A Simple Micromechanics-based Approach for Evaluating the Rutting

Potential of Asphalt Pavements

Prof. Björn BirgissonThe Royal Institute of Technology (KTH)

Transportforum 2009

• A test that reflects mixture rutting potential is required for:– Mixture optimization– Mixture design– Pass/Fail criteria

Problem Statement

• It would be nice to use a Superpave Gyratory Compactor to Evaluate the Rutting Potential of Mixtures – It’s readily available– It’s simple– It’s measures mixture parameters over a range

of volumetric conditions

Instability Rutting

• Question:– What are the key elements that are required to

assess mixture rutting performance using a gyratory compaction approach?

• Answer:– We need to induce conditions that are most

relevant to the mechanism of instability rutting and measure the relevant response under these conditions

Back to Basics

• Rutting instability is associated with plastic flow and formation of shear planes

Field Observations

Shear Planes

• Based on previous tire contact studies and associated finite element analyses – plot shear stresses and their directions:

• High shear stress in the presence of low confinement and even tension appears to be controlling – Defines condition of Impending Instability

Tire Contact Studies and Analyses

• Need to induce conditions associated with Impending Instability in mixtures and measure the relevant response under these conditions

• Using the gyratory compactor:– Cannot induce tension or low confinement– Can induce high shear stresses by changing gyration

angle – Can create the aggregate structural rearrangement that

appears associated with impending instability

Focus on Key Mechanism

• Create the aggregate structural rearrangement that appears associated with impending instability– Compact mixture to 7 percent air voids at a

gyratory angle of 1.25 degrees– Induce rearrangement of aggregate structure

using a high shear angle (2.5 degrees)– Monitor gyratory shear strength and vertical

strain

New Approach

• At condition of Impending Instability, gyratory shear strength peaks, followed by a rearrangement of aggregate structure

• Gyratory shear strength may or may not increase after rearrangement of aggregate structure

Observed Response

0100200300400500600700800900

1000

0 20 40 60 80 100 120 140

Number of Gyratory Revolutions (N)

Gyr

ator

y Sh

ear S

treng

th (k

Pa) .

1.25o2.5o

Three Possible Basic Characteristics of GyratoryShear Strength Curves at Impending Instability

Vertical Strain

• Failure strain - the strain at point of local minimum gyratory shear strength after increase in gyratory angle

Definition of “Failure Strain”

Proposed Framework for theEvaluation of Rut Resistance

• Use a total of 31 Mixtures– 10 oolitic limestone mixtures of different gradations– 6 Georgia granite mixtures of different gradations– 8 mixtures from a previous study on the effect of fine aggregate

angularity– 5 Superpave field mixtures– 2 HVS mixtures – PG 67-22 used for all mixtures except for an SBS modified HVS

mixture (PG 76-22)

• Asphalt Pavement Analyzer (APA) measurements obtained for all mixtures (at 7 percent Air Voids)

Evaluation of Proposed Framework

Evaluation of Proposed Framework

0

5

10

15

20

25

30

35

40

1 1.2 1.4 1.6 1.8 2 2.2 2.4

Failure Strain (%)

Gyr

ator

y Sh

ear

Slop

e (k

Pa)

Observed APA crackingAPA Rutting > 7.0mmAPA Rutting < 7.0mm

• A stepwise discriminant function analysis was performed using gyratory shear slope and failure strain as predictor variables to test the validity of the categories proposed

– Category 1 – optimal mixtures (shear slope > 15 kPa and failure strain between 1.4 and 2.0 %)

– Category 2 – Brittle mixes (failure strain < 1.4 %)– Category 3 – Mixtures with low shear slope (< 15 kPa)– Category 4 – Plastic mixtures (failure strain > 2.0 %)

• The results showed – The failure strain was more important than the gyratory shear slope

in determining the category of each mixture– The proposed categories were statistically significant

Statistical Evaluation of Results

Field Mixtures Only

0

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25

30

35

40

1 1.2 1.4 1.6 1.8 2 2.2 2.4

Failure Strain (%)

Gyr

ator

y Sh

ear

Slop

e (k

Pa)

Observed Field Instability Rutting

No Field Instability Rutting

Effects of SBS Modification

0

5

10

15

20

25

30

35

1 1.2 1.4 1.6 1.8 2 2.2 2.4

Failure Strain (%)

Gyr

ator

y Sh

ear

Slop

e (k

Pa)

Unmodified MixtureSBS Modified Mixture

EXPLANATION?A Conceptual Model for Mixtures

• Large enough aggregates should engage dominantly in the structure

(>1.18mm or bigger sieve size) to perform well in terms of cracking

and rutting

• Either single size or range of particle sizes could form the dominant

aggregate structure and result in good performance

• Sufficient volume between the dominant aggregate size particles

would be required for asphalt mastic, and air voids

• Stiffness of this volume should be optimal to prevent excessive

creep strain rate

Rutting Instability

• Excessive creep strain rate (rutting instabiilty) results when:– Excessively fine particles are the dominant

part of the aggregate structure.

– Inadequate interlock of dominant aggregate size range, even when the dominant range is composed of coarser particles.

Dominant Aggregate Size Range (DASR)

• Interactive range of particle sizes that forms the primary

structural network of aggregates. (either one size or a

range of sizes)

• DASR must be composed of coarse enough particles and

its porosity must be low enough for a mixture to

effectively resist deformation and cracking.

• Particles smaller than this range fill the gaps between

the DASR particles, along with the binder (Interstitial

Volume) and provide support to the DASR particle

network.

Dominant Aggregate Size Range (DASR)

• Particles larger than those within the DASR

essentially float in the DASR matrix.

• Particle size retained on 1.18mm sieve size were

considered as big enough to provide sufficient

interlock to help resist stress that induces rutting

and cracking.

Interstitial Volume (IV) & Interstitial Components (IC)

• The volume of material (AC, AV and aggregates) that exists within the interstices of the DASR.

• IV serves to hold together the DASR• IC are the components of IV.• The characteristics of IV and the properties of the IC

– durability and fracture resistance

(a) SMA (b) Coarse dense (c) Fine dense

Dominant Aggregate

IC, IV

Interstitial Volume (IV) & Interstitial Components (IC)

• Properties of the IC affect mixture

performance:

– Excessively low stiffness and/ or excessively

high volume may result in high creep rate

– Excessively high stiffness and/or insufficient

volume may result in a brittle mixture

DASR Porosity

• For granular materials, 45-50% maximum

porosity required for stone-on-stone contact

• Stone-on-Stone contact is critical for adequate

resistance to deformation.

• 50% was selected as a reasonable starting point

for evaluation.

Spacing Analysis

0.00

0.05

0.10

0.15

0.20

0.25

0 10 20 30 40 50 60 70 80 90 100

% passing for sections

Slop

e LargeSmall

• An approach was developed to determine the spacing between specified particle sizes on the Interstitial Surface (IS).

• Spacing slope increase steeply when % passing of any particle size increases 70% in a binary mixture.

• Spacing should be 30-70% for any two contiguous size particles to interact and behave as a unit.

0

1

2

3

4

100/

0

95/5

90/1

0

85/1

5

80/2

0

70/3

0

60/4

0

50/5

0

40/6

0

30/7

0

20/8

0

15/8

5

10-9

0

5/95

0/10

0

Large/Small Particle Proportion

Spac

ing,

cm

LargeSmall

APLICATION TO ANALYSIS OF FIELD PROJECTS

12 Superpave Projects were divided intothree groups based on their gradations characteristics

Well Performing Group 1: ηDASR < 50%

This included field gradation of projects 3, 4, 5, 7 and plant mix gradations of projects 8 and 11

• The DASR porosity was less than 50% along the section.

• Projects 3, 4, 5, and 7 resulted with little or no rutting in the field.

• Project 8 performed very well in the APA and Servopac.

• Project 11 performed well in the APA, Servopac results indicated

potentially marginal performance.

Poorly Performing Group 2: ηDASR > 50%

This included field gradation of projects 6 and 8, and plant mixgradation of projects 9 and 12

• The DASR porosity was greater than 50% along the section.

• Projects 6 and 8 exhibited relatively high rates of rutting in the field.

• Projects 9 and 12 exhibited relatively poor rutting performance in

the APA and Servopac tests.

Servopac Test Results

Superpave Servopac Results

0

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15

20

25

30

35

40

1 1.2 1.4 1.6 1.8 2 2.2 2.4

Vertical Failure Strain, %

Gyr

ator

y Sh

ear S

lope

, kPa

P9 L15A

P9 L25A

P9 L15B

P12 L15A

P12 L15BLow Shear Resistance

Optimal MixturesBrittle Mixtures Plastic MIxtures

DASR Porosity = 50 %

Group 3: Marginal Interaction

This included field gradation of projects 1 and 2 and plant mix gradations of project 10

• Marginal interaction @ 4.75-2.36 resulted in variable DASR porosity

along the section.

• Projects 1 and 2 resulted with relatively high rates of rutting in the

field and the Servopac.

• Projects 10 exhibited relatively poor rutting performance in the APA

and Servopac tests.

• For evaluating mixture rutting resistance, we need to induce conditions associated with Impending Instability in mixtures and measure the relevant response under these conditions

• Using the gyratory compactor, we can create the aggregate structural rearrangement that appears associated with impending instability

• This can be achieved by inducing high shear stresses by increasing the gyratory angle to 2.5 degrees and monitoring the gyratory shear strength and vertical strain

Conclusions

• The “failure strain” under the condition of impending instability along with gyratory shear slope provide the basis for a framework for evaluating the rutting resistance of mixtures using the gyratory compactor

• The proposed framework was evaluated and tested using 31 mixtures of different aggregate structure and aggregate properties – Appears to work

• The new framework has the potential for providing an index of the rutting potential of mixtures during mix design and optimization as well as for QC pass/fail purposes

Conclusions

• A simple micromechanics-based aggregate gradation framework appears to explain the observed rutting behavior in the field, APA, and the Servopac!

Conclusions

Questions ?