Airport Pavement Design and Evaluation Prof. Jie Han, Ph.D., P.E.
The University of Kansas
Outline of Presentation Introduction FAA Pavement Design Principles FAA Flexible Pavement Design FAA Rigid Pavement Design 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 The methods discussed are suitable for aircraft with gross weights of 30,000 lbs (13,000 kg) or more Design of flexible pavements: CBR method Design of rigid pavement: jointed edge stress analysis Layered elastic analysis Design service life = 20 years
AC 150/5320-6D
Aircraft Considerations Load (95% main landing gear, 5% nose gear) 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 Tire pressure: 75 to 200 psi (515 to 1,380 kPa) 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 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 The design aircraft is the aircraft type that produces the greatest pavement thickness 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 Minimum CBR value of 80 is assumed for base course 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 Minimum CBR value of 20 is for subbase course 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 As a general rule of thumb, the design CBR value should be equal to or less than 85% of all the subgrade CBR values 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 Based on Westergaard analysis of edge loaded slabs (modified to simulate a jointed edge condition) Determine k value for rigid pavement Concrete flexural strength Gross weight of design aircraft Annual departures of design aircraft
Subbase Requirements A minimum thickness of 4 in. subbase 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 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 Design strength of 600 to 650 psi is recommended for most airfield applications Strength at 28 days 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<6)
Effect of Subbase on K - Stabilized Subbase
Design Curves – Single Wheel Gear
Gross weight of design aircraft
Design Curves – Dual Wheel Gear
Design Curves – Dual Tandem Gear
Critical and Noncritical Areas Total critical pavement thickness = T Noncritical pavement thickness (for concrete slab thickness) = 0.9T For variable section of the transition section and thinned edge, the reduction applies only to the concrete slab thickness The change in thickness for the transitions should be accomplished over an entire slab length and width
Stabilized subbase required
Design Example
• Dual tandem aircraft: gross weight = 350,000 lbs, annual equivalent departures =6000 (including 1200 of B-747 weighing 780,000 lbs) • Subgrade k =100pci with poor drainage, frost penetration =18 in. • Primary runway, 100% frost protection • Subgrade soil is CL • MR = 650 psi
Design Steps • Several thickness of subbase thickness should be tried => 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
