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Session 30 Björn Birgisson
1. A Simple Micromechanics-based
Approach for Evaluating the Rutting
Potential of Asphalt Pavements
Prof. Björn Birgisson
The Royal Institute of Technology (KTH)
Transportforum 2009
2. Problem Statement
• A test that reflects mixture rutting potential is
required for:
– Mixture optimization
– Mixture design
– Pass/Fail criteria
3. Instability Rutting
• 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
4. Back to Basics
• 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
5. Field Observations
• Rutting instability is associated with plastic
flow and formation of shear planes
Shear Planes
6. Tire Contact Studies and Analyses
• 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
7. Focus on Key Mechanism
• 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
8. New Approach
• 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
9. Observed Response
• At condition of Impending Instability, gyratory shear
strength peaks, followed by a rearrangement of aggregate
structure
1000
Gyratory Shear Strength (kPa) .
900
800
700
600
500
400 1.25 o 2.5 o
300
200
100
0
0 20 40 60 80 100 120 140
Number of Gyratory Revolutions (N)
• Gyratory shear strength may or may not increase after
rearrangement of aggregate structure
10. Three Possible Basic Characteristics of Gyratory
Shear Strength Curves at Impending Instability
Vertical Strain
11. Definition of “Failure Strain”
• Failure strain - the strain at point of local minimum gyratory
shear strength after increase in gyratory angle
13. Evaluation of Proposed Framework
• 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)
14. Evaluation of Proposed Framework
40
Observed APA cracking
35 APA Rutting > 7.0mm
Gyratory Shear Slope (kPa)
APA Rutting < 7.0mm
30
25
20
15
10
5
0
1 1.2 1.4 1.6 1.8 2 2.2 2.4
Failure Strain (%)
15. Statistical Evaluation of Results
• 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
16. Field Mixtures Only
40
Observed Field Instability Rutting
35
No Field Instability Rutting
Gyratory Shear Slope (kPa)
30
25
20
15
10
5
0
1 1.2 1.4 1.6 1.8 2 2.2 2.4
Failure Strain (%)
18. 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
19. 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.
20. 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.
21. 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.
22. 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
Dominant
Aggregate
IC, IV
(a) SMA (b) Coarse dense (c) Fine dense
23. 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
24. 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.
25. Spacing Analysis
4 0.25
0.20
3
Spacing, cm
0.15
Slope
2 Large Large
Small Small
0.10
1
0.05
0
0.00
100/0
95/5
90/10
85/15
80/20
70/30
60/40
50/50
40/60
30/70
20/80
15/85
10-90
5/95
0/100
0 10 20 30 40 50 60 70 80 90 100
Large/Small Particle Proportion % passing for sections
• 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.
26. APLICATION TO ANALYSIS
OF FIELD PROJECTS
12 Superpave Projects were divided into
three groups based on their gradations
characteristics
27. 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.
28. Poorly Performing Group 2:
ηDASR > 50%
This included field gradation of projects 6 and 8, and plant mix
gradation 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.
30. 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.
31. Conclusions
• 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
32. 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
33. Conclusions
• A simple micromechanics-based aggregate gradation
framework appears to explain the observed rutting behavior
in the field, APA, and the Servopac!