1. In Tai Kim & Erol Tutumluer University of Illinois, Urbana-Champaign
Validated NAPTF Pavement P209/P154 Granular Base/Subbase Rutting Predictions
In Tai Kim & Erol Tutumluer
University of Illinois, Urbana-Champaign

2. Introduction Rutting of Aggregate Layers
The only failure mechanism of Unbound Aggregate base/subbase layers – The Performance Indicator!..
Knowledge is always required of the relative contribution of the aggregate layers to the total permanent deformation of the airport pavement structure
Current standard laboratory test procedures, such as the AASHTO T307-99, not adequate for evaluating permanent deformation behavior of granular geomaterials because
Heavier (aircraft) wheel loads applied
Actual moving wheel load conditions

3. FAA’s Full Scale Test Facility (NAPTF)
Low & Medium strength flexible sections (5 to 10 inches Asphalt & CBR 4 to 8 subgrade soils) failed with up to 4 inches ruts
Highest contribution to permanent deformations often from
4 to 30 inches thick P209 base, or
12 to 36 inches thick P154 subbase
FAA has constructed full-scale test facility, named National Airport Pavement Test Facility, at Atlantic city, New Jersey. It allows many combinations of aircraft gear loading. First round testing of the constructed pavement test sections was completed in Two gear configurations, b777 and b 747 and up to 65,000 lbs for single wheel were applied.
In Flexible pavement sections, rutting of up to 4 inches permanent deformations were observed on the surface.
Furthermore, multi depth deflectometer (MDD) data and test section forensic analyses showed significant rutting was found in both P209 base and P154 subbase layers
There were 9 sections tested in first round trafficking. 6 flexible pavement and 3 rigid pavement sections. Here are the crosssections of 6 flxible test sections.
6-wheel (B777) & 4-wheel (B747) Gear Assemblies

4. NAPTF Trafficking Results – LFC
Low-Strength Subgrade
5-in. P-401 Surface
8-in. P-209 Base
36-in. P-154
Subbase
LFC
Wheel Load: 45,000-lbs (20.4 metric tonnes) per wheel
After 20,000 passes : 65,000-lbs (29.5 metric tonnes) per wheel
(Garg, 2003)

5. FAA CEAT Project Objectives
Characterize Permanent Deformation Behavior Laboratory testing of FAA’s base and subbase materials, P209 and P154
Develop Prediction Models
Constant & Variable Confining Pressure (CCP & VCP) Test Conditions
Investigate Factors Affecting Permanent Deformation Accumulation
Validate Model Performances w/ NAPTF Data

6. Laboratory Investigation of Permanent Deformation Behavior20 40 60 80
100
0.01
0.1 1 10
Sieve Sizes (mm)
Percent Passing (%)
P154
P209
P209 Base Material
Friction Angle () = 61.7
Cohesion ( c) = 30 psi
P154 Subbase Material
Friction Angle () = 44
Cohesion ( c) = 26.4 psi
This is the gradation curve for the P209 and p154
By modified proctor test,
Material
Maximum Dry
Density, kN/m3
Optimum
Moisture
Content, %
P209
24.19 (154.9 pcf)
4.7
P154
20.04 (128.3 pcf)
6.5

7. FAA NAPTF Permanent Deformation Testing Program – Univ. of Illinois
► Advanced Test Equipment: UI-FastCell
Compression and Extension Stress States
Constant (CCP) & Variable (VCP) Confining Stress Paths

8. Laboratory Test Program P209 & P154
static
1d (dynamic)
CCP
► Constant Confining
Pressure (CCP) Tests - 13
Stress Ratio s1/s3 = 4
Stress Ratio s1/s3 = 6
Stress Ratio s1/s3 = 8
Stress Ratio s1/s3 = 10
s1d (kPa)
s3 (kPa)
62.1
20.7
96.6
144.9
186.3
103.5
34.5
172.5
241.5
310.5
165.6
55.2
276.0
386.4 
207.0
69.0
345.0
69.5
Typically 10,000 load applications at each stress state

9. Variable Confining Pressure (VCP) Test Program
Moving wheel load x
Stresses sv
Vertical stress t sh
Extension
Extension
So far, I talked about CCP test program which didn’t consider principal stress rotation.
Then, let’s move to VCP test program
Horizontal
stress
Time
Shear stress t
Typical pavement element z

10. VCP Test Program m 1d q s3d = 0 CCP Static failure VCP: s3d  0 3d 
VCP ( s3d & s1d ) 1 m
p = (s1d+2s3d)/3 + p0= q/3
Compression
q = s1d- s3d
In general, the stress path slope for the standard constant confining pressure(CCP) tests, such as AASHTO , takes a constant value of 3.0. For variable confining pressure (VCP) tests, the stress slope varies generally from -1.5 to 3. Various stress paths cause different loading effects on pavement elements, which are not yet fully studied and understood to explain permanent deformation accumulation p0
Extension
m = Dq / Dp
= slope of stress path -3 2
CCP: Constant Confining Pressure,
m = 3, s3d = 0
(SHRP P46)
s1d = 0
- q

11. Stress Path Slope (m) = -1
VCP Test Matrix – 39 tests
Stress Path Slope (m) = 1.5
(compression states)
Stress Path Slope (m) = 0
Stress Path Slope (m) = -1
(extension states) 3
(kPa)
1d
3d
20.67
72.62
18.12
65.46
15.43
61.73
120.8
30.18
108.9
25.63
102.7
168.9
42.24
152.3
35.9
143.6
217.9
54.43
196.4
46.3
185.1
34.45
201.8
50.43
181.9
42.86
171.5
282.1
70.48
254.2
59.94
239.7
362.3
90.6
326.6
76.96
307.9
55.12
193.4
48.37
174.3
41.06
164.3
322.6
80.61
290.8
68.56
274.2
451
112.7
406.5
95.84
383.3
68.9
241.6
60.36
217.7
51.33
205.3
402.9
100.7
363.1
85.57
342.4
39 stress states were used to evaluate the effects of stress path slopes (m) and lengths (L) on the permanent deformation. Two dynamic stresses are properly applied at the same time in the horizontal and also vertical (1d) directions according to the simulated field stress states. As such, the VCP tests offer the capability to apply a wide combination of stress paths by pulsing cell pressure and vertical deviator stress.

12. Permanent Deformation (Strain) Models (based on CCP & VCP test data)  f(s)
Permanent Strain Models Developed in the form
p = A * NB R2
for
All
Data
R2 values
Stress Path Slope (m)
m= -1
VCP
m = 0
m = 1.5
m = 3
CCP
P209 FAA Base Material
CCP :
p = A * 1dB * C * ND
(  = 33 + 1d )
0.56
0.24
0.70
0.47
0.86
VCP :
p = A * 3B * 1dC * 3dD * NE
0.80
0.38
0.78
0.53 -
P154 FAA Subbase Material
0.46
0.62
0.32
0.85
0.60
0.35
s3: Static confining pressure
s1d: Vertical dynamic stress
s3d: Horizontal dynamic stress
N: No. of load applications
m: Stress path slope
A, B, C, D, & E: regression parameters
Based on the extensive laboratory testing database established from UI-FastCell permanent deformation testing of the P209/P154 base and subbase course materials under the various constant and variable confining pressure (CCP and VCP) test conditions, seven models were developed to account for hydrostatic confining pressure (s), dynamic stresses in both axial (1d) and radial (3d) directions, stress path length (L), stress path slope (m), and number of load applications (N) as shown in Table 5–2. The model performances were compared to predict the axial permanent deformation (p) behavior of the P209 and P154 aggregates. Due to the complex loading regimes followed especially in VCP testing, models had to be analyzed simultaneously using the static and dynamic components of the applied stresses.

13. ep Model Validation w/ NAPTF Data
Load Wander
Patterns
Calculate stress
states for each
wander position

14. ep Prediction for NAPTF Load Wander
LFC P154 subbase layer
p = A * 1dB * C * ND

15. ep Accumulation for NAPTF Load Sequence – 66 passes
Calculate no. of load applications according to wander distribution
Odd-Numbered Passes: Carriage Moves West to East
Even-Numbered Passes: Carriage Moves East to West
Normal Distribution ( s = 30.5 in.)
63,64
64,66
61,62
51,52
59,60
53,54
57,58
55,56
43,44
45,46
41,42
47,48
39,40
49,50
37,38
19,20
35,36
21,22
33,34
23,24
31,32
25,26
29,30
27,28
1,2
17,18
3,4
15,16
5,6
13,14
7,8
11,12
9,10
Track No. : -4 -3 -2 -1 1 2 3 4
9.843 in
(250 mm)
typical

16. NAPTF Moving Wheel Stress Paths
FAA – National Airport Pavement Test Facility
Compression
LFS section
Interestingly, the actual stress regime experienced by pavement elements under a rolling wheel is consisting of the combination of several complex stress paths, not single stress path utilized in current laboratory test procedure, as a moving wheel is approaching and departing. Therefore, a proper laboratory test protocol is required to simulate the effect of principal stress rotation on permanent deformation development.
Extension

17. ep Prediction for NAPTF Moving Gear/Wheel Loads
VCP Model dp Prediction
“A moving wheel loading consists of five sequential (15) load locations”
Stress path slope = -1
4 1d = 3d
Stress path slope = 0
1d = 3d
Stress path slope = 3
3d = 0
Stress path slope = 0
1d = 3d
Stress path slope = -1
4 1d = 3d 1 2 3 4 5
The granular layer was divided into six sublayers and the permanent strain in each sublayer was computed by the model individually using the average stress states predicted at mid-depth of that sublayer, according to five different wheel positions. The predicted permanent strain in each sublayer, i.e., the summation of the strains obtained considering five load locations, was then multiplied by the thickness of that sublayer. The total granular layer permanent deformation was finally obtained as the summation of all the sublayer deformations, as explained in detail in Figure 7–12. }
P154 subbase
6 sublayers
* Layer 1: Top layer
Pavement elements

18. ep Prediction for NAPTF Moving Gear/Wheel Loads
Figure 7–13 shows the permanent deformations predicted from 6 sublayers using the VCP model 4. The deformations in the upper layers were usually higher than those obtained in the lower portion of the LFS P209 base layer. This is due to the fact that the upper part usually experiences higher stress states compared to the lower portion and also the prediction models were highly nonlinear stress dependent.

19. Other Major Factors Affecting Permanent Deformation Behavior
Laboratory Testing versus NAPTF Testing
Compacted with
vibratory compactor
Unconditioned virgin specimen
Loaded with 0.1-sec
(equivalent to 50 km/hr)
load duration
Trafficked at 8 km/hr (0.5-sec
load duration) with aircraft gear
Previous loading of base
and subbase layers during
pavement construction
and slow moving load test
(response test)
Load Pulse Duration and Stress History effects involved

20. Predictions Considering NAPTF Trafficking Speed & Loading Stress History Effects
A new set of specimens were tested to adequately account for NAPTF (1) pulse duration (trafficking speed) and (2) stress history effects
0.5-second load duration accumulates ~40% more permanent deformation compared to 0.1-second – viscoplastic ?
Slow Moving Response Tests with 36,000-lb wheel loads at 0.54 km/h
200 load cycles were applied to simulate slow moving response test for conditioning specimens
Stress history ratios used in computing model parameter adjustment factors
(36,000 lbs / 45,000 lbs = 0.8)

21. Permanent Deformation Predictions Validated with NAPTF Measured Ruts
S : Stress History Effects
L : Load Duration Effects
Measured Permanent Deformation
VCP Prediction considering S + L
VCP Prediction considering S
CCP Prediction considering S + L
CCP Prediction considering S
LFC P154
Subbase

22. Summary
Conducted Laboratory Permanent Deformation Testing on the FAA’s National Airport Pavement Test Facility (NAPTF) P209/P154 Unbound Aggregate Base/Subbase Materials
Power function form stress dependent permanent strain (ep) prediction models developed based on CCP (stationary repeated loading) & VCP (moving wheel loading) test data
Rut accumulations predicted in the NAPTF LFC P154 subbase layer by properly considering load pulse duration (trafficking speed) and previous stress history effects
VCP model predicted much closer to the measured NAPTF ruts

23. Research Findings/ Accomplishments
A new granular base/subbase permanent deformation test procedure was proposed to take into account the effects of
Heavy wheel loads: applying stresses up to 90% of the shear strength
Moving wheel loads: considering three different stress path slopes in VCP testing
Load pulse duration: in accordance with field trafficking speed
Previous stress history: preconditioning of specimens
Major Accomplishments
PhD Dissertation of Dr. In Tai Kim
– August/October 2005
Final Project Report – CEAT Report No. 28

24. Current/Future Research Focus
Investigate the NAPTF trafficking dynamic response database to understand complicated recovered & unrecovered pavement deformation behavior due to various combinations of applied
Load magnitudes and loading sequences (application order and stress history effects)
Trafficking speeds (load duration effects)
Traffic directions (shear stress reversals)
Gear spacing or interaction
Wander positions and wander sequences (order of 66 loadings)
Based on the proposed test procedure, fully develop a permanent deformation test procedure for evaluating airport pavement granular base/subbase layer rutting potential
Study CC3 test section subbase rutting performances
Establish granular layer thickness/performance equivalencies

25. NAPTF trafficking dynamic response
unrecovered !..

26. NAPTF trafficking dynamic response
Base/Subbase
Contractive &
Dilative
Behavior
A large scatter is observed in the permanent deformation accumulation in the P-154 subbase layer of MFC section, especially after 5,000 passes (for example, see Figure 9.24). This is due to the continual “contractive/dilative” response of the P-154 granular layer resulting from the effect of gear wander. This was pointed out in Chapter 8 while discussing the MDD transverse distribution results. This behavior is also reflected in the development of permanent deformation at the surface. Interestingly, the accumulation of permanent deformation in the subgrade shows less scatter than the granular layers even though the subgrade rutting magnitudes are higher.
In Figure 9.36, the Residual or Unrecovered displacements in the pavement and subgrade layers is shown during the first 1,400 passes. The Residual responses show higher magnitudes in the pavement layers than in the subgrade. Note that the pavement layer shows a strong contractive/dilative Residual response behavior. Even though the Subgrade exhibits contractive/dilative Residual response behavior, it is mainly a contractive response. Thus, the NET permanent deformation which is equal in magnitude to the differences between the sequential dilative and contractive Residual responses will be lower in the case of the pavement layers while the cumulative contractive Residual responses produces higher rutting magnitude in the subgrade (see Figure 9.37). A closer view of the contractive/dilative Residual response behavior in the pavement and subgrade layers is shown in Figure 9.38 for one wander cycle. The maximum Residual response occurs when one of the wheel groups passes directly over the MDDs.
The phenomenon of contractive/dilative permanent deformation behavior for a sequence of repetitive loads with different wander positions have been observed in previous full-scale airport flexible pavement trafficking tests (Ledbetter, 1977; Ahlvin et al, 1971; Crockford et al, 1990; Webster, 1992). Gomez-Ramirez and Thompson (2002) observed this phenomenon during the NAPTF response tests. It was also reported by Hayhoe and Garg (2002) based on their analysis of subgrade strains measured in the MFC section.

27. NAPTF trafficking dynamic response
Load path (stress history) effect

28. NAPTF trafficking dynamic response
Traffic Direction Effect