Analysis of Flexible Overlay Systems for Airport Pavements:

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 November9, 2005 University of Illinois at Urbana-Champaign

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

3. 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). Problem statement - Review

4. No. of Elements? AC Overlay Concrete Slab CTB Subgrade 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)

5. 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 Previous Work: Objectives:

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

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

11. 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 4 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

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-2 PCC-3 PCC-4 PCC-1 PCC-5

13. Details for Expanded Model Analyses Loading types Crack length: 0.5 inch 51F TAC=-1.5F/in Overlay=5”; AC=1.3888910-5 1/F 58.5F Concrete slabs=18” PCC=5.510-6 1/F TPCC=-1.25F/in Temperature profile Longitudinal Joint 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-2 PCC-3 PCC-4 PCC-1 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-2 PCC-3 PCC-4 PCC-1 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-3 Joint-1 Joint-2 (with a crack) Joint-4

18. y, v Crack-tip element (Singular Element)  r C2 B2 Crack faces x, u C1 B1 ℓ 4 ℓ 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

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!