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Characterization of Unbound Pavement Materials for MEPDG

Characterization of Unbound Pavement Materials for MEPDG. November 15, 2010. Shabbir Hossain, Ph.D., P.E. Research Scientist, VTRC. Objectives and Scope. Develop a predictive regression model for Resilient Modulus for Fine Soil using Unconfined Compression Test

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Characterization of Unbound Pavement Materials for MEPDG

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  1. Characterization of Unbound Pavement Materials for MEPDG November 15, 2010 Shabbir Hossain, Ph.D., P.E. Research Scientist, VTRC

  2. Objectives and Scope • Develop a predictive regression model for Resilient Modulus for Fine Soil using Unconfined Compression Test • Verify the low resilient modulus for coarse soil • Obtain typical resilient modulus values for VDOT base course aggregates: 21A and 21B

  3. Aggregate and Soil Samples • Nine VDOT construction Districts • Six Fine Soils • Two A-7-5, two A-4, one A-7-6, and one A-6 • Five Coarse Soils (plus five more samples from one construction project) • Six A-2-4, one A-2-6, one A-2-7 and two A-3 • Six Aggregate sources: • Two 21A: Granite gneiss and Schist • Four 21B: Dolomitic LS, LS, Diabase and Marble • Specific gravity and std proctor test (OMC & MDD)

  4. Laboratory Tests • Resilient Modulus Test – AASHTO T 307 • Fine Soil, Coarse Soil and Base Aggregate • Varied degree of saturation (Three, ranging 50 to 100%) • Calculation with MEPDG constitutive Model • K –values from regression (R2 > 0.9) • Resilient Modulus calculated using MEPDG model • Soil: confining pressure 2 psi and deviator stress 6 psi • Aggregate: confining pressure 3 psi and deviator stress 24 psi

  5. Constitutive Models • Models Considered • Model 1: -- Soils Lab • Model 2: -- AASHTO • Model 3: -- MEPDG • k – values from regression analysis (R2 > 0.9) • Mr values - assuming stresses • Confining 2 psi and deviator (cyclic) 6 psi

  6. Laboratory Tests • Quick Shear Test (Triaxial Compression) • At the end of RM test with 5 psi confinement • Unconfined Compression Test – AASHTO T 208 • Fine Soil • Stress-strain Diagram • Initial Tangent Modulus • Varied degree of saturation • Different compaction hammers • Static • Proctor (standard) • Harvard Miniature Compactor

  7. Base Aggregate • K- values for 21As and 21Bs • Resilient Modulus at 3 psi (confining) and 24 psi (dev) • 21A: 12,800 to 20,517 psi • 21B: 18,259 to 31,297 psi • Increase in degree of saturation reduces Mr • Two saturation only: OMC and a lower value • Sample could not be compacted at higher saturation • Good correlation with Quick Shear (Triaxial) test • Mr and stresses at 0.1% strain from quick shear (triaxial) test • Triaxial setup is also complex/ resource intensive

  8. Coarse Soil • K-values for stress dependent constitutive equation • Calculated Mr: confining 2 psi & Dev 6 psi • Resilient Modulus values are low compared to MEPDG recommended • Sample size effect • 3” diameter sample • 6” diameter sample • Comparable values but low • Good correlation with Quick Shear (Triaxial) test • Mr and stresses at 0.1% strain from quick shear (triaxial) test • Triaxial setup is also complex/ resource intensive • Mr is inversely proportional to degree of saturation

  9. Fine Soil • K- values for MEPDG model • Calculated Mr: Confining 2 psi and Dev. 6 psi • Mr is inversely proportional to degree of saturation • Different relation for different lab • VDOT lab and an outside vendor • Very specific to a particular soil but strong correlation • Strong correlation with Quick Shear (triaxial) test results • Unconfined compression test a special triaixal test

  10. Fine Soil • Unconfined Compression Test • Stress-strain plot • Initial Tangent Modulus as a measure of stiffness • Initial Tangent Modulus is inversely proportional to degree of saturation • Different relation for different sample preparation technique • Impact (proctor), Static and Harvard Miniature • Good correlation between resilient modulus and initial tangent modulus • OMC and MDD samples only • Interpolated to match the degree of saturation • Sample preparation

  11. Prediction Model

  12. Prediction Model

  13. Conclusions • Strong influence of degree of saturation on Mr • Both density and moisture • Dynamic modulus test • Reasonable estimate from static triaxial (quick shear) test • Coarse soil values are low compared to MEPDG • Irrespective of sample size • Actual measured value or Catalog of values • Coarse base aggregate is stiffer • Resilient Modulus of Fine soil could be estimated from unconfined compression test

  14. Recommendations • Implementation of resilient modulus for MEPDG • A database of resilient modulus values should be developed • Initial tangent modulus from unconfined compression test should be used to estimate resilient modulus for fine soil • Further data collection to update the model

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