Title: Characterization of Granular Base Materials for Design of Flexible Pavements
1Characterization 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
2Introduction
- 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.
3Introduction
- 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
4Test 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)
5Lab 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
6Sample 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
7Testing Apparatus
8Standard 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
9Example from Quick-Drained Triaxial Tests
10Mohrs 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
11Repeated 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
12Load Pulse and Response Pulse
13Stress-Strain Curves for 1 Load Increment
14Permanent Deformation
15Resilient Modulus Changes
16Failure Stress Example for 15 psi
17Crushed Limestone Base Permanent Strain
18Mohrs Circle for Repeated Load Testing
19Summary 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
20Shear Strength Comparison
21Resilient Modulus for Crushed Aggregate and
Limestone Fines
22Sample 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
23CH Clay
24Resilient Modulus for CH Subgrade
25Conclusions
- 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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