1. 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

2. Outline - Previous Work - Progress Since Last Review Meeting
- Current/Future Work

3. 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).

4. †Kim and Buttlar (2002); Bozkurt and Buttlar (2002); Sherman (2003)
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.
Concrete Slab
Subgrade
CTB
AC Overlay
No. of Elements?
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)

5. Previous Work: Objectives:
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.
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

6. 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

7. (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

8. 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

9. Starting with a 2-D Model
2-D Modeling
Reflective Crack
Subgrade
Subbase
PCC
AC Overlay
Boeing 777
3-D in reality
Starting with a 2-D Model

10. Extended Geometry and Loading
Loading Positions
Transverse Joint = 0.5in
Longitudinal Joint = 0.5in
240 in
225 in
Top view C L
Traffic Direction A B
Concrete
Slabs
ECTB = 2,000 ksi; CTB = 0.20
k = 200 pci
Subgrade
CTB
18 in
8 in
AC Overlay
5 in
EAC = 200 ksi; AC = 0.35
0.5 in
0.2 in
EPCC = 4,000 ksi
PCC = 0.15
Cross section

11. 2D Model Description--Loading
Boeing : 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 1
Slab 2
Slab 3 4
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

12. 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-1
PCC-2
PCC-3
PCC-4
PCC-5

13. Details for Expanded Model Analyses
Loading types
Crack length: 0.5 inch
51F
Overlay=5”; AC= 10-5 1/F
TAC=-1.5F/in
58.5F
Concrete slabs=18” PCC=5.510-6 1/F
TPCC=-1.25F/in
Longitudinal Joint
Temperature profile
225 in
81F
CTB=8”; CTB=7.5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 :

14. Expanded Model with One Gear Load
Position A – One gear Load
Undeformed
PCC-1
PCC-2
PCC-3
PCC-4
PCC-5
Deformed
Joint-1
Joint-2 (with a crack)
Joint-3
Joint-4
* Deformation Scale Factor = 100

15. Deformation Scale Factor on Crack Tip
* Undeformed
* Deformation Scale Factor = 1
Deformed
Exaggerated
* Deformation Scale Factor = 100

16. Expanded Model with Both Gears
Position A – Both gears Loading
Undeformed
PCC-1
PCC-2
PCC-3
PCC-4
PCC-5
Deformed
Joint-1
Joint-2 (with a crack)
Joint-3
Joint-4
* Deformation Scale Factor = 100

17. Stress Contour – von Mises
* Traffic + Temperature Loading
* Deformation Scale Factor = 1.0
Joint-1
Joint-3
Joint-2 (with a crack)
Joint-4

18. 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) ℓ 4 C2 C1 B2 B1
y, v
x, u  r
Crack-tip element
(Singular Element)
Crack faces
u = the sliding disp. at the crack flank nodes
= the opening disp. at the crack flank nodes

19. 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.

20. 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.

21. 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.

22. 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.

23. 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.

24. 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

25. 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

26. Thank you!