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					     In-Situ Validation of
    Mechanistic Pavement
   Finite Element Modeling

Imad L. Al-Qadi
Mostafa A. Elseifi
Pyeong J. Yoo
   Acknowledgement

             VTRC/VDOT
       Kevin McGhee and Tom Freeman

Michelin Research and Development
               Ibrahim Janajreh


                VTTI/VT
  Smart Road Graduate Students & Researchers
            Introduction
• The multi-layer elastic theory is the
  most popular method to predict
  pavement response to vehicular loading
• Various software utilizes the multi-layer
  elastic solution (e.g., BISAR3, VESYS,
  etc.)
• Many assumptions are questionable, but
  are usually accepted for simplicity
            Assumptions
• Homogeneous, isotropic, and elastic
  layers: E, n
• Weightless materials
• Infinite layers horizontally
• Finite layer thicknesses; except for
  subgrade
• Static loading over a circular area
• Compatibility of strains and stresses
  satisfied at interfaces
The Finite Element Method
• The FE method “approximates the
  behavior of a continuum by an assembly
  of finite elements”
• More powerful and flexible approach to
  simulate pavement response to
  vehicular loading
• Has been used successfully for
  pavement applications
    Theoretical Methodologies
Method         Layered
                           Finite Element
               Theory
Dimension       Axis           2D, 3D
Loading Area   Circular         Any
Pressure       Uniform    Unif. or Nonunif.
Bonding          Full       Full/Friction
Dynamic          No             Yes
Material       Elastic     Elastic/Visco-
                             elastic …
           Objectives
• Determine the accuracy and
  convergence of a typical FE model
• Compare the results against the
  elastic layered theory and field-
  measured pavement responses
• Modify the FE models to better
  simulate pavement response to
  loading
       Virginia Smart Road

• Adaptable Facility for Transportation
  Research and Evaluation
• 2.5km Long: 12 Flexible Pavement Sections
  and One CRCP
• Instrumentation during Construction
• Rain and Snow Towers
                   Truck Loading
•   3 Load Levels (L1, L2, L3)
•   3 Tire Inflation Pressures (80,
    95, 105 psi) (550, 655, 725 kPa)
•   4 Speeds (5, 15, 25, 45 mph) (8,
    25, 40, 72 km/h)
                                Instrument Responses
                    400
Transverse Strain




                    300
                                                                                     Time Retardation
    (mm/m)




                    200

                    100

                      0
                          0.0    0.5      1.0       1.5                      2.0
                                       Time (sec)
                                                    Longitudinal Strain   600

                                                                          450
                                                         (mm/m)

                                                                          300

                                                                          150

                                                                             0
                                                                              0.00   0.10   0.20   0.30   0.40   0.50
                                                                          -150
                                                                                            Time (sec)
                           Instrument Responses
                    600                                                             Strain
Transverse Strain




                    400                                                          Accumulation
    (mm/m)




                    200

                     0
                          0.0   0.5      1.0                 1.5        2.0
                                      Time (sec)
                                          Vertical Stress

                                                            100
                                                             80
                                              (kPa)


                                                             60
                                                             40
                                                             20
                                                              0
                                                                  0.0            0.5        1.0
                                                                              Time (sec)
      Pavement Responses
• Instrument responses indicate the
  viscoelastic nature of HMA: time
  retardation, relaxation with time, and
  asymmetry of the response
• Pavement response is different in the
  transverse direction than in the
  longitudinal direction
• Longitudinal strain was greater than
  transverse strain for the dual-tire
  configuration
      Theoretical Formulation
• Phase I: Sensitivity analysis to ensure
 convergence of the developed FE model
  – Compare the results to the layered theory
  – Compare the results to field measurements


• Phase II: Introduce modifications to the FE
 model to improve the prediction of
 pavement responses
                  Section B

                                 Surface Mix (SM-9.5D – 38mm)
• Typical interstate
  highway pavement                Base Mix (BM-25.0 – 150mm)

  design
                                 Asphalt-Treated Drainage Layer
• Heavy instrumentation:                (OGDL – 75mm)
  strain gages, pressure
                            21A Cement Stabilized Base layer
  cells, thermocouple, etc.        (21B – 150mm)

                                 21B Aggregate Subbase Layer
                                       (21B – 175mm)
       FE Model (ABAQUS)
      250     622     250

500




920         Loading
             Area



500
                            Y
              1122              X
           Sensitivity Analysis
• Two criteria for convergence:
     The finite element model converges to the
      continuum solution
     Jumps in stresses across inter-element
      boundaries are acceptable
   Case ID Thickness    dof      Elements Computation
                                           Time (sec)
     A       38.10     172,476     42,916     3145
     B       19.05     212,226     55,656     4871
     C        9.53     291,726     81,136     6599
     D        4.76     450,726    132,096    18565
     E        2.38     760,776    231,468    30551
                                         Sensitivity Analysis
                   100
Jump in Vertical Stress


                                            3D FE    Accuracy Limit
                          80
                                                                                               Jump in Vertical
                          60                                                                    Stress at the
        (kPa)




                          40                                                                     Interfaces
                          20

                           0
                               38.10    19.05   9.53    4.76    2.38
                                                            730
                                       Element Thickness (mm)
                                                                                 720
                                                         Vertical Stress (kPa)
                                                                                 710                             FE       Bisar 3.0
                                                                                 700
                           Convergence of the
                                                                                 690
                           Vertical Stress with                                  680
                            Mesh Refinement                                      670
                                                                                 660
                                                                                 650
                                                                                       38.10    19.05    9.53      4.76        2.38
                                                                                                Element Thickness (mm)
                                 Sensitivity Analysis
                                   Vertical Stress (kPa)
                   0           200           400           600   800
               0
              20       Kenlayer
              40
Depth (mm)




                       3D FE
              60
              80       Bisar
             100
             120
             140
             160
             180
             200
Measured vs. Calculated



                                                   Layered Theory = -0.18
                                                       3D FE = -0.995
                           200
                                                                     Measured
                           160
        Strain (mstrain)


                                                                     BISAR 3.0
                           120                                       3D FE

                            80

                            40

                             0
                                 Under BM-25.0 -   Under BM-25.0 -   Under OGDL - TS
                                       TS                LS
                                                       Depth
          Results of Phase I
• The sensitivity analysis indicate that an
  element thickness of 9.53mm provides an
  acceptable level of accuracy
• The layered theory and its analogous FE
  model poorly predict pavement responses
  to vehicular loading
• In order to improve pavement prediction,
  several modifications are necessary
    Suggested Modifications
• Simulate field pulse loading
• Quasi-static analysis (Speed = 8km/h)
• Friction contact between the layers
  (Mohr-Coulomb Theory)
• Linear springs to simulate subgrade
  support
• Viscoelastic time hardening model to
  simulate HMA constitutive behavior
         T1
692kPa




         T2
904kPa




         T3
859kPa


         T4
895kPa
         T5
692kPa
         T5




692kPa
         T4




904kPa
              Tire Contact Area




         T3




859kPa
         T2




895kPa
         T1




692kPa
                                  Loading Pattern


                                                                 Loading Pattern




                                                             1
                                                         0.9
Normalized Vertical Stress




                                                         0.8                      Dual Tire
                                                         0.7
                                                         0.6
                                                         0.5
                                                         0.4
                                                         0.3
                                                         0.2
                                                         0.1
                                                             0
                             -0.08 -0.06 -0.05 -0.03 -0.02       0   0.016 0.032 0.048 0.064 0.08
                                                         Time (sec)
Model Prediction
      Material Characterization
• Viscoelastic constitutive behavior based
  on variable creep load test:
  – Apply 10 different load levels while holding
    each load for 60sec
  – Impose a recovery rest period of 350sec
    between each load


     
        A t
         c

      c = Creep strain rate
                               m   n 1


        = Stress
   A, m, n = fitting parameters
                               Measured vs. Calculated
Vertical Stress (kPa)



                        800
                                             Measured
                        600                  BISAR 3.0
                                             Modified 3D FE
                        400

                        200

                          0
                              Under   Under   Under                   Under
                               WS     OGDL      21A                    21B
                                          Depth                                                  Measured
                                                  Strain (mstrain)   200
                                                                     160                         BISAR 3.0
                                                                                                 Modified 3D FE
                                                                     120
                                                                     80
                                                                     40
                                                                       0
                                                                           Under BM-    Under BM-    Under OGDL
                                                                            25.0 - TS    25.0 - LS      - TS
                                                                                          Depth
                                           Model Results
                    80              T1
                    70
                                    T2
Transverse Strain




                    60
                                    T3
    (mstrain)




                    50
                    40              T4
                    30              T5
                    20
                    10
                     0
                         0   0.05    0.1     0.15   0.2                    0.25    0.3   0.35
                                             Time (sec)

                                                                 100          T1
                                           Longitudinal Strain




                                                                              T2
                                                                              T3
                                               (mstrain)




                                                                 50
                                                                              T4
                                                                              T5
                                                                  0
                                                                       0    0.05   0.1   0.15   0.2   0.25   0.3   0.35
                                                                 -50
                                                                                         Time (sec)
Surface Transverse Strain (T=25°C)
            Section B
                              80.0
  Transverse Surface Strain


                              60.0

                              40.0

                              20.0

                               0.0
                                      0.0   200.0       400.0       600.0   800.0
                              -20.0

                              -40.0

                              -60.0

                              -80.0
                                                    Distance (mm)
                          HMA Rutting Damage
                          0.00




Surface Deflection (mm)
                          -0.02

                          -0.04                                   Approaching Load
                                                                  Maximum Load
                          -0.06                                   Leaving Load

                          -0.08

                          -0.10

                          -0.12
                                  0   48 96 144 192 240 288 336 384 432 480 528
                                       Distance from the left edge of the tire (mm)




                                                             Approaching Load


                                                               Maximum Load

                                                               Leaving Load
         Conclusions
• The multi-layer elastic theory
  poorly predicted pavement
  responses to vehicular loading
• Several modifications are
  necessary to improve FE
  simulation
            Conclusions
• Anisotropic characteristics of HMA are
  critical in simulating pavement
  responses in the lateral directions

• A time hardening model was
  successfully used to simulate time
  retardation of HMA in the transverse
  direction, and fast relaxation in the
  longitudinal direction

				
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posted:3/28/2011
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