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 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
4
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
5
ASPHALT CONCRETECONCRETE MIXTURES
Asphalt Concrete = binder + aggregate 3 stages of Life
mixing (fluid asphalt cement) curing (viscoelastic solid) aging (environmental effects & loading)
6
Behavior depends on: Temperature Time of loading (Traffic Speed) Aging (properties change with time)
Factors Influencing the Behavior
7
Permanent Deformation
Function of warm weather and traffic
Courtesy of FHWA
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
ability to conform to long-term variations in underlying layer elevations
settlement (clay), heave (frost, moisture)
open-graded aggregate
binderFlexibility
12
Fatigue Resistance
resistance to fracture caused by repetitive loading (bending)
fatigue (alligator) cracking
dense-graded aggregate binder degree of compactionFatigue Resistance
13
14
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
15
Low Temperature Behavior
Low Temperature Cold Climates Winter
Rapid Loads Fast moving trucks
16
Thermal Cracking
Courtesy of FHWA
17
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)
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
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
20
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
21
Mix Design
select aggregate blend determine optimum
binder content balance desired
properties
22
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 ----
23
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
24
Thermal Cracking
Courtesy of FHWA
25
Binder-Aggregate Bonding
wettability viscosity (temp) composition (oxygen) durability
surface chemistry (mineral composition)
surface texture porosity surface condition
(cleanliness, moisture)
Binder Aggregate
26
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
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
Function of warm weather and traffic
Courtesy of FHWA
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
The particulate and matrix carry the same strain.
mmppc VEVEE
Vp = volume of particulate
Vm = volume of matrix
Used to describe soft particles in a hard matrix
34
Series Model
The particulate and matrix carry the same stress.
mppm
mpc VEVE
EEE
Used to describe hard particles in a soft matrix
35
Hirsch’s Model
a
a
p
p
aappc E
V
E
VX1
EVEV
1X
E
1
X: represents the degree of bonding
36
to tr
Stress to trtime
Strain
to trtime
Strain
Elastic
Viscous
Viscoelastic Behavior of Asphalt Concrete
time
Viscoelastic response = Immediate elastic + Time dependent viscous
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
G
Viscous
to tr
Stressto trtime
Strain
to trtime
Elastic
time
Viscous
to trtime
Strain
Strain
39
Maxwell Model
total s d
total G
Constant Stress(Creep)
Constant Strain(Relaxation)
time
Strain
time
Stress
40
Kelvin Model
dstotal
Constant Stress(Creep)
Constant Strain(Relaxation)
time
Strain
time
Stress
Gtotal
41
Burger Model
Constant Stress(Creep)
time
Strain
42
Asphalt Binder Behavior
Viscoelastic behavior
Temperature Value depends on asphalt type
Elastic partis negligible
Viscous behavior
Temperature scale
Semi solid or solid fluid
43
Viscous Behavior of Fluids
yield
Shear Stress
Shear Rate
Slope = (Viscosity)
Shear Stress
Shear Rate
yield
Yield stress
NewtonianNon NewtonianBingham behavior
44
1n
A n
Shear Stress
Shear Rate
Shear Stress
Shear Rate
1n
A n
Viscous Behavior of Fluids
Non NewtonianShear Thinning
Non NewtonianShear 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
Function of warm weather and traffic
Courtesy of FHWA