Analysis of Flexible Overlay Systems for Airport Pavements:

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Analysis of Flexible Overlay Systems for Airport
Pavements

• Relative Contributions of Environmental and Load
  Related Factors to Reflection Crack Growth
  in Airport Flexible Overlays -

William G. Buttlar, Ph.D., P.E. Hyunwook Kim,
Research Assistant FAA COE Annual Review
Meeting November 9, 2005 University of Illinois
at Urbana-Champaign
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Outline

• - Previous Work
• - Progress Since Last Review Meeting
• - Current/Future Work

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Problem statement - Review

• Functions of Asphalt Overlays (OL)
• To restore smoothness, structure, and minimize
  moisture infiltration on existing airfield
  pavements.
• Problem
• The new asphalt overlay often fails before
  achieving its design life.
• Cause Reflective cracking (RC).

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Limitation of traditional FE modeling at joint
FEA ? applied on modeling of asphalt overlaid
JCP.

• Limitation
• The accuracy of the predicted critical OL
  responses immediately above the PCC joint was
  highly dependent on the degree of mesh refinement
  around the joint.

To seek reliable critical stress predictions,
LEFM will be applied in an attempt to arrive at
non-arbitrary critical overlay responses around a
joint or crack.
Kim and Buttlar (2002) Bozkurt and Buttlar
(2002) Sherman (2003)
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Previous Work
Objectives

• To investigate how the following parameters
  affect the potential for joint RC in rehab.
  airfield pavements.
• Bonding condition between slabs CTB
• Load transfer between the underlying concrete
  slabs
• Subgrade support
• Structural condition (modulus value) of the
  underlying slabs

• Introduce a robust reliable method (J-integral
  interaction-integral) to obtain accurate
  critical OL responses.
• Understand the effect of temp. loading by
  introducing temp. gradients in models.
• Identify critical loading conditions for rehab.
  airfield pavements subjected to thermo-mechanical
  loadings.

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Basic Concept of Fracture Analysis J-integral
Compute Path Integral Around Various Contours
Estimate Stress Intensity Factors (KI and KII) at
Tip of an Inserted Crack of Varied Length
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(2-Slab Modeling Results of K. Chou)
Mode I SIFs vs. 2 a/hAC ratios -- 11 positions --
Fine coarse meshes
Reduced contact tire pressure 69.7 ? 215 psi

• Tensile mode I SIFs are predicted starting from
  loading position 6, where the center of B777 main
  gear is at least 93.45 away from the PCC joint.
• Both mesh types give about the same predictions
  of mode I SIFs

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Ongoing Research

• Starting point Needed larger domain and to
  investigate the need to consider gear
  interaction, since counterflexure was found to be
  important for thick PCC pavements w/ overlay.
• Expand the model domain from 2 slabs to 5 slabs
• Compare one gear loading vs both for 777
• Compare with previous (2 slab) model
• Stress intensity factor (KI KII)
• J-Contour Integral
• Stress contour
• Deformation

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2-D Modeling
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Extended Geometry and Loading
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2D Model Description--Loading

• Boeing777-200 larger gear width (36 ft 432 in)
• The 2nd gear is about 2 slabs away from 1st gear
• Original assumption the distance between gears
  is large enough such that interactions may be
  neglected for the study of the OL responses ?
  Results of Chou suggested this assumption may not
  be valid

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Slab 3
1
Slab 2
Gear 1
Gear 2
55in
55in
57 in
57 in
240 in
432 in
16.32 in
6.82 in
225 in
225 in
Note Dimensions not drawn to scale
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Model Expansion (5 Slab model)
Position A One gear Loading
Previous Model
PCC-1
PCC-2
Position A Both gears Loading
Position A One gear Loading
New Expanded Model
PCC-2
PCC-3
PCC-4
PCC-1
PCC-5
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Details for Expanded Model Analyses
Loading types
Crack length 0.5 inch
51?F
?TAC-1.5?F/in
Overlay5 ?AC1.38889?10-5 1/?F
58.5?F
Concrete slabs18 ?PCC5.5?10-6 1/?F
?TPCC-1.25?F/in
Temperature profile
Longitudinal Joint
225 in
81?F
CTB8 ?CTB7.5?10-6 1/?F
81?F
Subgrade
Subgrade support 200 pci
100 load transfer efficiency
Traffic Temperature Loading Traffic loading
only Temperature loading only
Loading conditions
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Expanded Model with One Gear Load
Position A One gear Load
Undeformed
PCC-2
PCC-3
PCC-4
PCC-1
PCC-5
Deformed
Joint-1
Joint-3
Joint-2 (with a crack)
Joint-4
Deformation Scale Factor 100
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Deformation Scale Factor on Crack Tip
Undeformed
Deformation Scale Factor 1
Deformed
Exaggerated
Deformation Scale Factor 100
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Expanded Model with Both Gears
Position A Both gears Loading
Undeformed
PCC-2
PCC-3
PCC-4
PCC-1
PCC-5
Deformed
Joint-1
Joint-3
Joint-2 (with a crack)
Joint-4
Deformation Scale Factor 100
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Stress Contour von Mises
Deformation Scale Factor 1.0
Traffic Temperature Loading
Joint-3
Joint-1
Joint-2 (with a crack)
Joint-4
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Stress Contour at crack tip
Traffic Temperature Loading
Extracting KI KII using displacement
correction technique (DCT) based on singular
element
Joint-2 (with a crack)
u the sliding disp. at the crack flank nodes
the opening disp. at the crack flank nodes
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Stress Contour at Joint-3
Both gears
Traffic Loading Only
The tensile stress of both gears loading was much
larger than one gear loading.
One gear
The both gears loading is more critical than one
gear loading.
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Comparison of SIF (KI or KII)
The Interaction integral method and displacement
correction technique (DCT) based on singular
elements were applied to extract all SIFs and
J-Contours.
Traffic Temperature Loading
Tension ()
Compression (-)

• KI is dominant and SIFs in 2 PCC with one gear
  were larger than 5 PCC with one gear.
• SIFs increases if both gears loading is applied
  instead of one gear.

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Comparison of SIF (KI)
- If the traffic loading only is applied, then KI
has negative values. It means that the stress at
the crack tip becomes compressive.
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Comparison of SIF (KI)
- However, if the temperature loading only is
applied, then KI has a tensile stress value.
Therefore, the temperature loading condition is
more critical at the crack tip and the value in 2
PCC model was 20 higher than 5 slab model.
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Comparison of J-Contour Integral
In 2D elastic materials
- Energy release rate (G) is equal to J-Contour
integral if the material is elastic. The energy
concentrated on a crack tip of the 5 slab model
with both gears was about 50 higher than the one
gear, 5-slab model.
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Ongoing Research this Fall

• More loading positions to study critical
  positions
• Parametric studies with expanded models will be
  accomplished for
• Crack length
• Load transfer efficiency
• Subgrade support
• More temperature profiles
• More mesh refinement and evaluation of a larger
  domain extent models to assess convergence
• Summarize findings in major project report

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Possible Future Directions

• Viscoelastic modeling of AC Overlays
• New material model
• Comparing with field performance
• Validation with field data
• Cohesive element modeling with a notch
• New element and fracture modeling
• Combination of cohesive modeling with a bulk
  viscoelastic property.
• 3-D modeling
• Interlayer reflective crack control treatments
• Fresh look at design methodology and interlayer
  considerations/ guidelines using new modeling
  tools

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Thank you!