Airport Pavement Design and Evaluation

Airport Pavement Design and Evaluation Prof. Jie Han, Ph.D., P.E.

The University of Kansas

Outline of Presentation ƒ Introduction ƒ FAA Pavement Design Principles ƒ FAA Flexible Pavement Design ƒ FAA Rigid Pavement Design ƒ FAA Layered Elastic Pavement Design

Introduction

References • Principles of Pavement Design, Yoder and Witczak (1975) • Airport Pavement Design and Evaluation, FAA Advisory Circular 150/5320-6D • Airfield and Highway Pavements, Proceedings of 2006 Airfield and Highway Specialty Conference • Web seminar “FAA – LEDFAA V1.3 Layered Elastic Flexible Pavement Design for Airfield Pavements”, Rodney N. Joel, FAA

Websites • http://www.chet-aero.com/download/software.php • http://www.airtech.tc.faa.gov/naptf/download/index1.asp • Airport Pavement Structural Design System http://www.mincad.com.au/apsdsbr.htm

Airfield vs. Highway Pavements • Repetition of load • Distribution of traffic • Geometry of the pavement

Affected by pavement width and type of aircraft

Plan View of Basic Types of Wheel Configuration a) single trailer-truck unit b) tricycle landing gear with single tires c) twin-tandem landing gear d) double twin-tandem gear

Several Typical Aircrafts

Effect of Standard Deviation of Aircraft Wander on Pavement Damage

Predicted transverse Equivalent DC-8-63F Strain repetitions (taxiway) Np x 103

Measured transverse crack frequency (%)

Flexible Airport Pavement Design • Corps of Engineering (CBR) method (CBR method): CBR test for subgrade evaluation • FAA method: field performance data correlated to soil classification, also a CBR method • Canadian DOT method: plate-bearing tests to evaluate subgrade support/repeated load triaxial tests for fulldepth airport pavements • AI method: theoretically oriented design

Rigid Airport Pavement Design

– PCA method – Corps of Engineering method – FAA method: based on the Westergaard analysis of edge loaded slabs

FAA Pavement Design Principles

FAA Airport Pavement Design

Scope and Design Philosophy ƒ The methods discussed are suitable for aircraft with gross weights of 30,000 lbs (13,000 kg) or more ƒ Design of flexible pavements: CBR method ƒ Design of rigid pavement: jointed edge stress analysis ƒ Layered elastic analysis ƒ Design service life = 20 years

AC 150/5320-6D

Aircraft Considerations ƒ Load (95% main landing gear, 5% nose gear) ƒ Landing gear type and geometry • Single gear aircraft • Dual gear aircraft • Dual tandem gear aircraft • Wide body aircraft – B-747, B-767, DC-10, L-1011 ƒ Tire pressure: 75 to 200 psi (515 to 1,380 kPa) ƒ Traffic volume

AC 150/5320-6D

Equivalent Single Wheel Load (ESWL)

AC 150/5320-6D

AC 150/5320-6D

Increased Loading Gear Complexity

Loading Gear Design

Aircraft Grew in Size

Gross Aircraft Weight

Individual Wheel Load (lbs)

Gross Aircraft Weight

Equivalent Single Wheel Load

A New Design Procedure Needed

Efforts for New Design Procedure

Efforts for New Design Procedure

Design Procedure • Forecast annual departures • Select design aircraft that requires the thickest pavement • Transform other aircrafts to equivalent departures of design aircraft

Determination of Design Aircraft ƒ The required pavement thickness for each aircraft type should be checked using the appropriate design curve and the forecast number of annual departures for that aircraft ƒ The design aircraft is the aircraft type that produces the greatest pavement thickness ƒ The design aircraft is not necessarily be the heaviest aircraft in the forecast

Factors for Converting Annual Departures by Aircraft to Equivalent Annual Departures by Design Aircraft

Conversion of Equivalent Annual Departure of Design Aircraft W2 log R 1 = log R 2 ⋅ W1 R1 – equivalent annual departures of the design aircraft R2 – annual departures expressed in design aircraft landing gear configuration W1 – wheel load of the design aircraft W2 – wheel load of the aircraft being converted Each wide body as a 300,000-pound dual tandem aircraft

Example 727-200 requires the greatest pavement thickness and thus is the design aircraft

Aircraft

Gear type

Wheel load Equiv. ann. depart. design Avg. ann Max. takeoff Equiv. dual Wheel load Design aircraft depart. Weight (lbs).gear depart (lbs) aircraft (lbs)

727-100

Dual

3760

160,000

3760

38,000

45,240

1,891

727-200

Dual

9080

190,500

9080

45,240

45,240

9,080

707-320B

Dual tandem

3050

327,000

5185

38,830

45,240

2,764

DC-9-30

Dual

5800

108,000

5800

25,650

45,240

682

CV-880

Dual tandem

400

184,500

680

21,910

45,240

94

737-200

dual

2650

115,500

2650

27,430

45,240

463

L-1011-100

Dual tandem

1710

450,000

2907

35,625

45,240

1,184

747-100

Double dual

85

700,000

145

35,625

45,240

83

tandem

300,000x0.95/8 Wide body

Total = 16,241 1.7 x 85 190,500x0.95/4 35625 log R 1 = log(145) ⋅ Conversion 45240 factor

Final design: 16,241 annual departures of a dual wheel aircraft weighing 190,500lbs

Typical Design Section of Runway Pavement

FAA Flexible Pavement Design - CBR Method

Base Course ƒ Minimum CBR value of 80 is assumed for base course ƒ Types of base courses - Item P-208: aggregate base course - Item P-209: crushed aggregate base course - Item P-211: lime rock base course - Item P-304: cement treated base course - Item P-306: econocrete subbase course - Item P-401: plant mix bituminous pavements

Subbase Course ƒ Minimum CBR value of 20 is for subbase course ƒ Types of subbase courses - Item P-154: subbase course - Item P-210: cliché base course - Item P-212: shell base course - Item P-213: sand clay base course - Item P-301: soil cement base course Items P-213 and P-301 are not recommended where frost penetration into the subbase is anticipated

Subgarde Compaction Requirements

CBR Design Equations

MWHGL = multiple-wheel, heavy gear load

Alpha Factors – MWHGL Data 1.4

12-Wheel Failure 1.2

Alpha = 0.23 log C + 0.15

12-Wheel Nonfailure 50-kip Single Wheel Failure

Single Wheel

1.0

0.8

0.6

0.4

Load Repetition Factor, Alpha

30-kip Single Wheel Failure 30-kip Single Wheel Nonfailure Dual-Tandem Failure

Twin Tandem

12 Wheels

0.2

Aircraft Traffic Volume Factor, Coverages 0.0 1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

Hayhoe (2005)

Selection of Design CBR Value ƒ As a general rule of thumb, the design CBR value should be equal to or less than 85% of all the subgrade CBR values ƒ Corresponds to a design value of one standard deviation below the mean value

Design Chart for Single Wheel Gear

Design Chart for Dual Wheel Gear

Design Chart for Dual Tandem Gear

Pavement Thickness for High Departure Levels Annual Departure Level 50,000

Percent of 25,000 Departure Thickness 104

100,000

108

150,000

110

200,000

112

1-in of the thickness increase should be HMA surfacing The remaining thickness increases should be proportioned between base and subbase

Minimum Base Course Thickness

Critical and Noncritical Areas Total critical pavement thickness = T Noncritical pavement thickness (for base and subbase only) = 0.9T For variable section of the transition section and thinned edge, the reduction applies only to the base course 0.7T as the minimum for thickness of base can be applied

Example • A flexible airport pavement to be designed – Dual gear aircraft – Gross weight of 75,000 lbs – 6,000 annual equivalent departures of the design aircraft – Design CBR value for subbase = 20 – Design CBR value for subgrade = 6

Total Pavement Thickness Using Subgrade CBR to find total pavement thickness (23 in. in this example)

Subbase Thickness Using Subbase CBR to find: the combined thickness of HMA and base course needed over a 20 CBR subbase is 9.2 in. Subbase thickness = 23-9.2 =13.8 in. (14-in)

Design Pavement Sections Thickness of HMA surface (critical area) =4 in. Thickness of base course = 9.2-4 = 5.2 in (6-in). Thickness of subbase course = 14in. Thickness should be rounded off to even increments

Notes on Frost Effects and Stabilized Materials • The thickness determined from these design charts are for untreated granular bases and subbases • Frost effects and stabilized materials must be handled separately

Stabilized Base and Subbase • Required for new pavements and jet aircraft weighting 100,000 lbs or more • Subbase and base equivalency factors – Standard for granular/stabilized subbase is Item P154 with CBR of 20 – Standard for granular/stabilized base is Item P-209, crushed aggregate base course with CBR of 80 • Min. total pavement thickness calculated ≥ that required by a 20 CBR subgrade from design curve

Frost Effect • Thicker subbase courses • Determine soil frost group

• Determine the depth of frost penetration • Frost protection (complete, limited, reduced subgrade strength)

Design Air Freezing Indices 3500

2500

1500 750

250

50

Unit: degree days Fo

Depth of Frost Penetration Air freezing index, degree days Fo 0

600

1000

2000

3000

20 40

40.8 60 80

Meters

Frost penetration inches

0

100 120 140 160

(Degree days Co)

FAA Rigid Pavement Design

Principles of Rigid Airport Pavement Design ƒ Based on Westergaard analysis of edge loaded slabs (modified to simulate a jointed edge condition) ƒ Determine k value for rigid pavement ƒ Concrete flexural strength ƒ Gross weight of design aircraft ƒ Annual departures of design aircraft

Subbase Requirements ƒ A minimum thickness of 4 in. subbase ƒ Types of subbase courses - Item P-154: subbase course - Item P-208: aggregate base course - Item P-209: crushed aggregate base course - Item P-211: lime rock base course - Item P-304: cement treated base course - Item P-306: econocrete subbase course - Item P-401: plant mix bituminous pavements ƒ Stabilized subbase (aircraft weight > 100,000 lbs) - Item P-304: cement treated base course - Item P-306: econocrete subbase course - Item P-401: plant mix bituminous pavements

Exceptions for No Subbase

Concrete Flexural Strength ƒ Design strength of 600 to 650 psi is recommended for most airfield applications ƒ Strength at 28 days ƒ 5% less than the test strength used for thickness design

Effect of Subbase on K

(MN/m3)

K on top of subbase (lb/in3)

- Well-Graded Crushed Aggregate

Effect of Subbase on K

(MN/m3)

k on top of subbase (lb/in3)

- Bank-Run Sand & Gravel (PI most economical section • Assume P-304 (cement treated base course) to be used • Trial thickness of subbase = 6 in.

Slab Thickness • 16.6 in. round off to 17 in. • 17 + 6 =23 in. > 18 in. (frost depth) • Wide body aircraft did not control slab thickness but to be considered in establishment of jointing requirements and design of drainage structures

Rigid Pavement Joint Types and Details

Recommended Maximum Joint Spacing - Rigid Pavement without Stabilized Subbase

Recommended Maximum Joint Spacing - Rigid Pavement with Stabilized Subbase Joint spacing (unit: in.)/radius of relative stiffness < 5.0 to control transverse cracking Maximum joint spacing = 60 ft. Radius of relative stiffness: ⎡ Eh ⎤ l=⎢ ⎥ 2 12 1 − ν k ⎣ ⎦ 3

(

)

1/ 4

Dimensions and Spacing of Steel Dowels

Amount of Reinforcement for Reinforced Concrete Pavements 3.7 L Lt As = fs where As = area of steel per foot of width or length (in2) L = length or width of slab, ft. T = thickness of slab, in. fs = allowable tensile stress in steel, psi, 2/3 yield strength Minimum percentage of steel reinforcement = 0.05% to the area of concrete per unit length or width

Allowable Strengths of Various Grades of Reinforcing Steel Allowable

Dimensions and Unit Weights of Deformed Steel Reinforcing Bars

Sectional Areas of Welded Fabric

Jointing of Reinforced Rigid Pavements

Spreadsheet Programs • F806FAA for flexible pavement design • F805FAA for rigid pavement design

FAA Layered Elastic Pavement Design

LEDFAA –Layered Elastic Design • Heavier load + complex multiple-wheel, multiple truck landing gear systems • Complex wheel load interactions with pavement structures – B-777 or Airbus A-380 (TDT) – B-777: 2 six-wheel main landing gears (TDT: 3 pairs of wheels in a row) + a single nose gear (single dual wheel) to support gross weight up to 535,000 lbs • Compatible with conventional FAA design • Landing gear configuration and layered pavement structures can be modeled directly

Flexible Pavement Failure Modes

Layered Elastic Method vs. CBR Method

LEDFAA V1.3 Default Values

LEDFAA V1.3

Cumulative Damage Factor (CDF) for Traffic Model

Cumulative Damage Factor (CDF) for Traffic Model

Cumulative Damage Factor (CDF) for Traffic Model

Cumulative Damage Factor (CDF) for Traffic Model

Sample Aircraft Traffic Mix CDF Contribution

Sample Aircraft Traffic Mix CDF Contribution

Large Aircraft Traffic Mix Gear Locations

No More Design Aircraft in LEDFAA

From CBR Method to LEDFAA • Nomographs => computer program • ‘design aircraft’ => ‘cumulative damage factor’ using Miner’s rule for fatigue failure design • CBR or k-value => elastic modulus

• LEDFAA design should comply with detailed requirements and recommendations from Advisory Circular • Should follow Advisory Circular recommendations in selection of input parameters

Flexible Airport Pavement Design • Two modes of failures – Vertical strain in the subgrade – Horizontal strain in asphalt layer • For traffic mixture including aircraft with triple dual tandem (TDT) gears – Min. thickness =5 in. of hot mix surfacing – Min. thickness =5 in. of stabilized base (not containing TDT, 6 in.) – P-301 soil cement base not acceptable – Min. thickness =3 in. of subbase base – Subgrade: E=1500*CBR

Rigid Airport Pavement Design • One mode of failure (cracking of concrete slab) – Limiting horizontal stress at the bottom surface of the concrete slab • For traffic mixture including aircraft with TDT gears – Min. thickness =6 in. of concrete surfacing – Min. thickness =4 in. of stabilized subbase (bound materials) – Subgrade : logE=1.415+1.284logk

Design Software • LEDFAA 1.3