                                       38
                                      to    time         tr
                     Maxwell Model


                            total   s   d

                                       
                            total    
                                      G 
         Constant Stress                          Constant Strain
         (Creep)                                  (Relaxation)

Strain                                   Stress




                                                                    39
                 time                                    time
                     Kelvin Model



                      total  s  d

Constant Stress
                      total  G  
                                            Constant Strain
(Creep)                                      (Relaxation)



Strain                              Stress



                                                    time
                                                               40
              time
Burger Model




         Constant Stress
         (Creep)


Strain




                  time
                           41
Asphalt Binder Behavior

           Temperature scale



Elastic part          Temperature
is negligible         Value depends
                      on asphalt type
Viscous
                      Viscoelastic
behavior
                      behavior
  fluid               Semi solid or solid

                                            42
         Viscous Behavior of Fluids


                            Shear
Shear
                       Stress

                             
Stress
                                              yield  
 
              Slope =        yield
              (Viscosity)     Yield
                              stress
           Shear
           Rate      
                                          Shear
                                          Rate    
                                       Non Newtonian
         Newtonian                     Bingham behavior


                                                           43
         Viscous Behavior of Fluids

Shear                   A n   Shear                  A n
                                  Stress
Stress                n1                              n1
                                 
             Shear
             Rate    
                                               Shear
                                               Rate
                                                       
          Non Newtonian                      Non Newtonian
          Shear Thinning                     Shear Thickening

     Increase in viscosity with        Decrease in viscosity with
     increase in strain rate           increase in strain rate

                                                                   44
      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.



                                                   45
 Permanent Deformation




                           Courtesy of FHWA



Function of warm weather and traffic
                                              46

				
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