A Simple Micromechanics-basedApproach for Evaluating the Rutting Potential of Asphalt Pavements Prof. Björn Birgisson The Royal Institute of Technology (KTH) Transportforum 2009
Problem Statement• A test that reflects mixture rutting potential is required for: – Mixture optimization – Mixture design – Pass/Fail criteria
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
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
Field Observations • Rutting instability is associated with plastic flow and formation of shear planes Shear Planes
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
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
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
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
Three Possible Basic Characteristics of GyratoryShear Strength Curves at Impending Instability Vertical Strain
Definition of “Failure Strain” • Failure strain - the strain at point of local minimum gyratory shear strength after increase in gyratory angle
Proposed Framework for theEvaluation of Rut Resistance
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)
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 (%)
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
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 (%)
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 Dominant Aggregate IC, IV(a) SMA (b) Coarse dense (c) Fine dense
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 4 0.25 0.20 3Spacing, 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.
APLICATION TO ANALYSIS OF FIELD PROJECTS12 Superpave Projects were divided intothree groups based on their gradationscharacteristics
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 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.
Group 3: Marginal InteractionThis 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.
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
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!