Modeling Flexible and Rigid Pavements under Aircraft Loading

www.wrsc.unr.edu Making a world of difference.sm Slide No. 1
Modeling of Flexible and Rigid
Pavements under Aircraft Loading
Elise M. Mansour
Pavement Engineering and Science Program
University of Nevada, Reno
M.Sc. Thesis Defense
Under the Supervision of :
Dr. Peter E. Sebaaly
May 1 𝑠𝑡
, 2018

Outline
• Introduction
• Purpose
• Construction Cycle 1 Analysis
• Aircraft Loading
• Instrumentation for Flexible and Rigid Test Items
• Pavement Heavy Weight Deflectometer Testing
• Validity of Flexible Test Items Sensors
• Validity of Rigid Test Items Sensors
• Conclusion and Recommendations

National Airport Pavement Test Facility (NAPTF)
• Purpose Provide full-scale pavement performance data and failure information
for incorporation into the new pavement thickness design
• Facility 900-foot long and 60-foot wide fully instrumented test track

National Airport Pavement Test Vehicle (NAPTV)
• Adjustable up to 75 kips per wheel
• Supported on rails on both sides
• Approximately 75-foot long and 80-foot wide
• Able to accommodate all types of gear configurations
• Changeable wheel spacing, gear spacing, tire pressure, and loading speed
• Controlled aircraft wander simulation
• Divided into independent test items of different pavement structures =>
Construction Cycles (CC)

Purpose
• Evaluate various simple and complex models by comparing their calculated
responses to the measured responses from the instrumented sections at the
NAPTF . The evaluated models include;
1) The FAA LEAF model: Linear Elastic Theory (LET) ; all layers assumed to
be linear elastic + uniform surface loads over a circular area. Implemented in
ELMOD and BAKFAA software.
2) The 3D-Move model: Viscoelastic properties of the AC layer + non-
circular/non-uniform 2D tire pressure.

3) The Finite element (FE) model: Moving and impulse loads + many
material models… It was implemented in FAARFIELD software.
4) The Odemark-Boussinesq (OB) model: for both forward and
backcalculation analyses. It was implemented in ELMOD software.
Purpose

Construction Cycle 1 Analysis
• Aircraft Loading
Traffic Wander
Wheel Load and Speed
• Responses from Instrumentation of Flexible and Rigid Test Items
• Pavement Heavy Weight Deflectometer Testing
Backcalculated Layers Moduli

9 test items- 6 flexible (F) and 3 rigid (R) having:
• Conventional (C) or Stabilized (S) base.
• The construction of CC-1 test items was completed in May 1999, and the pavements were
trafficked in two phases:
February 14 to November 15, 2000
May 12 to November 5, 2001
3
Subgrade
Types
Low Strength County Sand and Sand Clay (CS & SC) Target CBR =4
Medium Strength DuPont Clay Target CBR =8
High Strength Well-graded sand Target CBR =20

• Layers were labeled in accordance with FAA standards:
- Item P -501: Portland cement concrete.
- Item P-401: Asphalt concrete (AC).
- Item P-306: Lean concrete base (Econocrete).
- Item P-209: Crushed aggregate base.
- Item P-154: Subbase.

Flexible Pavement Sections with Conventional (C) Base:
Item 1-3
LFC (St. 2+25 – 3+00)
Item 2-3
MFC (St. 3+25 – 3+87.5)
Item 3-3
HFC (St. 8+37.5 – 9+00)
5 1/8 (5.125) inch P-401 5 1/8 (5.125) inch P-401 5 1/4 (5.25) inch P-401
7 ¾ (7.75) inch P-209 7 7/8 (7.875) inch P-209 10 7/8 (10.875) inch P-209
3 feet – 0 3/8 (36.375) inch
P-154
1 foot – 0 1/8 (12.125) inch P-154
High CBR SG
Medium CBR SG
Low CBR SG

Flexible Pavement Sections with Stabilized (S) Base:
Item 1-2
LFS (St. 1+25 – 2+00)
Item 2-2
MFS (St. 4+12.5 – 4+75)
Item 3-2
HFS (St. 7+50 – 8+12.5)
5 inch P-401 5 inch P-401 5 1/8 (5.125) inch P-401
4 7/8 (4.875) inch P-401 4 7/8 (4.875) inch P-401 4 ½ (4.5) inch P-401
2 feet – 5 5/8 (29.625) inch
P-209
8 ½ (8.5) inch P-209
High CBR SG
Medium CBR SG
Low CBR SG

Rigid Pavement Sections with Stabilized Base:
Item 1-1
LRS (St. 0+00 –1+00)
Item 2-1
MRS (St. 5+00 –6+00)
Item 3-1
HRS (St. 6+25 –7+25)
11 inch P-501 9 3/4 (9.75) inch P-501 9 inch P-501
6 1/8 (6.125) inch P-306 5 7/8 (5.875) inch P-306 6 inch P-306
8 3/8 (8.375) inch P-154 8 5/8 (8.625) inch P-154 6 5/8 (6.625) inch P-154
Low CBR SG Medium CBR SG High CBR SG

CC-1 Aircraft Loading
Boeing 777 Boeing 747
• Single Wheel
Load=45 kips
• Speed of 5 mph 270 kips 180 kips

Traffic Wander:
Track
Gear’s Centerline Transverse Offset, feet
Pass NumbersNorth Wheel Track
(B777)
South Wheel Track
(B747)
-4 -16.204 8.921 1, 2, 19 & 20
-3 -15.351 9.774 17, 18, 35, 36, 43 & 44
-2 -14.498 10.627 3, 4, 21, 22, 45, 46, 51 & 52
-1 -13.645 11.480 15, 16, 33, 34, 41, 42, 59, 60, 63 & 64
0 -12.792 12.333 5, 6, 23, 24, 47, 48, 53, 54, 65 & 66
1 -11.939 13.186 13, 14, 31, 32, 39, 40, 57, 58, 61 & 62
2 -11.086 14.039 7, 8, 25, 26, 49, 50, 55 & 56
3 -10.233 14.892 11, 12, 29, 30, 37 & 38
4 -9.380 15.745 9, 10, 27 & 28

Traffic Wander:

CC-1 Instrumentation
Dynamic Sensors:
Flexible Sections
• Pressure Cells (PCs)
• Strain Gauges (SGs)
• Multi-Depth Deflectometers (MDDs)
Rigid Sections
• Strain Gauges
• Joint Gauges
Static Sensors:
• Temperature
• Moisture
• Relative Humidity

Sensors Installation:
Asphalt Strain Gauges Installation Pressure Cells Installation
Multi-Depth Deflectometers (MDD) Installation

Flexible Test Items Instrumentation:
a) Pressure Cells: Total of sixty (60) 6-inch and eighty-four (84) 2-inch
diameter pressure cells.
b) Strain Gauges: Total of 108 strain gauges; with 24 gauges in stabilized
base (S) test items and 12 gauges in the conventional base (C) ones.
c) Multi-Depth Deflectometers (MDDs): Each MDD consisted of 7
displacement transducers (DTs), for a total of 30 MDDs and 210 DTs.

Rigid Test Items Instrumentation:
a) Concrete Strain Gauges (CSGs): Around 154 CSGs and were installed
near both the top and bottom surfaces of the slabs. The gauges were
oriented in the longitudinal direction, transverse direction, or at an angle
in the corners.
b) Joint Gauges (JGs): A total of 9 JGs were instrumented in each of the test
tracks.

Concrete Strain Gauges Distribution:
Traffic Direction

Static Sensors:
• Measured temperature , moisture, and crack status on an hourly basis.
• Temperature sensors were installed at various locations and at multiple
depths within the AC layers and the PCC slabs.
For the AC Layer the in-place average temperature is the critical
parameter in the analysis.
For the PCC slab temperature and moisture differentials are important.

2 Types of HWD testing: Uniformity and Routine testing
Pavement Heavy Weight Deflectometer Testing

Backcalculated Layer Moduli:
1. OB and LET methods using Dynatest ELMOD 6 software,
2. LET method found in BAKFAA software,
3. New proposed dynamic FE model.

• P-401: AC, P-209: CAB, and P-154: Subbase
Layer OB [ELMOD]
LET [BAKFAA & ELMOD]
New FE Model
No Stiff Layer
Stiff Layer
at 20-Foot
Variable Depth
Stiff Layer
P-401 at 77F 320 to 502 ksi 272 to 720 ksi 378 to 687 ksi 397 to 650 ksi 416 to 708 ksi
P-209 30 to 119 ksi 15 to 94 ksi 22 to 183 ksi 24 to 101 ksi 20 to 35 ksi
P-154 --- --- --- --- 17 to 31 ksi
P-209 & P-154
Combined
25 to 53 ksi 15 to 30 ksi 19 to 43 ksi 21 to 39 ksi ---

• P -501: PCC, P-306: Econocrete, and P-154: Subbase
Layer OB [ELMOD]
New FE Model
No Stiff Layer
Stiff Layer
at 20-Foot
Variable Stiff
Layer
P-501 3,910 to 4,050 ksi 5,930 to 8,250 ksi 4,200 to 7,360 ksi 6,020 to 6,280 ksi 3,900 to 4,450 ksi
P-306 --- --- --- --- 1,200 to 1,900 ksi
P-306 & P-154
Combined
143 to 338 ksi 110 to 415 ksi 237 to 929 ksi 195 to 487 ksi ---

Layer OB [ELMOD]
New FE Model
No Stiff Layer
Stiff Layer
at 20-Foot
Variable Stiff
Layer
Subgrade
Low CBR
11.8 to 27.7 ksi
(CBR=10.9 to 41.4)
16.3 to 27.1 ksi
(CBR=18.1 to 40.0)
9.7 to 15.6 ksi
(CBR=8.0 to 16.9)
14.3 to 25.2 ksi
(CBR=14.7 to
35.7)
4.1 to 6.2 ksi
(CBR=2.1 to 4)
Medium
CBR
10.1 to 39.6 ksi
(CBR=8.6 to 72.4)
24.2 to 34.6 ksi
(CBR=33.5 to 58.6)
17.5 to 23.3 ksi
(CBR=20.2 to 31.6)
15.3 to 30.6 ksi
(CBR=16.4 to
48.4)
9.2 to 11.5 ksi
(CBR=7.4 to 10.5)
High
CBR
14.6 to 62 ksi
(CBR=15.2 to 145.9)
38.8 to 52.7 ksi
(CBR=70.1 to 113.2)
28.8 to 35.9 ksi
(CBR=44.0 to 62.1)
25.5 to 44.5 ksi
(CBR=36.4 to
86.9)
17.5 to 23.5 ksi
(CBR=20.2 to 32)
Subgrade Target CBR
Values
Target Moduli values during Construction
[E(psi) = 𝟐, 𝟓𝟓𝟓 × 𝐂𝐁𝐑 𝟎.𝟔𝟒]
Low CBR 4 6,205
Medium
CBR
8 9,669
High CBR 20 17,380

Validity of Flexible Test Items Dynamic Sensors
Verification Procedure (Sensors Elimination Steps):
Step 1: if no data or noisy data recorded.
Step 2: if sensors were found non-repeatable; at the same depth and same offset
distance, sensors should have similar or close responses.
Proceed to Step 3 to 5, if sensors have significant responses at the same
depth.
LFS Test Item Illustration

Verification Procedure (Sensors Elimination Steps):
Step 3: if responses did not decrease with an increase in depth.
Step 4: if the responses for the shallow sensors under the B777 and B747 gears
were not in close agreement.
Step 5: if the measured responses were not in close agreement with responses
calculated using a mechanistic model, 3D Move software.

LFS Sensors Validity Illustration:
In LFS Test Item
(-25.7 psi) (-27.2 psi) (-31.3 psi)

LFS Sensors Validity Illustration:
In LFS Test Item
(174 microns)
(206 microns)
(197 microns)

Step 5 of the Verification Procedure - 3D Move Modeling:
Main Reasons for selecting 3D-Move:
• Incorporates moving loads with non-circular and non-
uniform two-dimensional surface pressure.
• Considers the viscoelastic properties of the asphalt concrete
layer, and
• Considers a damping ratio for base and subgrade.

Transverse Offset Distance (TOD) concept was developed to determine the
relative position of the gear’s centerline and the sensor in every pass for the
flexible test items.
TOD
Sensor

Transverse Offset Distance (TOD) Illustration :

3D-Move Inputs:
• 45 kips wheel load, 190 psi uniform tire pressure, and 5 mph speed.
• For the asphalt concrete layers moduli:
 Dynamic modulus E* for the AC layers obtained from cores tested by
the NAPTF for the six flexible test items.
 Reference temperature 42ºF and Poisson’s Ratio µ=0.35
• For the remaining layers:
 Backcalculated layer moduli using FE model were used as inputs
 µ=0.40 (for base and subbase) and µ= 0.45 (for subgrade)
 Damping ratio=5%

• A one-to-one comparison of the peak responses measured by the PCs, SGs, and MDDs with
the 3D-Move calculated responses for the six flexible test item is performed, and the percent
error was determined.

• Based on the percent difference (absolute error) between the measured and 3D-Move
calculated responses acceptable, marginal, or inacceptable:
Abs. Response
Value
Marginal,
%
Inacceptable,
%
1 25 45
500 10 20

Strength Factor
1 Best
Worst
2
3
4

Marginal (M1)

Verification Procedure Summary:
Pressure Cells Strain Gauges Multi-Depth Deflectometers
Six Flexible Test Items Overall Conditions
Pressure Cells Strain Gauges MMDs
Acceptable 38 25 80
Marginal 12 18 31
Inacceptable 53 12 23
No Data or Bad Data 35 41 33
Total 144 96 168

Validity of Rigid Test Items Dynamic Sensors
Concrete Strain Gauges:
• Plots of measured strains by bottom and top slab sensors for a given location under
the B777 gear and B747 gear, respectively.
LRS Test Item Illustration

Three Rigid Test Items Overall Conditions
Concrete Strain Gauges:
LRS MRS HRS
• The rigid pavement responses were calculated by performing finite element
modeling for LRS, MRS, and HRS test items using FEAFAA software.

Rigid Sensors Verification – FEAFAA Modeling:
• FEAFAA was developed by the
FAA Airport Technology R&D
Branch.
• Is useful for computing responses of
rigid structures to individual
aircraft landing gear loads.

Rigid Sensors Verification – FEAFAA Output:
LRS Longitudinal CSGs Measured vs. Calculated Responses under
B777 Gear at Z=9.5-inch
Mismatching Causes:
1. Static vs. dynamic
effects,
2. The backcalculated
layer moduli,
3. The joint conditions,
4. The model itself,
etc…

Conclusion and Recommendations
• The CC-1 dynamic sensors were extensively evaluated ; PCs, SGs, and MDDs for
the flexible test items and CSGs for the rigid test items.
• 3 different subgrades having L , M and H-strengths.
• Backcalculation: performed to determine the in-situ layer properties using various
models:
1) Odemark-Boussinesq (OB) model in ELMOD software,
2) Linear Elastic Theory (LET) model in ELMOD and BAKFAA software, and
3) A new developed dynamic finite element model

• Since the new FE model produced reliable backcalculated layer moduli
recommended to further develop this model for use in the BAKFAA software;
 Evaluation of the current backcalculation model,
 FE code able to perform forward calculation,

• Validity of flexible test items sensors : a multistep procedure to identify the
reliable, marginal, and unreliable sensors.
A significant number of PCs and SGs had inacceptable data , or NBD.
The backcalculated layer moduli using the new FE model were used as
input in the 3D-Move to predict the pressures, strains, and deflections.
3D-Move responses were predicted with high reliability.
Recommendations for Future Flexible Test Items Instrumentation:
1) Minimum number of sensors : 4 of the same type installed at the same
depth.

2) Position of shallow and deep sensors relative to the gear’s positions:
position the sensors in a way to produce absolute peak responses, i.e.
shallow sensors under the center of the wheel & deep sensors
under the center of the gear directly.
3) Sensors positioning in the transverse direction: various transverse
offsets.

• Validity of rigid test items sensors : majority of evaluated CSGs had
acceptable signals.
The backcalculated layer moduli using the new FE model were used as
input in the FEAFAA software to predict the strains for the rigid test
items.
FEAFAA responses could be considered highly reliable.

Thank You
Any Question?

Modeling Flexible and Rigid Pavements under Aircraft Loading

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