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**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

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**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

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**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

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**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)

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**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

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**Laboratory Investigation of Permanent Deformation Behavior**

20 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

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**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

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**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

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**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

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**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

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**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.

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**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 ( = 33 + 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.

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**ep Model Validation w/ NAPTF Data**

Load Wander Patterns Calculate stress states for each wander position

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**ep Prediction for NAPTF Load Wander**

LFC P154 subbase layer p = A * 1dB * C * ND

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**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

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**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

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**ep Prediction for NAPTF Moving Gear/Wheel Loads**

VCP Model dp Prediction “A moving wheel loading consists of five sequential (15) 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

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**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.

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**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

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**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)

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**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

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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

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**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

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**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

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**NAPTF trafficking dynamic response**

unrecovered !..

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**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.

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**NAPTF trafficking dynamic response**

Load path (stress history) effect

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**NAPTF trafficking dynamic response**

Traffic Direction Effect

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