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1 Characterization of Granular Base Materials for Design of Flexible Pavements Lulu Edwards, Walter Barker, Don Alexander US Army Engineer Research and Development Center Vicksburg, MS 2010 FAA Worldwide Airport Technology Transfer Conference and Exposition Atlantic City, NJ

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2 Introduction Current method used to design flexible pavements was developed by the U.S. Army Corps of Engineers at the start of World War II Due to the increased tire loads and tire pressures of military vehicles, these design procedures have been increasingly challenged, particularly in the use of locally available materials for base and sub-base layers Main structural elements of such pavements are the granular base and sub-base layers Granular materials of increasing strength are used to protect the weaker natural subgrade PROBLEM: Current procedure for characterizing granular materials is based indirectly on strength characterization, relying on gradation and fractured faces.

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3 Introduction Performance of unbound, granular pavement layers Dependent on aggregate properties Poor performance results in premature pavement distresses Current characterization tests were developed empirically Shear strength is most important property that governs unbound pavement layer performance NEED: Performance-based procedures to characterize granular materials and predict the performance of flexible pavements (a direct method) Standard triaxial Repeated-load triaxial tests

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4 Test Section Full-scale test sections constructed to develop and validate flexible pavements criteria Minimum thickness Marginal materials New CBR criteria Test sections constructed with different granular materials in base and subbase Lab results are being investigated to predict performance of test sections (future work)

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5 Lab Testing 5 different granular materials tested in laboratory Sand Crushed stone Crushed aggregate Blend of sand and crushed aggregate Crushed stone fines and crushed aggregate Lab tests conducted: Standard Triaxial Repeated Load Triaxial

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6 Sample Preparation for Granular Materials Water was added to bring sample to optimal moisture content Sample compaction Porous stone with filter paper cover are placed at the bottom of split mold Compact samples in a split mold using vacuum to keep membrane expanded 5.5 lb drop hammer at height of 12 in. Compact in 1.5 in. lifts Measure height of 2 nd, 4 th, 6 th, and 8 th lifts to verify density Top is leveled, with sand if necessary Top filter paper, porous stone, and end cap are placed on sample Vacuum is disconnected and membrane is sealed Height and diameter are measured 2 nd membrane is placed over 1 st membrane

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7 Testing Apparatus

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8 Standard Triaxial Test Protocol 3 samples are tested per material Drained condition at confining stresses of 5, 15, and 30 psi Controlled rate of deformation (strain) mode 1% strain-per-minute Total deformation of 0.85 in. Measurements recorded during testing Cross-head movement LVDT movement Applied load Measurements recorded after testing Water content Dry density

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9 Example from Quick-Drained Triaxial Tests

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10 Normal Stress, PSI Q-Test Blend of CS Crushed Aggregate and Limestone Fines Mohrs Circle for Quick-Drained Triaxial Tests Shear Stress, PSI 30 PSI 15 PSI 5 PSI

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11 Repeated Load Triaxial Test Protocol 3 samples are tested per material Drained condition at confining stresses of 5, 15, and 30 psi Array of load increments applied Load increments estimated with strength from Q test Maximum strength was divided by 5 to determine load increment 1000 loading cycles Load duration is 1 second and no-load duration is 2 seconds Load waveform is offset sine curve Minimum load is 2-4 psi Load levels increase until sample fails Data recorded Time, load, crosshead movement, LVDT movement, chamber pressure, and cycle number Cycles: 1-10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 1000

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12 Load Pulse and Response Pulse

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13 Stress-Strain Curves for 1 Load Increment

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14 Permanent Deformation

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15 Resilient Modulus Changes

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16 Failure Stress Example for 15 psi

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17 Crushed Limestone Base Permanent Strain

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18 Normal Stress, PSI Shear Stress, PSI Mohrs Circle for Crushed Limestone From Repeated Load Triaxial Test Mohrs Circle for Repeated Load Testing

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19 Summary of Results for Granular Materials Quick-Drained / StandardRepeated Load Material Cohesion (psi) Angle of Internal Friction (Deg) Shear Strength a (psi) Cohesion (psi) Angle of Internal Friction (Deg) Shear Strength a (psi) Sand2431184016 Crushed Gravel0541455218 Crushed Limestone175330145528 Blend Crushed Gravel and Sand 2541605414 Blend Crushed Gravel and Limestone Fines 8492055117 a Based on an assumed normal stress of 10 psi

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20 Shear Strength Comparison

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21 Resilient Modulus for Crushed Aggregate and Limestone Fines

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22 Sample Preparation for Subgrade Materials Subgrade samples were taken from test sections using 3 in. diameter and 10 in. length Shelby tube samplers Wrapped with plastic and aluminum foil and dipped in wax for moisture retention Stored in humid room until testing Trimmed to cylinder size of 2.8 in. wide and 5.6 in. high Covered with rubber membrane and placed in triaxial chamber for testing CH subgrade clay tested: 4 CBR 10 CBR 15 CBR

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23 CH Clay

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24 Resilient Modulus for CH Subgrade

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25 Conclusions Standard triaxial test and repeated load triaxial test would be an improvement over Corps of Engineers current procedure for characterizing granular materials Good comparison for cohesion and angle of internal friction values for both standard and repeated load testing Repeated load triaxial test More accurately represents actual loading conditions and thus is an improvement over the standard triaxial test Resilient modulus can also be estimated Materials are stressed to reach permanent deformation to provide better understanding of material behavior Repeated load triaxial test is more complicated to execute than the standard triaxial test

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26 Questions?