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Field Validation and Parametric Study of a Thermal Crack Spacing Model

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 Lexington, Kentucky March 10 – 12, 2003.

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Field Validation and Parametric Study of a Thermal Crack Spacing Model

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  1. 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 Lexington, Kentucky March 10 – 12, 2003

  2. Cracking Characteristics • Thermal cracking common in cold climates • Features • Transverse cracks • Regular spacing

  3. Crack Spacing Focus of this Study is the question What features control the spaces between Cracks?

  4. E, n, r, H, a E, n, r, c, f Model Stress Profile in Thermally Cooled Asphalt Layer on Granular Base Modeled in Two ways

  5. Finite Difference Code--FLAC Grid Element Sizes Asphalt Concrete (Elastic Model) 50x250 mm 63x315 mm 313x1563 mm Granular Base (Mohr Coulomb Model) z x

  6. 1-D Semi-Analytical ModelElastic Layer with Elastic-Plastic Restraint q=kux t=ca+stanf xt Timm, Guzina and Voller Int J Solids and Structures, 2002

  7. Form of Stress Profile Rate of Strees Increase Curling Stress Distance from free end

  8. Comparison of Models

  9. Crack Spacing from Stress Curve Sliding On Rigid Base s1 St x Cracking will not occur xc Cracking may occur

  10. Crack Spacing from Stress Curve xc xc Average Spacing = 1.5·Xc s1 St x

  11. 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

  12. E, n, r, H, a E, n, r, c, f 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

  13. Field Validation Methodology • Select MnROAD sections • Analyze thermal crack spacing by section • Analyze in situ thermal conditions • Gather material property data for model • Simulate pavement, determine spacing • Compare predictions to measured • Assess validity

  14. Cell 1 Cell 2 Cell 3 Cell 4 0 150 155 160 231 200 102 102 400 Depth Below Pavement Surface, mm 838 600 711 838 LEGEND HMAC 800 Class 6 G.B. Class 5 G.B. Class 4 G.B. 1000 Class 3 G.B. 1200 MnROAD Sections • Similar thickness designs • Identical binders • Common subgrade • Different base layers

  15. Average Crack Spacing Avg Spacing Cell 1: 12 m Cell 2: 8 m Cell 3: 13 m Cell 4: 9 m

  16. Bottom of pavement Feb 2 Feb 1 Feb 3 Top of pavement Temperature Cycling

  17. E, n, r, H, a E, n, r, c, f Material Property Data • Backcalculation • Laboratory testing as part of Mn/ROAD project • Derived values • Thermal coefficient = fn (Volumetrics) • Model ‘tuned’ with friction and cohesion

  18. Resulting Friction and Cohesion Mohr-Coulomb Properties of Material Directly Beneath HMA

  19. 16 Cell 3 14 12 Cell 1 10 Predicted Spacing, m 8 Cell 4 Cell 2 6 Line of Equality 4 2 0 0 2 4 6 8 10 12 14 16 Measured Average Spacing, m ModelComparison

  20. Model Assessment • Crack spacings pass reasonableness check • Recently, model has been used to predict other crack spacing phenomenon TiN Coating

  21. Factors that Influence Stress Profile Rate of Stress Increase Max stress Curling Stress

  22. 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

  23. Input Parameters

  24. HMAC Stiffness (E1)

  25. HMAC Poisson Ratio (n1)

  26. HMAC Thickness (H1)

  27. HMAC Thermal Coeff. (a1)

  28. Base Stiffness (E2)

  29. Base Cohesion (c2) As c gets Large Only elastic resistance

  30. Base Friction Angle (f2) Note: c = 10 kPa

  31. Factors that Influence Stress Profile Rate of Stress Increase Max stress Curling Stress

  32. 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

  33. 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

  34. Potential Uses of Model • Plan mitigation strategies • Saw and seal • Material selection • Assess probability and expectation of cracking

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

  36. Thank You! Questions?

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