Investigation of Warm Mix Asphalt Additives Using the Science of Tribology to Explain Improvements in Mixture Compaction Sebastian Puchalski – Kraton Polymers Prof. Hussain Bahia – University of Wisconsin-Madison

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• Good HMA compaction commonly translates into high mix density, which is related to pavement quality and durability. • HMA compaction is a complex process. Yet, in terms of asphalt binder properties, only rotational viscosity is commonly associated with the mix compactibility. • Warm Mix Asphalt (WMA) made its way into industry as a way to produce asphalt pavements at reduced mixing/construction temperatures. • WMA effect can be achieved via:

Foaming asphalt Reducing asphalt viscosity Using chemical additives

• Chemical additive technologies may encompass categories such as surfactants or friction modifiers.

Introduction

• While foaming and viscosity reduction technologies are generally understood in terms of their effect on HMA compaction, the chemical additive technologies are not. • Objective of this presentation is to discuss a method which quantifies the lubricating

effect of some chemical additives.

Problem statement

INTRODUCTION AND PROBLEM STATEMENT

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Mix summary: • Wisconsin E-10 mix (for heavy traffic), • Coarse granite gradation, • 5% binder content (fixed)

2%

3%

4%

5%

6%

80 90 100 110 120 130 140 150

Air

Vo

ids

at N

de

sign

Compaction Temperature (°C)

PG64-22

+ Additive A

+ Additive B_low

+ Additive B_high

Source: Hanz, Andrew, Phd Thesis, 2010, Pouya Teymourpour, PhD

BACKGROUND DATA AND MATERIALS

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Viscosity of binders presented in the previous slide is not equal. Yet the difference is not large and goes both ways! Could the mentioned additives have a lubricating effect that is not captured via viscosity testing? To answer such question we must defer to the field of tribology.

0

1

2

(η + additive) (η )

Brookfield Viscosity (constant rate)

135°C

150°C

165°C

PG64-22 + Additive A +Additive B_low +Additive B_high

Source: Hanz, Andrew, Phd Thesis, 2010

VISCOSITY

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According to Bhushan (2002) Tribo in Greek means rubbing. According to Google rubbing is Τριβή /triví ̱/ in Greek… …hence trivialogy? Goal: to understand the nature of interactions between two materials rubbing or sliding past each other and to understand the interfacial phenomena The phenomena can be classified as: • Friction • Wear • Adhesion In order to minimize such phenomena lubricant of a sort is typically in order.

TRIBOLOGY

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Linking tribology to asphalt mix compaction let’s assume for now that: • wear in form of aggregate chipping off is not an issue • there are no appreciable adhesive tensile forces between aggregates (quite accurate) Then, the only phenomenon we must deal with is friction. Since we like to quantify things, let’s use a coefficient of friction, µ.

It is a simple ratio that can be easily measured. Keep in mind that the ratio is a system property (depends on substrates and lubricant)

P

N

µ=𝑃

𝑁

TRIBOLOGY

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Source: Rabinowicz, 2005, “Modern Tribology Text book, 2000.

Log of ZN/p Undefined Part

Log

Co

effi

cien

t o

f Fr

icti

on

STRIBECK CURVE

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STRIBECK CURVE

Source: Rabinowicz, 2005, “Modern Tribology Text book, 2000.

Log of ZN/p Undefined Part

Log

Co

effi

cien

t o

f Fr

icti

on

Hydrodynamic

N

P

Z

𝒁 ∗ 𝑵

𝑷

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Source: Rabinowicz, 2005, “Modern Tribology Text book, 2000.

Log of ZN/p Undefined Part

Log

Co

effi

cien

t o

f Fr

icti

on

Mixed

STRIBECK CURVE

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Source: Rabinowicz, 2005, “Modern Tribology Text book, 2000.

Log of ZN/p Undefined Part

Log

Co

effi

cien

t o

f Fr

icti

on

Boundary

STRIBECK CURVE

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STRIBECK CURVE

Hydrodynamic

Mixed

Boundary

Source: Rabinowicz, 2005, “Modern Tribology Text book, 2000.

𝒁 ∗ 𝑵

𝑷

N

P

Z

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Now the question is: What regime is most suitable to describe asphalt binder during mix compaction?

Source: Hanz, Faheem, Mohmoud, Bahia, 2010

ASPHALT LUBRICITY TEST

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0.00

0.05

0.10

0.15

0.20Hydrodynamic Coefficient of friction

85°C 115°C 145°C

0%

1%

2%

3%

4%

5%

6%

7%

0.00 0.05 0.10 0.15 0.20

% A

ir V

oid

s @

Nd

es

Coefficient of Friction (Asphalt Lubricity Test)

85C 115C 145C Linear (85C)

Seems to provide good indication of relative ranking at relatively low mixing temperatures. - hydrodynamic mode Ranking: 1. Additive B_high 2. Additive B_low 3 Additive A 4 PG64-22

Source: Hanz, Andrew, PhD Thesis, 2010, Pouya Teymourpour, PhD

ASPHALT LUBRICITY TEST

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Could it be because as mix temperatures rise, the aggregate/mastic particles start running into each other more often creating friction? Also, as viscosity is reduced due to temperature climb, the Sommerfeld number naturally pushes the lubrication mode to the left of the Stribeck curve!

Viscosity Reduction

Log of ZN/p Undefined Part

Co

effi

cien

t o

f Fr

icti

on

STRIBECK CURVE VS COMPACTION

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Source: Bhushan, Bharat. Introduction to Tribology. New York: John Wiley & Sons, 2002.

• Bulk flow properties of lubricant play little role

• Mono- and multimolecular films provide lubrication

• Important physical film properties: melting point, shear

strength, hardness.

• Boundary lubricants form easily sheared film

Log

film

th

ickn

ess

(nm

) Lo

g C

oef

f. O

f Fr

icti

on

1-3nm

<25 µm

BOUNDARY REGIME

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𝐶𝑜𝐹 =𝑇𝑜𝑟𝑞𝑢𝑒

𝑅𝑎𝑑𝑖𝑢𝑠 ∗ 𝑁𝑜𝑟𝑚𝑎𝑙 𝐹𝑜𝑟𝑐𝑒

ASPHALT BOUNDARY LUBRICITY TEST

Source: Puchalski, 2012

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1-3nm

<25 µm

0.1

1

10

0 10 20 30 40

Gap

(μ

m)

Rotational Velocity (rad/s)

0.001

0.01

0.1

1

0 10 20 30 40

Co

eff

icie

nt

of

Fric

tio

n

Rotational Velocity (rad/s)

Bo

un

dar

y Constant Test Speed: 0.05 rad/s

Source: Bhushan, Bharat. Introduction to Tribology. New York: John Wiley & Sons, 2002.

Log

film

th

ickn

ess

(nm

) Lo

g C

oef

f. O

f Fr

icti

on

REGIME VALIDATION

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Limestone Granite

Aggregate types

Temperature Levels

85°C, 115°C, 145°C

100kPa, 500kPa, 1MPa

Contact Pressure Levels

SUBSTRATE TYPES

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0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

80 90 100 110 120 130 140 150

Co

effi

cie

nt

of

Fric

tio

n

Temperature (C)

1 MPa Limestone

PG64-22 + Additive A + Additive B high + Additive C

RESULTS

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0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

80 90 100 110 120 130 140 150

Co

effi

cie

nt

of

Fric

tio

n

Temperature (C)

1 MPa Granite

PG64-22 + Additive B low + Additive B high

+ Additive B extra + Additive A + Additive C

RESULTS

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0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

80 100 120 140

Co

effi

cie

nt

of

Fric

tio

n

Temperature (C)

1 MPa Limestone

PG64-22 + Additive A

+ Additive B high + Additive C

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

80 90 100 110 120 130 140 150Temperature (C)

1 MPa Granite

PG64-22 + Additive B low

+ Additive B high + Additive B extra

+ Additive A + Additive C

RESULTS

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Temperature (°C) Variable PG64-22 Additive A Additive B

high Additive B

Low

145

Air Voids Ndes 4 3 2 1

CoF 100kPa 4 1 2 3

CoF 500kPa 4 1 3 2

CoF 1MPa 4 1 3 2

115

Air Voids Ndes 4 3 2 1

CoF 100kPa 4 1 3 2

CoF 500kPa 4 1 3 2

CoF 1MPa 4 2 3 1

85

Air Voids Ndes 4 3 2 1

CoF 100kPa 4 1 2 3

CoF 500kPa 4 3 2 1

CoF 1MPa 4 3 2 1

Gradation: Coarse Aggregate: Granite Base binder: PG64-22 Binder content: 5%

2%

3%

4%

5%

6%

80 100 120 140 160

Air

Vo

ids

at N

de

sign

Compaction Temperature (°C)

PG64-22

+ Additive A

+ Additive B low

+ Additive B high

MIXTURE CORRELATION

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Best Subsets Regression:

R² = 0.65

0

0.04

0.08

0.12

0.16

0.1 10 1000 100000

Bo

un

dar

y C

oF

Mastic Viscosity (0.01 sec^-1)

Mastic Viscosity – Boundary CoF 1 MPa

Log(Mastic Viscosity) = 5.23 + 0.829 Log(Binder Viscosity) - 33 Boundary CoF

+ Binder Viscosity =

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-0.5 0.5 1.5 2.5Log Predicted

Mas

tic

Vis

cosi

ty

(Pa.

s)

Log Measured Mastic Viscosity (Pa.s)

Measured – Predicted Mastic Viscosity

R2 (adj) = 0.85

Partial data source: : Sefidmazgi, N. R., Teymourpour, P., Bahia, H. U.

MASTIC VISCOSITY CORRELATION

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• Asphalt binder may act as a lubricant • Boundary lubrication regime has little dependence on lubricant’s viscosity as opposed to hydrodynamic regime Coefficient of friction • Boundary lubrication regime as evaluated in the test, showed to be sensitive to the presence of WMA additives • Coefficient of friction in boundary regime is dependent on aggregate substrate type • Boundary and hydrodynamic lubrication are factors in asphalt mix compaction • Boundary coefficient of friction cannot be directly related to improvements in mixture compaction . It may serve,

though, as an additional tool to help understand and evaluate WMA behavior. WMA additives • WMA additives may improve boundary coefficient of friction • Certain WMA additives may improve the boundary CoF regardless of aggregate substrate type, while others may

only work with a specific mineralogy

Mastic viscosity Mastic viscosity can be modeled as a function of boundary lubrication and binder viscosity.

OBSERVATIONS

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• Warm mix additives affect asphalt concrete compaction

• It is important to look at asphalt as a lubricant at compaction temperatures

• Lubricating effect of warm mix additives cannot be solely captured through viscosity testing. Tribology should be used instead.

CONCLUSIONS

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• Dr. Andrew Hanz • Dr. Nima Sefidmazgi • Dr. Pouya Teymourpour References Bhushan, Bharat. Introduction to Tribology. New York: John Wiley & Sons, 2002. Hanz, Andrew. "Quantifying the Impacts of Warm Mix Asphalt on Constructability and Performance." PhD Thesis. Madison: University of Wisconsin – Madison, 2012. Hanz, Andrew, Ahmed Faheem, Enad Mahmoud, and Hussain U Bahia. "Measuring Effects of Warm Mix Additives Using a Newly Developed Asphalt Binder Lubricity Test for the DSR." Transportation Research Record, 2010: 85-92. Rabinowicz, Ernest. Friction and Wear of Materials. New York City: John Wiley & Sons, Inc., 1995. Puchalski, S. Investigation of Warm Mix Asphalt Additives Using the Science of Tribology to Explain Improvements in Mixture Compaction. Master of Science Thesis, Madison, WI, 2012. Sefidmazgi, N. R., Teymourpour, P., Bahia, H. U. " Effect of Particle Mobility on Aggregate Structure Formation in Asphalt Mixtures, "Journal of the Association of Asphalt Paving Technologists, Vol. 82, 2013, pp. 16-34.“

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