Asphalt Cements

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Asphalt Cements
New Superpave Performance Graded Specification
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PG Specifications

• Fundamental properties related to pavement
  performance
• Environmental factors
• In-service construction temperatures
• Short and long term aging

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PG Specifications

• Based on rheological testing
• Rheology study of flow and deformation
• Asphalt cement is a viscoelastic material
• Behavior depends on
• Temperature
• Time of loading
• Aging (properties change with time)

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High Temperature Behavior

• High in-service temperature
• Desert climates
• Summer temperatures
• Sustained loads
• Slow moving trucks
• Intersections

Viscous Liquid
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Pavement Behavior(Warm Temperatures)

• Permanent deformation (rutting)
• Mixture is plastic
• Depends on asphalt source, additives, and
  aggregate properties

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Permanent Deformation
Courtesy of FHWA
Function of warm weather and traffic
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Low Temperature Behavior

• Low Temperature
• Cold climates
• Winter
• Rapid Loads
• Fast moving trucks

Elastic Solid
Hookes Law
s t E
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Pavement Behavior(Low Temperatures)

• Thermal cracks
• Stress generated by contraction due to drop in
  temperature
• Crack forms when thermal stresses exceed ability
  of material to relieve stress through deformation
• Material is brittle
• Depends on source of asphalt and aggregate
  properties

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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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Superpave Asphalt Binder Specification
The grading system is based on Climate
PG 64 - 22
Min pavement temperature
Performance Grade
Average 7-day max pavement temperature
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Pavement Temperatures are Calculated

• Calculated by Superpave software
• High temperature
• 20 mm below the surface of mixture
• Low temperature
• at surface of mixture

Pave temp f (air temp, depth, latitude)
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Tests Used in PG Specifications
Construction
BBR
DSR
RV
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Concentric Cylinder Rheometers

• Concentric Cylinder

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Rotational Viscometer (Brookfield)
Torque Motor
Inner Cylinder
Thermosel Environmental Chamber
Digital Temperature Controller
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Original Properties, Rutting, and Fatigue
DSR
BBR
RV
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Dynamic Shear Rheometer (DSR)
Shear flow varies with gap height and
radius Non-homogeneous flow

• Parallel Plate

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Oscillating Plate
B
C
A
Fixed Plate
B
Test operates at 10 rad/sec or 1.59 Hz 360o 2
p radians per circle 1 rad 57.3o
Time
A
A
C
1 cycle
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Elastic
Viscous
B
Strain
Time
A
A
C
Strain out-of-phase d 90o
Strain in-phase d 0o
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Complex Modulus, G
Viscous Modulus, G
d
Storage Modulus, G
Complex Modulus is the vector sum of the storage
and viscous modulus
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DSR Equipment
DSR Equipment
Computer Control and Data Acquisition
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Motor
Parallel Plates with Sample
Area for Liquid Bath
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25 mm Plate with Sample
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Rutting
BBR
RV
DSR
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Permanent Deformation
Addressed by G/sin d on unaged binder gt 1.00
kPa G/sin d on RTFO aged binder gt 2.20 kPa
For the early part of the service life
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Short Term Binder Aging

• Rolling Thin Film Oven
• Simulates aging from hot mixing and construction

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Inside of RTFO
Fan
Rotating Bottle Carriage
Air Line
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Bottles Before and After Testing
Opening in Bottle
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Testing

• Calculate mass loss after RTFO
• Determine G/sin d for RTFO aged material at same
  test temp. used for original asphalt cement

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Permanent Deformation

• Question Why a minimum G/sin d to address
  rutting
• Answer We want a stiff, elastic binder to
  contribute to mix rutting resistance
• How By increasing G or decreasing d

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Fatigue
BBR
RV
DSR
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Fatigue Cracking
Function of repeated traffic loads over time (in
wheel paths)
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Testing

• Aged binder
• Since long term performance problem, include
• Short term aging
• Long term aging
• Determine DSR parameters using 8 mm plate and
  intermediate test temperature

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Pressure Aging Vessel(Long Term Aging)

• Simulates aging of an asphalt binder for 7 to 10
  years
• 50 gram sample is aged for 20 hours
• Pressure of 2,070 kPa (300 psi)
• At 90, 100 or 110 C

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Pressure Aging Vessel
Rack of individual pans (50g of asphalt / pan)
Bottom of pressure aging vessel
Vessel Lid Components
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Pressure Aging Vessel
Courtesy of FHWA
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Fatigue Cracking

• G (sin d) on RTFO and PAV aged binder
• The parameter addresses the later part of the
  fatigue life
• Value must be lt 5000 kPa

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Fatigue Cracking

• Question Why a maximum G sin d to address
  fatigue?

Answer We want a soft elastic binder (to
sustain many loads without cracking) How By
decreasing G or decreasing d
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Thermal Cracking
BBR
RV
DSR
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Bending Beam Rheometer
Computer
Deflection Transducer
Air Bearing
Load Cell
Fluid Bath
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Bending Beam Rheometer Sample
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Bending Beam Rheometer Equipment
Fluid Bath
Loading Ram
Cooling System
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Bending Beam Rheometer

• S(t) P L3

4 b h3 d (t)
Where S(t) creep stiffness (M Pa) at time,
t P applied constant load, N L distance
between beam supports (102 mm) b beam width,
12.5 mm h beam thickness, 6.25 mm d(t)
deflection (mm) at time, t
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Bending Beam Rheometer

• Evaluates low temperature stiffness properties
• Creep stiffness
• Slope of response (called m-value)

Log Creep Stiffness, S(t)
8 15 30 60 120 240
Log Loading Time, t (sec)
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Is Stiffness Enough?

• No. Need to assess strain needed to break
  specimen.
• Thermal cracking occurs when strain is too great
• Direct tension test
• Currently (1998) in specification
• New equipment is now available

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Direct Tension Test
Load
Stress s P / A
D L
sf
D Le
ef
Strain
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Direct Tension Test
Courtesy of FHWA
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Direct Tension Test
Courtesy of FHWA
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Summary
Low Temp Cracking
Fatigue Cracking
Rutting
Construction
DTT
RV
DSR
BBR
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Questions - ?