Field Validation and Parametric Study of a Thermal Crack Spacing Model

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Field Validation and Parametric Study of a
Thermal Crack Spacing Model

• David H. Timm - Auburn University
• Vaughan R. Voller - University of Minnesota

Presented at the Annual Meeting of the
Association of Asphalt Paving Technologists Lexin
gton, Kentucky March 10 12, 2003
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Cracking Characteristics

• Thermal cracking common in cold climates
• Features
• Transverse cracks
• Regular spacing

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Crack Spacing
Focus of this Study is the question What
features control the spaces between Cracks?
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Model Stress Profile in Thermally Cooled Asphalt
Layer on Granular Base
Modeled in Two ways
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Grid Element Sizes
Asphalt Concrete (Elastic Model)
50x250 mm
63x315 mm 313x1563 mm
Granular Base (Mohr Coulomb Model)
z
x
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1-D Semi-Analytical ModelElastic Layer with
Elastic-Plastic Restraint
qkux
tcastanf
xt
Timm, Guzina and Voller Int J Solids and
Structures, 2002
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Form of Stress Profile
Rate of Strees Increase
Curling Stress
Distance from free end
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Comparison of Models
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Crack Spacing from Stress Curve
Sliding On Rigid Base
s1
St
x
Cracking will not occur
xc
Cracking may occur
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s1
St
x
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Objectives

• Validate thermal crack spacing model with field
  data
• Perform sensitivity analysis on length scale
• Help guide future laboratory work
• Develop more complete understanding
• Identify how material selection will affect
  spacing

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Scope

• Field Validation
• 4 similar sections at Mn/ROAD
• Parametric Study
• 10 input variables
• Layer 1
• Stiffness, Poisson, Density, Thickness, Thermal
  Coef.
• Layer 2
• Stiffness, Poisson, Density, Cohesion, Friction
  Angle

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Field Validation Methodology

1. Select MnROAD sections
2. Analyze thermal crack spacing by section
3. Analyze in situ thermal conditions
4. Gather material property data for model
5. Simulate pavement, determine spacing
6. Compare predictions to measured
7. Assess validity

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MnROAD Sections

• Similar thickness designs
• Identical binders
• Common subgrade
• Different base layers

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Average Crack Spacing
Avg Spacing Cell 1 12 m Cell 2 8
m Cell 3 13 m Cell 4 9 m
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Temperature Cycling
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Material Property Data

• Backcalculation
• Laboratory testing as part of Mn/ROAD project
• Derived values
• Thermal coefficient fn (Volumetrics)
• Model tuned with friction and cohesion

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Resulting Friction and Cohesion
Mohr-Coulomb Properties of Material
Directly Beneath HMA
Cell Friction Angle, o Cohesion, kPa
1 30 10
2 50 15
3 35 10
4 25 10
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Model Comparison
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Model Assessment

• Crack spacings pass reasonableness check
• Recently, model has been used to predict other
  crack spacing phenomenon

TiN Coating
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Factors that Influence Stress Profile
Rate of Stress Increase
Max stress
Curling Stress
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Parametric Investigation Methodology

• Uniform temperature change
• 2-layer structure
• 10 input parameters varied from low, medium, and
  high
• Maximum tensile stress curves plotted and
  evaluated
• Maximum Stress
• Rate of Stress Increase
• Curling Stress

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Input Parameters
Layer Input Units Low Medium (Baseline) High
1 E1 Pa 5109 1.41010 31010
1 n1 unitless 0.15 0.20 0.25
1 r1 kg/m3 2,200 2,300 2,400
1 H1 cm 7.6 15 30
1 a1 /?C 1.3310-5 2.1510-5 2.9710-5
2 E2 Pa 5.5107 5.5108 5.5109
2 n2 unitless 0.35 0.4 0.45
2 r2 kg/m3 1,800 2,000 2,200
2 c2 kPa 0, 0.1, 1, 10, 70, 140 0, 0.1, 1, 10, 70, 140 0, 0.1, 1, 10, 70, 140
2 f2 ? 20 40 60
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HMAC Stiffness (E1)
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HMAC Poisson Ratio (n1)
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HMAC Thickness (H1)
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HMAC Thermal Coeff. (a1)
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Base Stiffness (E2)
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Base Cohesion (c2)
As c gets Large Only elastic resistance
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Base Friction Angle (f2)
Note c 10 kPa
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Factors that Influence Stress Profile
Rate of Stress Increase
Max stress
Curling Stress
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Relative Influence on Each Criteria Relative Influence on Each Criteria Relative Influence on Each Criteria
Input Parameter Maximum Stress Rate of Stress Increase Curling Stress
E1 3 1 --
n1 2 -- --
r1 -- -- --
H1 -- -- 3
a1 3 1 --
E2 -- 3 --
n2 -- -- --
r2 -- -- --
c2 -- 3 3
f2 -- 2 --
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Conclusions

• Model compared favorably to field data
• Model is sensitive to base material properties
• Model is simple, yet provides length scale to
  thermal cracking problem
• Key input parameters are
• Stiffnesses of HMAC and Base
• Thermal coefficient
• Frictional properties of Base material

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Recommendations

• Further validation with field sections
• Model has compared favorable to other types of
  cracking
• Incorporate a fracture mechanics model to
  simulate crack propagation
• Examine viscoelastic constitutive models

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Potential Uses of Model

• Plan mitigation strategies
• Saw and seal
• Material selection
• Assess probability and expectation of cracking

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Acknowledgements

• Dr. Bojan Guzina
• Minnesota Department of Transportation
• Minnesota Road Research Project

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Thank You!

• Questions?