1 Contribution of Aircraft Gear Loads to Reflective Cracking in Airport Asphalt Overlays January 30 th, 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 is the geometric discontinuity which makes modeling much more complex and tedious. * Kim and Buttlar (2002)

5 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 More Physics, More Complexity, More Difficult to Implement Understand Physics of Problem (Distress Mechanism) Simplify Problem

6  J-Contour Approach (LEFM)  Cohesive Zone Model Approach  Can study mode-I and mixed-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  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.

7 #1 - J-Contour Approach

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

9 J-Integral: Path Independence 11 22 33 44 njnj y x njnj mjmj Crack faces  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  Rice (1968) showed a mathematical presentation to prove the path- independency of the J-integral. For a linear elastic, isotropic material

10 Airport Overlay System  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 Concrete Slabs E CTB = 2,000 ksi; CTB = 0.20 k = 300 pci Subgrade CTB 18 in 8 in AC Overlay 5 in E AC = 200 ksi; AC = in 0.2 in E PCC = 4,000 ksi PCC = 0.15 Cross Section Transverse Joint = 0.5in Longitudinal Joint = 0.5in 240 in 225 in Top View C L Traffic Direction

11 36 ft (10.97 m) Boeing in in in 55in One Boeing 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 Aircraft Gear Configuration

12 FE Model Assumptions 225 in Longitudinal Joint Overlay=5”;  AC =  /  F Concrete slabs=18”  PCC =5.5  /  F CTB=8”;  CTB =7.5  /  F 35  F 13  F Subgrade 5F5F  T PCC =-1.25  F/in  T AC =-1.5  F/in Thermal Loading  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) LTE

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

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

15 Mode-I Stress Intensity Factor (K I ) (a) Gear Loading Responses without Temperature Loading (b) Gear Loading Responses with Temperature Loading Tension Compression  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.

16 (a) Responses without Temperature Loading (b) Responses with Temperature Loading Mode-II Stress Intensity Factor (K II )  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.

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

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 GfGf Cohesive Zone Model

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

FE Model Input Elastic properties –Young’s modulus (E) –Poisson’s ratio (ν) Viscoelastic properties –Creep compliance Fracture properties –Fracture energy (G f ) –Tensile strength (S t ) 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)

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

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

Example of Thermal Analysis

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

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 Direction Current 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 !!