Understanding Binder Blending and Diffusion in RAP Mixes

2020 Asphalt Technical Symposium

June 16, 2020

Chris Campbell

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2020RTI20597

2

Reclaimed Asphalt Pavement (RAP) Basics

100% recyclable

Environmental & economic incentives

Average RAP content in new mix increased from 15% to 20% between 2009 and 2018 (NAPA)

Incorporating RAP in new mix requires caution

• Manage stiffness of aged binder in RAP

• Ensure proper aggregate gradation and effective binder content

• Achieve proper cohesion between virgin and RAP binders, and adhesion to aggregate

How do we properly account for aged binder?

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Step I : Understand blending between RAP and virgin binders

• Evaluate binder diffusion using change in viscosity

Step II: Demonstrate binder blending in mixtures

• Use findings to validate diffusion in mixtures

• Determine average diffusion distance / binder thickness

Step III: Understand impact of binder blending in practice

• Plant Trials / Field Validation

Understanding RAP-Virgin Binder Blending

4

RAP Binder, a “black rock” or “binder blend component”?

RAP Aggregate RAP BinderVirgin BinderVirgin Aggregate Binder Blend

Extent of blending is critically important for mixture rheological & failure properties

5

What is Diffusion?

• Migration of material in the direction of concentration gradient

• Moving from two separate liquids to one homogeneous blend

Virgin RAP Homogeneous Blend

6

Step I: RAP-Virgin Binder Diffusion in Experiment

1

10

100

1 100 10000

η*,

Pa.

s

Diffusion Time, s

1

10

100

1 100 10000

η*,

Pa.

s

Diffusion Time, s

1

10

100

1 100 10000

η*,

Pa.

s

Diffusion Time, s

1

10

100

1 100 10000

η*,

Pa.

s

Diffusion Time, sRAP

aggregate

RAP binder

Virgin binder

RAP-virgin

binder

diffusion

Test in RheometerDiffusion in Mix Results

RAP

50/50 Blend

Virgin

Diffusion

Results

corrected for

effects of

oxidation &

evaporation

7

Diffusion is Faster at Higher Temperature

Time to equilibrium

Diffusion at Varying Temperature

Homogenous Blend

Blending by Diffusion

• Faster Brownian motion at higher temperature increases diffusion rate

• Log of diffusion coefficient increase linearly with temperature

140130120110100

-11.5

-12.0

-12.5

-13.0

-13.5

-14.0

Temperature, °C

log

D,

m²/s

MS-58

MS-64

OTT-58

OTT-64

SAR-58

SAR-64

Blend

Dependence of Diffusion Coefficient On T

8

Binders behave like liquids & blend by diffusion

Diffusion rate increases with temperature

Key Learnings during Step 1

9

Film thickness & time at temperature define blending in the mix

• Distribution of thicknesses exists

• Proper binder contact may not be reached by mixing

Understanding effective binder thickness in mix is essential

Binder Blending in Mix is More Complex

10

Conditioning under N2 atmosphere at three temperatures (90, 120, 150 °C)

DSR testing in torsion, 5-10 repeats per sample

Step II: Evaluating Diffusion in Mix

11

Binder Diffusion Model Tracks Mix Property Evolution

12 h

Diffusion distance = 800µm

2 days

12

-14

-13

-12

-11

-10

-9

-8

-7

-6

-5

60

80

100

120

140

160

0 15 30 45 60 75 90 105 120

log

D, m

2 /s

Tem

pe

ratu

re, °

C

Time, min

HMA T Profile

D, HMA

Diffusion coefficient decreases with mix cooling

Winter paving conditions?

Simulating Diffusion Rate at Realistic Conditions

mixing storage transport placement service

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Binder film thickness & time at temperature are critical parameters

Diffusion may not be completed during mix production

• Effective binder content may be lower than expected

Silo storage at higher temperature can assist diffusion

Key Learnings during Step 2

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Hypotheses for extended silo storage:

• Heat & time helps blending between virgin & RAP binders in plant HMA

• Improves cohesiveness of final blend & its performance

Step III: Plant Trail / Field Validation

Promoted by

silo storage

Less-uniform binder film

Lower effective binder content

Homogenous binder film

Higher effective binder content

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• Two HMA plants in Ontario, Canada, participated

• Mix samples collected at 0, 1, 4, 8, 12 hours of storage

• 24-hour samples were also collected at Plant 2

Asphalt Plants & Samples

Plant Operating Conditions & Mix Properties

Plant

Asphalt Course Surface (HL-3) Base (HL-8) Surface (HL-3) Base (HL-8)

Production Temperature (°C) 160 150-170 165 170-180

Silo Temperature (°C) 140 140 147 147

Silo Status Off-loading Off-loading Stagnant Stagnant

Production Rate (tons/h) 178 180 180 150

Virgin Binder PG 58-28 PG 52-34 PG 58-28 PG 52-34

AC Content (%) 5.0 4.7 5 4.7

RAP content (%) 15 30 20 40

Plant 1 Plant 2

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Mix Modulus Evolves During Silo Storage

• Properties of mixes change with storage time counterintuitively, despite binder aging & absorption

Increase in silo durationIncrease in silo duration

17

Binder Aging During Silo Storage not Significant*

Virgin (tank)

Recovered (0h)

Recovered (12h)

Recovered (24h)

15% RAP 20% RAP

30% RAP 40% RAP

Recovered (RAP)

*PG 58-28 + Softener to PG 52-34

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Rutting Depth Reduced with Storage Time

Aging, absorption or diffusion?

0

1

2

3

4

5

RAP 15% RAP 20%

Rut de

pth

(m

m)

HL-3

0 8 12 24

0

1

2

3

4

5

RAP %30 RAP %40

Ru

t D

ep

th (

mm

)

HL-8

0 8 12 24

Plant 2Plant 2Plant 1 Plant 1

19

Fatigue Performance Improved with Storage Time

• Fatigue performance improves

• VMA slightly reduces & VFA slightly increases with storage time

0

10000

20000

30000

40000

50000

60000

70000

RAP 30% RAP 40%

No. of C

ycle

s

HL-8

0 8 12 24Plant 1 Plant 2

0

10000

20000

30000

40000

50000

60000

70000

RAP 15% RAP 20%

No. of C

ycle

s

HL-3

0 8 12 24Plant 1 Plant 2

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Binder & mix properties evolve during silo storage

Binder diffusion is dominant factor in evolution of mix properties within 12 Hrs.

• Fatigue & rutting both improve as the binder film homogenizes

Caution must be exercised on binder chemistry

Binder softeners may increase binder aging during silo storage

RAP mixes evolve over time due to on-going RAP-virgin binder diffusion

Key Learnings during Step III – Field Validation

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…It depends, time & heat helps.

Is RAP binder “a black rock” or “a binder blend component”?

Thank you for your time!