mechanical characterization of aggregates · mechanical properties of aggregates [email protected]...
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Royal Institute of Technology, Stockholm
Tatek Fekadu Yideti
PhD Student in Highway and Railway EngineeringDepartment of Transportation Science
AF2903 Highway Construction and Maintenance
Mechanical Properties of Aggregates
California Bearing ratio (CBR) Value
Basically a penetration test
Piston penetrates soil at constant rate 0.05 in/min
Pressure is recorded
Take the ratio to the bearing capacity of a standard rock
Range: 0 (worst) –100 (best) Type equation here.
CBR= Pressure to cause 0.1” penetration to the samplePressure to cause 0.1” penetration for standard rock
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California Bearing ratio (CBR) Value
California Bearing ratio (CBR) Value
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Stress
Strain
Stress and Strain
LoadStress
Area
DeformationStrain
Original length_
Elastic
Elasto-Plastic
Viscous
Visco-elastic
Types of materials responses
Load
AppliedLoad
Released
Def
orm
atio
n
Time
Recoverable
Unrecoverable
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strain
Elastic Stress – strain behavior
Str
ess
Fully recovered Strain
Loading
Unloading
Triaxial loading cell is used to test the cyclic loading behavior
Cyclic Loading Behavior
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The response of the materials is elasto-plastic
Under cyclic load application the aggregates experience:-
recoverable (resilient) strain
non-recoverable (permanent) strain
The stable resilient behavior obtained after a large number of load
cycles.
The accumulation of permanent strains, which is more complex to
describe
Cyclic Loading Behavior
Deviator Stress
Mean principal stress
Bulk stress
Stresses
1 3dq
p 1 2 3 1 3
2 3
2
3 3
1 32
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strain
Elasto-Plastic Stress – strain behavior
Str
ess
Permanent Strain Unrecovered
Resilient Strain recovered
r
stressM
Resilient _Strain
Loading
Unloading
Important mechanical property for pavement designis the elastic properties of aggregate recognizing certain non-linear characteristics (AASHTO T 307)Defined as applied stress divided by recoverable strain
Deviator stress
Resilient strain
Resilient modulus ≠ Strength
Resilient Modulus (Mr)
dr
r
M
d
r
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k1 and k2 are constants that represent the intercept and slope of resilient modulus against the bulk stress on logarithmic scales.Psi (Pounds per square inch)
K-θ Model for Resilient Modulus (Mr)
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krM k
Mr = 3,2906θ0,367
1
10
1 10 100
Resilient Modulus, M
r*103[psi]
Bulk Stress, θ, [psi]
k2
k1= 3290.6 psi (22.68Mpa)
k2=0.367
k1
The Table below shows the result of resilient modulus tests on an aggregate materials. Determine the nonlinear coefficient k1 and k2.
Example
Confining Deviatoric Recoverable Resilient Bulk
Pressure Stress Strain Modulus Stress
(psi) (psi) (x 10E-3) (x 10E3) θ, (psi)
2 6 0,58
5 15 0,74
10 30 0,95
15 45 1,14
20 50 1,2
10,34
20,27
31,58
39,47
41,67
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30
60
90
110
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Example
k1= 2201,5 psi (15,2 Mpa)
k2=0.639
k1
k2
Mr = 2,2015θ0,639
1
10
100
1 10 100
Resilient Modulus, M
r, *103[psi]
Bulk Stress, θ, [psi]
Deformation of Unbound Aggregate Materials
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Factors that influence the deformation and shear behavior
Particle shape
Particle size
Angularity
Grain size distribution
Surface texture
Packing (i.e. degree of compaction)
Density
Mineralogy
Mechanisms that contribute to deformation
Relative motion of particles due to sliding or rolling
Distortion and crushing of particles
•C•A
•B•C•A
•B
Before Loading After Loading
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When the shear force becomes larger than the shear resistance at the contact:
Relative sliding between particles
The overall deformation is partly from:
Individual particles, and
Relative sliding between particles
Position before loading
Position after loading
Mechanisms that contribute to deformation
Unbound Base layer Deformation
Subgrade
Aggregate
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Unbound Base layer Deformation
Present Approach
Use a high quality aggregate for surface and base layers
Scandinavian countries
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Royal Institute of Technology, Stockholm
Questions?