1. Behavior of Asphalt Binder and Asphalt Concrete

2. 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)

3. AC Mix Design WEIGH components in production
Asphalt Concrete = binder + aggregate
select & proportion components that provide adequate performance over design 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 fracture (tensile) strength thermal characteristics
adequate performance assessed based on MIXTURE PROPERTIES
stiffness
stability
durability
flexibility
fatigue resistance
fracture (tensile) strength
thermal characteristics
skid resistance
permeability
workability

5. ASPHALT CONCRETE MIXTURES
Asphalt Concrete = binder + aggregate
3 stages of Life
mixing (fluid asphalt cement)
curing (viscoelastic solid)
aging (environmental effects & loading)

6. Factors Influencing the Behavior
Behavior depends on:
Temperature
Time of loading (Traffic Speed)
Aging (properties change with time)
This specification uses tests which evaluate the fundamental material properties (stress, strain, and strain rate). Changes in asphalt properties due to temperature, rate of loading and the effect of aging are considered.
Asphalt is a viscoelastic material. That is, it can both exhibit elastic and viscous properties at the same time. To demonstrate, take a penetration tin of asphalt cement at room temperature. Press your thumb into the asphalt; this will leave a large depression in the surface. Then use a hammer to hit the a remaining flat section of the asphalt; this will not leave much of an impression. Under light but long term loads, the asphalt has a noticeable viscous behavior (it flows out from under the load). Higher, but much shorter duration loads result in a primarily elastic response.

7. Permanent Deformation
Ruts can be very visible in extreme cases such as the one shown in this photo. Other places where rutting can be observed are at stop lights. In many cases, the crosswalk lines can highlight this type of distress.
Courtesy of FHWA
Function of warm weather and traffic

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

9. 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

10. Stability

11. Flexibility Flexibility
ability to conform to long-term variations in underlying layer elevations
settlement (clay), heave (frost, moisture)
open-graded aggregate
 binder
Flexibility

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

14. Tensile (Fracture) Strength
resistance to thermal cracking
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

15. Low Temperature Behavior
Cold Climates
Winter
Rapid Loads
Fast moving trucks
At cold temperatures, or under very quick loads, the binder response is predominately elastic.

16. Thermal Cracking Courtesy of FHWA
Thermal cracks are transverse cracks, usually at relatively evenly spaced intervals. The spacing gets closer together with increasing binder stiffness the colder the temperatures.
Courtesy of FHWA

17. Aging Asphalt reacts with oxygen Short term Long term
“oxidative” or “age hardening”
Short term
Volatilization of specific components
During construction process
Long term
Over life of pavement (in-service)
Aging also needs to be considered in the specification as oxidation and heat hardening during tank storage, mixing and placement (short term aging) of the asphalt concrete change the properties of the original binder.
Long term aging refers to the changes in binder property after 7 to 10 years of exposure to environmental factors.

18. 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

19. Durability 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

20. Mix Design
select & proportion component materials to obtain desired reasonable cost
properties of component materials
properties of composite material
economic factors & availability of materials
construction methods

21. Mix Design select aggregate blend determine optimum binder content
balance desired properties

22. Mix Design

23. Mix Design selection of aggregate blend selection of binder content
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

24. Thermal Cracking Courtesy of FHWA
Thermal cracks are transverse cracks, usually at relatively evenly spaced intervals. The spacing gets closer together with increasing binder stiffness the colder the temperatures.
Courtesy of FHWA

25. Binder-Aggregate Bonding
wettability
viscosity (temp)
composition (oxygen)
durability
surface chemistry (mineral composition)
surface texture
porosity
surface condition (cleanliness, moisture)

26. Binder-Aggregate Bonding
ac wetting the aggregate surface
low surface energy
need dry aggregates
polar nature of ac / electrostatic interaction
mechanical bonding
failure
interface
stripping

27. Binder-Aggregate Bonding

28. 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

29. Composite Material

30. Permanent Deformation
Ruts can be very visible in extreme cases such as the one shown in this photo. Other places where rutting can be observed are at stop lights. In many cases, the crosswalk lines can highlight this type of distress.
Courtesy of FHWA
Function of warm weather and traffic

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

32. 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.

33. 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

34. Series Model The particulate and matrix carry the same stress.
Used to describe hard particles in a soft matrix

35. Hirsch’s Model gg
X: represents the degree of bonding

36. Viscoelastic Behavior of Asphalt Concrete
Viscoelastic response =
Immediate elastic +
Time dependent viscous
Strain
Elastic
Stress to
time tr
Strain to
time tr
Viscous to tr
time

37. 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.

38. Basic responses Strain Elastic to time tr Stress Strain Viscous to to

39. Maxwell Model Constant Stress (Creep) Constant Strain (Relaxation)
time
time

40. Kelvin Model Constant Stress (Creep) Constant Strain (Relaxation)
time
time

41. Burger Model
Constant Stress
(Creep)
Strain
time

42. Asphalt Binder Behavior
Temperature scale
Elastic part
is negligible
Viscous
behavior
Temperature
Value depends
on asphalt type
Viscoelastic
behavior
fluid
Semi solid or solid

43. Viscous Behavior of Fluids
Shear
Stress
Shear
Stress
Slope = 
(Viscosity)
yield
Yield
stress
Shear
Rate
Shear
Rate
Non Newtonian
Bingham behavior
Newtonian

44. Viscous Behavior of Fluids
Shear
Stress
Shear
Stress
Shear
Rate
Shear
Rate
Non Newtonian
Shear Thinning
Non Newtonian
Shear Thickening
Increase in viscosity with
increase in strain rate
Decrease in viscosity with
increase in strain rate

45. 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.

46. Permanent Deformation
Ruts can be very visible in extreme cases such as the one shown in this photo. Other places where rutting can be observed are at stop lights. In many cases, the crosswalk lines can highlight this type of distress.
Courtesy of FHWA
Function of warm weather and traffic