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Asphalt Cements. New Superpave P erformance G raded Specification. PG Specifications. Fundamental properties related to pavement performance Environmental factors In-service & construction temperatures Short and long term aging. PG Specifications. Based on rheological testing

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asphalt cements
Asphalt Cements

New Superpave Performance Graded Specification

pg specifications
PG Specifications
  • Fundamental properties related to pavement performance
  • Environmental factors
  • In-service & construction temperatures
  • Short and long term aging
pg specifications3
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)
high temperature behavior
High Temperature Behavior
  • High in-service temperature
    • Desert climates
    • Summer temperatures
  • Sustained loads
    • Slow moving trucks
    • Intersections

Viscous Liquid

pavement behavior warm temperatures
Pavement Behavior(Warm Temperatures)
  • Permanent deformation (rutting)
  • Mixture is plastic
  • Depends on asphalt source, additives, and aggregate properties
permanent deformation
Permanent Deformation

Courtesy of FHWA

Function of warm weather and traffic

low temperature behavior
Low Temperature Behavior
  • Low Temperature
    • Cold climates
    • Winter
  • Rapid Loads
    • Fast moving trucks

Elastic Solid

Hooke’s Law

s = t E

pavement behavior low temperatures
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
thermal cracking
Thermal Cracking

Courtesy of FHWA

aging
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)
slide11

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

slide12

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)

slide14

Mi

t Rq =

2 p Ri2 L

W R

g =

Ro - Ri

Concentric Cylinder Rheometers

  • Concentric Cylinder
slide15

Rotational Viscometer (Brookfield)

Torque Motor

Inner Cylinder

Thermosel Environmental Chamber

Digital Temperature Controller

dynamic shear rheometer dsr

2 M

p R3

R Q

h

tR =

gR =

Dynamic Shear Rheometer (DSR)

Shear flow varies with gap height and radius

Non-homogeneous flow

  • Parallel Plate
slide18

Oscillating Plate

B

C

A

Fixed Plate

B

Test operates at 10 rad/sec

or 1.59 Hz

360o = 2 p radiansper circle

1 rad = 57.3o

Time

A

A

C

1 cycle

slide19

Elastic

Viscous

B

Strain

Time

A

A

C

Strain out-of-phase

d = 90o

Strain in-phase

d = 0o

slide20

Complex Modulus, G*

Viscous Modulus, G”

d

Storage Modulus, G’

Complex Modulus is the vector sum of the storage and viscous modulus

slide21

DSR Equipment

DSR Equipment

Computer Control and Data Acquisition

slide22

Motor

Parallel Plates with Sample

Area for Liquid Bath

slide24

Rutting

BBR

RV

DSR

slide25

Permanent Deformation

Addressed by:

G*/sin d on unaged binder > 1.00 kPa

G*/sin d on RTFO aged binder > 2.20 kPa

For the early part of the service life

slide26

Short Term Binder Aging

  • Rolling Thin Film Oven
    • Simulates aging from hot mixing and construction
inside of rtfo
Inside of RTFO

Fan

Rotating Bottle Carriage

Air Line

testing

Original mass - Aged mass

Original mass

x 100

Mass loss, % =

Testing
  • Calculate mass loss after RTFO
  • Determine G*/sin d for RTFO aged material at same test temp. used for original asphalt cement
slide30

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

slide31

Fatigue

BBR

RV

DSR

fatigue cracking
Fatigue Cracking

Function of repeated traffic loads over time

(in wheel paths)

testing33
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
slide34

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
slide35

Pressure Aging Vessel

Rack of individual pans

(50g of asphalt / pan)

Bottom of pressure aging vessel

Vessel Lid Components

pressure aging vessel
Pressure Aging Vessel

Courtesy of FHWA

slide37

Fatigue Cracking

  • G* (sin d) on RTFO and PAV aged binder
  • The parameter addresses the later part of the fatigue life
  • Value must be < 5000 kPa
slide38

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

bending beam rheometer
Bending Beam Rheometer

Computer

Deflection Transducer

Air Bearing

Load Cell

Fluid Bath

bending beam rheometer equipment
Bending Beam Rheometer Equipment

Fluid Bath

Loading Ram

Cooling System

bending beam rheometer43
Bending Beam Rheometer
  • S(t) = P L3

4 b h3d (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

bending beam rheometer44
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)

is stiffness enough
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
direct tension test
Direct Tension Test

Load

Stress = s = P / A

D L

sf

D Le

ef

Strain

direct tension test47
Direct Tension Test

Courtesy of FHWA

direct tension test48
Direct Tension Test

Courtesy of FHWA

summary

PAV

Long Term Aging

RTFO

Short Term Aging

No aging

Summary

Low Temp

Cracking

Fatigue

Cracking

Rutting

Construction

[DTT]

[RV]

[DSR]

[BBR]