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Characterization of Granular Base Materials for Design of Flexible Pavements

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Characterization of Granular Base Materials for Design of Flexible Pavements Lulu Edwards, Walter Barker, Don Alexander US Army Engineer Research and Development Center – PowerPoint PPT presentation

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Title: Characterization of Granular Base Materials for Design of Flexible Pavements


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

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.

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

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)

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

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 2nd, 4th, 6th, and 8th 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
  • 2nd membrane is placed over 1st membrane

7
Testing Apparatus
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

9
Example from Quick-Drained Triaxial Tests
10
Mohrs Circle for Quick-Drained Triaxial Tests
Q-Test Blend of CS Crushed Aggregate and
Limestone Fines
Shear Stress, PSI
30 PSI
15 PSI
5 PSI
Normal Stress, PSI
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

12
Load Pulse and Response Pulse
13
Stress-Strain Curves for 1 Load Increment
14
Permanent Deformation
15
Resilient Modulus Changes
16
Failure Stress Example for 15 psi
17
Crushed Limestone Base Permanent Strain
18
Mohrs Circle for Repeated Load Testing
19
Summary of Results for Granular Materials
Quick-Drained / Standard Quick-Drained / Standard Quick-Drained / Standard Repeated Load Repeated Load Repeated Load
Material Cohesion (psi) Angle of Internal Friction (Deg) Shear Strengtha (psi) Cohesion (psi) Angle of Internal Friction (Deg) Shear Strengtha (psi)
Sand 2 43 11 8 40 16
Crushed Gravel 0 54 14 5 52 18
Crushed Limestone 17 53 30 14 55 28
Blend Crushed Gravel and Sand 2 54 16 0 54 14
Blend Crushed Gravel and Limestone Fines 8 49 20 5 51 17
a Based on an assumed normal stress of 10 psi
20
Shear Strength Comparison
21
Resilient Modulus for Crushed Aggregate and
Limestone Fines
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

23
CH Clay
24
Resilient Modulus for CH Subgrade
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

26
  • Questions?
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