Contribution of Aircraft Gear Loads to Reflective Cracking in Airport Asphalt Overlays

1. Contribution of Aircraft Gear Loads to Reflective Cracking in Airport Asphalt Overlays January 30th, 2007 FAA COE Project Review and Project Proposal Meeting William G. Buttlar, Associate Professor Hyunwook Kim, Research Assistant

2. Outline • Discuss Research Ideas • Review State-of-the-Art in RC Analysis • Overview • J-Contour (LEFM) Approach • Cohesive Zone Model Approach • Summary and Discussion • RC Research Ideas

3. Some Open Asphalt Research Areas • Reflective Cracking Analysis and Design • Analysis • Supporting Lab and Field Data • State-of-the Art on AC Stiffness Prediction Models (Witczak, Hirsch, DiBenedetto, You, Yin, etc.) • Thermal Cracking Test for Airfield Pavements • Performance Testing Suite for High Traffic HMA Designs

4. Reflective Cracking • Reflective cracking is very complex airfield distress mechanism; likely a result of combined environmental and gear loading effects. • Cracks can begin to appear as soon as the first winter after construction. • Can cause the acceleration of other pavement distresses through water infiltration, stripping in HMA layers, and loss of subgrade support. • The key challenge isthe geometric discontinuity which makes modeling much more complex and tedious. Reflective Cracking * Kim and Buttlar (2002)

5. More Physics, More Complexity, More Difficult to Implement RC Model Evolution • Empirical Models • “Simple” FEM Models – Strength of Materials Approach • LEFM Approach • Cohesive Zone Modeling Approach • Reduced Physics (Surrogate Model) • New Design Method Understand Physics of Problem (Distress Mechanism) Simplify Problem

6. Can study mode-I andmixed-mode loading conditions. Can model crack nucleation, initiation, and propagation. Compatible with elastic and viscoelastic bulk material behavior. Can be used in conjunction with realistic thermal gradients.  More realistic approach but needs more computational effort. Recent Fracture Modeling Approaches • J-ContourApproach (LEFM) • Can be used to study the mixed-mode loading condition (combined opening and shearing). • Provides reliable, numerical estimates of stress concentration at the crack tip. • Can not be used directly with viscoelastic material. • Does not directly predict crack propagation.  Simplified approach requiring less computational effort. • Cohesive Zone Model Approach

7. #1 - J-Contour Approach

8. Stress Intensity Factor (SIF) - LEFM • Three Failure Modes Mode I (Opening) Mode II (In-Plane Shear) Mode III(Out-of-Plane Shear) Traffic-induced Traffic & thermal Most common mode Unstable subgrade presents Less common cause of RC • The stress fields ahead of a crack tip for each mode in an isotropic linearelastic material are: K = Stress intensity factor Where, fij() = Dimensionless function of  Where,  (= I, II, or III): Mode of loading • A stress singularity occurs at the crack tip as r approaches zero.

9. 2 nj 1 mj y 4 nj Crack faces 3 x J-Integral: Path Independence • Rice (1968) showed a mathematical presentation to prove the path-independency of the J-integral. For a linear elastic, isotropic material • For any closed contour, J = 0. The J-integral is a conservation integral. • J-integral is independent of the contour taken around the crack tip

10. Airport Overlay System Cross Section EAC = 200 ksi; AC = 0.35 AC Overlay 5 in 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 = 300 pci C L Top View 225 in • A typical pavement section of an airport that serves Boeing 777 aircraft was studied • The selected model geometry and pavement cross sections are based on the structure and geometric information of un-doweled sections of runway 34R-16L at Denver international airport(DIA) in Colorado 240 in Traffic Direction Transverse Joint = 0.5in Longitudinal Joint = 0.5in

11. Aircraft Gear Configuration 57 in 57 in 55in 13.64 in 21.82 in 36 ft (10.97 m) One Boeing-777-200 aircraft: • 2 dual-tridem main gears • Gear width = 36 ft • Main gear (6 wheels; 215 psi) • Gross weight = 634,500 lbs • Each gear carries 47.5% loading = 301,387.5 lb Boeing 777-200

12. FE Model Assumptions • The 2-D FE model is a reasonable approximation of 3-D geometry for the purpose of fracture study. • Material properties: Elastic (Due to the limitation of J-integral approach) • Subgrade: Winkler foundation (Linear spring) • Thermal loading: Simplified thermal loading rates and thermal coefficients were used • Load transfer efficiency (LTE): 2 node spring element (Stiffness in the vertical direction) (Hammons 1998) Thermal Loading 5F TAC=-1.5F/in Overlay=5”; AC=1.3888910-5 1/F 13F Concrete slabs=18” PCC=5.510-6 1/F TPCC=-1.25F/in Longitudinal Joint LTE 225 in 35F CTB=8”; CTB=7.510-6 1/F 35F Subgrade

13. Description of FE Fracture Model Both Gears Loading y Undeformed Center Position = 236 in x PCC-2 PCC-1 PCC-3 PCC-4 PCC-5 PCC-6 Deformed Joint-1 Joint-3 Joint-5 Joint-2 (with a crack) Joint-4 AC Overlay PCC Subbase Crack Tip

14. Stress Contour around Crack Tip • The extent of the zone of material tension, along with the stress concentration at the crack tip can be observed. Stress Concentration Tension Zone Crack Tip AC Crack Tip Existing Crack Existing Crack PCC Joint

15. Mode-I Stress Intensity Factor (KI) • The most critical tensile conditions were when the gear was located 67” and 337” away from the crack, not directly over the crack. • The cases with longer existing crack lengths tended to be more sensitive to fracture responses. • When temperature loading is applied in combination with certain gear loading, the critical tensile responses were significantly increased. Tension Compression • Gear Loading Responses without Temperature Loading (b) Gear Loading Responses with Temperature Loading

16. Mode-II Stress Intensity Factor (KII) • In the case of Mode-II responses, the critical shear conditions are found to be under the edge loading condition, e.g., under the edge of one tire loading among the four wheels represented in the 2D model. (b) Responses with Temperature Loading • Responses without Temperature Loading

17. One Gear vs. Both Gears • While the fracture responses have similar trends, the differences between single and dual gear loading becomes larger at the critical loading positions. (b) Mode-II Stress Intensity Factor (KII) (a) Mode-I Stress Intensity Factor (KI)

18. Summary • J-integral approach was verified and implemented into FE fracture pavement model to consider the critical responses under various aircraft gear loadings combined with simplified thermal loading. • The critical tensile condition for the pavement studied occurred when the gear load was positioned away from the existing joint (counter flexure) • Thick PCC over CTB • Large Aircraft Tire Radius and Multiple Wheels

19. Summary (cont.) • J-integral approach has limitations (Elastic, Fixed-length cracks) • Cohesive Zone Modeling Approach Overcomes these Limitations

20. #2 - CZM Approach

21. Cohesive Zone Model Gf

22. Schematic of DC(T) Fracture Test Crack Mouth Opening Displacement (CMOD) Gage Used to Obtain Fracture Energy, and can be used to Estimate Material Tensile Strength

23. FE Model Input • The others • Layer thickness • LTE • Subgrade support • Thermal coefficient • Friction between PCC and granular subbase • Gear loading time (e.g., 0.1 sec = 50 mph) • Pavement temperature profiles (EICM) • Elastic properties • Young’s modulus (E) • Poisson’s ratio (ν) • Viscoelastic properties • Creep compliance • Fracture properties • Fracture energy (Gf) • Tensile strength (St)

24. Thermal Loading Only- 1:00pm – 7:00am, 18 Hours -(Fracture Energy = 200 J/m2, Tensile Strength = 3.56 MPa) Joint 2 (CZM) Joint 3 Joint 4 Joint 5 Joint 1

25. AC Overlay 4” Existing AC PCC 4” 9” Critical Pavement Cooling Cycle • Pavement temperature profiles were determined by the enhanced integrated climate model (EICM, 2003). 1:00pm(Jan. 4th) – 7:00am (Jan. 5th, 1999) : 18 hours O’Hare Airport (ORD) 4L-22R

26. Example of Thermal Analysis

27. Thermal + Gear Loading- 3:00am – 4:00am, 1 Hour + Gear Loading -(Fracture Energy = 200 J/m2, Tensile Strength = 3.56 MPa) Gear Loading Position Joint 2 (CZM) Joint 3 Joint 4 Joint 5 Joint 1

28. Example of Crack Propagation

29. Summary • Two different fracture models were applied to investigate the mechanism of reflective cracking in the airfield overlay system. • Both J-integral and CZM approaches can be used to obtain a non-arbitrary assessment of critical responses in airfield overly systems (HMA/PCC). • Both fracture approaches provide useful tools to study the complex mechanism behind reflective cracking in airfield overlay systems.

30. RC Research DirectionCurrent and Proposed • Complete CZM Simulations • Compile Major Comprehensive Report (with Recommendations) by April • Make Recommendations • Surrogate Model Feasibility and Promising Directions • Document Needs for Additional Field Data (and Supporting Lab Data)

31. Etc. • Post-Doc – Su Kai • Qinwu Xu (China Penn State)

32. Thank You !!