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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

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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

3Some 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

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Reflective Cracking

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)

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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

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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.

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#1 - J-Contour Approach

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Stress Intensity Factor (SIF) - LEFM

fij () = Dimensionless function of

K = Stress intensity factor

Where, (= I, II, or III): Mode of loading

)(2

lim )()(

0

ijijrf

r

K

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 & thermalMost common mode

Three Failure Modes

Mode II (In-Plane Shear)Mode I (Opening) Mode III (Out-of-Plane Shear)

Where,

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J-Integral: Path Independence

1

2

3

4

nj

y

x

nj

mj

Crack faces

4321

043211

1 JJJJdsnx

uWnJ jij

i

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.

2

2

'

2

'

2

IIIIII K

E

K

E

KGJ

For a linear elastic, isotropic material

10Airport 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

ECTB = 2,000 ksi; CTB = 0.20

k = 300 pciSubgrade

CTB

18 in

8 in

AC Overlay 5 in EAC = 200 ksi; AC = 0.350.5 in0.2 in

EPCC = 4,000 ksiPCC = 0.15

Cross Section

Transverse Joint = 0.5in

Longitudinal Joint = 0.5in

240 in

225 in

Top View CL

Traffic Direction

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36 ft (10.97 m)

Boeing 777-200

57 in 57 in

21.82 in

13.64 in

55in

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

Aircraft Gear Configuration

12FE Model Assumptions

225 in

Longitudinal Joint

Overlay=5”; AC=1.3888910-5 1/F

Concrete slabs=18” PCC=5.510-6 1/F

CTB=8”; CTB=7.510-6 1/F35F

35F

13F

Subgrade

5F

TPCC=-1.25F/in

TAC=-1.5F/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

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Description of FE Fracture Model

PCC-1 PCC-2

Both Gears Loading

Undeformed

Deformed

PCC-3 PCC-5 PCC-6

Joint-1 Joint-4Joint-2 (with a crack) Joint-5

PCC-4

Joint-3

Crack Tip

AC Overlay

PCC

Subbase

x

y Center Position = 236 in

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Stress Contour around Crack Tip

Crack Tip

Joint

Tension Zone

Stress Concentration

PCC

AC

Existing Crack

Existing Crack

Crack Tip

The extent of the zone of material tension, along with the stress concentration at the crack tip can be observed.

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Mode-I Stress Intensity Factor (KI)

-1200

-1000

-800

-600

-400

-200

0

200

400

600

0 100 200 300 400 500

Center Position of Gear Loading from a Crack Tip (inch)

SIF

(K

I)

0.1 in 0.3 in0.5 in 1.0 in1.5 in 2.5 in3.5 in 4.5 in

(a) Gear Loading Responses without Temperature Loading

(b) Gear Loading Responses with Temperature Loading

0

500

1000

1500

2000

2500

3000

3500

0 100 200 300 400 500

Center Position of Gear Loading from a Crack Tip (inch)

SIF

(KI)

0.1 in 0.3 in0.5 in 1.0 in1.5 in 2.5 in3.5 in 4.5 in

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.

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(a) Responses without Temperature Loading

(b) Responses with Temperature Loading

Mode-II Stress Intensity Factor (KII)

-200

-100

0

100

200

0 100 200 300 400 500

Center Position of Gear Loading from a Crack Tip (inch)

SIF

(K

II)

0.1 in 0.3 in0.5 in 1.0 in1.5 in 2.5 in3.5 in 4.5 in

-200

-100

0

100

200

0 100 200 300 400 500

Center Position of Gear Loading from a Crack Tip (inch)

SIF

(KII)

0.1 in 0.5 in1.0 in 1.5 in2.5 in 3.5 in4.5 in

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.

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One Gear vs. Both Gears

0

400

800

1200

0 100 200 300 400 500

Center Position of Gear Loading from a Crack Tip (inch)

SIF

(K

I)

One Gear - KI

Both Gear - KI

(a) Mode-I Stress Intensity Factor (KI) (b) Mode-II Stress Intensity Factor (KII)

-200

-100

0

100

0 100 200 300 400 500

Center Position of Gear Loading from a Crack Tip (inch)

SIF

(K

II)

One Gear - KII

Both Gear - KII

While the fracture responses have similar trends, the differences between single and dual gear loading becomes larger at the critical loading positions.

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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

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Summary (cont.) J-integral approach has limitations

(Elastic, Fixed-length cracks)

Cohesive Zone Modeling Approach Overcomes these Limitations

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#2 - CZM Approach

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sep

d

0

)(Gf

Cohesive Zone Model

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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 (Gf)

– Tensile strength (St)

• 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/m2, Tensile Strength = 3.56 MPa)

Joint 1 Joint 2(CZM)

Joint 3 Joint 4 Joint 5

Critical Pavement Cooling Cycle

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

-10 0 10 20 30 40

Temperature (Fahrenheit)

Dep

th (

inch

)

1:00 PM 2:00 PM3:00 PM 4:00 PM5:00 PM 6:00 PM7:00 PM 8:00 PM9:00 PM 10:00 PM11:00 PM 12:00 AM1:00 AM 2:00 AM3:00 AM 4:00 AM5:00 AM 6:00 AM7:00 AM

1:00pm(Jan. 4th) – 7:00am (Jan. 5th, 1999) : 18 hours

AC Overlay

PCC

4”

9”

Existing AC4”

Pavement temperature profiles were determined by the enhanced integrated

climate model (EICM, 2003).

O’Hare Airport (ORD) 4L-22R

Example of Thermal Analysis

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Thermal + Gear Loading

- 3:00am – 4:00am, 1 Hour + Gear Loading -

(Fracture Energy = 200 J/m2, Tensile Strength = 3.56 MPa)

Joint 1 Joint 2(CZM)

Joint 3 Joint 4 Joint 5

Gear Loading Position

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Example of Crack Propagation

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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.

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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)

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Etc.

Post-Doc – Su Kai

Qinwu Xu (China Penn State)

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Thank You !!