session 30 björn birgisson
TRANSCRIPT
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A Simple Micromechanics-based Approach for Evaluating the Rutting
Potential of Asphalt Pavements
Prof. Björn BirgissonThe Royal Institute of Technology (KTH)
Transportforum 2009
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• A test that reflects mixture rutting potential is required for:– Mixture optimization– Mixture design– Pass/Fail criteria
Problem Statement
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• 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
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• 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
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• Rutting instability is associated with plastic flow and formation of shear planes
Field Observations
Shear Planes
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• 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
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• 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
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• 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
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• 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
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Three Possible Basic Characteristics of GyratoryShear Strength Curves at Impending Instability
Vertical Strain
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• Failure strain - the strain at point of local minimum gyratory shear strength after increase in gyratory angle
Definition of “Failure Strain”
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Proposed Framework for theEvaluation of Rut Resistance
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• 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
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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
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• 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
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Field Mixtures Only
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 Field Instability Rutting
No Field Instability Rutting
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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
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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
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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.
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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.
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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.
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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
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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
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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.
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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
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APLICATION TO ANALYSIS OF FIELD PROJECTS
12 Superpave Projects were divided intothree groups based on their gradations characteristics
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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.
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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.
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Servopac Test Results
Superpave Servopac Results
0
5
10
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 %
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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.
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• 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
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• 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
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• A simple micromechanics-based aggregate gradation framework appears to explain the observed rutting behavior in the field, APA, and the Servopac!
Conclusions
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Questions ?